老鼠愛上貓？其實是寄生蟲操縱宿主的一個典例。

When a healthy rat smells a cat, it flees. But rats infected with the Toxoplasma brain parasite actually follow cat odors, often presumably to their doom, red in tooth and claw. Now researchers have found out exactly what’s going on in the rat’s infected brain—it’s in love. The study is in the journal Public Library of Science ONE. [Patrick K. House, Ajai Vyas and Robert Sapolsky, "Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats"]

That Toxo-infected rats approach cats has long been a well-known example of a parasite manipulating the behavior of its mammalian host.

The protozoan needs to be in a cat’s intestine to reproduce. It then gets pooped out and has to find its way into another cat to continue its fabulously glamorous lifestyle. Rats that touch the poop and then get eaten by a cat can complete the circuit.

The new research reveals that when a Toxo-infected rat smells cat urine, it has increased activity in brain regions associated with sexual attraction. Which the researchers say is “compelling evidence that Toxoplasma overwhelms the innate fear response by causing, in its stead, a type of sexual attraction to the normally aversive cat odor.” That’s right, the rat is turned on. Before it’s permanently turned off.

—Steve Mirsky