ORANGUTANS SEXUAL

Orangutans Sexual - research has estimated that, in the wild, infant mortality is low and that the majority of orang utans live for at least 40 years. In captivity, where the potential lifespan is 50 to 60 years, 80% die before 25 years and 90% before 30 years.



In size, the orangutans is the second largest ape and is markedly sexually dimorphic. Males are approximately twice the weight of females (Mackinnon 1979). In the immature animals and the adult females, the hair is always straight, but in the adult males the hair on the shoulders, back and upper arms may become extremely long and wavy and even curly in some individuals (Markham 1990). On average wild females weigh approximately 38 kg and are 1.16m in height. Wild males weigh approximately 86 kg and are 1.40m in height. Some years after maturity, the gular sack of the adult males, which is present in all orang utans from birth, becomes very large and pendulous and large cheek flanges develop. The gular or throat sack plays a role in the production of the male 'Long Call', which is possibly used to attract females as well as advertise territories to rival males. A sagittal crest develops in most adult males, but not in females. Study on Orangutans showed adult female orang utans have a core range of two to three km². (The core area is an area a female orang utan nests in and uses more intensively and to the exclusion of any other female). The adult female's home range is between 5-6km². This home range overlaps with home ranges of other females and provides sufficient food resources for a year-round supply for her and up to two dependent offspring. Because daughters move out gradually into adjacent ranges to their mothers, the neighbouring females in any given area are usually related. Adolescent males leave their mothers’ home range and travel long distances into territories new to them. This results in the adult males in an area being unlikely to be related to the resident females.



In a particular area, adult males are either resident or nomadic. Studies also found resident adult males may move away from their home ranges when all the resident females have young infants (i.e. unable to become pregnant) and there were seasonal food shortages. The adult resident male's home range (at least 10km²) takes in part the home ranges of up to five adult females and occasionally may overlap with home ranges of other resident adult males. Adult resident males do not appear to maintain a core range.
The extreme sexual dimorphism in orang utans suggests a polygynous mating system with inter-male competition for both territory and females. The resident adult male tries to maximise his reproductive potential by following a ranging pattern, which contacts as many females as possible, while excluding other adult males from his home range.
As young males leave the maternal range, they are subject to an increased mortality in unfamiliar territory. Young females are exposed to fewer risks by moving gradually into neighbouring territories. This difference in potential risk reduces the ratio of males to females by the time they reach adulthood. Subadult males will follow adult females to gain access to their food resources, while becoming familiar with the territory and the resident females. They engage in forcible copulation with the adult females as a haphazard attempt at reproduction and a method of establishing dominance. As most copulations are performed by subadult orangutans males concluded that subadult males were the main sires in the population and the less sexually active adult males acted as guardians to the offspring within their home ranges. At the subadult stage females begin to establish a relationship with the resident adult male through periodic consortships. Although subadult orangutans males perform most copulations, adult females enter consortships with adult males for a few days during periods of receptivity. Adult females are usually either suckling an infant or pregnant, in which case they are not ovulating, or they are receptive and consorting with an adult male. Subadult males, therefore, have little chance of impregnating an adult female. In forced copulations, it is unlikely that transmission of semen is achieved, as this requires cooperation from the adult female.



In areas of overlapping home ranges, close kinship of neighbouring female orang utans allows them to tolerate each other while exploiting seasonal food sources, such as in large fruiting trees. It has been observed that in such gatherings of females and their offspring in large fruiting trees, there is never more than one adult male present and the only physical contact is between infants. Usually seasonal food sources are likely to be exploited by consecutive visits of female orang utans and their dependent off spring.
As adult males are unrestricted by dependent young, they are generally able to forage over larger areas than females. Competition for resources between adult males and females is restricted, because of temporal and spatial separation. Males generally forage lower in the tree canopy. The restriction of competition for resources from resident adult males may maximise the survival opportunities of infants the male has sired within the region. In addition the resident male's presence restricts the utilisation of food resources by other adult males.
In periods of scarcity the greater mobility of the adult male allows him to exploit distance resources away from his home ranges, possibly leaving the area entirely. In the opposite case when resources are plentiful, there is often an influx of nomadic males into the area. Adult males eat more animal protein (such as ants and termites), are less selective and spend far more time foraging on the ground than adult females. The exploitation of slightly different ecological niches again reduces competition and effectively reduces the time adult males and females can associate with each other without reducing their foraging efficiency. Adult males may leave their resident ranges for a few years when all the females in their range have infants. This also removes pressure on female resources.

In the wild menarche (first menstruation) for female orang utans usually does not occur until twelve years of age. There appears to be a period of infertility after the onset of menarche and wild females on average do not give birth to their first offspring until fifteen years of age. In captivity female orang utans have been known to enter menarche as early as seven years of age with pregnancy and birth of their first offspring shortly after. Wild female orang utans have an interbirth interval of six to eight years. The period of postpartum amenorrhoea, which lasts for five to seven years appears to correspond to the period of infant suckles. Due to hand-raising of infants in zoos and the practice of housing males and females in the same enclosure, the interbirth interval of captive females is usually reduced.

Copyright - orangutans sexual