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In 2013, while studying Guadalupe fur seals on San Benito Island in the Mexican state of Baja California, Fernando Elorriaga-Verplancken spotted something unusual. Through his binoculars he saw that some of the animals had bright yellow patches on their bodies, ranging from a few square inches in size to more than half of their otherwise gray-ish coats. “What is that?” wondered Elorriaga-Verplancken, a seal ecologist at Mexico’s National Polytechnic Institute.  

When he and his colleagues got closer, they saw what was wrong. The seals had lost some of their guard hair—an outer layer of coarse, long fur that lends fur seals their coloration—revealing their yellow undercoat.

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Sporadic hair loss is rare in any mammal and especially worrisome in fur seals. They have relatively little blubber, and to stay warm in cool waters, they rely on air trapped between the layers of their guard hairs and undercoat. Any gap in that insulation will waste precious energy, weakening the animals and making it harder for them to hunt, mate, care for their young, and survive. 

Elorriaga-Verplancken has now seen afflicted seals on the island every year since his first observation. “It’s very concerning,” he says—and it’s not an isolated issue. 

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These species really reflect the quality and the health of the oceans.

Within the past few decades, scientists have documented a rise in coat thinning and hair loss—known as alopecia—in marine mammals around the world, including fur seals, gray seals, ringed seals, and even polar bears. While many individuals fully recover and regrow their coat, in others the condition has been linked to strandings, dangerous levels of fat loss, or death. For already-endangered animals like the Guadalupe fur seal, the rise in alopecia may signal greater, existential threats to the species.¹

Scientists have studied these animals, searching for a cause, but so far they’ve been largely stumped. The usual suspects—parasites and infections that cause hair loss—have been mostly ruled out, pushing suspicion to larger-scale environmental stressors like pollution or climate change, which can have complex cascades of impacts on animals’ physiologies.

Finding the cause is not only important for understanding what threatens these mammals, but also for untangling threats to ocean life more generally. As marine mammals live at the top of ocean food chains, “these species can really reflect the quality and the health of the oceans,” says marine ecologist Katherine McKenna of the New England Aquarium. “Paying close attention to any changes that are going on, or any new and emerging conditions, is really important.”

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One of the earliest known outbreaks of marine mammal alopecia was documented in the late 1990s during annual surveys of polar bears along the Southern Beaufort Sea in Alaska. Every spring, scientists from the United States Geological Survey would approach bears via helicopter, sedate them with a dart, and inspect them. The scientists first noticed alopecia in 1998, when a handful of bears were afflicted with the condition. One year later, about 16 percent of bears had dinner plate-sized patches on their heads, necks, or shoulders. 

Alopecia likely causes the bears’ bodies to lose heat—no small thing for Arctic creatures who plunge in and out of icy waters to catch seals during the winter hunting season. Alopecic bears tended to be thinner than healthy animals, which might mean that they’re burning more precious fat to stay warm, or that whatever is causing the fat loss is also causing the alopecia—or both. 

In Body Image
BARELY BEARING IT: A polar bear’s thick coat (like the one pictured here) helps insulate it against the chill of arctic waters. But if patches of fur go bare, which seems to happen more frequently during years of low ice coverage, it can further stress bears by taxing their energy reserves to keep warm. Image by Chlorophylle Photography / Shutterstock.

Following a decade of relatively low alopecia occurrence, prevalence spiked again in 2012, this time affecting nearly one-third of surveyed polar bears along the Southern Beaufort Sea—levels that haven’t been seen in the population since.² Curiously, ringed seals in the area suffered alopecia, too. Unlike the bears, many of them died. The researchers wondered if the bears caught a disease from seals they ate, but an exhaustive search for mites, fungi, bacteria, and viruses in samples taken from the bears found no obvious culprits.

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“It pointed toward alopecia, at least in polar bears, not being a consequence of any one agent,” says Todd Atwood, a wildlife biologist with the U.S. Geological Survey. Rather, it might be caused by a general stress response driven by several kinds of stressors in their environment. Notably, 2011 and 2012 were bad years for Arctic sea ice, which reached its lowest recorded extent in the summer of 2012. Less ice would have made it harder for bears to find and hunt seals; when they succeeded, the seals they ate were themselves less nutritious, having likely been emaciated by alopecia and its related stressors.  

In humans, chronic stress can lead to hair loss—perhaps, some studies suggest, by increasing production of hormones that damage hair follicle stem cells. The same may be true of bears. The loss of hair may in turn cause even more physiological stress. As a further indicator that the bears are stressed, alopecic bears had reduced immune system function compared to healthy animals.³ Stress is known to impair immune function.

Climate change may also have contributed to the alopecia of Guadalupe fur seals on San Benito Island. In the summers of 2017 and 2018, Elorriaga-Verplancken and his colleagues managed to examine 13 affected seals. Like Atwood, they couldn’t find any evidence for parasites or infections, and the researchers blame an unusual marine heat wave that was first noticed in the northeastern Pacific in 2013 and grew throughout 2014 and 2015, disrupting many ocean ecosystems. California sea lions and some other pinniped species were hard hit; thousands of malnourished animals washed up along the coast of western North America. The Guadalupe fur seal population at San Benito plummeted by around 50 percent. As for the survivors, said Elorriaga-Verplancken, many of them had alopecia.

His team thinks that ecological upheaval caused by the heat wave may have forced fur seals to travel farther to feed, which caused stress by exhausting them.⁴ Scat examinations suggest that they switched prey from small jumbo squid and opalescent squid to the neon flying squid, which are likely less nutritious: the seal equivalent of junk food. By disrupting their metabolism, this could have hampered the seals’ ability to synthesize key proteins, causing hair damage and loss.

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Alopecia likely causes polar bears’ bodies to lose heat.

It’s still too early to rule out other possible causes like pollution, Elorriaga-Verplancken says—but if nutritional stress is implicated, that’s especially troubling. It may pose an even greater threat to the survival of the species, which only recently rebounded after being hunted to near-extinction. Alopecia may also be only the first symptom of environmental changes, adds Karina Avecedo-Whitehouse, a wildlife disease expert at the Autonomous University of Queretaro. She fears that if environmental stressors worsen, other organ systems might become affected, eventually leading to even more health problems.

Interestingly, the baldness seen in San Benito’s Guadalupe fur seals is quite different from what has been found on Deen Maar Island in southern Australia, where fur seals with alopecia have been spotted since 2004. Among the former, alopecia can occur on either side of the animal’s body; in Australian fur seals, the bald patches tend to begin on the back and are symmetrical on both sides. And whereas the alopecic seals Elorriaga-Verplancken examined on San Benito were male, on Deen Maar the condition mainly affects females. 

At first glance, the pattern seen in the Australian seals is typical of hormonal disruption caused by exposure to toxic chemicals that can mimic, block, or otherwise interfere with a body’s hormones, says veterinarian and marine mammal scientist Rachael Gray at the University of Sydney. In a recent study, Gray and colleagues found significantly higher concentrations of PFAS—long-lasting, possibly hormone-disrupting pollutants—in alopecic seals.⁵

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That could suggest that Deen Maar’s seals are exposed to a specific cocktail of alopecia-triggering chemicals—but it could also mean that seals already exhausted by the heat-wasting effects of alopecia are forced to forage in near-shore areas that happen to contain more pollutants. Gray suspects the baldness is likely driven by several stressors, which could include pollution, heatwaves, and nutritional deficiency caused by declines in prey, although these haven’t been investigated. 

Alopecia may then put yet another strain on the already-declining Deen Maar population. Although many fur seals with mild alopecia were spotted in subsequent years with new hair—albeit shorter and finer than usual—the condition could contribute to an early death, Gray says. 

For other alopecia outbreaks—such as in Massachusetts, where bald patches were seen in about 7 percent of surveyed gray seals between 2004 and 2013—researchers are still only scratching the surface of possible causes.⁶ McKenna and her colleagues suspect that, because alopecia was more common in crowded colonies, it may be caused by competition for prey, increased stress from living in packed congregations, or a disease that spreads easily in dense populations. Due to funding constraints, however, the team hasn’t been able to investigate the causes.

Understanding what’s happening with marine mammal alopecia will likely take years. What’s already becoming clear, though, is that many of the recent outbreaks are probably not caused by any one thing, but rather by everything all at once: the constellation of stressors humans are creating in the ocean. Alopecia is just an especially visible symptom—and an important one, appearing as it has in some of the seas’ top predators. “There’s so much more going on,” says McKenna, “of which we just have no idea.” 

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Lead photo by Fernando Elorriaga-Verplancken


1. Guzmán-Solís, A.A., Elorriaga-Verplancken, F.R., & Acevedo-Whitehouse, K.A. Guadalupe fur seal alopecia: A metabolic syndrome associated to climatic anomalies? bioRxiv (2023).

2. Atwood, T., et al. Prevalence and spatio-temporal variation of an alopecia syndrome in polar bears (Ursus maritimus) of the southern Beaufort Sea. Journal of Wildlife Diseases 51, 48-59 (2015).

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3. Bowen, L.A., Miles, K., Stott, J., Waters, S., & Atwood, T. Enhanced biological processes associated with alopecia in polar bears (Ursus maritimus). Science of the Total Environment 529, 114-120 (2015).

4. Amador‐Capitanachi, M.J., et al. Ecological implications of unprecedented warm water anomalies on interannual prey preferences and foraging areas of Guadalupe fur seals. Marine Mammal Science 36, 1254-1270 (2020).

5. Taylor, S., et al. Per and polyfluoroalkyl substances (PFAS) at high concentrations in neonatal Australian pinnipeds. Science of the Total Environment 786, 147446 (2021).

6. Pugliares-Bonner, K., McKenna, K., Sette, L., Niemeyer, M., & Tlusty, M. Prevalence of alopecia in gray seals Halichoerus grypus atlantica in Massachusetts, USA, 2004-2013. Diseases of Aquatic Organisms 131, 167–176 (2018).

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