Image Credit: David Adam Kess, Wikimedia Commons // CC BY SA-3.0

Galapagos sea lions aren’t the most independent children in the animal kingdom. They nurse on their mothers’ milk until they’re two or three years old, and sometimes even longer. Scientists have seen sea lions as old as seven, with pups of their own, still relying on mom for food. 

Clearly, motherhood is an investment for sea lions, but male pups present a special challenge. From birth, they’re larger than females and need extra calories to meet their energy needs. Scientists assumed that mothers simply fed their sons more, but a group of German researchers wasn’t so sure about that. Recent research showed that sea lion pups begin hunting when they’re about a year old and supplement their milk diet with fish, and ecologist Fritz Trillmich thought that male sea lions might take some strain off their moms and get their extra calories on their own by hunting more than female pups. 

After studying groups of sea lions in the Galapagos, though, he found that male pups really are just mama’s boys

Trillmich and his research team at Bielefeld University have been monitoring a colony of sea lions on the tiny island of Caamaño since 2003. For this new study, they tracked more than 100 pups in three age groups (1 year olds, 1.5 year olds, and 2 year olds) that were both nursing and foraging on their own. Glue-on tags that recorded location and depth told them how far the sea lions roamed from the colony, and how often and how deep they dove while looking for food. The scientists also figured out what the pups were eating by analyzing skin samples taken from some of the tracked animals and their mothers and comparing chemical signatures that come from their diets. 

That’s My Daughter in the Water

The results showed that female pups in all three age groups ventured farther out and were more active divers, while the males stuck close to home and rarely went diving. Female pups wandered as far as 18 miles away from their moms, while the males never got much farther than 650 feet. Out on their own, the female pups spent more time in the water, and dove more often than the males. They also had more fish and other solid food in their diets, while the males fed mostly on milk. 

The researchers didn’t see any difference in diving skill between males and females. The males that did go out into the ocean showed they could dive just as deep as the females and spend about as much time underwater. The difference between the sexes, the team says, isn’t one of diving ability, but effort. For some reason, female pups are more self-reliant when it comes to food, while males—contrary to what the researchers expected—mostly rely on mom to feed them even though they could hunt for themselves. 

Beasts of Burden

Providing for a stay-at-home son can cost a sea lion mother dearly, the researchers say. Keeping a male pup supplied with milk requires more energy and could prevent the mother from having additional pups that she couldn’t support. The heavy investment in male pups might pay off if they mate with lots of females and spread their genes around, but the researchers think this is unlikely since males don’t produce that many more offspring than females. 

While it seems that young male sea lions have it easy, their lazy ways can catch up with them in the long run. The researchers think that female pups’ early independence may help them mature faster, giving them more years to breed and have pups on their own, while late-blooming males have to wait longer until they can mate. The extra hunting experience may also make female sea lions more resilient when the going gets tough. During lean times when their mothers have to spend more time hunting or can’t supply as much milk, female pups can fend for themselves, while males are left high and dry and are less likely to survive.