Sex differences, but no seasonal variations in the hippocampus of food-caching squirrels: A stereological study

Recent studies have described sex differences in the relative size of the h ippocampus that are associated with sex differences in space use in birds a nd short-lived mammals. A correlation between spatial learning and increase d hippocampal volume has also been demonstrated in food-caching animals. Su ch results suggest that sexually dimorphic spatial learning (sex difference s in space use during the breeding season) and seasonal variations in food- caching behavior (spatial memory for cache locations) might correlate with morphological changes in the hippocampus of adult long-lived mammals. We us ed modern stereological techniques to examine the volume and neuron number of the structures forming the hippocampal complex identate gyrus, CA3, and CA1) of wild adult eastern gray squirrels (Sciurus carolinensis) throughout the year. We observed differences in brain size between samples collected at different times of the year (October, January, and June). Our analysis s howed sex differences, but no seasonal variations, in the volume of CA1 str atum oriens and stratum radiatum. There were no sex differences or seasonal variations in the relative volume or the number of neurons of any other la yer of the structures forming the hippocampal complex. These results confir m the existence of sex differences in the structure of the hippocampus; how ever, this sexual dimorphism does not vary seasonally in adulthood and is l ikely to result from developmental processes. These results do not support the hypothesis that seasonal variations in food-caching behavior might corr elate with morphological changes, such as variations in volume or neuron nu mber, in the hippocampal complex of adult long-lived mammals. J. Comp. Neur ol. 425:152-166, 2000. (C) 2000 Wiley-Liss, Inc.