Huddling in groups leads to daily energy savings in free-living African Four-Striped Grass Mice, Rhabdomys pumilio

1. Free-living animals make complex decisions associated with optimizing energy and nutrient intake. In environments where ambient temperatures fall below the thermoneutral zone, homeotherms must choose whether or not to forage, how long and what to forage for, and whether or not to perform activities that conserve energy.

2. Huddling in groups has long been thought of as a possible means of conserving energy. Laboratory studies have shown that at low ambient temperatures individuals in groups expend less energy than individuals by themselves. However, studies have yet to demonstrate that thermoregulatory savings can have an impact on the overall daily energy expenditure (DEE) of free-living animals.

3. Here we show that, in the laboratory, African Four-Striped Grass Mice (Rhabdomys pumilio) expend less energy per individual in large groups than smaller groups. We also show that when free-living groups were experimentally reduced to one-half of their original size, DEE and water turnover increased by 19% and 37%, respectively.

4. The magnitudes of the reduction in free-living DEE were comparable with calculated energy savings from the laboratory. One of the reasons why this species may sometimes occur in groups is that energetic benefits can be gained through huddling in habitats in which food and water are scarce.