Seasonal changes of hippocampus volume in parasitic cowbirds

Clayton, N.S., Reboreda, J.C., & Kacelnik, A. (1997). Seasonal changes of hippocampus volume in parasitic cowbirds. Behavioral Processes, 41, 237-243.

This study compared a set of phylogenetically closely related species with striking differences in natural behavior, offering the opportunity for seeking neural differences in association with these behavioral changes. Notable differences exist in these South American cowbirds of the genus Molothrus related to parasitic nesting. In one species, the birds are especially active during the breeding season, searching and locating potential hosts' nests. In another two species, only females are active during the breeding season in locating potential hosts' nests (sexual dimorphism). A fourth species is non-parasitic and no sexual differences are known with respect to spatial memory use in either season. Results show that the first species shows significantly larger relative hippocampal volume in the breeding season. The sexual dimorphic species show significantly larger relative hippocampal volume in the breeding season, but only in females. The evidence suggests seasonal variation in neuroanatomy associated with specific changes in behavior, namely spatial memory demands, as opposed to purely seasonal fluctuations per se.