Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Creagh W. Breuner

Commitee Members

Douglas J. Emlen, Erick Greene, Thomas E. Martin, Hubert G. Schwabl


behavioral polymorphism, corticosterone, corticosterone binding globulin, testosterone, White-throated sparrow, Zonotrichia albicollis


University of Montana


In this body of work, I examine how testosterone (T) physiology mediates the life-history trade-off between mating effort and parental care in the White-throated Sparrow. This species exhibits a behavioral polymorphism that occurs in both sexes. White-striped (WS) morphs are more territorially aggressive, sing more frequently and seek more extra-pair copulations. Tan-striped (TS) morphs provision nestlings more frequently. Thus this species roughly illustrates the trade-off between mating effort and parental care. I examine T physiology on three levels: plasma titres, binding globulins and response to the social environment. I ask whether levels of T correlate with morph-specific behavior and does this relationship change with stage in the nesting cycle. I found that WS males have significantly higher plasma levels of T than TS males. This difference is small, but it persisted through the parental stage of the nesting cycle. This suggests that T may mediate differences in mating effort and parental behavior in males, but is likely not the only factor. Female morphs did not differ in plasma T, thus T does not appear to play a similar role in females. Next I ask how corticosterone binding globulin (CBG) modulates T action. I found that CBG binds over 90% of T and is an important modulator of T action in this species. However CBG capacity did not differ between morphs, nor did morphs differ in baseline levels of corticosterone (CORT, a stress hormone that competes with T for binding sites on CBG.) Therefore interactions with CBG and CORT do not affect T action differently in the morphs, and patterns of free T (T not bound to CBG) mirror patterns of total T. Finally I investigated how T physiology responds to a change in the social environment- the establishment of a dominance relationship. WS males exhibited aggression more frequently and tended to dominate TS males. Levels of total T, CBG, CORT and free T were not predictive of future dominance status. Nor did these measures show persistent changes once the dominance relationship was established. The response of T physiology to the formation of a dominance relationship did not differ between morphs.



© Copyright 2007 Meredith Bettencourt Swett