Animals tend to get smaller as their populations grow because of increased competition for food resources among members of the same species. That’s not what has happened with California sea lions, however, according to a new study led by scientists at UC Santa Cruz.

Published April 27 in Current Biology, the study found that male California sea lions have gotten bigger as the population grew over the past 50 years, while female body size has remained stable.

“It’s counterintuitive. You would expect that their body size would decrease as dietary resource competition intensified,” said coauthor Paul Koch, professor of Earth and planetary sciences at UCSC.

The number of California sea lions has increased dramatically since the Marine Mammal Protection Act was passed in 1972. In parts of their range, the sea lions may now be approaching the ecological “carrying capacity,” the largest number of animals an ecosystem can support.

In other marine mammal species, including northern fur seals, South American sea lions, and harbor seals, declines in adult body size have been observed as their population size increased, according to first author Ana Valenzuela-Toro. She led the study as a graduate student in ecology and evolutionary biology at UCSC, working with Koch and Daniel Costa, professor of ecology and evolutionary biology and director of the Institute of Marine Sciences at UCSC.

Valenzuela-Toro looked at the size and morphology of sea lion skulls collected between 1962 and 2008 in central California and now held at the California Academy of Sciences. She also analyzed bone samples for clues to changes in the animals’ diets. Stable isotopes of carbon and nitrogen in bone samples can yield information about where in the ocean the animals were foraging and what kinds of prey they 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," Valenzuela-Toro said. “Apparently they are now going farther north than they used to, which is consistent with observations reported by other researchers.”

By expanding the breadth of their diets, the sea lions have been able to get bigger even as their numbers increased. In theory, larger sea lions should be able to travel further, dive deeper, and handle larger prey. In addition, as their breeding sites became more crowded, increased competition between males during the breeding season may have favored larger males over time.

“Body size is very important in competition with other males to control territory at breeding sites. Being bigger also means they can fast longer and stay on the beach to defend their territory,” Valenzuela-Toro said.

Male and female California sea lions differ substantially in body size and foraging behavior. The males generally congregate in colonies only during the breeding season, after which they leave on long foraging trips. Females, meanwhile, stay in the colony to give birth and nurse their pups, so their foraging is restricted to areas near the colony.

“This creates different selection pressures on females and males,” Valenzuela-Toro said.

She noted that prey was abundant during the period covered in this study. That may not continue to be the case in the future as marine ecosystems respond to a changing climate.

“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,” she said. “Then we might see their population size start to plateau or decrease, and we could even see body size start to decline.”

Costa noted that the study was only possible because of the 167 California sea lion skulls collected over 44 years and maintained at the California Academy of Sciences. “These results are important to help us understand how marine mammals are adapting as their habitat changes in response to a changing climate,” he said.

In addition to Valenzuela-Toro, Costa, and Koch, the coauthors of the paper include Rita Mehta at UC Santa Cruz and Nicholas Pyenson at the Smithsonian Institution’s National Museum of Natural History.