As adults, they’re Africa’s most successful predators. But as cubs, spotted hyenas are a favorite snack for lions. For that reason, hyena cubs usually steer clear of the big cats, spending most of their time near their parents’ dens.
That’s unless the young hyenas are infected with the parasiteToxoplasma gondii. Those unfortunate cubs get closer to lions and are four times more likely to be killed by the big cats than their healthy peers, according to decades’ worth of data collected in Kenya’s Masai Mara National Reserve.
“I was stunned to see the big difference in how close the infected versus uninfected cubs actually got to lions,” says Kay Holekamp, a behavioral ecologist at Michigan State University and co-author of a new study on the topic published in Nature Communications. “I’m always surprised when something that incredibly clear jumps out at me.”
Toxoplasma is a single-celled parasite that infects at least one-third of the world’s human population. It’s famous for its ability to manipulate its hosts, such as mice, into acting recklessly around felines, such as house cats. But this is the first time scientists have documented such effects in large wild mammals. (Read how Toxoplasma takes over human brains.)
The research also shows that the generally nonfatal parasite, which can infect a wide range of animals with a disease called toxoplasmosis, plays a bigger role than previously thought in how wild animals behave.
“This parasite doesn't just affect domestic cats and their mouse prey, but it's potentially a much wider-spread phenomenon,” says Holekamp, who has studied hyenas since 1988.
Cat and hyena game
The Toxoplasma parasite can infect many host species, including rodents, birds, and other prey animals, if they ingest contaminated meat or feces. But the parasite can only sexually reproduce in feline intestines. That can be challenging—after all, why would a prey animal approach a predator?
Over millions of years of evolution, this distant cousin of malaria has acquired a neat trick: Rodents with toxoplasmosis find the smell of cat urine irresistibly alluring, and that can draw them closer to a hungry feline.
“This has the benefit of not only shuffling the parasite's genome, but also leads to the production of environmentally stable spores that can infect many additional hosts,” study co-author Zach Laubach, a postdoctoral fellow at the University of Colorado in Boulder, says by email.
Since the parasite reproduces in lion intestines, and hyenas are known to be Toxoplasma carriers, Laubach and Holekamp wanted to know whether the parasite would make its hyena hosts behave differently. (Read how toxoplasmosis is harming endangered seals in Hawaii.)
The researchers turned to the multi-decadal Mara Hyena Project, which records data on individual hyenas’ locations—including their proximity to other animals—as well as cubs’ age, sex, and blood samples, which would show whether they had ever been infected by Toxoplasma, which causes a lifelong infection.
Their analysis revealed that a third of cubs studied had been exposed to Toxoplasma, as had 71 percent of juveniles and 80 percent of adults.
While uninfected cubs stayed an average of 300 feet away from lions, cubs that had Toxoplasma antibodies in their blood had ventured within an average of 142 feet from the predators, a dangerous proximity. These differences disappeared after the cubs turned one, perhaps because the survivors learned not to get too close to the felines.
One of the study’s limitations, Holekamp and Laubach say, is that it’s unknown whether the hyena cubs were also bolder around other predators, feline or otherwise—a question they’re already investigating.
The study “is a game-changer,” says Stefanie Johnson, a researcher at the University of Colorado who studies how Toxoplasma impacts people and wasn’t involved in the hyena research. “It confirms that Toxo has pretty strong effects on mammal behavior”—possibly including ours.
Most people who get toxoplasmosis have a mild fever and recover quickly, though the parasite can cause severe birth defects in fetuses, which is why pregnant people are urged not to clean their cat’s litter box. But there’s also intriguing if controversial evidence that the disease can make people take more risks, such as driving more dangerously or starting a new business.
Johnson is among those who believe these effects are part of a broad suite of changes that Toxoplasma uses to control its hosts—and that the parasite could be influencing how people act in ways we’re not aware of yet. (Learn about more parasites that mind-control their hosts.)
“It's a parasite that people think is like pretty benign, especially in humans,” Johnson says. “But when you look at some of these effects, Toxo could be having pretty big potential impacts on human behavior, even at a societal level.”