Social thermoregulation and socio-physiological effect in the subterranean Mashona mole-rat (Fukomys darlingi).

A strict underground style of life is one of the greatest challenges for mammals partly due to the high energetic cost of obtaining food by digging through a mechanically resistant substrate. Any energy saving adaptation, for example the effect of social thermoregulation, is thus very important for subterranean mammals. It has also been suggested that social mammals may suffer from "isolation stress" if measured alone, because the presence of other family member(s) may decrease the stress levels and thus their metabolic rates. This phenomenon known as a socio-physiological effect should be conspicuous when the metabolism of huddling individuals is measured within a species΄ thermoneutral zone (TNZ), where no energetic costs for body warming or cooling exist. In our study, we measured the resting metabolic rates of a social species of African mole-rat, the Mashona mole-rat (Fukomys darlingi), in individuals, pairs and groups of three to eight individuals. Measurements were carried out at ambient temperature below the species' TNZ (20 °C) to test the effect of social thermoregulation and at a temperature within the species' TNZ (30 °C) to test the socio-physiological effect. In pairs, the Mashona mole-rat saved 25% of its individual energetic expenses at the temperature below the TNZ. With increasing group size, energetic savings rose up to four animals, but no savings were found in larger groups. At the temperature within the TNZ, mole-rats saved 10% of individual energetic expenses in pairs, but the difference was not significant. Also, no energetic savings were found in larger groups within the TNZ. Our results on thermoregulatory savings in the TNZ are in contrast with extremely high energetic savings found by other authors in different mole-rat species.