LATITUDINAL VARIATION IN TESTIS SIZE IN 6 SPECIES OF NORTH-AMERICAN SONGBIRDS

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