LYNN, S. E.; COOMBS-HAHN, T. P. ; BREUNER, C. W. ; The College of Wooster; University of California, Davis; University of Texas, Austin: Testosterone, corticosterone, and binding globulins: plasma interactions and aggressive behavior in mountain white-crowned sparrows (Zonotrichia leucophrys oriantha)
Male-male aggression over territory boundaries is common during the breeding season for many birds. In some species, expression of aggressive behavior is accompanied by a rise in testosterone (T) secretion, but in others aggressive behavior is expressed while T levels remain unchanged. Which pattern males exhibit has been hypothesized to relate to certain ecological variables such as the length of the breeding season and/or necessity of paternal care. Recent evidence in birds indicates that corticosteroid binding globulin (CBG), which binds corticosterone (CORT) in circulation, also binds T. Thus, fluctuations in CORT and CBG may result in fluctuations in unbound (�free�) T that is available to bind receptors at target tissues, effectively increasing the biological activity of T without an increase in T secretion. We were interested in determining whether territorial male mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) modulate total and/or free T in response to a simulated territorial intrusion (STI). We conducted all STI�s early in the breeding season (incubation) and compared T, CORT, and CBG levels of males captured immediately following STI with those of males captured passively in potter traps (no STI). All experimental males showed aggressive behavior in response to the STI, but T, CORT, and CBG (and consequently, free T levels) remained unchanged relative to controls. Thus, males of this species apparently did not require an increase in T for activation of territorial aggression in this context. We compare these data to existing data in closely related species, and suggest that the lack of T modulation in response to STI may relate to maximizing reproductive success under specific environmental constraints.
