Scientists Find Crude Oil Destroys Sea Otters’ Buoyancy

Sea otters are known for their luxurious fur, but it nearly drove them to extinction. In 1938, only a small population of about 50 remained on the central coast of California. Since then, these mammals have fought back. However, these charismatic creatures are still threatened by crude oil spilled from offshore platforms.

But no one knew how much crude oil affected the buoyancy of sea otter fur, or how well it recovered after cleaning. Kate Riordan of California Polytechnic State University, San Luis Obispo (Cal Poly SLO), in the US, adds that the fur of newborn sea otter pups was also thought to be particularly buoyant, but no one had checked.

Curious to answer these questions, Riordan teamed up with Nicole Thometz (University of San Francisco, USA), Francesca Batac (California Department of Fish and Wildlife (CDFW), USA) and Heather Liwanag (Cal Poly SLO) to investigate.

They published their discovery in the Journal of experimental biology that crude oil pollution significantly reduces the buoyancy of sea otter fur, putting pups at particular risk because of their greater surface area relative to their body mass, and that buoyancy does not fully recover even after cleanup.

Sea otters in the wild die naturally for many reasons and are collected by the CDFW, which monitors the health of the population. So they passed on sections of some precious southern sea otter pelts, which range in age from tiny pups to nine-year-old adults, to Riordan, Annika Dean and Sarah Kerr (both of Cal Poly SLO).

After cleaning the skin, the team then weighed a 25cm animal² The researchers measured a portion of each fur in air and water to determine the fur’s buoyancy. However, when they compared the buoyancy of the young’s fur with that of adult adults, they found no difference. All sea otter fur had a buoyancy of about 0.3 N, about the same as a 10 g piece of cork, so a baby sea otter’s fur is no more buoyant than its parents’ fur.

However, when the team calculated the impact of fur buoyancy on the pups’ entire bodies, the pups’ fur was nearly three times more buoyant relative to their body mass than that of adults, thanks to the pups’ relatively large surface area relative to their size. This allows the pups to trap more air for their smaller body mass, making them extremely buoyant and keeping them afloat while mom is out hunting.

But how did spraying the crude oil affect the fur’s buoyancy? This time, the team massaged the fur with crude oil, mimicking the way sea otters naturally lick themselves—inadvertently rubbing pollution into their fur—before measuring the buoyancy of the oil-soaked fur.

Indeed, the buoyancy of the fur dropped by nearly 55% to 0.145 N, making it much harder for the animals to stay afloat. The reduction in buoyancy combined with the loss of insulation would almost certainly prove fatal in the wild.

Fortunately, conservationists respond quickly to major crude oil spills, drenching the soaked creatures in detergent to rid them of the oil, but how well does the buoyancy of sea otter fur recover after a cleanup? Riordan and his colleagues gently washed each oil-stained pelt with dish soap, then rinsed it thoroughly, dried it with a hair dryer, and measured the pelt’s buoyancy again. As they had hoped, the fur’s buoyancy improved, but only by 36% to 0.197 N, still about 40% less than fur’s natural buoyancy.

The team suspects that live sea otters may recover better after cleaning because they are continually grooming themselves, but they suggest that thoroughly rinsing off the detergent after cleaning is essential to ensure that oiled sea otters recover fully.