Sexual dimorphism has been a major topic of discussion among anthropologists in recent years, particularly debating reasons of occurrence in nonhuman primates. Sexual dimorphism originated from Charles Darwin’s theory on sexual selection, presuming females ‘“choose” their male mates’ (Plavcan, 2001: 33), contingent on sexual characteristics and attractiveness (Stanford et al., 2011). In contrast, sexual dimorphism assumes differences between genders dependent on the ability to protect individuals, attract mates and locate resources. Walter Leutenegger’s theory on allometric relationships between primate body size and breeding systems, describes how the development of physical appearance in nonhuman primates is influenced by the environment and mating systems within a species (Leutenegger and Lubach, 1987). There are various primate mating systems: monogamy; polyandry, and polygyny; polygynandry, and fission-fusion communities. All these mating systems depend heavily upon niche divergence and territoriality. Species living in similar conditions, will become phylogenic (the development of similar traits and behaviours) as ecological conditions are almost identical. The tendency of sexual dimorphism depends on the mating system within a nonhuman primate species and vice versa. Categorised by breeding systems, the relationship between sexual dimorphism — craniofacial structure; tooth size; and body size — and species mating will be analysed.
In monogamous mating systems, individuals interact to become a society. Monogamy occurs when one male and one female mate and produce offspring, becoming what is nowadays called a nuclear family. Resources are evenly distributed, meaning no need for competition, resulting in a low degree of sexual dimorphism. As there is no contest for food, shelter or mates, no need for a surplus in offspring persists, thus monogamous pairs have low differential success (Plavcan, 2001). Co-dominance between genders allows for pairs to unanimously decide offspring quantity beneficial to them and their community. Genera classed as monogamous include: Hylobates, Cheracebus, Lemur, Tarsius, and Perodicticus. Scarce levels of dimorphism are found among the above-mentioned genera, thus craniofacial structure, tooth size and body size do not differ. Nevertheless, as nature is diverse, an exception to this statement exists: Hylobates craniofacial structure of the male is slightly larger than that of the female (Flores and Casinos, 2011). However, unknown to anthropologists is why these minor differences occur. Sexual dimorphism and mating systems depend upon one-another, as the niche divergence around species will declare whether or not competition persists.
A similar result can be seen in the Polyandry mating system, where one female mates with multiple males. Although this system is rare, two genera are classed under this name: Saguinus and Callithrix. This classification begins with a monogamous pair; another male will advance into the territory and mate with the female. As both males will have mated with the female in her estrus state, neither will know the father of the offspring produced, thus both remain with the female (O’Neil, 2000). Here, sexual dimorphism is nonessential, as in small communities resources are in surplus.
Territoriality becomes a greater influencer of sexual dimorphism in polygynous societies, where one male mates with multiple females (Lowie, 1920). This results in higher volumes of primates in one area, meaning availability of resources becomes competitive (Emlen and Oring, 1977). Consequently, females will protect food resources, whilst males — well equipped with physical and mental intelligence — will protect the females and territory. Representative of increased physical ability to protect and defend is the difference in craniofacial structure, tooth size and body size between genders in Papio and Gorilla. The increased crainofacial volume allows for a strengthened ‘head-butt’ in dispute, particularly if the dimorphism influences the skull stratum. Similarly, elongated canines permits more harm inflicted on the opposing primate when defending females and territory (Plavcan and Ruff, 2008). Lastly, body size is a general advantage to the terrestrial primate, as predators will be less likely to attack. Although males show the benefits of sexual dimorphism in this case, one should not assume male domination among polygynous communities (O’Neil, 2000). The stereotype appears in Papio and Gorilla, which are male dominated: Papio live in grassland areas where predators are likely to hunt, yet Gorilla are male dominated despite the fact their body size wards off any competition. Nevertheless, Theropithecus also belong to the relevant mating system, but are known for female control in societies. Therefore, a paradox remains regarding why sexual dimorphism generally benefits the male gendered primates, even if they are not the dominating party.
Contrasting the polygynous paradox of dominance, one can perceive hierarchal structure in the polygynandry communities. Such a society is multimale and multifemale, with a male rule. The dominance hierarchy allows members to be ranked according to gender, supposedly reducing violence and increasing submission. Females will work together to protect and provide nutrition for all, whilst males will regulate social conflicts and territorial protection. This is evident in Papio cynocephalus and Macaca as tooth size dimorphism is greatest, enabling males to harm predators. The craniofacial dimorphism in Macaca is larger in males than females. Saimiri show body size differences between males and females, however temporarily; males are known to gain body fat during mating season, as the largest male will be chosen (Plavcan, 2001). Thus, sexual dimorphism depends on the mating system and the mating system will depend on resources.
Fission-fusion communities present changes in group composition throughout the year. Primates in this category, leave a group to find mates or due to scarce resources. Pan males generally remain the core of the society, whilst females leave for mating (O’Neil, 2000). Conducive to females joining or returning to a male, there must be interest through attraction and intelligence.
Hereby, sexual dimorphism has aided primates such as Homo, Macaca, and Papio. Macaca males have an advantaged craniofacial structure, as well as canine size, compared to females of the same genus. Silmilarly, Homo and Papio reveal sexual dimorphism in terms of body size, where males are larger than females, aiding in regaining female members. However, Homo follow Machiavellian Intelligence, through political scheming and intimidation to gain power therefore, sexual dimorphism of the body is not necessarily required (Whiten and Byrne, 1997).
The relationship between sexual dimorphism and mating systems enables anthropologists to establish theories on the evolution of primates. Ultimately, through analysis of mating systems and their sexual dimorphism one can presume sexual dimorphism is more dependent on mating systems than vice versa. However, several considerations should be made: the solitary mother mating system, nor all traits composing sexual dimorphism were analysed. Furthermore, ‘noncanine tooth size dimorphism is largely a correlate of body mass dimorphism’ (Plavcan, 2001: 29), indicating the larger the primate the bigger its individual physical characteristics. Thus, this may be an unreliable property of sexual dimorphism.
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