The effects of season and sex on dentate gyrus size and neurogenesis in a wild rodent, Richardson’s ground squirrel (Urocitellus richardsonii)

• Tested if sex and season affect dentate gyrus size and hippocampal neurogenesis in a wild rodent.
• Dentate gyrus is larger in breeding than non-breeding seasons, but no sex differences were found.
• The number of DCX-positive cells varied with both sex and season.
• Ground squirrels could be a model for studying hippocampal plasticity in the wild.

Sex and reproductive status affect hippocampal neurogenesis and dentate gyrus (DG) size in rodents. Relatively few studies, however, address these two effects simultaneously and even fewer studies address this issue in wild populations. Here, we examined seasonal and sex differences in neurogenesis and DG size in a wild, polygynous and social rodent, Richardson’s ground squirrel (Uriocitellus richardsonii). Based on the behavioral ecology of this species, we predicted that both neurogenesis and DG size would be sexually dimorphic and the degree of dimorphism would be greatest in the breeding season. Using unbiased stereology and doublecortin (DCX) immunohistochemistry, we found that brain volume, DG size and number of DCX cells varied significantly between breeding and non-breeding seasons, but only brain volume and the number of DCX labeled cells differed between the sexes. Both sex and seasonal differences likely reflect circulating hormone levels, but the extent to which these differences relate to space use in this species is unclear. Based on the degree of seasonal differences in neurogenesis and the DG, we suggest that ground squirrels could be considered model species in which to examine hippocampal plasticity in an ecologically valid context.