#965 - Robert Sapolsky

Joe Rogan (0:09)

All right, well first of all, thanks for doing this.

I found out about you several years back. I had heard something about toxoplasmosis, am I saying it right?

Yeah, and I had seen a speech that you had given on it where you were talking about how many people had been infected by this cat parasite. I've had cats my whole life and I even had feral cats. And I've always wondered, like I should probably get tested and I'm worried about the result. Perhaps not. Well, it was just fascinating to me that literally, I mean, what is the number that you estimate in Americans alone that might have been infected?

Robert Sapolsky (0:48)

It's on the order of, I'm not sure with Americans, but worldwide it's something like 50% of humans is the best guess.

Joe Rogan (0:56)

50% of humans worldwide.

Robert Sapolsky (0:57)

Something on that scale.

Joe Rogan (0:59)

For people who have never heard of this, would you mind explaining what this parasite is and how it affects rats and then cats and then people?

Robert Sapolsky (1:06)

Okay, totally bizarre. So it's this protozoan parasite, Toxoplasma gondii, and it's got one of these weird parasitic lifestyles. The only place on earth where it could reproduce sexually is in the gut of a cat.

I don't know why there are people who know this, but so it reproduces there, comes out in the cat feces, feces are eaten by rodents, and now Toxo's evolutionary challenge is to get that rodent into a cat's stomach. So what Toxo has evolved is this ability, it slowly migrates to the brain of rodents, and basically wipes out the innate fear that rodents have of cat smells.

Like you take a lab rat who's been like the descendant of lab rats for like a thousand years and have never seen a cat and put like a little puddle of cat pee in its cage, and the rat's going to go on the other side of the cage. Just hardwired, instinctual aversion to cat pheromones, and then put Toxo in a rat and it loses that aversion. And in fact, in a subset of rats, they like the smell. So out in the natural setting, you now go and approach a cat and assume the rodents inside the cat's stomach and Toxo is complete in its life cycle.

Joe Rogan (2:24)

Well, I had heard that it was, so it's a subset of rats that actually are gravitated towards it, because I've heard it actually rewires them sexually, right?

Robert Sapolsky (2:32)

Yes, that's actually work that we did in my lab, that it basically crosses some of the circuitry in the brain and hypothalamus so that cat pheromones that used to be activating every alarm circuit in your like limbic system in these rodents, now instead sort of taps into sexual arousal pathways. And in male rats, when they smell cat pheromones, they increase testosterone production.

So Toxo has just figured out the most brilliant way of doing it, it makes cat pee smell sexy.

Joe Rogan (3:08)

Is there any understanding at all of the mechanism of how a parasite can figure out how to rewire an animal's sexual reward system, the fear of the predators? Like, how does that work?

Robert Sapolsky (3:22)

Well, that's something my lab spent a bunch of years on, trying to figure out. I mean, you look at some of these parasites, this taps into this whole world of behavioral manipulation of hosts by parasites. And turns out they've like evolved unbelievably brilliant mechanisms for manipulating hosts for their own benefit. I mean, think about it, you get rabies, you get a rabid dog, and what that's about is a virus that has affected the nervous system of that dog so that it's now rabid and more likely to bite somebody with viral particles in its saliva, which it now passes on to the next individual. Like, you take 10,000 neuroscientists and stick them in a convention on the neurobiology of aggression, and rabies knows more about the neural wiring of aggression than we do. And toxo knows, quotation marks, something about fear and aversion and the neurobiology of attraction. Part of what it seems to involve is toxo somewhere along the way has picked up a gene that is pertinent to the dopamine system in mammals. Dopamine is this neurotransmitter. It's about pleasure. There's no protozoan parasite for buineers that's had any use for this stuff, except it's part of how toxo seems to be manipulating the reward system in rodents.

And then, a couple years ago, there's a paper showing that in chimpanzees, toxo makes you less afraid of the smell of leopards.

So this appears to be a parasite that just has evolved like this spectacular insight into fear circuitry and attraction circuitry. And it's all for its own benefit to wind up in a cat's gut.