Hormonal Correlates of Divergent Growth Trajectories in Wild Male Anubis (Papio anubis) and Hamadryas (P. hamadryas) Baboons in the Awash River Valley, Ethiopia

Abstract

Comparative investigations of hormone concentration and pattern during ontogeny can offer insight regarding the evolution of growth trajectories. Anubis (Papio anubis) and hamadryas (P. hamadryas) baboons exemplify primate populations at a crucial stage of phylogenetic divergence. Though not reproductively isolated, the species are distinguished by consistent morphological, behavioral, and physiological differences, including trajectories of growth and maturation associated with divergent male reproductive strategies. As a step toward understanding the proximate causes of these differences, we tested several hypotheses regarding the relationship of growth-regulatory hormones and binding proteins (insulin-like growth factor-I, insulin-like growth factor binding protein-3 [IGFBP-3], growth hormone binding protein, and testosterone) to growth in several measurements. We collected samples (N = 559) across 13 field seasons, from 7 different social groups. Samples came from 398 different individuals. We sampled 285 once; 76, twice; 29, three times; 5, four times; and 3, five times. Although ages at peak hormone concentrations were not significantly different, concentrations of all hormones and binding proteins measured, except testosterone, were higher in hamadryas than in anubis. All factors measured correlated positively with growth in both species, and IGFBP-3 and testosterone in particular correlated significantly with growth in all measurements. Overall, our findings suggest a role for the growth hormone/insulin-like growth factor axis in producing distinctive patterns of growth in these species.