Differences in plasma testosterone levels related to social status are more pronounced during mating than nonmating season in the tropical bat Saccopteryx bilineata (greater sac-winged bat)

Testosterone (T) concentrations often covary strongly with mating activities in temperate-zone vertebrates. Recent studies suggest that males of tropical species experience lower and year-round constant T levels. Here we studied plasma T concentrations in a tropical mammal with a seasonally invariant social system, the greater sac-winged bat ( Saccopteryx bilineata (Temminck, 1838)). Colony composition remains almost stable throughout the year, although mating is restricted to only a few weeks each year. Males follow one of two mating strategies: they either defend a territory with a female group (harem) or live peripheral without females. First, we compared T within the same individuals between seasons, asking whether T levels are higher during mating season. Second, we tested whether T levels are related to social status, and third, whether T correlates with parasite infection because of an immunosuppressive effect. We found high individual T variations. Testosterone was elevated during the mating season only in harem males and was, on average, higher than in peripheral males. We found no correlation between T and parasite infestation. Although reproduction is seasonal in S. bilineata, T concentrations showed no pronounced seasonal pattern, probably because colony members form long-term relationships and live in a year-round stable social system.