Hormonal Mediation of a Unique Behavioral Polymorphism in the White-throated Sparrow (Zonotrichia albicollis)

by Swett, Meredith Bettencourt

Abstract (Summary)
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.
Bibliographical Information:

Advisor:Dr. Creagh W. Breuner; Dr. Douglas J. Emlen; Dr. Erick Greene; Dr. Thomas E. Martin; Dr. Hubert G. Schwabl

School:The University of Montana

School Location:USA - Montana

Source Type:Master's Thesis

Keywords:organismal biology and ecology


Date of Publication:02/06/2008

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