Summer's almost here, and you know what that means? A whole lot of bears are going to be dragging tapeworms out of their asses. Ahh, yes. The best season there is.
I never knew about this phenomenon until Tommy Leung, a parasitologist and evolutionary biologist, shared the following video. It shows a black bear trailing a 7-foot-long tapeworm from her butt like a piece of toilet paper. She doesn't seem to care or notice.
Here's another video of a bear extruding a tapeworm from its behind:
And one more, which seems to have been cruelly filmed by chasing the bear with a vehicle:
As it turns out, this is a fairly common thing toward the end of summer and fall. In North America, bears may contract a variety of tapeworms belonging to the genus Diphyllobothrium, but mainly a species called the "broad fish tapeworm."
Dangling tapeworms can sometimes be seen on bears through the Katmai National Park & Preserve's famous bear cam, though infrequently.
"When there is a bear around with an obvious tapeworm, it generates a lot of curiosity and revulsion in us humans. Tapeworms can trail out of a bear's anus for a few inches to a few feet. It's pretty gross," Cathy Bell, who is Chief of Interpretation & Education at the park, told me in an email.
"For us park rangers, gross things are a great hook for getting people to learn more about nature. Not just 'red in tooth and claw,' nature can be downright disgusting, too. And just like kids are fascinated by poop, a lot of grownups harbor a secret fascination with the ickier side of biology."
Hey, that's me!
The vectors for tapeworm infections are usually salmon, which the bears eat as they swim en masse to spawn. Once the larvae—originally eaten as eggs by small crustaceans, which are, in turn, eaten by fish—are ingested by the bears, they'll latch onto the stomach's mucousy membrane layer. Here, the worms will siphon nutrients from their hosts for the rest of their lives. (Oh, and they can sometimes grow to lengths reaching 30 feet.) Then, the bears will poop out their eggs, and the cycle repeats.
"For Alaska's coastal brown bears, fish are an important food resource, and of course the bears eat the fish raw. They ingest the larval tapeworms and the tapeworms grow into their adult form; then the cycle begins again," Bell added.
Bears can occasionally expel the worms by pooping them out. But afflicted bears can become sick as well. Adult tapeworms can cause intestinal blockage, anaemia, and a loss of proteins, vitamins, and carbohydrates. Some necropsied bears with tapeworm infestations have shown intestinal irritation and general poor health.
We reached out to several veterinarians about whether bear anus tapeworms are GOOD, BAD, or NEUTRAL, and will update this post if we hear back.
Humans, too, can harbor tapeworms in their guts. In 2006, people were discouraged from eating raw salmon after several people became infected from North American stock. Freezing and cooking the meat can decrease the risk of infection, according to the US Department of Agriculture and Centers for Disease Control and Prevention.
"[Tapeworms are] a real conversation starter about some of the realities faced by wildlife," Bell added. "And not a bad reminder that it's a good idea to cook our food thoroughly, too."