African striped mice have relatively smaller brains in the food deprived dry season than in the wet season

Abstract

The ability to alter the mass of energetically consumptive organs in response to seasonal variation in nutritional access has been demonstrated in several species from temperate climates, but less so from other climate zones. We predicted that adult striped mice (Rhabdomys pumilio) from the Succulent Karoo semi-desert in South Africa have lower organ masses during the hot summer dry season with low food availability (n = 28) when compared to the food-rich wet season (n = 25) as a possible adaptation to reduced energy expenditure. Food availability in the wet season was more than twice than that of the dry season. Body mass was positively correlated with most organ masses considered, except for the spleen. Mandible length, as a non-plastic measure of body size, was positively correlated with the mass of heart and stomach. Relative to body mass and to mandible length, kidneys and the small intestine were heavier in the wet season than during the dry season in both sexes. Liver masses were greater in females (but smaller in males) during the wet season, possibly due to increased female reproductive investment during this season. Both sexes had relatively heavier brains (by 9.6% on average) during the wet season than during the dry season, which is the first indication of the Dehnel phenomenon in a rodent, in a subtropical climate, and in the southern hemisphere. Future studies will have to test whether this change in brain size is reversible. Having relatively smaller brains during the dry season could be a mechanism to reduce energy consumption. In conclusion, our study indicates that striped mice may save energy during the food restricted dry season by reducing energetically expensive organ masses, including brain mass.