Reading Between the Bones: New Research Reveals an Unexpected Growth Spurt in California Sea Lions

As climate change alters ocean ecosystems, scientists look to marine mammal ecology and morphology to predict how recovering species will fare in an uncertain future

A multitude of sea lions sit very close together on a crowded beach.
California sea lion populations have been on the rise for the past 50 years, leading to overcrowded breeding grounds and increased competition for food and mates. Daniel Costa, Año Nuevo Island

The rocky beaches of California's golden coast would not be complete without herds of sea lions basking in the morning sunlight.  Since the 1970s, these sea-faring giants have been rebounding from the effects of human hunting.  Their population has skyrocketed, and scientists expected to observe a common ecological trend seen across marine mammals: as a species recovers and competition for food intensifies, individuals experience a decrease in body size. But researchers were shocked to discover that male California sea lions have consistently gotten bigger.  And over the course of recent decades, they have seemed to show no signs of stopping.  

The passing of the Marine Mammal Protection Act in 1972 put an end to centuries of commercial hunting that decimated sea lion populations.  As recovering herds established new breeding grounds and repopulated California’s coastline, an ecosystem that had seen plummeting numbers of predators for generations experienced a rapid transformation.

Paleoecologist Ana Valenzuela Toro analyzed over 300 California sea lion skulls from the California Academy of Sciences’ collection to learn about the ecology of this recovering species. Ana Valenzuela Toro

For Ana Valenzuela Toro, a Smithsonian research collaborator and an ecologist and paleobiologist at the University of California, Santa Cruz, investigating the dynamics of the recovering sea lion population is key to building a conservation plan that will support the species in the future.  “I realized that California sea lions are a species that almost everyone living on the west coast will recognize, but as scientists we know remarkably little about them,” says Valenzuela Toro.  “We are just starting to understand the ecological consequences of their rapid recovery in a world that is now facing climate change.”

One of the most essential indicators of the health and ecology of recovering marine mammal populations is body size.  Studies analyzing other surging marine populations, like northern fur seals and South American sea lions, have found that body size regularly decreases as food resources dwindle in increasingly crowded habitats.  Eager to support this promising line of new research, Valenzuela Toro turned to the maze of sea lion skulls housed in the California Academy of Sciences.  She predicted that California sea lions would fit right in with the trend of decreasing body size.

“I see specimens as windows to a past life that would have otherwise been forgotten,”  — Ana Valenzuela Toro, Paleoecologist and Smithsonian Research Collaborator

While some may shudder at the thought of working in a sterile lab environment with only rows of sea lion skulls to keep you company, Valenzuela Toro found it to be an incredible experience. She spent months analyzing over 300 sea lion skulls collected between 1962 and 2008, taking careful measurements and tiny samples that would allow her to unlock the biological secrets hidden within the bone.  “I see specimens and skulls as windows to a past life that would have otherwise been forgotten,” says Valenzuela Toro.  “If I listen closely to these remains, they start to tell me stories about the ecology and evolution of organisms that lived years ago.”
Valenzuela Toro took 6 measurements of each skull, using the skeletal dimensions to estimate the overall body size of the marine mammals when they were alive. Gabe Santos, Natural History Museum of Los Angeles County

Valenzuela Toro discovered that despite their increasing population, male sea lions were growing larger while female size remained stable. This sparked her effort to understand why these sea lions bucked the ecological trend. What was making these male sea lions react differently to their recovery circumstances than other marine mammals?

It turned out it all came down to mating. California sea lions congregate once a year in their breeding colonies. Here, testosterone runs high as males mate with as many females as possible to ensure that their genetic lineages will live on.  As population sizes increase, males must engage in fights to secure breeding females, and a large body size is a powerful advantage.  However, males can only achieve these immense physiques if there is enough food to sustain them.

To take a closer look at sea lion foraging habits, Valenzuela Toro fed tiny bone samples into a refrigerator-sized machine that analyzed the carbon and nitrogen isotopes inside the biological material.  All organisms essentially ‘are what they eat,’ meaning that everything an organism consumes leaves chemical traces inside of its body in the form of isotopes.  While the analysis cannot tell precisely whether the sea lions are eating rockfish versus sardines, these isotopes can provide researchers with an idea of how big or small their meal was and where it came from. 

California sea lion skull samples are prepared for a biochemical analysis that will help researchers understand how their diets have changed over time.  Carolyn Lagattuta, Earth and Marine Sciences Building at the University of California Santa Cruz

Valenzuela Toro determined that California sea lions have diversified their diets and foraging ranges in recent decades, likely traveling further North to find prey that provides them with the calories needed to maintain a larger body size.  This was the final piece of the puzzle, illustrating how male sea lions are able to facilitate these sexually selected growth spurts without running out of resources.  In April, Valenzuela Toro and her colleagues published their findings in Current Biology, pulling back the curtain on the complex ecological dynamics that drive recovering marine mammal populations.

While male California sea lions increasing their size seems to be a conservation and recovery success story, Valenzuela Toro stresses that these are the best of times.  Climate change is expected to decrease the availability of sea lions’ preferred food of pelagic fish like sardines and anchovies as cyclical warm climate events known as El Niño stimulate large-scale changes in oceanic conditions.  In the near future, researchers predict that sea lion body size, and even their total population numbers, could decrease drastically.

“As sea lions and other marine mammal populations recover, humans are going to continue to grow and urbanize the oceans,” says Nicholas Pyenson, the museum’s curator of fossil marine mammals and a co-author of the study.  “Uncovering the hidden ecological dynamics at play is extremely important if we want to be prescriptive in our conservation approaches.”

Both Pyenson and Valenzuela Toro believe that utilizing museum collections and researching the marine mammals of the past is key to creating solutions that will aid in future conservation efforts.  Valenzuela Toro plans to continue her research into recovering mammal populations, looking to bridge the gap between paleo-biological research and the active preservation of modern oceans.
Skeletal material acts as a window into the lives of past organisms, providing detailed information about their morphological and ecological characteristics that can be used to strengthen marine conservation. Illustration by Sarah Gutierrez

“I am very passionate about promoting interactions between museum-based research with more applied conservation and ecological efforts,” says Valenzuela Toro.  “These are multi-disciplinary conversations that scientists need to have in order to create a better and brighter future for all species.”

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