Testis size increases with colony size in cliff swallows

By using a sample of over 800 male cliff swallows (Petrochelidonpyrrhonota) that died during a rare climatic event in our Nebraskastudy area in 1996, we investigated how testis size was relatedto body size, age, parasite load, a bird's past colony-sizehistory, and spleen size. Testis volume increased with bodysize. After correcting for body size, testis volume was lowestfor birds age 1 and 2 years but did not vary with age for males3 years old or more. Birds occupying parasite-free (fumigated)colonies had significantly larger testes than did birds at nonfumigatedsites. Testis volume increased significantly with the size ofthe breeding colonies a bird had used in the past. Testis volumehad no relationship with spleen volume after correcting forbody size. The results show within a species that larger testesare favored in more social environments, probably reflectinga response to increased rates of extrapair copulation (and thussperm competition) among cliff swallows in large colonies. Thepresence of ectoparasites, by inflating levels of plasma corticosterone,may in turn reduce testis mass. These data provide no supportfor the hypothesis that large testes, perhaps by producing moretestosterone, are immunosuppressive and thus costly for thatreason.