After my post about why cats are so useful to us, a related post about why rats sometimes don't fear the smell of our cats. The idea is explained by, again, the great thinker and researcher of mammalian emotions Jaak Panksepp, but this time he is interviewed on Ginger Brown's other podcast, Brain Science Podcast. (I highly recommend it to those who are interested in the new developments in neuroscience.) The thing is this: Rats can become fearless if they are infected by a particular parasite.
Dr. Campbell: Is that toxoplasmosis?
Dr. Panksepp: Toxoplasma gondii... which is very prevalent in the world; many people are infected with it. France seems to have the highest level of infection. We get it from our cats. That's why mothers should not be cleaning cat poop out of a litter box.
It turns out that Toxoplasma gondii, in order to reproduce, the only place it reproduces is the stomach of a cat. So, how does Toxoplasma get into the stomach of a cat? Obviously by things cats eat. Cats like to prey on little rodents. Little rodents get exposed to Toxoplasma gondii. The cat eats it. And when the cat eats it, it eats rodents that aren't too scared of it. It turns out that Toxoplasma makes rats less fearful of cat smell.
Toxoplasma actually gets into the brain, as Sapolsky has shown, and blocks the cat smell from activating fear. Wow! That means these rats are more liable to end up in the stomach of a cat. So, that makes a wonderful evolutionary story: how a little bug outwitted the rat emotional system to get into the cat's stomach.
When reading passages like this, one has to wonder if lots of the things that we do as animals (things that are good or bad for us) have their roots in pathogens? Dawkins called us robots for the genes, but what if we are more robots (or better yet zombies) for parasites?