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The present research investigates the effects of hybridization between Macaca maurus and M. tonkeana on adult male form and patterns of growth allometry. Comparisons of adult hybrid mean phenotypic values with the adult averages of the parental species indicate a condition of heterosis for cranial vault length and crown-rump length. Negative heterosis is indicated for body mass. Regression parameters describing growth allometry are generated for four craniofacial measurement variables and one body measurement using both least squares and reduced major axis regression. Comparisons of hybrid and parental regression slopes and intercepts using analysis of covariance and t-tests suggest that there is a hybrid pattern of growth allometry characterized by an increase in regression slope values coupled with lower intercept values compared to those of the parental species and the parental averages for most regression parameters. Multivariate analyses of the adult and ontogenetic morphometric data indicate significant differences across species taxa in form and shape during development and adulthood. Our finding of significant differences between hybrids and their parental taxa in growth allometry and craniofacial form and shape during development challenges the assumption often made regarding the reproductive and taxonomic significance of observed ontogenetic divergence between Neandertals and modern humans. We propose that anthropological primatology, with its goal of developing nonhuman primate models for investigating human evolution, can provide a biologically relevant means by which to empirically estimate the taxonomic significance of morphological and ontogenetic divergence observed in the hominid fossil record.

Many female catarrhine primates possess visually conspicuous organs that apparently function to increase the sexual interest of adult male conspecifics around the time the female is ovulating-i.e. sexual swellings. The hypothesized functional benefits for both sexes of these sexual swellings are reviewed (honest signaling; paternity confusion; paternity confidence and paternal investment; protection; incitement of precopulatory male-male competition; and postcopulatory sexual selection), as well as an additional hypothesis that has not yet been applied to this problem (sensory exploitation). Currently available evidence is presented that supports or fails to support each of these hypotheses. Predictions associated with broad groupings of these hypotheses, which could be tested in noninvasive field studies, are then presented. Ecological circumstances are discussed that could have led to differential mating success among female primates, and hence to sexual selection on females and directional evolution of sexual swellings. It is concluded that the available evidence does not support the paternity confidence-paternal investment hypothesis; that the paternity confusion hypothesis lacks empirical support, but could still be viable; and that insufficient data exists at present to rigorously test the other hypotheses. The ecological factors that may have led to differential reproductive success among females as a function of mating frequency or mate choice likewise require further empirical investigation.