It’s official, the sea lions in San Francisco are getting bigger

“When we started the project, we predicted the opposite,” lead author Ana Valenzuela-Toro, a paleoecologist at UC Santa Cruz, said in an email.

Since hunting sea lions became a federal crime in 1972, the mammal population has tripled. Typically, an increase in population in any animal results in a reduction in body size as competition for food increases. This has been observed in polar bears, harbor seals and many other species, Valenzuela-Toro said. But sea lions — or at least the males — only get bigger as the species continues to thrive.

“Our results are very unexpected and show that something special is happening in California sea lions, likely related to their dietary flexibility and prey abundance along the US West Coast,” said Valenzuela-Toro.

By analyzing bone samples for clues as to where and what the mammals ate, the researchers concluded that sea lions are finding food farther off the California coast and deeper in the ocean, and are diversifying their diets. Basically, California sea lions will eat anything, anywhere.

“I see the California sea lions as the raccoons of the sea,” said Valenzuela-Toro. “They can eat a wide variety of prey, from small pelagic fish like sardines and anchovies to larger rockfish, salmon, squid, or even squid.”

This no-frills diet means sea lions can make up for sparse prey with another snack, partly explaining the species’ spectacular population growth in recent decades.

The increase in height is scientifically significant, but may not be too obvious to those facing Pier 39.

“Our study shows that male sea lions increased their skull size by a few millimeters, corresponding to an average increase in male body size of just over 10 cm over the study period,” said Valenzuela-Toro. No increase was observed in females, likely due to different evolutionary selection pressures – males benefit from being larger when competing with other males for territory, while females would see no benefit from being larger.

California sea lions are apex carnivores, meaning they have no natural predators. They’re also wilder than they might look. “I think sea lions are less scary to humans because of their weird looking looks and clumsy movements on land,” Valenzuela-Toro said.

The study analyzed more than 300 sea lion skulls collected at the California Academy of Sciences in Golden Gate Park. They were found off the coast of central and northern California between 1962 and 2008.

“I spent a lot of time surrounded by sea lion skulls,” said Valenzuela-Toro. “It was a spectacular and unique experience. Each skull is a window into the past and tells a story that would otherwise be lost forever.”

The full study can be found in the scientific journal Current Biology.