1. Energy can be limiting, especially for small animals with high metabolisms, particularly if they rely on ephemeral resources. Some energy-saving strategies, such as torpor, can impair physiological processes. Alternatively, group living can reduce energetic costs through social thermoregulation. This may allow individuals to maintain a high metabolism as well as processes such as gamete production. Although group living is common, its energetic benefits for heterothermic individuals during the season of sperm production have yet to be investigated.
2. We remotely quantified the daily energy expenditures of individual parti-coloured bats (Vespertilio murinus) kept solitarily and in groups during the period of spermatogenesis, using high-resolution heart rate monitoring.
3. Data showed that the energetic benefits of group living are complex. In groups, individual daily energy expenditures were more than 50% lower. Group roosting reduced the cost of thermoregulation during normothermia and allowed for a decrease in the depth, but not the duration, of torpor.
4. Group living may enable bats to buffer unfavorable environmental conditions. Energy saved this way can then be invested into fitness-relevant processes potentially making this a driver of the evolution of male sociality.
