Energetics of burrowing, running, and free-living in the Namib Desert golden mole (Eremitalpa namibensis)

Abstract

The Namib Desert golden mole (Eremitalpa granti namibensis) is a small (c. 20 g), blind, sand-swimming, chrysochlorid insectivore that inhabits the sand dunes of one of the driest and least productive areas of the world. Its food, largely termites, is sparse and occurs in widely distributed patches, and free water is unavailable. The moles forage by running on the surface and burrowing below the sand. We estimated their daily energy expenditure in the field to be 11.8 kJ d^-1 by constructing a ‘distance-energy budget’ based on measurements of tracks in the sand and the energy cost of running (4.2 kJ d^-1), burrowing (3.2 kJ d^-1), and resting (4.4 kJ d^-1). We also measured field metabolic rate (12.5 kJ d^-1) and water turnover (2.3 ml d^-1) independently with doubly-labelled water. The resting metabolic rate (0.5 ml O₂ g^-1 h^-1 at 35 °C) is about a fifth of that predicted for a normal insectivorous mammal, and the daily field energy expenditure and water turnover are about a half. The low daily energy expenditure stems mainly from the low resting metabolic rate, which is associated with low body temperatures and metabolic depression. Moles save more energy during foraging by running on the surface of the sand, rather than burrowing under it. The gross energy cost of sand-swimming (80 J m^-1) is 26 times more expensive than running on the surface (3.0 J m^-1), but is less than a tenth of the energy required by mammals that tunnel through compact soil. Nevertheless, it would be energetically impossible for the moles to obtain enough food by foraging only underground at our study site. The mean track length was 1.4 km, but only 16 m of it was below the surface. There is evidence that the track length and fraction underground depend on food abundance which is influenced by rainfall.