High Seasonal Variation of Plasma Testosterone Levels for a Tropical Grassland Bird Resembles Patterns of Temperate Birds.

AbstractTestosterone (T) is a sexual hormone capable of modulating several traits in birds, including aggressiveness and reproductive behavior. Although variation in T-related traits is well-known for temperate zone birds, this variation has not been extensively studied in tropical species. The campo miner (Geositta poeciloptera) is a threatened bird endemic to the grasslands of the South American Cerrado. We investigated the seasonal variation in plasma T levels and associated behavior in the campo miner, addressing the following questions: (1) Does the species exhibit seasonal variation in T profile? (2) Do males have higher plasma T levels than females, irrespective of season? (3) Are males with higher plasma T levels more aggressive than males with lower T levels? (4) Do males' plasma T levels decrease after females lay eggs? We found that T levels are higher during the breeding season than during the nonbreeding season and that males present higher T levels than females throughout the year. Such high T levels are associated with a higher probability to engage in aggressive behavior; however, T levels decline toward the egg-laying date and keep decreasing afterward. Higher T levels before egg laying are apparently related to territorial defense against invaders and extrapair copulations. With the beginning of parental care, T levels decrease, which is in line with previous observations that the species becomes less aggressive after egg laying. This study contributes to the understanding of environmental endocrinology of tropical birds, filling some knowledge gaps about the diverse Neotropical avifauna.