Highly context-specific activation of the HPG axis in the dark-eyed junco and implications for the challenge hypothesis

• When and why do males socially modulate testosterone (T)?
• We staged interactions varying in sensory modality, timecourse, and sex of stimuli.
• We found that male juncos largely did not elevate T to staged social interactions.
• But males exposed to a social challenge when re-nesting had highly elevated T.

One of the best studied hormone-behavior interactions is the transient rise in testosterone (T) associated with male–male aggression. However, recent research on songbirds has demonstrated numerous exceptions to this pattern. One species previously thought to elevate T in response to a simulated territorial intrusion is the dark-eyed junco (Junco hyemalis). Here, we show that under most circumstances male juncos do not elevate circulating T or CORT levels in response to social stimuli, despite being physiologically capable of elevating T as indicated by their response to GnRH. The lack of hormonal response was found regardless of the sex of the social stimulus (singing male vs. soliciting female), its sensory modality (song only, song + live lure, song + taxidermic mount), or the timecourse of sampling. Notably, males did elevate T levels when exposed to a simulated territorial intrusion in the days following simulated predation of their chicks. Whether the high T seen in these narrow circumstances represents stage-dependent social modulation of T or re-activation of male reproductive physiology in preparation for re-nesting (i.e. socially independent T modulation) remains to be determined. It is clear, however, that activation of the HPG axis is highly context-specific for male juncos. These results highlight important and unresolved issues regarding the socially mediated component of the challenge hypothesis and how it relates to the evolution of hormone-mediated traits.