DeKoster, Paige (2004)
Potential Causes of Testosterone Increase in Male White-Crowned Sparrows.
Bachelor of Arts, Pomona College, Biology.
Advisor: Rachel Levin.
In vertebrate species the androgen testosterone (T) has been associated with both aggression and sexual behavior. With the onset of spring, male passerines exhibit marked increases in their levels of circulating T. Although past research has attributed this T elevation to the change in day length between winter and spring, androgen concentration is also influenced by social cues. This is demonstrated by the peak in male T levels that coincides with the arrival of females to the breeding territory. Two hypotheses offer explanations for this effect of female presence on male physiology. The female receptivity hypothesis suggests that exposure to sexually receptive females causes an increase in male T-levels. The mate-guarding hypothesis proposes that T levels increase because males must guard their prospective mates from competitive males. In my experiment I sought to determine which hypothesis better accounts for the springtime T elevation in male white-crowned sparrows (Zonotrichia leucophrys gambelii). In order to test between these hypotheses male sparrows were housed with either receptive females (experimental) or non-receptive females (control) and exposed to photostimulatory long days (20L: 4D). A conspecific intruder was then introduced. Blood samples were taken prior to intrusion to assess the influence of female receptivity on male androgen concentration. Two more samples were taken 24 hours and 7 days post-intrusion. Androgen concentrations were determined via radioimmunoassay and compared between and within treatment groups. There was no effect of female receptivity on male androgen levels, suggesting that the female receptivity hypothesis does not fully explain springtime T elevation. Increases in androgen concentration in both control and experimental males following intrusion provide potential support for the mate-guarding hypothesis. Further research of this hypothesis is necessary in order to clarify the effect of aggression on androgen concentration.