Researchers have shared adorable footage of mother Weddell seals teaching their babies to swim in Antarctica’s McMurdo Sound.
Adult Weddell seals, which live in Antarctica, can stay underwater for more than an hour and dive to over 300m. But babies don't automatically know how to swim: they have to learn how from mum.
In the delightful video of these swimming lessons, taken in 2015, these little seals can be seen getting used to being in the water by a dive hole in the ice, with mum watching closely.
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“Once the pup is in the water the pair stays pretty close by the dive hole,” says Kaitlin Allen, a postdoctoral scholar at Woods Hole Oceanographic Institution (WHOI) who studies how marine mammals manage oxygen while they're diving. “If the mother were foraging, we’d likely see her take off pretty soon after entering the water, but instead she’s spending time with her pup as the pup gets used to being in the water.”
When the little one grows up, it will be able to spend much longer in the water – “this species is one of the best marine mammal divers, with the longest recorded dive of around 96 min,” she adds – but the baby’s swims are short and sweet for now.
“The pups are not fully 'waterproof' at this stage,” Allen explains. "These swimming lessons are relatively brief to begin with.”
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Living in this extreme environment is part of the reason mothers teach their babies to swim, but it may also be because the pups have a longer lactation period than other species and so spend more time with mum.
Other species aren’t instinctively better swimmers, Allen explains. “For example, northern elephant seal mothers don’t spend any time in the water with their pups,” she says. “But once the pups have been weaned, they’ll start to venture into the surf or ponded areas on the beach where they practice swimming and holding their breath before they take off for a big months-long foraging trip.”
Diving under the ice can be difficult and dangerous. “It’s really tricky to hop in and out of the water with around two metres of ice to jump over,” says Allen. “Mothers will 'rake' the ice on the inner surface of the dive hole with their teeth to make it less slippery, which should help the pups to get back out of the water onto the ice surface where they can rest.”
Drowning is a common cause of death for babies that can’t get back out of the water at the dive hole. Before they’ve built up their breath-holding ability, young pups can only hold their breath for a few minutes at a time.
The video also captures an otherworldly soundtrack of what sounds like spaceships in the water. This is the sound of Weddell seals calling to each other. Researchers think the species use vocalisations for navigation under the ice as well as communication.
For a mother, these swimming lessons aren’t the only investment she makes into raising a healthy pup. “Weddell seal mothers lose about a third of their body mass during lactation, whereas the pups triple in size,” says Allen. Mum is fasting while raising her cub, “so all that energy and nutrient going to the pup is having to come from her own body stores.”
As well as losing lots of fat during this nursing period – “Weddell seal milk is around 55% fat!” Allen points out, adding: “for reference, whole milk at the grocery store is only around 4% fat.” – she also passes huge amounts of iron onto her baby.
Iron is critical for Weddell seals’ diving ability. Their milk is around 200 times more iron-rich than most mammals, according to Allen. This iron helps haemoglobin and myoglobin proteins in their blood bind lots of oxygen and store it in their body.
“Pups need iron to start building these proteins, and it has to come from their mothers,” she says. “As a result, mothers have significantly decreased body iron stores after lactation, which negatively impacts their diving.” This means Weddell seal mums are less able to forage effectively after weaning a baby because they’re not able to dive at their usual capacity.
Allen and her colleagues are interested in finding out how these high levels of iron aren’t harmful to the seals (too much of the mineral can cause iron poisoning in humans).
She’s also concerned about how resilient they might be as Antarctic temperatures continue to increase, adding: “the breeding colony is really dependent on the seasonal sea ice coverage in this area, which has declined substantially in recent years.”
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