The same brain region that triggers a mating response in male rats also lights up when rats smell cat urine—if those rats are infected with the parasite Toxoplasma. The reaction doesn’t necessarily mean the rats find cats sexually appealing. It’s a trick that Toxoplasma plays to have the rats eaten by cats, a clever manipulation of rat behavior that is part of the parasite’s reproduction scheme. For a rat, fear of cats is rational. A cat’s small intestine is the only environment in which the parasite, Toxoplasma, can reproduce sexually, so it is critical for the parasite to get itself into a cat’s digestive system in order to complete its life cycle.
