Hormonal correlates of natal dispersal and rank attainment in wild male baboons

Abstract

In many mammals, maturational milestones such as dispersal and the attainment of adult dominance rank mark stages in the onset of reproductive activity and depend on a coordinated set of hormonal and socio-behavioral changes. Studies that focus on the link between hormones and maturational milestones are uncommon in wild mammals because of the challenges of obtaining adequate sample sizes of maturing animals and of tracking the movements of dispersing animals. We examined two maturational milestones in wild male baboons-adult dominance rank attainment and natal dispersal-and measured their association with variation in glucocorticoids (fGC) and fecal testosterone (fT). We found that rank attainment is associated with an increase in fGC levels but not fT levels: males that have achieved any adult rank have higher fGC than males that have not yet attained an adult rank. This indicates that once males have attained an adult rank they experience greater energetic and/or psychosocial demands than they did prior to attaining this milestone, most likely because of the resulting participation in both agonistic and sexual behaviors that accompany rank attainment. In contrast, natal dispersal does not produce sustained increases in either fGC or fT levels, suggesting that individuals are either well adapted to face the challenges associated with dispersal or that the effects of dispersal on hormone levels are ephemeral for male baboons.

Keywords: Baboons; Dispersal; Glucocorticoids; Rank attainment; Testosterone.

Fig. 1

Average residual values of hormone concentrations as a function of age in male baboons. Residuals were calculated from a model containing season and number of adult males as predictors and male identity, social group, and year as random variables; thus, the pictured trends include the effects of both the maturational milestone in question, and the dominance rank that a male had attained. Each value represents the mean + SE or mean − SE across males of the residuals of log transformed hormone concentrations for a 6-month period. Numbers above the data points represent the numbers of males sampled for each age period. A) Comparison of residual fGC levels for males who had attained rank (i.e., males in the ‘ranked’ plus the ‘both’ category) and males who had not attained rank (males in the ‘neither’ plus the ‘dispersed’ category). B) Comparison of residual fGC levels for males who had attained dispersal (males in the ‘dispersed’ plus the ‘both’ category) and males who had not attained dispersal (males in the ‘neither’ plus the ‘ranked’ category). C) Comparison of residual fT levels for males who had and had not attained rank (categories as in panel A). D) Comparison of residual fT levels for males who had and had not attained dispersal (categories as in panel B). The dashed lines in A and C represent median age at rank attainment (7.45 years). The dashed lines in B and D represent median age at dispersal (7.67 years).