As the California sea lion population got bigger, so did male sea lions
Unlike other marine mammals, male California sea lions have gotten bigger over the past 50 years as their population has grown
Peer-Reviewed PublicationIMAGE: CALIFORNIA SEA LION SKULLS USED FOR THIS STUDY AT THE ORNITHOLOGY AND MAMMALOGY COLLECTION OF THE CALIFORNIA ACADEMY OF SCIENCES. view more
CREDIT: ANA VALENZUELA TORO
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.
Male and female California sea lions differ substantially in body size and foraging behavior.
CREDIT
Illustration by Sarah Gutierrez
A male California sea lion at Año Nuevo Reserve.
CREDIT
Ana Valenzuela Toro
JOURNAL
Current Biology
ARTICLE TITLE
Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche
ARTICLE PUBLICATION DATE
27-Apr-2023
Male California sea lions are becoming bigger and better fighters as their population rebounds
IMAGE: A PHOTO OF CALIFORNIA SEA LIONS RESTING AT AÑO NUEVO ISLAND. view more
CREDIT: DANIEL COSTA
California sea lions have managed to maintain—and, in the case of males, increase—their average body size as their population grows and competition for food becomes fiercer. This is in contrast to other marine mammals, whose average body size tends to decrease as their numbers increase. Researchers report April 27 in the journal Current Biology that sexual selection was a strong driving force for males to grow bigger and to strengthen muscles in their neck and jaw that help them fight for mates. Both male and female sea lions evaded food shortages by diversifying their diets and, in some cases, foraging further from the shore.
“Body size reduction is not the universal response to population increase in marine predators,” says lead author Ana Valenzuela-Toro (@Avalenzuelatoro), a paleoecologist at the University of California Santa Cruz and the Smithsonian Institution. “California sea lions were very resilient over the decades that we sampled and were able to overcome increasing competition thanks to prey availability. They’re like the raccoons of the sea: they can consume almost everything, and they can compensate if something is lacking.”
While many marine mammal species have rebounded to some extent since the Marine Mammal Protection Act was passed in 1972, California sea lions are notable for the size and duration of their population increase: the number of breeding females, who have been most consistently counted, has more than tripled in the US since the 1970’s—from around 50,000 to nearly 170,000—and their population growth is only now beginning to plateau.
To explore how California sea lion ecology has changed as their population has grown, the researchers analyzed museum specimens of adult male and female California sea lions collected in central and northern California between 1962 and 2008. To estimate changes in body size, they compared the overall size of more than 300 sea lion skulls collected over the years. They also measured other skull features, such as the size of muscle attachment points, which allowed them to assess changes in sea lion neck flexibility and biting force.
The team additionally took tiny bone samples from some of the skulls and measured their stable carbon and nitrogen isotope composition, which allowed them to make inferences about where the sea lions were foraging and what they were consuming. “Carbon provides information about habitat use—whether they’re foraging along the coast or offshore—and nitrogen provides insights about the trophic level of their prey, for example if they’re consuming smaller or larger fish,” says Valenzuela-Toro.
Overall, the researchers found that male sea lions have increased in size, while female sea lion size has remained stable. This sex difference is probably due to the fact that size matters for male, but not female, mating success. “One male can breed with many females, and males in the breeding colony fight with each other to establish their territory,” says Valenzuela-Toro. “Bigger males are more competitive during physical fights, and they can go longer without eating, so they can stay and defend their territory for longer.”
Male sea lions also increased their biting force and neck flexibility over this same time period. “The neck muscles are really important because they allow them to move their head and neck more agilely, bite harder, and, eventually, win when they are fighting other males in the breeding colony,” says Valenzuela-Toro.
The isotopic analyses indicated that both male and female sea lions managed to meet their nutritional needs by diversifying their diet and feeding on a broader range of prey. Male sea lions also foraged further afield. “Over time, some male sea lions were foraging further north,” says Valenzuela-Toro. “This is consistent with some anecdotal records that they’ve even been seen in Alaska, where they were not known to go in the past.”
Female sea lions consistently had a more diverse diet compared to male sea lions. The authors suggest that this flexibility in food choice is what allowed females to maintain their average body size without foraging further away.
“They remain in a narrow zone around their breeding colony, but they still show a lot of flexibility in what they eat,” says Valenzuela-Toro. “We believe that skull morphology in the rostrum—which is related to the size and shape of the mouth—probably has something to do with this flexible foraging behavior. We found that the size and shape of the mouth of females is related to the size of the prey that they consume.”
However, this flexibility in diet can only take the sea lions so far, and the authors warn that the sea lions’ future may not be so rosy.
“All these dynamics occurred in an environment that was rich in prey: full of anchovies and sardines, two species that are super important for their diet,” says Valenzuela-Toro. “But over the last few years, the populations of these two fish have collapsed, so California sea lions are diversifying their diet to compensate, and apparently they are not doing so well.”
“As climate change progresses, prey availability of sardines and anchovies will decrease even more, and eventually we will have more permanent El Niño-like warming conditions, reducing the size and causing a poleward shift of these and other pelagic fishes,” she says. “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.”
A photo of California sea lion skulls used for this study, taken at the Ornithology and Mammalogy Collection at the California Academy of Sciences.
CREDIT
Ana Valenzuela Toro
This research was supported by funding from the University of California Santa Cruz Division of Physical and Biological Sciences, the Graduate Student Association, and the Rebecca and Steve Sooy Graduate Fellowship; the Peter Buck Predoctoral Fellowship of the National Museum of Natural History, Smithsonian Institution; and ANID PCHA/Becas Chile.
Current Biology, Valenzuela-Toro et al. “Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche” https://www.cell.com/current-biology/fulltext/S0960-9822(23)00474-8
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.cell.com/current-biology. To receive Cell Press media alerts, contact press@cell.com.
JOURNAL
Current Biology
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche
ARTICLE PUBLICATION DATE
27-Apr-2023
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