Lemur behavior is as variable as lemur morphology. Differences in diet, social systems, activity patterns, locomotion, communication, predator avoidance tactics, breeding systems, and intelligence levels help define lemur taxa and set individual species apart from the rest. Although trends frequently distinguish the smaller, nocturnal lemurs from the larger, diurnal lemurs, there are often exceptions that help exemplify the unique and diverse nature of these Malagasy primates.

Diet

Lemur diets are highly variable and demonstrate a high degree of plasticity, although general trends suggest that the smallest species primarily consume fruit and insects (omnivory), while the larger species are more herbivorous, consuming mostly plant material. As with all primates, hungry lemurs might eat anything that is edible, whether or not the item is one of their preferred foods. For instance, the Ring-tailed Lemur eats insects and small vertebrates when necessary and as a result it is commonly viewed as an opportunistic omnivore. Coquerel’s Giant Mouse Lemur (”Mirza coquereli”) is mostly frugivorous, but will consume insect secretions during the dry season.

A common assumption in mammalogy is that small mammals cannot subsist entirely on plant material and must have a high-calorie diet in order to survive. As a result, it was thought that the diet of tiny primates must be high in protein-containing insects (insectivory). Research has shown, however, that mouse lemurs, the smallest living primates, consume more fruit than insects, contradicting the popular hypothesis.

Plant material makes up the majority of most lemur diets. Members of at least 109 of all known plant families in Madagascar (55%) are exploited by lemurs. Since lemurs are primarily arboreal, most of these exploited species are woody plants, including trees, shrubs, or lianas. Only the Ring-tailed Lemur, the bamboo lemurs (genus ”Hapalemur”), and the Black-and-white Ruffed Lemur (”Varecia variegata”) are known to consume herbs. While Madagascar is rich in fern diversity, these plants are rarely eaten by lemurs. One possible reason for this is that ferns lack flowers, fruits, and seeds—common food items in lemur diets. They also occur close to the ground, while lemurs spend most of their time in the trees. Lastly, ferns have an unpleasant taste due to the high content of tannins in their fronds. Likewise, mangroves appear to be rarely exploited by lemurs due to their high tannin content. Some lemurs appear to have evolved responses against common plant defenses, however, such as tannins and alkaloids. The Golden Bamboo Lemur (”Hapalemur aureus”), for instance, eats giant bamboo, which contains high levels of cyanide. This lemur can consume twelve times the typically lethal dose for most mammals on a daily basis; the physiological mechanisms that protect it from cyanide poisoning are unknown. At the Duke Lemur Center (DLC) in the United States, lemurs that roam the outdoor enclosures have been observed eating poison ivy (”Taxicodendron radicans”), yet have shown no ill effects.

Some lemurs exhibit female philopatry, where females stay within their natal range and the males migrate upon reaching maturity, and in other species both sexes will migrate. In some cases, female philopatry may help explain the evolution of female-bonded multi-male groups, such as those of the Ring-tailed Lemur, Milne-Edwards’ Sifaka (”Propithecus edwardsi”), and the Verreaux’s Sifaka. Their ancestors may have been more solitary, with females that lived in mother-daughter pairs (or dyads). Over time, these dyads may have allied themselves with other neighboring mother-daughter dyads in order to defend more distributed resources in a wide home range. If this is true, then multi-male groups in lemurs may differ fundamentally in their internal structure from those in catarrhine primates (Old World monkeys and apes).

The presence of female social dominance sets lemurs apart from most other primates and mammals; in most primate societies, males are dominant unless females band together to form coalitions that displace them. However, many ”Eulemur” species are exceptions and the Greater Bamboo Lemur (”Prolemur simus”) does not exhibit female dominance. When females are dominant within a group, the way they maintain dominance varies. Ring-tailed Lemur males act submissively with or without signs of female aggression. Male Crowned Lemurs (”Eulemur coronatus”), on the other hand, will only act submissively when females act aggressively towards them. Female aggression is often associated with, but not limited to, feeding.

There have been many hypotheses that have attempted to explain why lemurs exhibit female social dominance while other primates with similar social structures do not, but no consensus has been reached after decades of research. The dominant view in the literature states that female dominance is an advantageous trait given the high costs of reproduction and the scarcity of resources available. Indeed, female dominance has been shown to be linked to increased maternal investment. However, when reproductive costs and extreme seasonality of resources were compared across primates, other primates demonstrated male dominance under conditions that were similar to or more challenging than those faced by lemurs. In 2008, a new hypothesis revised this model using simple game theory. It was argued that when two individuals were equally matched in fighting capacity, the one with the most need would win the conflict since it would have the most to lose. Consequently, the female, with higher resource needs for pregnancy, lactation, and maternal care, was more likely to win in resource conflicts with equally sized males. This, however, assumed monomorphism between sexes. The following year, a new hypothesis was proposed to explain monomorphism, stating that because most female lemurs are only sexually receptive for a day or two each year, males can utilize a more passive form of mate guarding: copulatory plugs, which block the female reproductive tract, preventing other males from successfully mating with her, and thus reducing the need for aggression and the evolutionary drive for sexual dimorphism.

Locomotor behavior in lemurs, both living and extinct, is highly varied and its diversity exceeds that of anthropoids. Locomotor postures and behaviors have included vertical clinging and leaping (including saltatory behavior), seen in indriids and bamboo lemurs; slow (loris-like) arboreal quadrupedal locomotion, once exhibited by ”Mesopropithecus”; fast arboreal quadrupedal locomotion, seen in true lemurs and ruffed lemurs; partially terrestrial quadrupedal locomotion, seen in the Ring-tailed Lemur; highly terrestrial quadrupedal locomotion, once exhibited by monkey lemurs such as ”Hadropithecus”; and sloth-like suspensory locomotion, once exhibited by many of the sloth lemurs, such as ”Palaeopropithecus”. The Lac Alaotra Gentle Lemur (”Hapalemur alaotrensis”) has even been reported to be a good swimmer. Sometimes these locomotor types are lumped together into two main groups of lemurs, the vertical clingers and leapers and the arboreal (and occasionally terrestrial) quadrupeds.

The jumping prowess of the indriids have been well documented and are popular among ecotourists visiting Madagascar. Using their long, powerful back legs, they catapult themselves into the air and land in an upright posture on a nearby tree, with both hands and feet tightly gripping the trunk. Indriids can leap up to 10& m (33& ft) rapidly from tree trunk to tree trunk, an ability referred to as “ricochetal leaping”. Verreaux’s Sifaka (”Propithecus verreauxi”) manages to do this in the spiny forests of southern Madagascar. It is unknown how it avoids impaling its palms on the thorn-covered trunks of large plants such as ”Alluaudia”. When distances between trees are too great, sifakas will descend to the ground and cross distances more than 100& m (330& ft) by standing upright and hopping sideways with the arms held to the side and waving up and down from chest to head height, presumably for balance. This is sometimes described as a “dance-hop”.

Communication

Lemur communication can be transmitted through sound, sight, and smell (olfaction). The Ring-tailed Lemur, for instance, uses complex though highly stereotyped behaviors such as scent-marking and vocalizations. Visual signals are probably the least used by lemurs, since they lack many of the muscles used in common primate facial expressions. Given their poor vision, whole-body postures are probably more noticeable. However, the Ring-tailed Lemur has demonstrated distinct facial expressions including a threat stare, pulled back lips for submission, and pulled back ears along with flared nostrils during scent-marking. This species has also been observed using yawns as threats. Their ringed tails also communicate distance, warn off neighboring troops, and help locate troop members. Sifakas are known to exhibit an open-mouth play face as well as a submissive teeth-baring grimace used in agonistic interactions.

Olfaction is particularly important to lemurs, except for the Indri, which lacks most common lemur scent glands and has a greatly reduced olfactory region in the brain.) Olfaction can communicate information about age, sex, reproductive status, as well as demarcate the boundaries of a territory. It is most useful for communication between animals that rarely encounter each other. Small, nocturnal lemurs mark their territories with urine, while the larger, diurnal species use scent glands located on various parts of their anatomy. The Ring-tailed Lemur engages in “stink fights” by rubbing its tail across scent glands on its wrists, and then flicking its tail at other male opponents. Some lemurs defecate in specific areas, otherwise known as latrine behavior. Although many animals exhibit this behavior, it is a rare trait among primates. Latrine behavior can represent territorial marking and aid in interspecies signaling.

Compared to other mammals, primates in general are very vocal, and lemurs are no exception. Some lemur species have extensive vocal repertoires, including the Ring-tailed Lemur and ruffed lemurs. Some of the most common calls among lemurs are predator alarm calls. Lemurs not only respond to alarm calls of their own species, but also alarm calls of other species and those of non-predatory birds. The Ring-tailed Lemur and a few other species have different calls and reactions to specific types of predators. With mating calls, it has been shown that mouse lemurs that cannot be discerned visually respond more strongly to the calls of their own species, particularly when exposed to the calls of other mouse lemurs that they would encounter normally within their home range. Lemur calls can also be very loud and carry long distances. Ruffed lemurs use several loud calls that can be heard up to 1& km (0.62& mi) away on a clear, calm day. The loudest lemur is the Indri, whose calls can be heard up to 2& km (1.2& mi) or more and thus communicate more effectively the territorial boundaries over its 34 to 40& hectares (0.13 to 0.15& sq mi) home range. Both ruffed lemurs and the Indri exhibit contagious calling, where one individual or group starts a loud call and others within the area join in. The song of the Indri can last 45 seconds to more than 3 minutes and tends to coordinate to form a stable duet comparable to that of gibbons.

Tactile communication (touch) is mostly used by lemurs in the form of grooming, although the Ring-tailed Lemur also clumps together to sleep (in an order determined by rank), reaches out and touches adjacent members, and cuffs other members. Reaching out and touching another individual in this species has been shown to be a submissive behavior, done by younger or submissive animals towards older and more dominant members of the troop. Allogrooming, however, appears to occur more frequently between higher ranking individuals, a shared trait with other primate species. Unlike anthropoid primates, lemur grooming seems to be more intimate and mutual, often directly reciprocated. Anthropoids, on the other hand, use allogrooming to manage agonistic interactions. The Ring-tailed Lemur is known to be very tactile, spending between 5 and 11% of its time grooming.

Predator avoidance

All lemurs experience some predation pressure. Common defenses against predation include the use of alarm calls and predator mobbing, mostly among diurnal lemurs. The leaping abilities of lemurs may have evolved for predator avoidance rather than for travel, according to a study in kinematics. Nocturnal lemurs are difficult to see and track at night and decrease their visibility by foraging alone. They also try to avoid predators by using concealing sleeping locations, such as nests, tree holes, or dense vegetation, and alternating between multiple sleeping locations. Even torpor and hibernation states among cheirogaleids may be partly due to high levels of predation. Infants are protected while foraging by either leaving them in the nest or by stashing them in a hidden location, where the infant remains immobile in the absence of the parent.

Diurnal lemurs are visible during the day, so many live in groups, where the increased number of eyes and ears helps aid in predator detection. Diurnal lemurs use and respond to alarm calls, even those of other lemur species and non-predatory birds. The Ring-tailed Lemur has different calls and reactions to different classes of predators, such as predatory birds, mammals, or snakes. Some lemurs, such as the Indri, use crypsis to camouflage themselves. They are often heard but difficult to see in the trees due to the dappled light, earning them the reputation of being “ghosts of the forest”.

Reproduction

Except for the Aye-aye and the Lac Alaotra Gentle Lemur, lemurs are seasonal breeders with very short mating and birth seasons influenced by the highly seasonal availability of resources in their environment. Mating seasons usually last less than three weeks each year, with the female vagina opening up only during a few hours or days of her most receptive time of estrus. These narrow windows for reproduction and resource availability appear to relate to their short gestation periods, rapid maturation, and low basal metabolic rates, as well as the high energy costs of reproduction for females. This may also relate to the relatively high mortality rate among adult females and the higher proportion of adult males in some lemur populations—both unusual traits among primates. In both the Aye-aye and Lac Alaotra Gentle Lemur, birth (parturition) occurs over a six-month period.

Lemurs time their mating and birth seasons so that all weaning periods are synchronized to match the time of highest food availability. Weaning occurs either before or shortly after the eruption of the first permanent molars in lemurs. Mouse lemurs are able to fit their entire breeding cycle into the wet season, whereas larger lemurs, such as sifakas, must lactate for two months during the dry season. Infant survival in some species, such as Milne-Edwards’ Sifaka, has been shown to be directly impacted by both environmental conditions and the rank, age, and health of the mother. The breeding season is also affected by geographical location. For example, mouse lemurs give birth between September and October in their native habitat in the southern hemisphere, but from May through June in the captive settings in the northern hemisphere.

Adapted from the Wikipedia article Lemur, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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In the 260-day cycle 20 day names pairs with 13 day numbers, totalling a cycle of 260 days. This cycle was used for divination purposes, it foretold lucky and unlucky days. The date of birth was also used to give names to both humans and gods in many Mesoamerican cultures, some cultures used only the calendar name whereas others combined it with a given name. Each day sign was presided over by a god and many had associations with specific natural phenomena.

History

The exact origin of the 260-day count is not known, but there are several theories. One theory is that the calendar came from mathematical operations based on the numbers thirteen and twenty, which were important numbers to the Maya. The numbers multiplied together equal 260. Another theory is that the 260-day period came from the length of human pregnancy. This is close to the average number of days between the ”first missed” menstrual period and birth, unlike Naegele’s rule which is 40 weeks (280 days) between the ”last” menstrual period and birth. It is postulated that midwives originally developed the calendar to predict babies’ expected birth dates.

A third theory comes from understanding of astronomy, geography and paleontology. The mesoamerican calendar probably originated with the Olmecs, and a settlement existed at Izapa, in southeast Chiapas Mexico, before 1200 BCE. There, at a latitude of about 15° N, the Sun passes through zenith twice a year, and there are 260 days between zenithal passages, and gnomons (used generally for observing the path of the Sun and in particular zenithal passages), were found at this and other sites. The sacred almanac may well have been set in motion on August 13, 1359 BCE, in Izapa. Vincent H. Malmström, a geographer who suggested this location and date, outlines his reasons:

(1) Astronomically, it lay at the only latitude in North America where a 260-day interval (the length of the “strange” sacred almanac used throughout the region in pre-Columbian times) can be measured between vertical sun positions — an interval which happens to begin on the 13th of August — the day the peoples of the Mesoamerica believed that the present world was created;

(2) Historically, it was the only site at this latitude which was old enough to have been the cradle of the sacred almanac, which at that time (1973) was thought to date to the 4th or 5th centuries B.C.; and

(3) Geographically, it was the only site along the required parallel of latitude that lay in a tropical lowland ecological niche where such creatures as alligators, monkeys, and iguanas were native — all of which were used as day-names in the sacred almanac.

Malmström also offers strong arguments against both of the former explanations.

A fourth theory is that the calendar is based on the crops. From planting to harvest is approximately 260 days.

Trecenas

The 260-day period was divided into periods of 13 days called in Spanish a ”trecena” (no indigenous word for this period is known). The days of a trecena were usually numbered from 1 to 13. There were some exceptions, such as in the Tlapanec area where they were counted from 2 to 14. The first day of the trecena, and the god who was its patron, ruled the following thirteen days. If the first day of a trecena was auspicious then so were the next twelve days.

Adapted from the Wikipedia article Mesoamerican calendars, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The 260-day calendar spread throughout the Mesoamerican cultural region, and it is regarded as being the oldest and most important of the calendar systems attested in the region, with an origin pre-dating its first appearances in Maya inscriptions. It is uncertain which Mesoamerican culture first developed this calendar. Stelae with the earliest known Long Count dates come from this general area. Some of the oldest calendric inscriptions are from early strata of Zapotec in the Oaxacan highlands at sites such as Monte Albán, dating from mid 1st-millennium BCE. A few earlier-dated inscriptions and artifacts have what appear to be calendric glyphs, such as at San José Mogote and in the Olmec Gulf Coast region. However, either the dating method or the calendric nature of the glyphs are disputed by scholars.

The original purpose of such a calendar, with no obvious relation to any astronomical or geophysical cycle, is not securely known, but there are several theories. One theory is that the calendar came from mathematical operations based on the numbers thirteen and twenty, which were important numbers to the Maya, (Thompson 1950: An Introduction to Maya Hieroglyphic Writing). The number twenty was the basis of the Maya counting system, taken from the total number of human digits. (See Maya numerals). Thirteen symbolized the number of levels in the Upperworld where the gods lived, and is also cited by modern daykeepers as the number of “joints” in the human body (ankles, knees, hips, shoulders, elbows, wrists, and neck). The numbers multiplied together equal 260.

Barbara Tedlock, studied this system in the contemporary K’iche Maya community of the municipality of Momostenango in highland Guatemala. She underwent a formal apprenticeship in calendar divination with a local adept, and was initiated as a diviner in 1976. She says: “The Momostecan calendar embraces both the 260-day cycle and the 365-day solar year, with the four Classic Maya Year-bearers, or Mam, systematically linking the two. The 260-day cycle is conceived as linked firmly to worldly or earthly affairs, mirroring no astronomical period but rather the period of human gestation. Past ethnographic accounts of this cycle contain various conflicting opinions as to what its first day is, but a comparison of the present results and those of previous studies indicates that there is no fixed first day.”

Anthony Aveni seerts, “Once a Maya genius may have recognized that somewhere deep within the calendar system lay the miraculous union, the magical crossing point of a host of time cycles: 9 moons, 13 times 20, a birth cycle, a planting cycle, a Venus cycle, a sun cycle, an eclipse cycle. The number 260 was tailor made for the Maya”. Others have observed that the “Venus Table” in the Dresden Codex, is an accurate ephemeris for predicting Venus positions. Others have also observed a basis for the 260 day cycle in the agricultural cycle of highland Guatemala, which is also about 260 days. There may also be a relation to the average length of time it takes between appearances of the planet Venus as morning or evening star, which is in round numbers 263 days. Aveni notes that “the average duration between successive halves of the eclipse season, at 173 ½ days, fits into the tzolkin in the ratio of 3 to 2.” This may seem contrived, but the Maya did employ the tzolkin to predict positions of Venus and eclipses.

Another theory is that the 260-day period is the length of human pregnancy. This is close to the average number of days between the ”first missed” menstrual period and birth, unlike Naegele’s rule which is 40 weeks (280 days) between the ”last” menstrual period and birth. It is postulated that midwives originally developed the calendar to predict babies’ expected birth dates.

Vincent Malmström identify a correlation between the 260-day cycle and the 260-day gap between zenith and transits of the sun. According to this hypothesis, the 260-day cycle originated in the narrow latitudinal band (14º42′N to 15ºN) in which the sun is vertically overhead about 12-13 August and again 260 days later about 30 April-l May (Malmström identifies the proto-Classic Izapan culture as one suitable candidate at this latitude). This period may have been used for the planting schedule of maize. However, others object to this conception, noting that while the 260-day calendar runs continuously the interval between autumn-spring and spring-autumn positions alternates between 260 and 105 days, and that the earliest-known calendric inscriptions are from considerably further north of this zone. Consequently this theory is not widely supported.

It is of course also possible that the number 260 has multiple sources.

Adapted from the Wikipedia article Tzolk’in, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Human reproduction takes place as internal fertilization by sexual intercourse. During this process, the erect penis of the male is inserted into the female’s vagina until the male ejaculates semen, which contains sperm, into the female’s vagina. The sperm then travels through the vagina and cervix into the uterus or fallopian tubes for fertilization of the ovum. Upon successful fertilization and implantation, gestation of the foetus then occurs within the female’s uterus for approximately nine months, this process is known as pregnancy in humans. Gestation ends with birth, the process of birth is known as labor. Labor consists of the muscles of the uterus contracting, the cervix dilating, and the baby passing out the vagina. Human’s babies and children are nearly helpless and require high levels of parental care for many years. One important type of parental care is the use of the mammary glands in the female breasts to nurse the baby.

Humans have a high level of sexual differentiation. In addition to differences in nearly every reproductive organ, numerous differences typically occur in secondary sexual characteristics.

Male reproductive system

The human male reproductive system is a series of organs located outside of the body and around the pelvic region of a male that contribute towards the reproductive process. The primary direct function of the male reproductive system is to provide the male gamete or spermatozoa for fertilization of the ovum.

The major reproductive organs of the male can be grouped into three categories. The first category is sperm production and storage. Production takes place in the testes which are housed in the temperature regulating scrotum, immature sperm then travel to the epididymis for development and storage. The second category are the ejaculatory fluid producing glands which include the seminal vesicles, prostate, and the vas deferens. The final category are those used for copulation, and deposition of the spermatozoa (sperm) within the male, these include the penis, urethra, vas deferens, and Cowper’s gland.

Major secondary sexual characteristics includes: larger, more muscular stature, deepened voice, facial and body hair, broad shoulders, and development of an adam’s apple. An important sexual hormone of males is androgen, and particularly testosterone.

Female reproductive system

The human female reproductive system is a series of organs primarily located inside of the body and around the pelvic region of a female that contribute towards the reproductive process. The human female reproductive system contains three main parts: the vagina, which acts as the receptacle for the male’s sperm, the uterus, which holds the developing fetus, and the ovaries, which produce the female’s ova. The breasts are also an important reproductive organ during the parenting stage of reproduction.

The vagina meets the outside at the vulva, which also includes the labia, clitoris and urethra; during intercourse this area is lubricated by mucus secreted by the Bartholin’s glands. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes. At certain intervals, typically approximately every 28 days, the ovaries release an ovum, which passes through the fallopian tube into the uterus. The lining of the uterus, called the endometrium, and unfertilized ova are shed each cycle through a process known as menstruation.

Major secondary sexual characteristics include: a smaller stature, a high percentage of body fat, wider hips, development of mammary glands, and enlargement of breasts. Important sexual hormones of females include estrogen and progesterone.

Production of gametes

The production of gametes takes place within the gonads through a process known as gametogenesis. Gametogenesis occurs when certain types of germ cells undergo meiosis to split the normal diploid number of chromosomes in humans (n=46) into haploids cells containing only 23 chromosomes.

In males this process is known as spermatogenesis and takes place only after puberty in the seminiferous tubules of the testes. The immature spermatozoon or sperm are then sent to the epididymis where they gain a tail and motility. Each of the original diploid germs cells or primary spermatocytes forms four functional gametes which is each capable of fertilization.

In females gametogenesis is known as oogenesis which occurs in the ovarian follicles of the ovaries. This process does not produce mature ovum until puberty. In contrast with males, each of the original diploid germ cells or primary oocytes will form only one mature ovum, and three polar bodies which are not capable of fertilization.

It has long been understood that in females, unlike males, all of the primary oocytes ever found in a female will be created prior to birth, and that the final stages of ova production will then not resume until puberty. However, recent scientific data has challenged that hypothesis. This new data indicates that in at least some species of mammal oocytes continue to be replenished in females well after birth.

Development of the reproductive system

The development of the reproductive system and urinary systems are closely tied in the development of the human fetus. Despite the differences between the adult male and female reproductive system, there are a number of homologous structures shared between them due to their common origins within the fetus. Both organ systems are derived from the intermediate mesoderm. The three main fetal precursors of the reproductive organs are the Wolffian duct, Müllerian ducts, and the gonad. Endocrine hormones are a well known and critical controlling factor in the normal differentiation of the reproductive system.

The Wolffian duct forms the epididymis, vas deferns, ductus deferens, ejaculatory duct, and seminal vesicle in the male reproductive system and essentially disappears in the female reproductive system. For the Müllerian Duct this process is reversed as it essentially disappears in the male reproductive system and forms the fallopian tubes, uterus, and vagina in the female system. In both sexes the gonad goes on to form the testes and ovaries, because they are derived from the same undeveloped structure they are considered homologous organs. There are a number of other homologous structures shared between male and female reproductive systems. However, despite the similarity in function of the female fallopian tubes and the male epididymis and vas deferens, they are not homologous but rather analogous structures as they arise from different fetal structures.

Diseases of the human reproductive system

Like all complex organ systems the human reproductive system is affected by many diseases. There are four main categories of reproductive diseases in humans. They are: 1) genetic or congenital abnormalities, 2) cancers, 3) infections which are often sexually transmitted diseases, and 4) functional problems cause by environmental factors, physical damage, psychological issues, autoimmune disorders, or other causes. The best known type of functional problems include sexual dysfunction and infertility which are both broad terms relating to many disorders with many causes.

Specific reproductive diseases are often symptoms of other diseases and disorders, or have multiple, or unknown causes making them difficult to classify. Examples of unclassifiable disorders include Peyronie’s disease in males and endometriosis in females. Many congenial conditions cause reproductive abnormalities but are better known for their other symptoms, these include: Turner syndrome, Klinefelter’s syndrome, Cystic fibrosis, and Bloom syndrome.

It is also known that disruption of the endocrine system by certain chemical adversely affects the development of the reproductive system and can cause vaginal cancer. Many other reproductive diseases have also been link to exposure to synthetic and environmental chemicals. Common chemicals with known links to reproductive disorders include: lead, dioxin, styrene, toluene, and pesticides.

Examples of congenital abnormalities

*Kallmann syndrome – Genetic disorder causing decreased functioning of the sex hormone-producing glands caused by a deficiency of a hormone.

*Cryptorchidism – Absence of one or both testes from the scrotum.

*Androgen insensitivity syndrome – A genetic disorder causing people who are genetically male (i.e. XY chromosome pair) to develop sexually as a female due to an inability to utilize androgen.

*Intersexuality – A person who has genitalia and/or other sexual traits which are not clearly male or female.

Examples of cancers

*Prostate cancer – Cancer of the prostate gland.

*Breast cancer – Cancer of the mammary gland.

*Ovarian cancer – Cancer of the ovary.

*Penile cancer – Cancer of penis.

*Uterine cancer – Cancer of the uterus.

*Testicular cancer – Cancer of the testicles.

*Cervical Cancer – Cancer of the cervix.

Examples of infections

*HIV – Infection by the retrovirus known as human immunodeficiency virus.

*Genital warts – Sexually transmitted infection caused by some sub-types of human papillomavirus (HPV).

*Herpes simplex – Sexually transmitted infection caused by a virus called herpes simplex virus (HSV) type 2

*Gonorrhea – Common sexually transmitted disease caused by the Gram-negative bacterium ”Neisseria gonorrheae”

*Yeast infection – Infection of the vagina by any species of the fungus genus ”Candida”.

*Pelvic inflammatory disease – Painful infection of the female uterus, fallopian tubes, and/or ovaries with associated scar formation and adhesions to nearby tissues and organs.

*Syphilis – Sexually transmitted infection caused by the bacterium ”Treponema pallidum”.

*Pubic lice – Infection of the pubic hair by crab lice, ”Phthirius pubis”.

*Trichomoniasis – Sexually transmitted infection by the single-celled protozoan parasite ”Trichomonas vaginalis”.

Examples of functional problems

*Impotence – The inability of a male to produce or maintain an erection.

*Hypogonadism – A lack of function of the gonads, in regards to either hormones or gamete production.

*Ectopic pregnancy – When a fertilized ovum is implanted in any tissue other than the uterine wall.

*Hypoactive sexual desire disorder – A low level of sexual desire and interest.

*Female sexual arousal disorder – A condition of decreased, insufficient, or absent lubrication in females during sexual activity

*Premature ejaculation – A lack of voluntary control over ejaculation.

Adapted from the Wikipedia article Reproductive system, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Sexual differentiation begins during gestation, when the gonads are formed. General habitus and shape of body and face, as well as sex hormone levels, are similar in prepubertal boys and girls. As puberty progresses and sex hormone levels rise, differences appear, though puberty causes some similar changes in male and female bodies.

Male levels of testosterone directly induce growth of the testicles and penis, and indirectly (via dihydrotestosterone (DHT)) the prostate. Estradiol and other hormones cause breasts to develop in females. However, fetal or neonatal androgens may modulate later breast development by reducing the capacity of breast tissue to respond to later estrogen.

In males, testosterone directly increases size and mass of muscles, vocal cords, and bones, deepening the voice, and changing the shape of the face and skeleton. Converted into DHT in the skin, it accelerates growth of androgen-responsive facial and body hair, but may slow and eventually stop the growth of head hair. Taller stature is largely a result of later puberty and slower epiphyseal fusion.

In females, breasts are a manifestation of higher levels of estrogen; estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus, proliferation of the endometrium, menses, and makes the skin clearer and gives it a pink to red hue (increases pheomelanin, reduces eumelanin).

In humans, secondary sex characteristics include:

* Male

** Growth of body hair, including underarm, abdominal, chest, and pubic hair. Loss of scalp hair androgenic alopecia can also occur

** Greater mass of thigh muscles in front of the femur, rather than behind it as is typical in mature females

** Growth of facial hair

** Enlargement of larynx [Adam's apple] and deepening of voice

** Increased stature; adult males are taller than adult females, on average

** Heavier skull and bone structure

** Increased muscle mass and strength

** Broadening of shoulders and chest; shoulders wider than hips

** Increased secretions of oil and sweat glands, often causing acne and body odor

** Coarsening or rigidity of skin texture, due to less subcutaneous fat

** Higher waist to hip ratio than prepubescent or adult females or prepubescent males, on average

* Female

** Enlargement of breasts and erection of nipples.

** Growth of body hair, most prominently underarm and pubic hair

** Greater development of thigh muscles in back (behind the femur) than in front of it

** Widening of hips; lower waist to hip ratio than adult males, on average

** Increased secretions of oil and sweat glands, often causing acne and body odor

** Upper arms approximately 2& cm longer, on average, for a given height

** Changed distribution in weight and fat; more subcutaneous fat and fat deposits mainly around the buttocks, thighs and hips

Adapted from the Wikipedia article Secondary sex characteristic, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Structure

The basic components of a mature mammary gland are the ”alveoli” (hollow cavities, a few millimetres large) lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as ”lobules”, and each lobule has a ”lactiferous duct” that drains into openings in the nipple. The myoepithelial cells can contract under the stimulation of oxytocin thereby excreting milk secreted from alveolar units into the lobule lumen toward the nipple, where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) mediated “let down reflex” ensues and the mother’s milk is secreted – not sucked from the gland – into the baby’s mouth.

All the milk-secreting tissue leading to a single lactiferous duct is called a “simple mammary gland”; a “complex mammary gland” is all the simple mammary glands serving one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10–20 simple glands. The presence of more than two nipples is known as polythelia and the presence of more than two complex mammary glands as polymastia.

To keep the correct polarized morphology of the lactiferous duct tree requires another essential component – mammary epithelial cells extracellular matrix (ECM), which together with adipocytes, fibroblast, inflammatory cells etc. constitute mammary stroma. Mammary epithelial ECM mainly contains myoepithelial basement membrane and the connective tissue. They not only help to support mammary basic structure, but also serve as a communicating bridge between mammary epithelials and their local and global environment throughout this organ’s development.

Development and hormonal control

Mammary glands develop all along during different growth cycles. They exist in both sexes during embryonic stage, forming only a rudimentary duct tree at birth. In this stage, mammary gland development is systemic hormone independent, but under the regulation of paracrine communication between neighboring epithelial and mesenchymal cells by parathyroid hormone-related protein(PTHrP). This local secreted factor gives rise to a series of outside-in and inside-out positive feedback between these two types of cells, so that mammary bud epithelial cells can get to proliferate and sprout down into the mesenchymal layer until they reach the fat pad to begin the first round of branching. At the same time, the embryonic mesenchymal cells around the epithelial bud get secrecting factors activated by PTHrP, such as BMP4, can transform into a dense, mammary-specific mesenchyme, which later develop into connective tissue with fibrous threads, forming blood vessels and the lymph system. Basement membrane, mainly containing laminin and collagen, formed thereafter by defferentiated myoepithelial cells keeps the polarity of this primary duct tree.

Secondary duct tree development occurs in females in response to circulating ovarian hormones from puberty. Estrogen promotes branching differentiation, whereas in males testosterone inhibits it. A mature duct tree reaching the limit of the fat pad of the mammary gland comes into being by bifurcation of duct terminal end buds (TEB), secondary branches sprouting from primary ducts and proper duct lumen formation. These processes are tightly modulated by components of mammary epithelial ECM interacting with systemic hormones and local secreting factors. However, for each mechanism the epithelial cells’ “niche” can be delicately unique with different membrane receptor profiles and basement membrane thickness from specific branching area to area, so as to regulate cell growth or differentiation sub-locally. Important players include beta-1 integrin, epidermal growth factor receptor (EGFR), laminin-1/5, collagen-IV, matrix metalloproteinase(MMPs), heparan sulfate proteoglycans etc. Elevated circulating level of growth hormone and estrogen get to multipotent cap cells on tip of TEB through a leaky thin layer of basement membrance and promote specific gene expression. Hence cap cells can differentiate into myoepithelial and luminal (duct) epithelial cells, and the increased amount of activated MMPs can degrade surrounding ECM helping duct buds to reach further in the fat pads. Lumen is formed when branching by inner body cells apoptosis for lack of survival signals. On the other hand, basement membrane along the mature mammary ducts is thicker with strong adhesion to epithelial cells via binding to integrin and non-integrin receptors. When side branches develop, it is a much more “pushing-forward” working process including extending through myoepithelial cells, degrading basement membrane and then invading into a periductal layer of fibrous stromal tissue. Degraded basement membrane fragments (laminin-5) roles to lead the way of mammary epithelial cells migration. Whereas, laminin-1 interacts with non-integrin receptor dystroglycan negatively regulates this side branching process in case of cancer. These complex “Yin-yang” balancing crosstalks between mammary ECM and epithelial cells “instruct” healthy mammary gland development until adult.

True secretory alveoli only develop in pregnancy, when rising levels of estrogen and progesterone cause further branching, together with an increase in adipose tissue and a richer blood flow. In gestation, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce more neighboring cells branching. When the lactiferous duct tree is almost ready, “leaves” alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted. Milk secretion (lactation) begins a few days later due to reduction in circulating progesterone and the presence of another important hormone prolactin, which mediates further alveologenesis and milk protein production. Laminin and collagen in myoepithelial basement membrane interacting with beta-1 integrin on epithelial surface again, is essential in this process. Their binding ensures correct placement of prolactin receptors on basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts. Suckling of the baby causes release of hormone oxytocin which stimulates contraction of the myoepithelial cells. In this way of combined control from ECM and systemic hormones, milk secretion can be reciprocally amplified so as to provide enough nutrition for the baby.

After lactation, decreased prolactin level and stop of baby suckling cause mammary involution. All alveoli and secretory duct structure collapse by programmed cell death (apoptosis) and autophagy for lack of growth promoting factors either from the ECM or circulating hormones. At the same time, apoptosis of blood capillary endothelial cells speeds up the regression of lactation ductal beds. Shrinkage of the mammary duct tree and ECM remodeling by various proteinase is under the control of somatostatin and other growth inhibiting hormones and local factors. This big structure change leads loose fat tissue to fill up the empty space thereafter. But a functional lactiferous duct tree can be formed again when a female is pregnant again.

Breast cancer

Tumorigenesis in mammary glands can be induced biochemically by abnormal expression level of circulating hormones or local ECM components, or from a mechanical change in the tension of mammary stroma. Under either of the two circumstances, mammary epithelial cells would grow out of control and eventually result in cancer. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands.

Adapted from the Wikipedia article Mammary gland, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The mandatory subjects that are taken in this exam include:

*Malay language (Bahasa Malaysia)

*English language

*Mathematics

*Science

*Geography

*History

*Living Skills

*Islamic Studies (mandatory for Muslim students, optional for others)

Malay language

The Malay language is a mandatory subject. Before the PMR examination during October, there are oral examinations and listening comprehension examination which contribute marks to the actual PMR examination, as well as a certificate. These examinations are taken three times throughout Form 3, with the best results being selected for the PMR examination. The Malay language examination consists of two papers, that is Paper One, and Paper Two.

In Paper One, 40 multiple choice questions are given to test the student’s comprehension of the written language being tested, and lasts for one hour. Paper One is usually tougher, with results above 30 considered distinctive ones.

Paper Two comprises four sections and it is two hours long. For the first section, the candidates are required to write a summary based on the passage given, which also contains three comprehension questions on the same passage. For the second section, the candidates are expected to write an essay of not more than 120 words based on visual aids (such as graphs, charts, images, multiple images, tables and cartoons) that are provided to the candidates. For the third section, the candidates must write an essay on one of five topics given to them. The composition must contain more than 180 words, and for that reason, this section carries the most number of marks. For the fourth and final section of the second paper, the candidates have to write a description for any one of the three novels studied by them in lower secondary school based on the instructions given. The questions asked differ from year to year.

English language

Similar to the Malay language, the English language usually has an oral examination, testing the students’ proficiency in speaking the language, a listening comprehension examination, testing the students’ ability to comprehend speech in daily situations, an examination to test the student’s composition skills, and finally an examination testing the student’s knowledge in grammar and vocabulary.

;Oral and listening examination

The oral and listening comprehension examination is taken before the PMR examination, which will later contribute marks to the actual PMR examination, as well as a certificate. The oral examination is taken 3 times throughout the year with the best results selected for the PMR examination. The oral and listening comprehension examination are usually taken together. This examination for the English language usually lasts about 10 to 15 minutes per student. The maximum score for this examination is 40. The oral examination is divided into 2 sections. The first section is to interpret an illustration given as thoroughly and detailed as possible, and giving comments about their actions in a formal way and predict the outcome of such a situation, this being graded on a score of 10. It is advised that students do not point to the picture. No names should be given and everything is to be said in present tense. The next section is to give a speech in front of the class. This part of the examination is different for each of the 3 oral examination per year. For the first oral examination, this part of the test requires the student to present an impromptu speech based on a topic given for more than 3 minutes. For the second oral examination, this part of the test requires the student to memorise a passage and present it in front of the class as interesting as possible for about 5 minutes. For final oral examination, this part requires 2 students to strike a conversation in front of the class for about 5 minutes which is relevant to the topic given. The maximum score for this part of the oral test is 10. The final section of the English oral examination requires the student to answer questions spontaneously the examiners asks of them related to the previous 2 sections, which often require their opinion and inference, this being graded on a score of 10.

The listening comprehension examination follows once the oral examination has finished for the particular class. This examination will then test the students’ ability to comprehend the spoken English language in various daily situations. This examination requires the student to answer subjective questions which is based on the information contained in the audio played to the students. This examination provides the final 10 marks.

;Written examination

For Paper One of the English language, students are required to answer 40 multiple choice questions in 1 hour. Questions based on grammar, vocabulary, phrases and idioms are tested. Students are also required to interpret information based on graphical stimuli such as statistical charts, memos, signs, short texts, notices and pictures. A rational cloze passage with a total of 8 questions is provided to the student; the passage tests grammar and vocabulary specifically. There is also a section which tests the student’s knowledge in English literature, such as poems, short stories and novels studied throughout the lower secondary English lessons.

For Paper 2 of the English language, students are required to write a long essay and a summary, as well as to answer a literature component. Section A of Paper 2 tests the student’s ability in functional or situational writing. If a functional writing question is provided, students are required to write an informal or formal letter. If a situational writing question is provided, students are required to write an essay in the form of a narrative or third person drama. Generally, this part of Paper 2 is tough and difficult to score. Section B of Paper 2 requires students to write a summary based on a passage given. The final section of Paper 2 is the literature component, where students are required to write an essay based on their knowledge in the novels studied in Form 2 and Form 3. The novels being tested in the literature component include Robinson Crusoe, Dr Jekyll and Mr Hyde and The Phantom of the Opera. The time limit for this paper is 1 hour and 30 minutes.

Mathematics

The mathematics examination in PMR is divided into two papers, that is Mathematics Paper 1 and Mathematics Paper 2. Paper 1 consists of 40 multiple choice questions and is worth 40 marks. The time limit for this paper is 1 hour and 15 minutes. The Mathematics Paper 1 has faced complaints from students and parents who complain about its very short duration to answer and its extreme toughness. Students usually score lower for Paper 1, with scores above 30 being a distinctive one. The usage of calculator is allowed for this paper.

Mathematics Paper 2 requires open-ended input, and comprises 20 questions in escalating difficulty. This paper is worth 60 marks. Long answer questions in Paper 2 are worth 2 to 6 marks. The time limit for this paper is 1 hour and 45 minutes. The usage of calculator is not allowed for this paper.

For both papers, the questions are usually in the form of:

* Whole numbers

* Real numbers

* Fractions and decimals

* Approximation and estimation

* Number patterns and sequences

* Basic mensuration

* Percentages

* Financial mathematics

* Lines and angles

* Bearing

* Squares, square roots, cubes and cube roots

* Indices

* Surds

* Polygons

* Statistics

* Pythagoras’ theorem

* Perimeter and area

* Volume and surface area

* Ratio and proportion

* Rates

* Coordinates

* Linear, simultaneous and quadratic equations

* Linear inequality

* Algebraic expression, formulae and manipulation

* Plane and solid geometry

* Circles

* Arc length and sector area

* Transformations

* Geometrical constructions and loci in two dimensions

* Scale drawings

* Graphs of functions

* Trigonometry

Science

The science examination in PMR is also divided into 2 papers, that is Science Paper 1 and Science Paper 2. Paper 1 consists of 40 multiple choice questions in escalating difficulty and is worth 40 marks. The time limit for this paper is 1 hour. The Science Paper 1, similar to Mathematics Paper 1, is usually very tough to score above 30. The usage of calculator for this paper is allowed, this is to help the students to answer the questions based on physics.

Science Paper 2, similar to the Mathematics Paper 2, requires open-ended input. This paper consists of 8 to 10 subjective questions. The marks allocated for the questions in Paper 2 vary from 1 mark to 6 marks, each measure proficiency in several units of the science syllabus, with a total of 60 marks. The time limit for this paper is 1 hour and 30 minutes and the usage of calculator is not allowed for this paper. The last 2 questions are usually experimental ones, which requires the student to formulate a hypothesis, determine the variables of the experiment and tabulate the data for the experiment. The marks allocated for this section of Paper 2 are usually more because it requires the student to explain further based on their knowledge in science. The syllabus covers various aspects of chemistry, biology and physics. These distinctions into different fields are not made in the examination format but can be derived based on the different themes:

;Chemistry

* Matter and materials science. Chemical and physical properties. The phases of matter and the changes it undergoes.

* The variety of resources on earth. Chemical elements, compounds and mixtures.

* Electrochemistry.

* Testing for results of biological processes.

* The composition of air. Combustion.

* Water and solution. Acids and bases.

* Silicon compounds and calcium compounds. Reactions of metals with non-metals.

* Pollution and steps to combat pollution.

* Manufactured substances in industries. Chemicals for consumers.

;Biology

* Cellular biology. Unicellular and multicellular organisms.

* Adaptation of life to the environment.

* The evolutionary theory.

* Scientific classification of life.

* The sensory organs.

* Biodiversity and the interdependence among living organisms and the environment.

* Biological production and population growth: recognising reasons for an exponential and logistic function in a graph.

* Animal gestation and plant germination. Life cycles. Photosynthesis.

* Harms and uses of different plants and animals, overall knowledge of role each organism plays in an ecosystem.

* Human growth

* Nutrition. The classes of food and a balanced diet. The human digestive system. Absorption of digested food and reabsorption of water and defaecation. The habits of healthy eating.

* The human anatomy.

* Respiration in humans, animals and plants.

* Blood circulation and transport in humans and plants.

* Support and movement in humans, animals and plants.

* Excretion in humans, animals and plants.

* Asexual reproduction in organisms.

* Sexual reproduction and organs in male and female. The menstrual cycle, fertilization, pregnancy and pre-natal care.

* Sexual intercourse and safe sex. Research in human reproduction and cloning.

* Pollination, flowers and dispersal of fruits. The development of fruit and seeds. Vegetative reproduction in flowering plants.

;Physics

* The scientific method. Physical quantities and their units. The use of measuring tools. The concept of mass and the importance of standard units in measurements.

* Energy. Its forms such as heat, thermodynamics in a system and the conservation of energy.

* Biogeochemical cycles: water cycle, nitrogen cycle, atmosphere, hydrosphere, biosphere.

* Air pressure and its application.

* Dynamics. Forces, work and power.

* Stability.

* Simple machines.

* Reflection and refraction of light. Concave and convex lens. Vision and optical illusions.

* Sound waves.

* Electricity and electrostatics. Ohm’s law. Concept of series and parallel circuits. Current, voltage and resistance.

* Magnetism and electromagnetism.

* The generation of electricity. Electronics. Transformers. Electrical supply and wiring system at home. Fuses and Earth wire.

* Astrophysics. The solar system, stars, galaxies and the universe.

* The history and developments of space exploration and the field of astronomy.

Geography, History and Living Skills

The format of the Geography, History and Living Skills examination in the PMR are the same. It has only 1 paper which consists of 60 multiple choice questions in escalating difficulty. The time limit for these 3 papers are 1 hour and 15 minutes. The questions in the Geography and History papers are very tricky and are not easy to score distinctions.

;Geography

The Geography paper focuses more on human geography rather than physical geography. It also features environmental geography, geomatics and regional geography. The usage of calculator is allowed for this examination. The Geography examination is widely considered as the hardest subject to score “A”. The topics covered in the examination include:

*Basic geography: Map reading, bearing, interpretation of topographical map and other basic techniques in geography.

*Physical geography: Weather and climate, natural vegetation, plate tectonics, weathering, rivers, coasts, climatic, manmade and natural disasters.

*Human geography: Population, settlements, agriculture and aquaculture, natural resource management, industrialisation, tourism, physical and human resources.

;History

For the History paper, it features both national history and international history. However, it focuses more on Malaysia’s road to independence during the British colonial times.

;Living Skills

For the Living Skills paper, the subject is categorized into 4 elective groups where students can choose any one. Then there is the mandatory section where students must take engineering drawing, technology, invention, domestic piping, electronics, electrical engineering, basic economics, home decor and safety, tailoring, horticulture and gardening, telecommunication, cooking, consumerism, and signs. The 4 elective groups are:

*Choice 1: Technical Skills (such as engine, electromechanics, motor and technical drawing)

*Choice 2: Home Economics (such as sewing, baking, catering and fashion)

*Choice 3: Agricultural Science (such as landscape, pets, gardening and plantation)

*Choice 4: Business and Entrepreneurship (such as marketing, entrepreneurship, accounting and commerce)

Students are also required to complete three projects, that is folios, for these 3 subjects in order to receive their PMR slip and certificate. Similar to the Malay and English language examination which requires the students to have their oral and listening comprehension examination, these 3 folios will contribute marks to the actual PMR examination during October. This project is to help the students to score distinctions as these papers are tough.

Optional subjects

Optional subjects are:

*Chinese language

*French language

*Spanish language

*Basic Arab Communication

*Higher Arabic language

*German language

*Japanese language

*Tamil language

*Punjabi language

*Iban language

*Kadazandusun language (From 2009)

Adapted from the Wikipedia article Penilaian Menengah Rendah, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The ”tzolk’in” (in modern Maya orthography; also commonly written ”tzolkin”) is the name commonly employed by Mayanist researchers for the Maya Sacred Round or 260-day calendar. The word ”tzolk’in” is a neologism coined in Yucatec Maya, to mean “count of days” (Coe 1992). The various names of this calendar as used by Precolumbian Maya peoples are still debated by scholars. The Aztec calendar equivalent was called ”Tonalpohualli”, in the Nahuatl language.

The tzolk’in calendar combines twenty day names with the thirteen numbers of the ”trecena” cycle to produce 260 unique days. It is used to determine the time of religious and ceremonial events and for divination. Each successive day is numbered from 1 up to 13 and then starting again at 1. Separately from this, every day is given a name in sequence from a list of 20 day names:

Some systems started the count with 1 Imix’, followed by 2 Ik’, 3 Ak’b'al, etc. up to 13 B’en. The ”trecena” day numbers then start again at 1 while the named-day sequence continues onwards, so the next days in the sequence are 1 Ix, 2 Men, 3 K’ib’, 4 Kab’an, 5 Etz’nab’, 6 Kawak, and 7 Ajaw. With all twenty named days used, these now began to repeat the cycle while the number sequence continues, so the next day after 7 Ajaw is 8 Imix’. The repetition of these interlocking 13- and 20-day cycles therefore takes 260 days to complete (that is, for every possible combination of number/named day to occur once).

Origin of the Tzolk’in

The exact origin of the Tzolk’in is not known, but there are several theories. One theory is that the calendar came from mathematical operations based on the numbers thirteen and twenty, which were important numbers to the Maya. The numbers multiplied together equal 260. Another theory is that the 260-day period came from the length of human pregnancy. This is close to the average number of days between the ”first missed” menstrual period and birth, unlike Naegele’s rule which is 40 weeks (280 days) between the ”last” menstrual period and birth. It is postulated that midwives originally developed the calendar to predict babies’ expected birth dates.

A third theory comes from understanding of astronomy, geography and paleontology. The mesoamerican calendar probably originated with the Olmecs, and a settlement existed at Izapa, in southeast Chiapas Mexico, before 1200 BC. There, at a latitude of about 15° N, the Sun passes through zenith twice a year, and there are 260 days between zenithal passages, and gnomons (used generally for observing the path of the Sun and in particular zenithal passages), were found at this and other sites. The sacred almanac may well have been set in motion on August 13, 1359 BC, in Izapa. Vincent H. Malmström, a geographer who suggested this location and date, outlines his reasons:

(1) Astronomically, it lay at the only latitude in North America where a 260-day interval (the length of the “strange” sacred almanac used throughout the region in pre-Columbian times) can be measured between vertical sun positions–an interval which happens to begin on the 13th of August–the day the peoples of the Mesoamerica believed that the present world was created;

(2) Historically, it was the only site at this latitude which was old enough to have been the cradle of the sacred almanac, which at that time (1973) was thought to date to the 4th or 5th centuries B.C.; and

(3) Geographically, it was the only site along the required parallel of latitude that lay in a tropical lowland ecological niche where such creatures as alligators, monkeys, and iguanas were native–all of which were used as day-names in the sacred almanac.

Malmström also offers strong arguments against both of the former explanations.

A fourth theory is that the calendar is based on the crops. From planting to harvest is approximately 260 days.

Adapted from the Wikipedia article Maya calendar, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Intertidal ecology

Intertidal ecology is the study of intertidal ecosystems, where organisms live between the low and high water lines. At low water, the intertidal is exposed (or ‘emersed’) whereas at high water, the intertidal is underwater (or ‘immersed’). Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as among the different species. The most important interactions may vary according to the type of intertidal community. The broadest classifications are based on substrates — rocky shore or soft bottom.

Intertidal organisms experience a highly variable and often hostile environment, and have adapted to cope with and even exploit these conditions. One easily visible feature is vertical zonation, in which the community divides into distinct horizontal bands of specific species at each elevation above low water. A species’ ability to cope with desiccation determines its upper limit, while competition with other species sets its lower limit.

Humans use intertidal regions for food and recreation. Overexploitation can damage intertidals directly. Other anthropogenic actions such as introducing invasive species and climate change have large negative effects. Marine Protected Areas are one option communities can apply to protect these areas and aid scientific research.

Biological rhythms

The approximately fortnightly tidal cycle has large effects on intertidal organisms. Hence their biological rhythms tend to occur in rough multiples of this period. Many other

Adapted from the Wikipedia article Tide, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The formation of a gender identity is a complex process that starts with conception, but which involves critical growth processes during gestation and learning experiences after birth. There are points of differentiation all along the way, but language and tradition in many societies insist that every individual be categorized as either a man or a woman, although there are societies, such as the Native American identity of a two-spirit, which include multiple gender categories.

When the gender identity of a person makes him/her a woman, but his/her genitals are male, (s)he will likely experience what is called gender dysphoria, i.e., a really deep unhappiness caused by his/her experience of him/herself as a woman and her lack of female genitals and breasts.

Some research has been done that indicates that gender identity is fixed in early childhood and is thereafter static. This research has generally proceeded by asking transsexuals when they first realized that the gender role that society attempted to place upon them did not match the gender identity that they found in themselves and the gender role that they chose to live out. These studies estimate the age at which gender identity is formed at around 2-3. Such research may be problematic if it made no comparable attempt to discover when non-transsexual people became aware of their own gender identities and choice of gender roles.

Some critics question this research, claiming that the studies suffer from a sampling bias. The acquisition of Hormone replacement therapy (female-to-male or male-to-female) and sexual reassign

Adapted from the Wikipedia article Gender identity, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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