Biometrical studies upon hominoid teeth: The coefficient of variation, sexual dimorphism and questions of phylogenetic relationship

Abstract

Sexual dimorphism as a function of variation in hominoid tooth metrics has been investigated for four groups of taxa: Recent great apes (two subfamilies), Dryopiths (one subfamily), Ramapiths (one subfamily) and hominids (one family). Gorilla, and to a lesser extent Pan, appear characterized by very high levels of sexual dimorphism and meet several criteria for statistical outliers. Recent great apes are the only group exhibiting consistently high levels of sexual dimorphism. Ramapiths are the only group characterized by low levels of sexual dimorphism and their relative canine length is most similar to Dryopiths. Both Dryopiths and hominids contain taxa with low and intermediate levels of sexual dimorphism. The Gingerich and Shoeninger hypothesis relating coefficients of variation to occlusal complexity is supported. Non-parametric statistics suggest that homogeneity of coefficient of variation profiles over most of the tooth row is characteristic of only the Dryopiths and a composite data set composed of the Dryopith plus Ramapith tooth measurements. Oxnard's model for the multifactorial basis of multiple sexual dimorphisms is also supported. The Dryopith and hominid patterns of sexual dimorphism are similar, an observation that suggests phylogenetic relationship. At the taxonomic level of subfamily or family, sexual dimorphism is a character of cladistic usefulness and possible phylogenetic valence. Assuming that breeding system and sexual dimorphism are functional correlates as many workers suggest, then Ramapithecus sp. China, Sivapithecus indicus and possibly Australopithecus boisei are good candidates for having possessed monogamous breeding/social structures. All Dryopith taxa, S. sivalensis, Sivapithecus sp. China, A. afarensis, Homo habilis and H. erectus emerge as the best candidates for having possessed a polygynous breeding/ social structure. No biometrical affinities of Ramapiths with hominids can be demonstrated and some phylogenetic relationship with Dryopiths is suggested. Kay's interpretation of Ramapith sexual dimorphism and taxonomic affinity is not supported. The lack of control over temporal and geographic range variation is discussed and the loose association of these variables with differences in tooth morphology is noted. The high heritability of tooth size also suggests that assignments of “high” or “low” index values to extinct taxa as a measure that describes evolving clades at discrete points in evolutionary time is appropriate. A preliminary model is sketched out that suggests sexual dimorphism and body size reflect taxa specific genetic responses to a set of loosely linked natural selection pressures (sexual selection, predator defense, foraging challenges, etc.). Sexual dimorphism (morphological and behavioral) and body weight are believed to covary within a large set of variates, all flexibly but inextricably linked. The entire set represents an ever-changing, overall phenotypic response on the Wright landscape to the macro-scale selection pressures of whole biotic communities, both in terms of individual life history and the phylogenesis of clades over evolutionary time. Lande's mathematical model is consistent with some of these suggestions. Body weight is not the fundamental variate from which sexual dimorphism derives; it is one member of a set of functionally reciprocally linked variates that respond to natural selection throughout a species' evolutionary trajectory. Over evolutionary time, changes in the genetic programs occur when molecular level stochastic genetic flux produces an adaptive variant upon pre-established themes of development, both in terms of ontogenic allometries and other variates as well.