Northern fur seals have large, bare flippers which they use to help regulate their body temperatures. Dense fur, a mixture of permanent dense under-fur and longer top hairs that are shed once a year, covers the rest of their body. Northern fur seals can dive up to six hundred feet deep, much deeper than other seals, to hunt for fish like herring, anchovy, and squid. They tend to keep to themselves at sea, but their breeding grounds on land get pretty packed. Most head for the breeding ground by May and babies are born in June. Males, or bulls, are polygamous, which means they mate with more than one female. Substantially larger than females, bulls are also pretty forceful; they set up their own territories and aggressively herd females and drive away other bulls. Northern fur seals have lifespans reaching thirty years.