Abstract
Six songbird species were studied to determine how testis size varies in relation to latitude, using data from breeding areas at latitudes 7°N to 69°N. Three of the species (Carpodacus mexicanus, Vireo olivaceus, Geothylpis trichas) exhibited a significant positive relationship, one (Passerculus sandwichensis) a significant negative relationship, and two (Agelaius phoeniceus, Spizella passerina) no significant association between relative testis mass and latitude. These results suggest that sperm competition may also vary latitudinally, likely in response to geographic variation in breeding season length, extrapair mating intensity, and social mating system.
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