Male southern elephant seals only feed on their favourite prey | Polarjournal
The massive Southern elephant seal males do not chase after every prey in their food spectrum. In fact, they are even more specialised than the females and feed almost exclusively on their favourite prey. Photo: Michael Wenger

Male Southern elephant seals are extremely picky when it comes to food, with each animal apparently having its own favorite food.

A new study by an international research team led by the University of New South Wales (UNSW) in Sydney, Australia, has found that male Southern elephant seals have distinct culinary preferences. In the current issue of the journal Marine Ecology Progress Series, the researchers provide the first insights into the feeding habits of adult male Southern elephant seals, which have rarely been studied compared to females.

“They could have the pick of the buffet, and yet each male southern elephant seal eats a lot of the same food, which is just a fraction of what’s on offer,” Andrea Cormack, a PhD student at UNSW and lead author of the study, said in a university press release. “So, they are extremely picky eaters, each with their own unique favourite foods they go after, whether it be fish, squid species, crustaceans or octopus.”

The researchers obtained this detailed information about the seals’ dietary preferences from the whiskers of 31 male elephant seals, which contain stable isotopes of the food they have previously eaten. The team analysed the longest whisker from each animal, though collecting samples from these enormous creatures weighing up to four tonnes was only possible after light anaesthesia.

Each of the whisker samples analysed contained up to a year’s worth of data on the culinary preferences of the individual bulls. The results show that almost all male Southern elephant seals are specialists who feed on the same food over a long period of time – especially in the months before the exhausting breeding season. Only one of the elephant seals was apparently not picky and fed on many different prey.

During the breeding season, elephant seals mainly stay on land and do not go out to sea to forage. The large males are then busy defending their territory on the beach. Photo: Michael Wenger

“These guys are out in the water foraging for months, and then fasting for two to three months on land during breeding season, so it’s hard to gather a lot of information about their diet through study methods like stomach analysis,” Cormack said. “But by analysing hard tissues that store an inert chemical record of what they’ve eaten, we can start putting together the pieces about their eating habits.”

A striking finding was that the body size of the elephant seals correlated strongly with the size of the prey. The heaviest bulls favoured large prey animals that were higher up the food chain. Especially before the breeding season, they specialised in energy-rich food such as large squid.

However, size was not the decisive factor for specialization, as smaller seals also showed extreme food preferences in early adulthood, but then for smaller prey further down the food chain. “They were all consistently picky on their food type regardless of size,” Cormack explained. “For these guys, who can lose up to 50 per cent of their body weight during the breeding season when they’re fasting on land, what you choose to eat could be very important.”

Although the exact reasons for selective feeding are still unclear, possible factors include the size of the mouth, which determines the ideal prey size and feeding technique, and fluctuations in the seasonal and annual food supply.

Southern elephant seals are the largest seals on earth. The males can reach a length of up to six meters and a weight of up to four tons. Photo: Heiner Kubny

The researchers suspect that specialization reduces competition during foraging and increases the success rate, with some animals choosing subpar prey to avoid conflict and save energy. In this way, they can still reach the size required to compete with other males for breeding rights. Long-term studies and further research are needed to decipher the exact causes of specialization and its effects on breeding success.

Despite the recovery of Southern elephant seal populations, researchers warn of the looming dangers of climate change. Climate-related shifts in the Southern Ocean, which affect the availability of krill, and newly emerging pathogens, such as the currently rampant bird flu, pose a threat to the seals’ favourite food sources.

“The Western Antarctic Peninsula, where these incredible animals live, is one of the areas experiencing the greatest changes from ocean warming,” Professor Tracey Rogers said, a marine ecologist at UNSW Science and senior author of the study. “The ice cliffs are almost completely gone, and the periods where new ice forms are getting shorter, changing the whole ecosystem. This could be problematic, with climate change affecting food resource availability across the Southern Ocean, which is why we need more research.”

Julia Hager, PolarJournal

Link to the study Cormack A, Slavich E, Negrete J, Bornemann H, Daneri GA, Rogers TL (2023) Extreme dietary specialization in adult male southern elephant seals: determining variation between individual trophic diets. Mar Ecol Prog Ser 725:185-199. https://doi.org/10.3354/meps14472

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