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Linda C. Smitha, Samrrah A. Raoufb, 1, Mary Bomberger Brownc, John C. Wingfieldb and Charles R. Brownc, ,
The Richard Stockton College of New Jersey, Pomona, NJ 08240, United States
bDepartment of Zoology, University of Washington, Seattle, WA 98195, United States
cDepartment of Biological Sciences, University of Tulsa, Tulsa, OK 74104, United States
Received 17 May 2004; revised 25 July 2004; accepted 18 August 2004. Available online 26 October 2004.
Abstract
The “challenge hypothesis” states that increases in testosterone levels of male animals during the breeding season are directly related to the extent of intrasexual competition for resources or mates that they experience. Although often tested in territorial species, the challenge hypothesis has not been evaluated for colonial animals that live in groups of different sizes and that thus experience different intensities of intrasexual competition. We measured circulating testosterone levels of male and female cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, where these birds nest in colonies of widely different sizes. Males had significantly higher testosterone levels than females, as expected. For males especially, there was a seasonal rise in testosterone levels early in the nesting cycle, corresponding to the period when birds were establishing nest ownership and egg laying, and then a fall as they switched to parental duties. Testosterone levels varied significantly with colony size; for both sexes, birds in larger colonies had higher levels of testosterone than those in smaller colonies when controlling for date. Age and body mass were not related to testosterone levels. Higher levels of testosterone for birds of both sexes in larger colonies probably reflect greater competition for matings, often extra pair, in the more social nesting situations. The results support the predictions of the challenge hypothesis.
Keywords: Coloniality; Ectoparasitism; Reproduction; Sexual selection; Social behavior
