CRF Blog


by Bill Hayes

In Mindsuckers, National Geographic reports on how parasites can change an animal’s behavior.

The best known mindsucking parasite plays out a similar manipulation on land. Along with other mammal species, rats and mice can be infected with Toxoplasma gondii, a single-celled relative of malaria-causing Plasmodium. The parasite can form thousands of cysts in the brain of its host. To take the next step in its life cycle, Toxoplasma has to get inside the gut of a cat. Toxoplasma doesn’t have the means to transport itself from a rat’s brain to a cat’s gut. But if its rat host gets eaten by a cat, the parasite can reproduce. Scientists have discovered that rats infected with Toxoplasma lose their normal fear of the smell of cats. In fact some infected rats become downright curious about the odor of cat urine, making themselves easy targets for a swipe of a cat’s paw — and thus raising the odds that Toxoplasma will advance through its life cycle.

How mutations and natural selection could give rise to such creepy powers is a particularly intriguing puzzle for evolutionary biologists. One useful concept for thinking about it comes from biologist Richard Dawkins, author of the landmark book The Selfish Gene.

In that book Dawkins argued that genes evolve to make copies of themselves more successfully. Our bodies may be important to us, but from our genes’ point of view, they are nothing more than vehicles to get themselves intact into the next generation. The entire collection of the genes that make up you or me is called our genotype. The sum total of all the bodily parts and functions that our genotype creates to advance its cause — you or me — is called our phenotype. [more]