California sea lions are getting bigger as their numbers grow

Broader diets are making for bigger males

California sea lion populations have been growing—in more ways than one. Not only have their numbers been on the rise. Their body size has increased, too. That’s unusual, since animals tend to get smaller as their populations grow because of increased competition for food.

“It’s counterintuitive. You would expect that their body size would decrease as dietary resource competition intensified,” said Paul Koch, professor of Earth and planetary sciences at the University of California Santa Cruz and a coauthor of the study published in Current Biology.

While female body size has remained, researchers found, male California sea lions (Zalophus californianus) have grown. Researchers believe that as they expand their ecological niche, they’re foraging on more diverse prey, allowing them to bulk up.

“Body size is very important in competition with other males to control territory at breeding sites,” said first author Ana Valenzuela-Toro, a graduate student in ecology and evolutionary biology at UCSC. “Being bigger also means they can fast longer and stay on the beach to defend their territory.”

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Header Image: The body size of male California sea lions has grown as their numbers have grown. Credit: Rhododendrites