MOORE, Ignacio T; GOYMANN, Wolfgang; SCHEUERLEIN, Alexander; HIRSCHENHAUSER, Katharina; GRAFEN, Alan; WINGFIELD, John C; Virginia Tech; Max Planck Research Centre for Ornithology, Germany; University of Missouri, St Louis; Unidade de Investiga��o em Eco-Etolog�a, Portugal ; University of Oxford, UK; University of Washington, Seattle; ; : Testosterone in tropical birds: How high and why?

Recent studies have suggested that low testosterone levels are a conserved physiological phenomena in male tropical birds, relative to northern temperate species. To test whether this generalization is true, as well as to try and explain species differences, we gathered all currently available plasma testosterone data from tropical birds. We focused our analysis on peak breeding testosterone levels in males, using both phylogenetic and conventional statistics. Male tropical birds had, on average, lower mean peak testosterone levels than northern temperate birds. However, in several tropical species, testosterone levels were well within the range of northern latitude birds. Without controlling for phylogeny, length of the breeding, duration of territoriality, and elevation explained a significant proportion of the variance in testosterone levels. The shorter the breeding season the higher the testosterone levels. Tropical birds that defend a breeding season territory had higher testosterone levels than birds that were year-round territorial or colonial, and testosterone levels were positively correlated with elevation of the breeding site. When controlling for phylogeny, only breeding season length predicted testosterone levels. In conclusion, we propose to refine previous notions of low plasma testosterone levels in tropical birds: short breeding seasons and perhaps environmental conditions at high elevations precipitate conditions under which high testosterone levels occur in male tropical birds.