QUANTITATIVE GENETICS OF RELATIVE ORGAN WEIGHT VARIATION IN CAPTIVE BABOONS

Anthropoid phylogeny has been characterized by dramatic increases in r elative brain size. Given the importance of genetic mechanisms in evol ution, quantitative genetic analyses of the biological concomitants of relative brain size variation should greatly augment our understandin g of this phylogenetic phenomenon. In humans the brain is often linked metabolically with three other organs-heart, kidneys, and liver-that together account for most of the total basal metabolic rate. Because t he weights of these four organs are proportional to their individual o rgan metabolic rates, their summed weights have been used by previous researchers as a proxy measure for their composite organ metabolic rat e. We have conducted a quantitative genetic analysis of variation in t he relative weights of these four organs in a population of captive ba boons from the Southwest Foundation for Biomedical Research. These ana lyses were performed on log(e)-transformed organ weights available for 601 animals, 307 of which were assigned to 25 pedigrees containing 2 to 49 members; the remaining 294 animals were treated as independent i ndividuals. Moderate but statistically significant (p < 0.005) heritab ilities were estimated for all four organ weights: h(brain)(2) = 0.409 +/- 0.147, h(heart)(2) = 0.386 +/- 0.184, h(kidneys)(2) = 0.468 +/- 0 .152, and h(liver)(2) = 0.600 +/- 0.160. Significant (p < 0.05) additi ve genetic correlations were estimated between brain and liver weights (rho(G) = 0.568) and between liver and kidney weights (rho(G) = 0.858 ). Significant (p < 0.05) environmental correlations were identified f or heart and kidney weights (rho(E) = 0.551) and for liver and kidney weights (rho(E) = 0.684). Our results clearly demonstrate that the fou r organ weights have substantial heritable components that, because of their similar magnitudes, are probably equally susceptible to selecti on. However, the patterns of intercorrelation revealed by our analyses of the genetic and environmental correlation matrices indicate that, of the four organs composing the proxy measure of organ metabolic rate , only the liver and the kidneys are likely to exhibit correlated resp onses to selection for increased relative brain size such as that obse rved in the anthropoid fossil record.