Cost of digging is determined by intrinsic factors rather than by substrate quality in two subterranean rodent species

Searching for food by extensive digging is one of the most important aspects of life of subterranean rodents. We studied the effect of extrinsic (substrate quality) and intrinsic factors (sex and body mass) upon the cost of burrowing, expressed as digging metabolic rate (DMR) in two African mole-rat species (Bathyergidae, Rodentia) with distinct social structures. The sexually dimorphic giant mole-rat (Fukomys mechowii) is a highly social species, whereas the almost monomorphic silvery mole-rat (Heliophobius argenteocinereus) is a solitary bathyergid. Burrowing in F. mechowii was more costly (DMR was greater) than in H. argenteocinereus, but there was no difference in burrowing speed between both species. DMR within a particular species was dependent upon body mass, but independent of sex. Different substrate quality had no effect upon DMR in either species, yet it affected burrowing speed. We conclude that less effective digging in F. mechowii can be compensated by the joint workforce of other family members. Alternatively, H. argenteocinereus, being a more effective digger, can afford a solitary way of life.