Why would an apex ocean predator eat gloves? Or rope? Or plastic cups? How does a whale end up with more than 200 pounds of waste in its stomach?

Last week, a ten-year-old whale was found dead on a beach in Scotland. A necropsy revealed 220 pounds of plastic and other trash congealed in clumps in his digestive system. The tragedy grabbed headlines—the sheer quantity of debris eclipsed that found in a growing number of similar cases: large whales discovered dead on beaches around the world with stomachs full of garbage.

View Images In November 2019, a young sperm whale was found dead on a beach on the Isle of Harris, in Scotland. A necropsy found a 220-pound tangle of litter in his stomach.

Photograph by Scottish Marine Animal Stranding Scheme

It’s unclear if these sightings are becoming more common, or if we’re simply more attuned to them now that the public is aware of the plastic crisis, but plastic production is increasingly exponentially—In 1950, we produced 2.3 million tons of it. In 2015, we produced 448 million tons. Production is expected to double by 2050.

There is so much we still don’t know about what eating plastic and other refuse does to marine animals, or why they eat it, or how it makes them feel. While the necropsies reveal a shocking bounty of inedible material, ingesting plastic isn’t generally a fast killer. More often, the toll comes in a slow creep, harming some species more than others, in ways both stealthy and subtle. Here’s what we do know.

Why do marine animals eat plastic?

Scientists struggle with this answer, says Matthew Savoca, a postdoctoral researcher at Hopkins Marine Station at Stanford University and a National Geographic Explorer. We know that plastic is everywhere. Some 18 billion pounds of it flow into our oceans every year. We know that animals are eating it. But finding the why behind the what is really tough. “We know shockingly little about what’s actually happening in the ocean,” Savoca says.

Conventional wisdom suggests that animals eat plastic because it’s there and they don’t know any better (to some animals, like anchovies, plastic may smell like food). But that doesn’t explain why only certain types of whales—deep-diving toothed whales, such as sperm whales, pilot whales, and beaked whales—turn up dead on beaches with stomachs full of plastic.

These species hunt deep in the ocean, sometimes more than 1,600 feet below the surface, where it’s pitch black. They use echolocation to hunt for food—typically squid. It’s possible, says Savoca, that plastic trash sounds like food to toothed whales.

Doesn’t plastic float?

In fact, many types of plastic, including water bottles, naturally sink. Other plastic that would otherwise float may have algae or barnacles grow on its surface, changing its mass and dragging it down. Tiny bits of plastic have even been found in the Mariana Trench—at seven miles below the surface, the world’s deepest point—where shrimp-like creatures eat it.

Why aren’t other species of whales turning up with plastic-filled stomachs?

Baleen whales, such as humpbacks and blue whales, have natural filters for their food. The brush-like baleen they have in place of teeth and their narrow throats keep them from ingesting anything much larger than the krill—small crustaceans that swarm in large, mobile packs—that forms the basis of their diet. That helps explain why they’re not ending up beached with stomachs full of debris, but Savoca and his team are currently studying how and whether baleen may be letting through smaller plastic particles. “There are so many unanswered questions,” he says.

Are whales eating more plastic than other species?

Not necessarily. Flesh-footed shearwaters—large, sooty brown seabirds that nest on islands off the coasts of Australia and New Zealand—eat more plastic as a proportion of their body mass than any other marine animal.

But whale deaths are always noteworthy because we see so few of them. The vast majority of whales die at sea, sinking to the ocean floor far from land. A lot of animals, including shearwaters, albatross, and fish, don’t get noticed when they wash up on a beach, says Savoca. “But a 50- or 60-foot whale washes up? It makes news.”

How does eating plastic actually hurt animals?

Sometimes, death by plastic is obvious—if, for instance, albatross chicks are found dead with only plastic and no food in their stomachs, or if a whale’s necropsy shows intestines perforated by sharp plastic.

But most of the time, the harm is more stealthy, likely manifesting as a chronic, unrelenting hunger or lethargy.

How We Can Keep Plastics Out of Our Ocean Plastic pollution poses one of the biggest known threats to the ocean, influencing all ecosystems from beautiful coral reefs to abyssal trenches, eventually accumulating in our own food.

Whales have to surface to breathe, which means deep-dive foraging trips are time-sensitive. “Let’s say a sperm whale can grab 30 pieces of food during a dive,” Savoca says. “If five or 10 of those is trash with no value, that’s possibly 10 to 30 percent less food than you would get otherwise.”

This deficiency, Savoca says, would make it tough for an animal to have the energy to do everything it needs to do, such as breeding, migrating, and continuing to forage.

And plastic comes on top of other stressors affecting life in the ocean—climate change, overfishing, shipping traffic, noise pollution. “It’s a real shame because their lives are challenging enough even without the additional pressure we put onto them,” says Savoca. Especially at the rate we’re altering their environment, he says.