Allometry and evolution of tertiary Hominoids

Various aspects of scaling to body size, physiological time and life history parameters are reviewed. The White & Gould (1965) scale factor, significance of allometric residuals, and intraspecific, interspecific and ontogenetic allometry are discussed within a context of an adaptationist paradigm, as well as the Katz (1980) hypothesis that relates the allometric equation to aspects of cellular development. Comments about LSR and RMA methods introduce an algorithm library of 38 empirically derived relationships that describe ECV, BW, neonatal growth, metabolism and life history. Several individual equations are assessed for preditive value against independently obtained data from living primates. Their predictive utility is judged acceptable and therefore of value. This portion of the analysis also provides indirect support for the reasonableness of predictions of other variates that scale to BW and therefore their potential usefulness to evolutionary discussions. Allometric analysis reveals A. zeuxis to be a considerable outlier with respect to ECV and possibly a monogamous folivore with a small home range. P. africanus ECV lies slightly below the allometric prediction. Allometric estimates for P. africanus and S. indicus BW are in accord with preliminary assessments derived from postcranial material. Allometric predictions of Gigantopithecus BW may be too high as judged against such data. The adaptive significance of pongid and hominid body size is discussed within an evolutionary context of sympatry and foraging strategy differentiation. Hominid ECV is considered an allometric outlier and the implications for foraging behavior and sympatry are discussed, particularly with reference to A. boisei and H. erectus and several hypotheses from the realm of theoretical ecology. A recent sociobiological hypothesis ( Zeveloff & Boyce, 1982 ) about neonate altriciality is not supported by allometric data. Realized early hominid lifespan appears to be far below allometric predictions. Pilbeam & Gould's (1974) scaling relationships for the Plio-Pleistocene hominids are confirmed and expanded using a revised data set. A single best-fit data set for all regressions can be identified and the results of the analysis related to the Katz (1980) model. Various lines of evidence suggest that the increase in brain sizc that characterizes the gracile hominid lineage is not a simple consequence of an allometric increase in body size. ECV, BW and embryonic growth rate exhibit significant scaling to time for both pongids and hominids. Time to sexual maturity and lifespan scale with significance only for pongids and the family Hominidae. Total metabolic potential does not scale with significance for either subfamily although typical values differ widely between the two groups. H. erectus appearance was marked by a large rapid increase in total metabolic potential. The White & Gould (1965) scale factor highlights the different evolutionary rates that characterize the two subfamilies. The scaling of hominid ECV to time cannot be described by a bivariate regression model and represents an example of a punctuated pattern of evolutionary change. High evolutionary rates are determined to be a real evolutionary phenomenon. Hominoid taxa whose appearance was accompanied by a large increase in overall adaptive potential can be identified. H. habilis and H. erectus stand out within this group. Evolutionary rates associated with individual variates do not vary within hominid phyletic lines but do vary within pongid ancestor-descendent transitions in that evolution of total metabolic lifetime proceeded at a markedly slow pace. All variate associated evolutionary rates are statistically less for pongids than hominids. A major difference in the (genetic) capacity for evolutionary change between the two groups is thereby identified. The heuristic value of the approach employed in this study to evolutionary discussions of adaptation and natural selection is noted.