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Why Are Male California Sea Lions Getting Bigger?

Last Modified Date: February 15, 2024

It's a well-known evolutionary pattern that as a species becomes more abundant, the members of that species will gradually decrease in size due to competition for food. Yet sometimes there are odd exceptions to even the most accepted rules of nature. It appears that California sea lions, whose population has skyrocketed since they were nearly hunted to extinction by the 1960s, are one such exception to the rule.

Researchers from the University of California, Santa Cruz, and the Smithsonian National Museum of Natural History discovered that during the past 50 years, male California sea lions have become larger, while females have stayed the same size. The researchers looked at over 330 sea lion skulls from natural strandings in Central and Northern California between 1962 and 2008. Their measurements reveal that the average male sea lion skull has increased by a few millimeters, equivalent to an increase of around 4.72 inches (12 cm) in overall body size.

Although animals usually decrease in size when their population grows more abundant, California sea lions have grown larger – but only the males.

Evolutionary pressure may be one reason for the male sea lion growth spurt. Males must compete for mates, and a stronger, larger male will be more successful in defending his turf and finding mates for his "harem." Female sea lions, on the other hand, have no evolutionary motivation to grow larger. Habitat expansion could also help explain why male California sea lions are getting bigger. While female sea lions tend to stay in a specific area for an entire year taking care of their offspring, male sea lions leave their breeding colonies and go on feeding trips. Venturing out into the North Pacific allows them to expand their diet, so they are less likely to be affected by disruptions to their food supply, giving them a size advantage.

California sea lions (including those that hang out at Pier 39 in San Francisco) are clearly flourishing at the moment, but threats from climate change could change that. Despite their dietary flexibility, much of their diet consists of sardines and archovies, which could decline in population in warming seas. Other dangers include pollution, boats, and entanglement in fishing nets.

The 700-lb raccoons of the sea:

California sea lions were once plagued by hunting to the extent that only about 10,000 existed in the United States in the 1950s. When it became illegal to hunt, kill, or capture sea lions in 1972 — thanks to the Marine Mammal Protection Act — their population recovered.

According to the Marine Mammal Center, the California sea lion population has rebounded to 257,000. They are now listed as a species of least concern by the International Union for Conservation of Nature.

Sea lions are not fussy eaters. According to study co-author Ana Valenzuela-Toro, sea lions are the "raccoons of the sea." In addition to sardines and anchovies, they have been known to eat salmon, squid, rockfish, and octopus.

According to the National Oceanic and Atmospheric Administration (NOAA), female sea lions generally weigh around 240 pounds (109 kg) and are approximately 6 feet (1.8 m) long. Males, on the other hand, can weigh up to 700 pounds (317.5 kg) and can be up to 7.5 feet (2.3 m) long.

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