LANDYS, M.M.*; DECENCIERE, B.; GOYMANN, W.; SLAGSVOLD, T.; Univ. of Oslo; Univ. of Oslo; Max Planck Institute for Ornithology, Andechs; Univ. of Oslo: Effects of cross-fostering on territorial aggression and plasma testosterone in the blue tit: testing the �challenge hypothesis�
Gonadal testosterone plays an important role in the support of territorial aggression in many breeding male birds. In turn, territorial aggression in socially monogamous species may stimulate the secretion of testosterone, as modelled in the �challenge hypothesis�. To explore the predictions of this hypothesis, we evaluated territorial aggression and plasma testosterone in breeding male blue tits Parus caeruleus using simulated territorial intrusions (STIs). Aggressive responses and testosterone levels were compared between cross-fostered (CF) males (i.e., birds raised by heterospecifics) and controls. CF blue tits strongly imprint upon their foster species but maintain the ability to recognize conspecifics. We found that CF blue tits displayed robust territorial responses toward both heterospecific and conspecific decoys during STIs. In fact, overall aggressive responses of CF birds were significantly greater than those of controls, possibly because CF birds recognize and interact with more competitors during breeding. The challenge hypothesis predicts that strong social stimulation during breeding should be accompanied by increased plasma testosterone. However, we found that baseline testosterone levels were similar between treatment groups, suggesting that testosterone is not responsible for observed behavioral differences. Even more suprisingly, we found that plasma testosterone dramatically decreased after 30 minutes of STI. Such effects appear to be rapid: testosterone decreased even in controls only briefly exposed to a conspecific decoy for purposes of capture (after heterospecific STIs). These results support recent findings in the great tit and suggest that the relationship between aggression and testosterone requires further study.
