Some California sea lions have been getting larger and larger, but not all of them. Strangely, it’s only the males.
Those sea lions have been steadily growing larger over time while females have stayed the same size, according to a study published April 27 in the journal Current Biology, which looked at populations of California sea lions over the past 50 years.
This is especially strange considering that other species of animals usually get smaller as their populations get larger as a result of increased competition for food.
“Our study shows that male sea lions increased their skull size by a few millimeters, equivalent to an average of a bit more than 12-centimeter increase in male body size, including other body parts and flesh,” Ana Valenzuela-Toro, the paper’s lead author and a seal researcher at the University of California, Santa Cruz, told Newsweek.
“Specifically, we analyzed skulls of female and male sea lion skeletons from natural strandings along the coastlines of Central and Northern California,” she said.
“Until the mid-2010s, female sea lions were rare in this area, which, combined with their skulls resulting from local strandings, allow us to conclude that their body size stayed stable, likely because they experienced low competition for food in Central and Northern California.”
The reasons for males growing larger but females staying the same may be that the males have widened their habitats, perhaps as a result of their increasing population.
Since the federal Marine Mammal Protection Act was passed in 1972, the number of California sea lions has soared.
“In the case of pinnipeds like sea lions and elephant seals, they compete for mates on the beach, increasing their sexual dimorphism and increasing pressure for larger body sizes and skulls as reported in this study,” Elliott Hazen, a research ecologist at the National Oceanic and Atmospheric Administration, told Newsweek. “Larger males are simply more successful on the beaches.”
Male and female California sea lions have different selection pressures acting upon them. This essentially means they have different evolutionary forces driving them toward different things to maximize the number of offspring they have.
“Unlike males that leave their breeding colonies and perform long feeding trips along the North Pacific once the reproductive season is over, females stay in a narrow zone around their colony all year round to nurse their pup,” Valenzuela-Toro said.
“These differences are reflected in the spatial distribution of their natural strandings, the source of the skulls we used in our study.”
She suggested that as the populations increase, being large favors the males, which they have managed to achieve by expanding their diets.
“In any case, it seems like sexual dimorphism has indeed increased. However, it is necessary to complete further studies, including more skulls and perhaps running additional biochemical analyses, to be sure,” she said.
Sexual dimorphism is when the males and females of a species are different in appearance or size, outside of the usual difference in genitalia.
Stark examples of this include some species of spider, where the female is drastically larger than the male and often eats him after mating, and peacocks, where the male is known for its characteristically spectacular green fan of tail feathers and the female usually is less colorful.
With the male California sea lions getting larger and their increased populations, they may pose more of a threat to California residents.
“While male sea lions have increased their size and biting strength, their increasing population counts might become a more important risk to consider regarding human interactions,” Valenzuela-Toro said.
She continued: “As their numbers increase, so is the likelihood of encountering them where they were unusually seen in the past. Sea lions are top predators. As such, they can be aggressive, especially during their breeding seasons and when searching for food.”
“For many people, sea lions appear to be less threatening or even friendly because of their funny-looking appearance and clumsy movements.”
“However, they are very fast on land, and their bite or close contact can lead to several serious health issues, so people must be careful and never get close to them in the wild,” Valenzuela-Toro said.
In time, with the effects of climate change, these changes in male body size may decrease again as a result of drops in the numbers of prey.
Many of the preferred prey of the sea lions may decrease because of increased ocean temperatures, Toro said, which may cause the sea lion population, or the male body size, to decline.
“Climate models predict widespread shifts for the California Current system because of global warming,” the authors wrote in the paper. “These new conditions will likely impact marine predators, such as California sea lions, increasing their foraging effort and depressing their energy budget by decreasing foraging performance.”
The authors said that “energetic and ecological tradeoffs resulting from density-dependent selection pressures like those presented here may be unattainable, lowering their capability to overcome resource competition and reducing species’ carrying capacity, leading to a steep population decrease.”