An interspecific comparison of variance in sex-based developmental markers

Abstract

Sexual dimorphism varies with the degree of male-male competition among primates. Changes in body size of both sexes are well known during ontogeny, but less is known about how osteological developmental markers vary under differing levels of sexual selection. Male-male competition is reflected in a species’ body size sex ratio: humans are reported to have a 1.2 ratio, while rhesus macaques have a 1.6 ratio. We predict greater results for larger bodies and canines in macaque males compared to macaque females and humans as well as greater growth marker variation among macaque males than in these other groups. We documented dental eruption and epiphyseal fusion in 292 macaque skeletal specimens and compared the data to over 25,000 individuals using published human population data. Two-way ANOVAs without replication were used to test whether species had similar variation in dental eruption and fusion time. The two species had significantly different eruption variation (males F=33.71, df=15,1, p <0.0001; females F=119.06, df =15,1, p <0.0001) with macaques more variable than humans. Both species also had different ranges in fusion time (F=7.28, df=13,1, p <0.05) with macaque males more variable than human males. The results support our prediction that macaque males show the greatest variation in these growth markers. Interspecies comparisons of developmental plasticity, such as this study, allow for inferences on how growth variation is affected by sexual selection.