Having claws instead of smooth flippers lets ‘true seals’ grasp prey
D.P. Hocking et al/Roy. Soc. Open Science 2018
Some seals still eat like landlubbers.
Just like lions, tigers and bears, certain kinds of seals have claws that help the animals grasp prey and tear it apart. X-rays show that the bones in these seals’ forelimbs look like those found in the earliest seals, a new study finds.
Ancestors of these ancient seals transitioned from land to sea at some point, preserving clawed limbs useful for hunting on land. But clawed paws in these northern “true seals,” which include harbor and harp seals, seem to be more than just a holdover from ancient times, says David Hocking, a marine zoologist at Monash University in Melbourne, Australia. Instead, retaining the claws probably helps northern true seals catch a larger meal than they could with the stiff, slippery fins of other pinnipeds such as sea lions and fur seals, Hocking and his colleagues report April 18 in Royal Society Open Science.
Hocking and his colleagues spent 670 hours observing wild harbor and gray seals hunting salmon in Scotland. Tests with three captive seals, two harbor seals born in captivity and one spotted seal born in the wild allowed the team to observe eating behaviors at closer range.
A look at bone structure in a fur seal, which has stiff flippers, and a harbor seal, which has bendable paws, reveals an anatomical basis for their different feeding styles. The harbor seal’s longer digits, which extend beyond its skin, help it bend its paws around its prey and sink in its claws.
While some of the captive seals seemed to prefer swallowing their prey whole, both the wild and captive animals relied heavily on their claws overall, the scientists found. The critters were frequently spotted using their slashers to hold onto prey and rip off smaller bites, much as a land animal like a wolverine or a bear might. Up-close observations revealed seals caught prey underwater, but ripped it apart at the surface. That probably lets them breathe while eating without inhaling gulps of seawater — a challenge when devouring a large meal underwater.
Northern true seals have flexible joints that allow the animals to curl their claws to grasp prey. These flexible joints are also seen on early pinnipeds such as Enaliarctos mealsi, a seal that lived 23 million years ago, Hocking and his colleagues found. Fur seals and sea lions, however, “have inflexible fingers that help them to maintain a stiff flipper,” Hocking says.
The evolution of flipperlike forelimbs helped some pinnipeds propel themselves through the water more efficiently. But slippery flippers aren’t as useful for grasping prey. That could explain why fur seals and sea lions tend to target smaller fish that they can swallow whole underwater without needing to grasp, Hocking says.
But this fully aquatic feeding style might have been a challenge for the earliest pinnipeds, who probably used their clawed paws to hunt more like today’s true seals, the researchers say. Catching prey underwater and then shredding it at the surface was probably a smaller behavioral leap from full-on land feeding than other aquatic hunting strategies.
Documenting seals using their paws to grasp food is a “nice observation,” says Frank Fish, a biologist at West Chester University in Pennsylvania. Without knowing what early seals ate, though, it’s hard to say for sure whether they actively used their claws to hold onto large prey, he says.
Other scientists have documented true seals using their pawlike forelimbs in stereotypically terrestrial ways, too, such as using the claws to dig out lairs in ice or uncovering buried fish from the seafloor.
D. Hocking et al. Clawed forelimbs allow northern seals to eat like their ancient ancestors. Royal Society Open Science. Published online April 18, 2018. doi:10.1098/rsos.172393.
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