Primate Size and Sexual Dimorphism

Sexual Dimorphism in Primate Evolution by J. Michael Plavcan and Correlates of Sexual Dimorphism in Primates: Ecological and Size Variables by Walter Leutenegger and James Cheverud are very similar articles for their respective journals, but do come to a major disagreement within their articles. The agreements come when discussing which primates are monogamous and polygynous, why this is the case, and why sexual dimorphism will continue at a forever-growing rate through time. Plavcan disagrees with Leutnegger and Cheverud on one key fundamental issue, saying that the study of sexual dimorphism as it relates to the size of primates, their mating system, and how this came to be.
Similarities
Plavcan, Leutnegger, and Cheverud reached the same conclusion when correlating the size of the primate and the likelihood of that primate being monogamous or polygynous. Quite simply, the bigger the primate, the more likely they are to be polygynous. The three cite a few reasons for this. The first reason is because of sexual selection. Polygynous males often fight for the affection of female primates and the bigger the primate, the more likely they are to succeed. This is why hominoids such as apes are polygynous, whereas smaller primates like Gibbons stay with one mate throughout their lives. The three scientists also do not consider it a coincidence that males are much larger and more advanced in their evolution of dimorphism. Consider the dental work of male and female Macaca Fascicularis. Though the mass of the head is similar in size, (still advantage male) the male canines are much greater, with the length and mass roughly tripling the size of their female counterpart. All three researchers, Plavcan, Leutnegger, and Cheverud believe sexual dimorphism will continue to expand, quite possibly at a more rapid rate. One reason is because of sexual selection and the need to reproduce among males. Another reason is because of the slow growing rate of these bigger primates like the hominoids and their polygynous activity. Simple calculations tell them and anyone not lacking common sense, is that more reproduction breeds more offspring, increasing the population of the next generation, creating more chances of reproduction.
Differences
Although they agree on many of the key issues involving sexual dimorphism among primates, there are some disagreements as well. For example, Leutnegger and Cheverud are firm believers that the size of the primate cannot be contributed to sexual dimorphism, wheres Plavcan does. Leutnegger and Cheverud say that the diet of a primate is also a factor, claiming that frugivorous species are more dimorphic than folivorous species. “This is in accordance with the hypothesis that energetic constraints-which presumably limit the increase in male size may be more restricting in some dietary groups than in others,” states Leutnegger and Cheverud. Plavcan sees the diet of a species collaborate with the mindset of a primate regarding monogamy/polygynous relations. He claims that species with a frugivorous diet do so to build strength and increase their chances for sexual selection.

Conclusion
Overall, Plavcan, Leutnegger, and Cheverud saw eye to eye on many of the key issues involving sexual dimorphism among primates. Yet this does not mean their theories are absolute. All three scientists have had many challengers with Gaulin and Sailer claiming any correlation between size and dimorphism is “completely unrealistic” to Kappeler and Godfrey pointing out the exclusion of strepsirrhine primates due to their misfit in the size determines sexual dimorphism conversation.
From the looks of it, sexual dimorphism is a real occurrence, but the factors and hypotheses remain highly debated amongst researchers. However, if Leutnegger, Cheverud, and Plavcan’s theory on rapid sexual dimorphism holds true, next century’s researchers will have a lot more to analyze and will be more likely to draw a more definitive conclusion.

Work Cited

Leutenegger, W., & Cheverud, J. (n.d.). Correlates of sexual dimorphism in primates: ecological and size variables . (1982). International Journal of larimatology, 3(4), 387-401. Plavcan, J. (n.d.). Sexual dimorphism in primate evolution. (2001). American Journal of Physical Anthropology, 116(33), 26-50.