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Advisor(s)
Abstract(s)
The challenge hypothesis proposes that testosterone (T) elevation above what is needed for breeding is
associated with social factors, and males possibly modulate their hormonal response to variations in
population density and sex ratio. We investigated the role of social environment in altering testosterone levels
and aggression in a tropical, seasonally breeding grassquit (Volatinia jacarina). We exposed males to three
social conditions during 1 year: all-males treatment (six males), mixed treatment (three males–three
females), and paired treatment (one male–one female). We quantified aggressiveness among males and T
plasma concentration for each individual in each treatment monthly. We found that more aggressive
interactions occurred in the all-males treatment than in the mixed treatment. The data also revealed that,
coincident with these behavioral changes, the patterns of T variation through time in each treatment were
markedly different. The all-males treatment exhibited an early increase in T concentration, which was
sustained for a lengthy period with two distinctive peaks, and subsequently declined sharply. The mixed
treatment presented an intermediate pattern, with more gradual increase and decrease in T levels. At the
other extreme, the paired treatment presented a later rise in T concentration. We conclude that the more
competitive environment, with higher density of males, caused the early and higher elevation in T level, thus
the presence of competitors may influence the decision of how much a male should invest in reproduction.
We suggest that the male's perception of his social environment ultimately mediates hormonal production
and alters his reproductive strategy.
Description
Keywords
Aggressiveness Challenge hypothesis Group density Sex ratio Testosterone Volatinia jacarina
Citation
Hormones and Behavior, 59, 51-55