The male sea lion has become larger and his fighting ability has become stronger

As populations grew and competition for food intensified, the California sea lion population maintained its average size while the average size of male sea lions increased. This is in stark contrast to other marine mammals, whose average body size typically decreases with increasing numbers.

Now, scientists have found that sexual selection is a powerful driver for males to grow bigger and strengthen their neck and chin muscles, which helps them win mates. Both male and female sea lions avoid food shortages by eating a varied diet, and in some cases, they forage far from the coast. Related research was recently published in Contemporary Biology.

“Shrinking in size is not a universal response to the increase in the number of marine carnivores.” Lead author Ana Valenzuela-Toro, a paleoecologist at the University of California, Santa Cruz and the Smithsonian Institution, said, “In the decades we sampled, California sea lions were very resilient, and thanks to their hunting ability, they were able to overcome increasing competition. They are like raccoons in the ocean: they eat almost everything. ”

To explore how California sea lions change as numbers grow, the researchers analyzed museum specimens of adult male and female 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 neck flexibility and bite force in sea lions.

The team also took tiny bone samples from some skulls and measured their stable carbon and nitrogen isotopic composition, which allowed them to infer where sea lions forage and what they eat. Valenzuela-Toro said: “Carbon provides habitat information – whether they feed along the coast or offshore, nitrogen provides information about the nutritional level of prey, such as whether they are eating smaller or larger fish. ”

Overall, the researchers found that male sea lions increased in size, while female sea lions remained stable. This sex difference may be due to the fact that body size is important for mating success rates in males and not for females. “A male can mate with many females, and males will fight each other to establish their territory.” Valenzuela-Toro said, “Larger males are more competitive in fights, and they can hold out longer without eating, so they can stay there and defend their territory longer.” ”

During the same period, male sea lions also increased their bite force and neck dexterity. Valenzuela-Toro said: “The neck muscles are very important because it allows them to move their head and neck more flexibly, bite harder and ultimately win when fighting other male animals. ”

Isotopic analysis has shown that both male and female sea lions meet their nutritional needs by diversifying their diets and preying on a wider range of prey. Male sea lions also travel farther to feed. “Over time, some male sea lions began to forage north.” Valenzuela-Toro said, “This is consistent with some anecdotal records that were even seen in Alaska, and in the past people didn’t know they were going to go there.” ”

Female sea lions have always had a more varied diet than male sea lions. The authors believe that this flexibility in food selection allowed females to maintain their average size without having to travel farther to feed.

“They still live in a narrow area around their breeding grounds, but they still show a lot of flexibility in what they eat.” Valenzuela-Toro said, “We think that the morphology of their skull in their snout—which is related to the size and shape of the snout process—may be related to this flexible foraging behavior.” We found that the size and shape of the female snout process correlated with the size of the prey they eat. ”

However, this dietary flexibility has only allowed sea lion populations to stay where they have been so far, and the authors warn that the future of sea lions may not be so rosy.

“Anchovies and sardines populations have shrunk dramatically over the past few years (two species that are essential to the diet of sea lions), so California sea lions are diversifying their diets to compensate, and apparently they’re not doing well enough.” Valenzuela-Toro said, “As climate change progresses, sardines and anchovies will decline further, and a longer-lasting El Niño will reduce the size of these and other pelagic fish and cause them to move towards the poles.” This will be a very bad situation for California sea lions, and eventually we expect their population size to stop growing and start declining. (Source: Feng Weiwei, China Science News)

Image courtesy of Foto-RaBe/Pixabay

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