Why Male California Sea Lions Are Getting Bigger

When a species grows more abundant, the animals typically become smaller as they face more competition with each other for food. But surprisingly, male California sea lions appear to be bucking this trend.

Since the marine behemoths received federal protection 50 years ago, their population has flourished. But instead of getting smaller, sea lion males have grown even larger, according to a paper published last week in the journal Current Biology.

California sea lions faced a dire future in the 1950s—hunting, pollution and bounties decimated the species, and only an estimated 10,000 remained in the United States. In 1972, the passage of the Marine Mammal Protection Act made it illegal to hunt, kill, capture or harass the pinnipeds, and in the decades that followed, their numbers rebounded.

Today, an estimated 257,000 California sea lions live in the wild, per the nonprofit Marine Mammal Center, including the well-known group that regularly draws tourists to San Francisco’s Pier 39. The International Union for Conservation of Nature lists California sea lions as a species of “least concern.”

Researchers from the University of California, Santa Cruz, and Smithsonian’s National Museum of Natural History wanted to understand what this population growth has meant for the animals’ physical size. They examined more than 330 sea lion skulls collected between 1962 and 2008 along the coast of Central and Northern California and preserved in museum collections. On average, males’ skulls have gotten a few millimeters bigger, which the researchers say translates to a roughly ten-centimeter increase in body size.

Females, meanwhile, have not grown. They have no evolutionary incentive to be larger, but males are polygamous and must fight each other for territory and harems. Bigger, stronger males would undoubtedly have an advantage when defending their turf and staking a claim to breeding partners—and with a greater stockpile of blubber, they can go longer without needing to feed.

“Larger males are simply more successful on the beaches,” Elliott Hazen, a research ecologist at the National Oceanic and Atmospheric Administration (NOAA) who was not involved with the study, tells Newsweek’s Jess Thomson. “They compete for mates on the beach, increasing their sexual dimorphism and increasing pressure for larger body sizes and skulls as reported in this study.”

Males can stretch up to 7.5 feet long and can weigh up to 700 pounds; females are typically 6 feet long and weigh just 240 pounds, per NOAA.

By examining the animals’ bone samples, researchers gained insights into their diets. The analyses suggest male California sea lions have expanded their foraging regions and have been eating more diverse types of food. This so-called “dietary flexibility” makes the creatures more resilient to disruptions to any one food source, as study co-author Ana Valenzuela-Toro, an ecologist at the University of California Santa Cruz, tells SFGate’s Andrew Charmings.

In other words, they’re not picky eaters and that’s given them a size advantage.

“I think of California sea lions as the raccoons of the sea,” says Valenzuela-Toro to SFGate. “They can eat many different prey, ranging from small pelagic fishes like sardines and anchovies to larger rockfish, salmon, squid or even octopus.”

Still, the researchers warned that though California sea lions are thriving right now, they could struggle to survive in the future because of human-caused climate change. Though they’ll happily gobble down many different types of fish, they rely on anchovies and sardines. But if ocean temperatures continue to rise, the number of anchovies and sardines could decline.

“It will be a really hostile environment for California sea lions, and eventually we expect that their population size will stop growing and actually decline,” says Valenzuela-Toro in a statement.

Meanwhile, the animals still face other human-derived threats, such as entanglement in fishing gear, pollution, algae bloom toxins, disease, boat strikes and human activities. For example, NOAA is currently offering a reward for anyone with information about a boat driver who drove through a group of California sea lions in the Columbia River in Oregon last month. The federal agency is also investigating a viral video that showed a man harassing the creatures that live on Pier 39.

Sarah Kuta | READ MORE

Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.