Mating Systems in Primate Species.

Body, Canine, Testes Size and Female Choice Impact on Primate Social Organization and Reproduction.

The process animals find sexual mates and care for their offspring varies from species to species. Each species has undergone an elaborate evolutional process, developing attributes that are best suited to cater for reproduction enhancing its survival. The size and nature of physical attributes in members of a species reflects on its social organization, affecting particularly the mating system of the particular species. In particular, polyandrous mating, which is a mating process where several males compete for the females within a group, causes sperm competition and confrontation since each male strives to dominate breeding in its environment. It is in such situations that physical attributes become essential for a male to establish dominance in the social and reproductive order within a group (Boehm, 1999). This essay explains the relationship between body size, testes size and canine size in monogamous, polygamous (one male, many female) and promiscuous (many males, many females) societies in primates. It also discusses the impact of female choice of mating partners on male reproductive success, also in primate groups.

In a monogamous social system, a female will have only one mating partner. Competition for females occurs when acquiring a new partner. There is thus less sexual dimorphism females have body sizes same as those of the males. Canines in primates are used for self defense and in aggression while competing for mating partners. Since competition for mates is minimal in monogamous societies, both males and females have relatively shorter and weaker canines when compared to promiscuous societies. The relative size of testicles in primates, like in most other mammals, is directly proportional to the quantity of sperm produced and the rate of sperm replenishment between consecutive ejaculations. Reproduction in monogamous societies is not characterized by sperm competition or the need for extra sperm production. Males in such societies therefore have smaller testes as compared to males of the same body weight in a promiscuous society (Harvey  Harcourt, 1984).

In single-male many-female societies, a male acquires dominance on the basis of bodily strength and fighting ability. Such society exhibit well pronounced sexual dimorphism males have larger bodies and long canines to defend their mating group from external competition and to dominate females in mating, foraging and other sexual contexts (Nelson, 2006). Even though reproduction in such societies are not characterized by sperm competition as only one male serves all the females in a group, there is a need to replenish sperm quickly between successive ejaculations so that mating can be sustained. Males therefore have larger testes as compared to males of similar body weight from a monogamous society.
Reproduction in promiscuous groups (many males, many females) is characterized by extreme competition for mating privileges. Body size becomes significant males have to exert authority to get any chance of breeding. Larger males with long canines stand a better chance of mating. In promiscuous societies therefore, sexual dimorphism is well pronounced, with the males bigger in size, strength and canine length. Breeding in such societies is characterized by sperm competition. However, there has not been established a link between testes size and reproductive success. Once intercourse has occurred, genetic data has shown that sperm from all males stand an equal chance of fertilizing the ovum. To prevent the risk of sperm depletion however, males have larger testes size-to-body ratio (Harvey,  Harcourt, 1984).

Originally, sexual competition was thought to preserve the male person within a social group. However, research has shown that female choice of mating partners has a significant impact on male reproductive success (Schrier, 1977). In primate species exhibiting extremity in sexual dimorphism, there is a tendency by receptive females in the mating season to choose the dominant males as mating partners, therefore increasing their rates of male reproductive success. I fact, it is impossible to separate the effect of female choice of a mating partner from other variables that directly impact on reproductive success in primate groups (Boehm, 1999). Competition between males for dominance and female choice of mating partners are the two main determinants of mating and therefore reproductive success in primates, just like in most vertebrates.

Even though the relative importance of female choice in primates is not clearly understood due to difficulty in managing sufficient research subjects in their natural habitats, studies carried out in captive populations have indicated that female primates prefer mates that have a high dominance rating within their society. In promiscuous primate groups where every male can breed, males that are preferred by the females have higher reproductive success than their less dominant counterparts (Schrier, 1977). Female primates, especially in chimpanzees, choose their mates according to rank, group and phase. In chimpanzees and other primate species where females can choose their mating partners strategy and aggression towards less dominant males when they are ready to copulate (Nelson, 2006). However, in other groups like Orangutans and gorillas, female resistance is hardly successful thus female choice is not a significant determinant of male reproductive success. The same applies to species with dominant females or species where the females are co-dominant to their male counterparts.


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