WACKER, DW*; WALKER, BG; COVERDILL, AJ; O’BRIEN, S; WINGFIELD, JC; Univ. of Washington; Univ. of Washington; Univ. of Washington; Univ. of Washington; Univ. of Washington: Plasma testosterone and aggression in breeding sooty fox sparrows, Passerella unalaschcensis

The Challenge Hypothesis states that males may up-regulate plasma testosterone (T) secretion in response to socially unstable situations. This transient up-regulation of T occurs in many male songbirds that display paternal care, especially during the late breeding season when maintaining continually high levels of T can interfere with this care. Whether a bird socially modulates T often correlates with breeding latitude. Studies of multiple subspecies of white-crowned sparrow, Zonotrichia leucophrys, have demonstrated that, during the late breeding season, males of subspecies at low to mid latitudes can transiently up-regulate T, while those at higher latitudes do not. To better understand how this trait arose and whether it is linked with environmental variables, male sooty fox sparrows, Passerella unalaschcensis, representing a closely related genus to Zonotrichia, were challenged by simulated territorial intrusion (STI). The STI mimics an invasion by a male conspecific, thus creating a socially unstable situation. Birds were captured in the early (territory formation) and late (paternal care) breeding season and bled for T at the northernmost extent of their range in South Central Alaska. Male fox sparrows had significantly higher average circulating T levels during early territory formation (8.6 +/- 3.6 ng/ml) than the late breeding season (3.0 +/- 1.0 ng/ml). Consistent with previous data in Z.l.gambelii, a high latitude breeder, male fox sparrows did not up-regulate T in response to STI in the late breeding season. Male fox sparrows were similarly aggressive in early and late breeding stages, but those in the late breeding season spent significantly more time within five meters of the decoy during the STI.