When they're out of the ocean, California sea lions hang out - technically "haul out" - on the shore, gathering in groups to breed, moult, or just take a breather after a hard week's fishing. But the way they gather changes with the seasons, with sea lions spreading out over summer and sticking together over winter.

A team led by Heather Liwanag took to monitoring the inhabitants of St Nicholas Island - a spot just off the California coast - to find just what makes them huddle. By taking photos throughout the year, and keeping detailed records of how toasty or chilly the beaches were, Liwanag's team were able to see when and how the sea lions huddled on shore.





During the winter, they found many more sea lions huddled together rather than resting alone, and that the huddles were much bigger over the winter than in warmer months. What's more, the sea lions snuggled closer together in the winter than they did throughout the rest of the year.

So what do they get out of these close encounters?

Heat. Snuggling sea lions were capable of keeping their bodies some 6°C warmer than their environment - and were much better at keeping cosy than the solitary seals, who only managed to be a mere 3°C warmer than the land they lounged on.

Throughout the animal kingdom huddling for heat is not uncommon. In fact, such behaviour is key to survival in cold places. During times of trouble and hibernation honeybees huddle around their queen to keep her warm, and when colonies of Emperor penguins care for their eggs over winter, they shuffle in waves to make sure no-one is on the outside of the huddle for too long. It seems sea lions are also making the most of this heat-conserving trick.

Pinnipeds (a group encompassing seals, sea lions and walruses) often congregate in large groups on the shore - and they don't just huddle for heat. There's a whole host of benefits to chilling out with your friends on the rocks - from strengthening family relationships to finding a mate. All in all, it's a very social activity and the heat gained from huddling is a highlight - something to help them out on the shore. So when temperatures drop they cuddle together more.

References

Liwanag, H. E. M., Oraze, J., Costa, D. P. and Williams, T. M., Thermal benefits of aggregation in a large marine endotherm: huddling in California sea lions. Journal of Zoology (2014)

Encyclopedia of Life: Zalophus californianus, California Sea Lion (accessed April 2014)

Zitterbart, D.P., Wienecke, B., Butler, J.P. and Fabry, B., Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle. PLoS ONE 6(6): e20260. (2011)

Images

1) Snuggling sea lions. Aww. Credit: Teddy Llovet via Flickr

2) It's pretty uncommon for sea lions to call a pier home, but after 24 happy years San Francisco's sea lions are famous for their residence on Pier 39. Here they are having a natter. Credit: Brandi Korte via Flickr