Energetic and Thermoregulatory Aspects of Clustering Behavior in the Neotropical Bat Noctilio albiventris

Communal roosting has been proposed as a mechanism for facilitating thermoregulation in bats. We examined the effects of cluster size on body temperature Tb and metabolism (measured as O₂ consumption, V̇o2) in Noctilio albiventris, a colonial insectivorous Neotropical bat with an adult body mass of 33-45 g. Measurements were made for group sizes of one, two, three, four, eight, and sixteen individuals at ambient temperatures Ta of 10°, 20°, and 30° C. At 30° C, V̇o2 is minimal (0.016 mL/[g · min]) and is not significantly affected by group size. At 10° and 20° C, V̇o2 is considerably higher than at 30° C, but it declines rapidly as group size increases from one to four. Increasing the group size beyond four individuals has little additional effect on V̇o2. By switching from solitary roosting to a cluster offour or more, individual bats can reduce their V̇o2 from 0.082 to 0.051 mL/(g min) at a Ta of 10°C (a 38% savings) and from 0.048 to 0.025 mL/(g · min) at a Ta of 20° C (a 47% savings). Mean body temperature was lowest for groups of four at all Ta's. At Ta's of 10° and 20° C there was more variance in Tb and a greater tendency for low Tb in groups of four or more than in smaller groups. The minimum Tb in groups of one to three was 32.3° C, but we measured Tb's as low as 27.4° C in larger groups. We conclude that clustering lowers the energy cost of roosting in N. albiventris because (1) heat loss rates are reduced in groups and (2) individuals in groups are more likely than solitary bats to further decrease metabolism by becoming hypothermic. Noctilio albiventris may be selected to avoid hypothermia when solitary because low Tb probably increases their vulnerability to predation (the minimum Tb for flight is 35° C). Presumably, risks of predation are lower for bats roosting in groups.