A SAME-SEX STEPPARENT SHORTENS A PREBREEDERS DURATION ON THE NATAL TERRITORY - TESTS OF 2 HYPOTHESES IN FLORIDA SCRUB-JAYS

Prebreeders of the Florida scrub-jay (Aphelocoma coerulescens) are les s likely to be found on their natal territories with a same-sex steppa rent than with parents or an opposite-sex stepparent. We tested two mo dels that had been proposed to account for this sexual asymmetry. The dominance hypothesis states that stepparents perceive same-sex prebree ders as competitors, primarily for a mate, so behave aggressively towa rd them. The pair-formation hypothesis states that prebreeders remain home to pair eventually with the opposite-sex stepparent. Predictions from these two models were tested by analyses from a quarter-century o f records on a study population at Archbold Biological Station and by new behavioral field observations at the Station. Results clearly reje cted the latter and strongly supported the former hypothesis. No predi ction from the pair-formation hypothesis was confirmed: no pairing by a prebreeder of either sex has ever occurred with a stepparent; remain ing home was equally frequent in age-one males with stepmothers and ge netic mothers, and in females with stepfathers and genetic fathers; an d the same results were found in age-two prebreeders. By contrast, the data strongly supported the dominance hypothesis. Field data showed h igher aggression rates by stepfathers to male prebreeders and stepmoth ers to female prebreeders than by parents to their same-sex genetic of fspring. Fewer age-one males remained home with a stepfather than with the genetic father, and the same was found for age-one females with a stepmother and genetic mother; at age two, the effect occurred only i n female prebreeders. The two hypotheses make different predictions ab out prebreeders with two stepparents versus with both parents, thus pr oviding a critical test. The dominance hypothesis correctly predicted decreased duration at home by age-one males and females; data for age- two females were in the direction of predicted difference but not sign ificant, and, as in other tests, no effect was found for age-two males . We propose that age-two male prebreeders remain home despite elevate d aggression from stepfathers because these prebreeders retain the pos sibility of budding from or inheriting the natal territory.