Reproductive biology

Make Him a Monogamy Junkie

The Monogamy Method

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A number of reproductive features are typical of primates. Male primates are characterized by permanent descent of the testes into a scrotum that is always located behind the root of the penis (postpenial position). Although several other mammal groups exhibit such descent of the testes, primates are unusual in that it occurs very early in life, usually by the time of birth. Female primates are characterized by the absence of a urogenital sinus, which is a shared canal for the urinary and reproductive systems that is primitively present in mammals. In all female primates, the urethra and the vagina have separate external openings. In all primates, placentation is relatively advanced in that involvement of the yolk sac in the circulation of the placenta has been partially or completely eliminated. Relative to maternal body size, primates typically have long pregnancies (gestation periods), and they produce a small number of well-developed (precocial) neonates that are characteristically born with a covering of fur and with their eyes and ears open. Both fetal and postnatal growth are characteristically slow in relation to maternal size, and lactation periods are also relatively long. Sexual maturity is attained late and life spans are correspondingly long relative to body size. In a nutshell, primates are adapted for slow reproductive turnover and intensive, long-term investment in individual offspring.

Another defining feature of primates is that the non-pregnant cycle of females is typically quite long, usually lasting about a month. (The only striking exception is the squirrel monkey, which has a cycle lasting only nine days or so.) Furthermore, ovulation during the female cycle occurs spontaneously and is not induced by the act of mating as in many other mammals. Lasting bonds between individual males and females are generally typical of primates, and the process of bonding may be quite intense and drawn out. However, the frequency and duration of mating show great variation between species. As a rule, mating is seen relatively rarely in monogamous primate species, whereas in multimale species mating may be very frequent, often involving several males for any individual female. One conspicuous feature associated with the female cycle and mating that is found in several Old World monkey species and in chimpanzees is the occurrence of sexual swellings, which reach a peak of size and coloration at about the time of ovulation.

It has often been assumed that primate mating systems are directly related to the patterns found in social groups. For instance, with species living in social groups with a single adult male (monogamous or harem groups), it has been widely assumed that that male is the father of all offspring born in the group. However, in most cases such restricted paternity has not yet been confirmed with genetic tests. Furthermore, there are some harem-living species in which incursions by extra-group males are known to occur quite regularly. This has, for example, been reported for patas monkeys and certain guenons. It has also been widely assumed that in multi-male groups of primates often showing a relatively clear hierarchy among males, paternity is related to male rank. In some cases (e.g., long-tailed macaques and plains baboons), this expectation has been confirmed with genetic tests, but in others (e.g., Barbary and Japanese macaques; hanuman langurs) it has been found that paternity is unrelated to rank.

Intensive parental care is also a hallmark of the primates. In most cases, there is a single offspring, although some prosimian species and marmosets and tamarins typically give birth to two or three infants at a time. All primates have frequent suckling bouts, long lactation periods, and intensive physical contact between the infant(s) and the mother, in some cases because they spend much time together in a nest but usually because the mother carries her infant(s) around with her, clinging to her fur. Incidentally, the characteristic grasping foot of the primates also plays an important role in infant clinging during parental carriage. In many monogamous primate species, the father (sometimes along with other group members) also plays a part in infant carriage. This is seen most conspicuously in certain New World monkeys (marmosets, tamarins, Goeldi's monkey, and owl monkeys), but it is also seen in some monogamous lemurs.

Primates show all possible patterns of breeding over the annual cycle, ranging from year-round breeding with only mild fluctuations right through to strict seasonal breeding, with mating and births restricted to tightly constrained periods of the year. In a few cases, as with the Moholi bushbaby (Galago moholi), there are two mating periods and two birth periods during the year. Primate species living in rainforests with year-round rainfall generally show little seasonal restriction in mating and births, although there are some notable exceptions (e.g. squirrel monkeys, Saimiri). By contrast, primates living in forests characterized by a marked dry season tend to show some seasonal restriction of breeding. Unusually, almost all lemurs on Madagascar show strictly seasonal breeding patterns, regardless of whether they live in rainforests or in dry forests. The only two exceptions seem to be the aye-aye and the gentle lemurs.

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