Don't let their hulking mass fool you: Male sea lions are actually mama's boys.

In the first couple of years after birth, sea lion sons seem to be more reliant on their mothers—consuming more milk and sticking closer to home—than sea lion daughters are, according to a study on Galápagos sea lions published in the December issue of the journal Animal Behaviour.

The young males venture out to sea on occasion, but their female counterparts dive for their own food much more often.

The curious thing is, it's not like the young males aren't capable of diving. As one-year-olds, males can dive to the same depth as females (33 feet, or 10 meters, on a typical dive).

It's also not like their mother's milk is always on hand. Sea lion moms frequently leave their growing offspring for days at a time to find food at sea. (Watch a video of a Galápagos sea lion giving birth.)

And yet, despite all this, for some reason sons are far less likely than daughters to take to the sea and seek out their own food.

"We always saw the [young] males around the colony surfing in tide pools, pulling the tails of marine iguanas, resting, sleeping," said Paolo Piedrahita, a Ph.D. student at Bielefeld University in Germany and the lead author of the study.

"It's amazing. You can see an animal—40 kilograms [88 pounds]—just resting, waiting for mom."

Daughters Go Diving

Piedrahita and his colleagues discovered this by tracking the movements of sea lions on and around a tiny island called Caamaño, which lies at the center of the Galápagos Islands (map). (Watch a National Geographic video of the Galápagos.)

They glued special recording devices onto the backs of 93 juvenile sea lions that ranged in age from 1 to 2 years old. For one to three weeks, the devices collected detailed information about the location of the animals in three-dimensional space and whether or not they were wet (at sea) or dry (on land). (See pictures of animal mothers and babies.)

This provided an unprecedented glimpse into the behavior of Galápagos sea lions during a critical period of development, at a time when "they are still drinking milk but they start foraging themselves," said Piedrahita.

The scientists found that females won the award for the most active divers, not just overall but in every age category (1-year-olds, 1.5-year-olds, and 2-year-olds).

The most striking disparity between genders was seen in 1.5-year-olds: Females dove to a depth of at least 16 feet (5 meters) 52 times per day, on average—more than seven times the rate for males in the same age category (seven dives per day). Many of the males (14 of 16 individuals) in that age category didn't even make any attempts at diving.

The females also ventured farther afield, sometimes traveling upwards of 19 miles (30 kilometers) out to sea. In comparison, males were never found more than 300 yards (274 meters) from their home colony. (See National Geographic's photos of animals on the Galápagos Islands.)

Sons More of a Burden?

All this diving activity wasn't just for the sake of diving practice either—the juvenile sea lions that dove were actively hunting fish.

Juveniles that spent more time at sea diving had diets that were composed of more fish and less milk. (The scientists figured this out by measuring a chemical signature of diet—the ratio of different isotopes of nitrogen—in the flippers of 23 juveniles and their mothers.)

The scientists did not test the diet of every sea lion in the study. But since more diving meant more fish-eating in this sample, they felt comfortable surmising that sons—due to their relative inactivity when it comes to diving—likely place a greater energetic burden on their mother by relying more on her milk. This may put the mother's survival at risk or reduce her ability to invest in subsequent offspring, said Piedrahita. (See "Sea Otter Moms Risk Lives to Raise Babies.")

But the mother may also benefit from investing heavily in sons. Once mature, daughters are virtually guaranteed to produce one pup annually, but sons may sire anywhere from none to four pups in a given year. Given this, mothers may have more "grandpups" if they produce large, dominant sons that are able to mate with many females.

"This study provides strong empirical evidence" that sea lion mothers invest differently in sons versus daughters, noted Nicole Thometz, a marine mammal biologist at the University of California, Santa Cruz, who was not involved in the study.

She added that it also raises a host of other questions about the family dynamics of these marine mammals. For instance, why do juvenile males opt to stay high and dry when they could go out and find food? For now the answer remains elusive, but Piedrahita hopes that—with further research—a more complete story will emerge.