Variation in brain volume in nine populations and three taxa of the African striped mouse Rhabdomys

Abstract

Brain size can vary between populations in different environments because of different selection pressures on behaviours, such as learning and memory, related to spatial, social and environmental differences. We investigated the variation in total and broad‐scale regional brain size in the murid rodent genus Rhabdomys from different environments. We assessed taxon‐, population‐ and sex‐level differences in total and regional brain volume in three populations each of three taxa (arid‐occurring Rhabdomys pumilio and mesic‐occurring Rhabdomys dilectus chakae and R. d. dilectus) originating across southern Africa. We μCT‐scanned crania obtained from museums in South Africa and used digital software to create endocasts and extract total endocranium and regional volumes: olfactory bulb, anterior cerebrum, posterior cerebrum, cerebellum volume and petrosal volume. Total endocranial volume scaled with basal skull length and all‐region volumes scaled with total endocranial volume. We found taxon‐, and population‐level differences in total endocranial volume. Relative anterior and posterior cerebrum volume did not differ significantly between taxa or populations, but relative cerebellum volume was larger in arid populations than mesic populations. Relative olfactory bulb volume was larger in mesic R. dilectus than in the R. pumilio, but petrosal lobule volume was larger in R. pumilio populations than in R. dilectus. Males had larger total endocranial volumes than females. Drivers of larger total endocranial volumes in R. pumilio are not immediately clear from our results. Environmental seasonality of food availability, cognitive buffering and locomotion may all correlate with total endocranial volume size, whereas the influence of sociality cannot be excluded. The environment and degree of semi‐arboreality are likely driving variation in cerebellum, olfactory bulb and petrosal lobule volumes.

Counter-gradient variation and the expensive tissue hypothesis explain parallel brain size reductions at high elevation in cricetid and murid rodents

“…In contrast, smaller brains have also been reported in African molerats (Bathyergidae), gophers (Geomyidae), fossorial squirrel-like rodents and South African porcupines and cane rats (Ctenohystrica 42 ) and some mice (mouse-related clade) 25,30,46 . Relative brain size was not found to be associated with social system in African mole-rats 50 , nor in African striped mice (Rhabdomys) 51 , although histological differences in the amygdala and hippocampus regions of the neocortex of different mesic and arid species of striped mice (Rhabdomys) were found to be associated with sociality rather than spatial memory in lab tests 52 . Nevertheless, although some previous studies have examined brain size in association with various factors, few have distinguished the potential genetic and environmental contributions to brain size variation.…”