Thermal benefits of aggregation in a large marine endotherm: Huddling in California sea lions

Abstract

Huddling is an important behavior for many endotherms, and it is associated with both social interactions and physiological regulation. Sea lions are highly communal pinnipeds that often congregate in large numbers on coastal rookeries. While this behavior serves a social role, it also has the potential to change the microhabitat and thus the local thermal conditions experienced by the animals. However, the thermal consequences of huddling in pinnipeds have yet to be quantified despite a propensity for close proximity in some species. To investigate this, we quantified the huddling behavior of California sea lions, Zalophus californianus, by measuring the proximity of individuals from digital photographs, and determined the thermal microhabitat of huddles using an infrared temperature monitor. All animals were measured on San Nicolas Island (California, USA) for 6 days in winter (Tair=13.2±2.1°C) and 7 days in summer (Tair=21.1±3.4°C). We found that sea lion huddling behavior increased in colder weather, as determined from three indices. First, a larger proportion (up to 97%) of the animals participated in huddles rather than resting alone during the winter season (P=0.010). Second, the number of animals per huddle was larger (reaching 172 animals) during the colder season (P=0.019). Lastly, sea lions participating in this behavior huddled more tightly in cold temperatures (P=0.023). The temperature differential between the animals' skin surface and that of the surrounding substrate was significantly greater (P<0.001) for huddling sea lions (6.0±3.6°C) than for animals resting alone (3.0±2.8°C). Furthermore, this differential was inversely proportional to ambient temperatures. These results are consistent with huddling behavior in California sea lions providing a significant thermal benefit that likely shapes their social behavior on land. © 2014 The Zoological Society of London.