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Being infected with a parasite is bad, right? So why are wolves in Yellowstone National Park infected with Toxoplasma gondii some of the most successful individuals

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Toxoplasma seroprevalence vs cougar density :

Predicted probabilities of infected wolf behaviour:

Feedback loop of toxoplasma in wolves and cougars :

No one wants to play host a parasite.

Kind of by definition, right? Like, if it works out well for you, it’s mutualism instead.

But there is one parasite that, even though it’s overall pretty bad, might help wolves find their way to the top. They become the literal leader of the pack… all while infected with a parasite you might find in your kitty litter. [ ♪ intro ♪] Toxoplasma gondii is a parasitic protozoan that can infect just about any bird or mammal, but its definitive host is the cat family. A definitive host is the final one a parasite uses in its adult stage, and the one it needs in order to reproduce.

For this reason, Toxoplasma is sometimes called the ‘kitty litter parasite’. Cute as that name sounds, it does also cause the illness known as toxoplasmosis. Most people wouldn’t notice much  more than flu-like symptoms, if they feel anything at all, but that’s not all the parasite is up to.

Because like the infamous  ant-controlling Cordyceps fungus, this parasite can influence its host’s behavior. Perhaps the most well known example of this comes from rodents. Rodents with toxoplasmosis no longer avoid the scent of cats, so they’re a lot more likely to get eaten by them.

And thus the parasite gets back into the cat to mature and breed -- you know, circle of life and so on stuff. So it’s easy to think ‘sure, but anyone can fool a rat!’, but it’s not just rats. Chimpanzees with Toxoplasma infections are no longer averse to the scent of leopards.

It seems like any cat goes! Including your beloved Mr. Fluffykins.

Up to 30% of humans might be infected with an unnoticed case of toxoplasmosis, though that varies hugely with where you live. But while it’s less clear with us than the rats, it is suggested that humans may experience behavioral changes with infection. Thanks, Mr.

Fluffykins. Yes, I’ll have your crystal goblet of tuna in just a moment. one second. So it does seem like being infected with a behavior-affecting parasite is something that’s going to be bad for the infected creature.

But there may be a bizarre upside for one animal in particular. Grey wolves are as susceptible to Toxoplasma as anything else, but when they get it, it sometimes appears to benefit them. They’re a lot more likely to become breeding individuals – and often that means they’re the leaders of the pack.

After all, wolf packs are  typically nuclear families with the parents in charge. And the change is thought to result from the same  kinds of changes in behavior that affect any other infected creature. Toxoplasma may increase testosterone levels, which might lead to heightened aggressive  behavior and increased appeal as a mate for those individuals.

And that makes a wolf a better leader, or at least more likely to breed. These behaviors then bleed into a lot of the wolves’ ecology. A study done in Yellowstone National Park tested for the frequency of Toxoplasma infection in the wolves in that part of the park.

The researchers wanted to know if infection affected three crucial risk taking behaviors. These were: dispersing from a pack, becoming a pack leader, and approaching people or vehicles. They also asked whether infected wolves were more likely to be killed by other wolves… as well as by humans.

Dispersing from the pack is important because it’s effectively the first step to becoming a socially dominant, breeding individual. You can’t do that if you  stay in your parents’ pack, since you’re going to be related to everyone else in it. I mean, you could try, but let’s not go full House Targaryen here.

And dispersing is risky. It involves leaving the  advantages of pack behavior and the territory you know to strike out on your own and do everything yourself. Dispersers have higher mortality than non-dispersing wolves.

But despite that risk, the study showed wolves with Toxoplasma were eleven times more likely to disperse than uninfected wolves. So you’ve left home and gotten the first step out of the way, but what about actually succeeding at becoming the top dog, the head honcho themself? The researchers found an even bigger difference here.

Wolves testing positive for Toxoplasma were 46 times more likely to become pack leaders than uninfected wolves! At this rate, you’d think the wolves would be lining up to get themselves infected. But being a pack leader is risky too.

Studies from the same study  population in Yellowstone show that the wolves most likely to die in a clash between packs are the breeding individuals. It’s tough being at the top, y’know? But like I said before, the parasite just isn’t completing its life cycle until it ends up in a cat.

In Yellowstone, that means cougars, who have fairly high rates of Toxoplasma – they’re the park’s main host for it. The researchers showed that the more the wolves in Yellowstone overlapped with cougar territory, the more likely they were to get Toxoplasma. They may get infected  through eating cougar scats - yup, your dog got that habit from his ancestors.

They may also scavenge dead cougars and pick up the parasite that way. But the two species can also come into conflict while they’re alive. And if infected wolves are more aggressive, they might end up in conflict with cougars more.

Not all predators care to eat each other, but cougars have sometimes been shown to eat wolves given the chance. So if young, infected wolves are both more likely to set off on their own, and more likely to engage in risky behaviors, so that they wind up close enough to cougars to pass the parasite back to them… then Toxoplasma would get back to its definitive host, and all those convoluted  behavior changes paid off for it. While the Yellowstone study only suggested that wolves ultimately infected cougars, there’s a similar study of Toxoplasma done on lions and spotted hyenas.

And that research found that hyenas, especially cubs, with Toxoplasma are much bolder around lions, and much more likely to be killed by lions as a result – so that the parasite gets to return to its definitive host, the lions. Now, it’s not all sunshine and rainbows for wolves infected with Toxoplasma. In pregnant wolves, it can cause entire litters to be lost.

And as far as the success of the individual passing on its genes goes, well… that’s bad. But even so, this parasite has happened upon a secondary host that can make the best of the behavioral changes it inflicts. And that’s not going to be the case most of the time… so if you ever start feeling indifferent to the scent of cat pee, you might want to postpone that trip to Yellowstone.

But if you do go to Yellowstone, you’ll need to be prepared with all the best science-themed supplies. Like our new James Webb Space Telescope calendar, which you can use to… mark the date of your trip. I mean Look, they told me to shout out space merch on a video about wolves.

I’m doing my best here. But if calendars with breathtaking space images aren’t your thing… aren’t they everyone’s thing?... you can find plenty of stuff, like our hat with an orca  wearing a hat of it’s own. You can find it all at or the link in the description, and thanks for watching. [♪ outro ♪ ]