Social correlates of androgen levels in a facultatively monogamous ape (Symphalangus syndactylus): a test of the challenge hypothesis

Abstract

The challenge hypothesis posits a correlation between male androgen levels and mating system, male–male aggression in a sexual context, and parental effort. This model has received support across a variety of taxa, including primates. Most primate studies have focused on multi-male societies characterized by relatively high levels of male–male aggression and limited paternal care. To expand this dataset, predictions of the challenge hypothesis were tested in a population of wild siamangs (Symphalangus syndactylus), small apes characterized by intense territoriality, monogamous/polyandrous grouping patterns, and varying amounts of paternal behavior. Behavioral data were collected on 11 study groups (five two-male groups and six one-male groups). Seven hundred thirty-four fecal samples were collected from 18 adult males to quantify concentrations of fecal immunoreactive androgens (fiA) by radioimmunoassay. As predicted by the challenge hypothesis, males involved in the aggressive replacement of a resident male had significantly higher fiA concentrations than control males, while males displaying active parental care had significantly lower fiA concentrations than average. Results showed no association between male androgen concentration and group composition, male rank, or rate of intragroup male–male aggression. These latter findings, seemingly inconsistent with the hypothesis, could be explained by the clear and stable dominance relationships between co-resident males: given the very low rates of physical aggression, a low baseline testosterone is to be expected. Furthermore, the effects of increased intragroup conflict experienced by males in two-male groups might be offset by the higher frequency of intergroup aggression experienced by males in one-male groups.