Meeting Abstract
Brown-headed cowbirds (Molothrus ater; hereafter “cowbirds”) show a reversal of sex-typical space use often seen in mammals. Females search for, revisit, and parasitize host nests and show greater spatial memory than males on some laboratory tasks. Previous research reported a female-biased sex difference in the volume of the hippocampus, a brain region involved in spatial memory. New neurons produced by neurogenesis may also help form new memories and replace old neurons that contain information from the past that cause interference. We tested for sex and seasonal differences in hippocampal volume and neurogenesis of cowbirds and the closely related non-brood-parasitic red-winged blackbird (Agelaius phoeniceus; hereafter “blackbirds”) to determine if there were differences in the hippocampus that reflected space use in the wild. Females had a larger relative hippocampus than males in both species, but hippocampal neurogenesis, measured by doublecortin immunoreactivity (DCX+), was greater in female than in male cowbirds in the absence of any sex difference in blackbirds, supporting the hypothesis of hippocampal specialization in female cowbirds. Cowbirds of both sexes had a larger relative hippocampus and greater hippocampal DCX+ than blackbirds, indicating that sex-specific selection on female cowbirds may have indirectly affected males. Relative hippocampus volume remained stable between breeding and post-breeding conditions for both species. However, DCX+ in cowbirds was greater in post-breeding condition, which suggests old memories are lost through hippocampal reorganization following breeding. Our results support, in part, the hypothesis that the cowbird hippocampus is specialized for brood parasitism.
