New insights into morphological adaptation in common mole‐rats ( Cryptomys hottentotus hottentotus) along an aridity gradient

Abstract

Morphological adaptation is the change in the form of an organism that benefits the individual in its current habitat. Mole‐rats (family Bathyergidae), despite being subterranean, are impacted by both local and broad‐scale environmental conditions that occur above ground. Common mole‐rats ( Cryptomys hottentotus hottentotus) present an ideal mammalian model system for the study of morphological variation in response to ecology, as this species is found along an aridity gradient and thus can be sampled from geographically non‐overlapping populations of the same species along an environmental longitudinal cline. Using the mass of five internal organs, ten skeletal measurements and 3D morphometric analyses of skulls, we assessed the morphology of wild non‐breeding individuals from five common mole‐rat populations in South Africa. We found that the body mass and mean relative mass of the spleen and kidneys in arid populations was larger, and individuals from arid regions possessed shorter legs and larger inter‐shoulder widths compared to individuals from mesic regions. Additionally, arid populations demonstrated greater skull depth, and shape change of features such as angular processes of the lower jaw than mesic individuals, indicating that these distinct geographic populations show differences corresponding to the aridity gradient, potentially in response to environmental factors such as the variation in food sources found between different habitats, in addition to different soil compositions found in the different regions. Arid populations potentially require a stronger jaw and neck musculature associated with mastication to chew xeric‐adapted plants and to dig through hard soil types, whereas mesic populations excavate through soft, looser soil and may make use of their front limbs to aid the movement of soils when digging. Aridity influences the morphology of this species and could indicate the impact of environmental changes on speciation and mammalian skull morphology.

Abstract

Distinct geographic populations of South African mole‐rats showed morphological variation that matched the predictions of the Aridity Index theory in response to factors such as the variation in food sources found in different biomes, as well as soil composition found in the different regions. Arid populations require a stronger jaw and neck musculature associated with mastication of xeric‐adapted plants, and to dig through the harder and rocky soil types. In contrast, mesic populations excavate through soft, looser soil and may make use of their front limbs to aid the movement of soil when digging. These findings are novel and suggest that ecological factors can influence the morphology within a species and could suggest the impact of environmental changes on speciation and mammalian skull morphology.