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California sea lions are getting bigger as their population increases

As their populations grow, animals usually tend to get smaller due to increased competition for food resources among conspecifics. For instance, in marine mammal species such as northern fur seals, South American sea lions, and harbor seals, significant declines in body sizes have been observed as their population size increased. 

Surprisingly, this has not happened in the case of California sea lions: as their populations increased over the past half a century, males have gotten bigger, while female body size has remained stable, according to a recent study led by the University of California, Santa Cruz (UCSC).

“It’s counterintuitive. You would expect that their body size would decrease as dietary resource competition intensified,” said study co-author Paul Koch, a professor of Earth and Planetary Sciences at UCSC.

The experts examined the size and morphology of sea lion skulls collected between 1962 and 2008 in central California, and analyzed bone samples for clues about where and what these animals were eating. 

“We found that male California sea lions have expanded their ecological niche, which means they are now foraging on a more diversified group of prey and expanding the places where they are foraging,” explained lead author Ana Valenzuela-Toro, who conducted the research during her graduate studies at UCSC. “Apparently they are now going farther north than they used to, which is consistent with observations reported by other researchers.”

According to the scientists, expanding the breadth of their diets could have helped male sea lions to get bigger even as their numbers increased. At the same time, larger sea lions could travel further, dive deeper, and handle larger prey, thus boosting even more their growth. Finally, as their breeding sites became more crowded, increased competition among males during the breeding season may have favored larger males over time.

The fact that female body size remained stable can be explained by the significant differences between male and female foraging behavior, which creates different selection pressures for the two sexes. While males leave on long foraging trips after the breeding season, females stay in the colony to give birth and raise their pups, so their foraging is restricted to nearby areas, lacking the diversity of resources males have access to.

Although prey was abundant during the period covered in this study, the situation may soon change due to global warming. 

“This has been a good period for sea lions, but if warm conditions become more frequent, we could see lower availability of their preferred prey, such as sardines and anchovies. Then we might see their population size start to plateau or decrease, and we could even see body size start to decline,” Valenzuela-Toro concluded.

The study is published in the journal Current Biology.

By Andrei Ionescu, Staff Writer

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