Sea Lion Survival

      Marine mammals have been vulnerable to the effects of human activity. This has forced their decline and, in some cases, their extinction. Still, some marine mammals have overcome these harmful effects and recovered. Like other marine mammals, California sea lions were hunted in the past for their fat and meat. This caused a severe reduction in their population along the eastern North Pacific. Nevertheless, as a result of decades of protection, they are now one of the most abundant and rapidly growing animals in the region. Their population growth is only now slowing down. California sea lions hang out on docks and other locations and bark loudly, making them a popular attraction for locals and visitors. Despite their popularity, scientists are still learning about them. Scientists are particularly interested in how their population has successfully recovered over the last decades.

Previous studies have shown that as marine mammal populations increase, competition for resources with other animals will likely intensify. This eventually results in less food to eat. Lower available food negatively affects growth, causing declines in adult body size, which has been observed in other marine mammals like polar bears, northern fur seals, South American sea lions, and even harbor seals. Given the continued increase in the number of California sea lion, we were interested in learning how they have managed to grow their population. We also wanted to know whether they have experienced similar body size declines as seen in other marine mammals.

  We predicted that California sea lions would decrease their body size as their population increased. To test this, we analyzed skulls of female and male sea lion skeletons from the California Academy of Sciences in San Francisco. These skulls were gathered from central and northern California beaches between the 1960s and early 2000s. For each skull, we recorded its size and shape. We also took tiny bone samples from the skulls to chemically analyze them to investigate whether their diet changed over time.

  Our work showed that California sea lions have managed to maintain their average body size over a 46-year (1962-2008) period. Males were even able to increase their average body size during this period. They were able to do this as their population grew and competition for food intensified. Male sea lions also diversified their diets and expanded their feeding areas. We also found that they increased their biting force and neck flexibility as shown by the shift in the shape and size of the areas for the muscle. 

  Our results suggest that sexual selection, a process by which individuals compete for mating opportunities, was a strong driver for males to grow bigger and strengthen certain muscles. One male California sea lion can breed with many females every year. These males will also fight with each other to establish their territory. As sea lions recovered, males at breeding sites increased, and therefore fights for territory increased as well. This increase in fighting favored individuals with larger sizes and stronger biting forces. Bigger males are at the advantage during physical fights. They can go longer without eating, so they can stay and defend their territory for longer. Neck muscles are also important because they allow them to bite harder and eventually win the fight.

  Over decades, sexual selection has favored individuals with larger sizes and stronger biting forces. Increased body size also results in higher energy needs. For instance, individuals of larger sizes have to eat more to maintain their weight. It was observed that larger individuals could search for food using less energy than smaller individuals. Our work indicated that male sea lions had met their higher energy needs by adding variety to their diet and feeding on a broader range of animals. We also found that male sea lions have eaten further north over time, reaching even the Gulf of Alaska. They were not known to go this far north in the past.

  On the other hand, we did not find changes in females' body size. This is likely because we studied fewer female skulls than males. Additionally, females likely experienced different environmental pressures, such as food shortages, during the study period. Males have occupied resting sites off the coast of central California (where our specimens were collected) for several decades. Unlike males, females were rare until the mid-2010s, when new breeding sites were established at certain coastal islands. During the study period (1983-2007), female sea lions inhabiting central California were sparse, experiencing low competition for food and space. This allowed them to keep a constant body size. Our results also show that female sea lions had a more diverse diet than males, which is surprising because females remain in a narrow area around their breeding colony. However, they still show a lot of flexibility in their feeding preferences. We believe that the size and shape of their mouth probably have something to do with this flexible eating behavior. This could allow them to choose diverse prey.

  Our study reveals how sexual and natural selection (the processes that result in the survival of some individuals over others) have influenced population recovery in California sea lions in recent decades. It also exposed how changes in energy and bodily needs underlie the California sea lions’ population recovery. Still, we predict that global climate change will negatively impact this success in the future. As climate change intensifies, depletion in food sources, such as certain open-sea fish, may reduce the California sea lions’ ability to overcome competition for resources. This could ultimately lead to their population decline. Finally, this study demonstrates how historical data can provide insights into an animal’s past, help us understand their present, and allow us to make more informed decisions to protect their future.

Written By: Ana Valenzuela-Toro

Academic Editor: Physicist 

Non-Academic Editor: NICU nurse

Original Paper

• Title: Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche

• Journal: Current Biology

• Date Published: 22 May 2023

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