The study is a fine example of an underappreciated part of science: replication. In 2000 British researchers carried out a study in which they put healthy and Toxoplasma infected rats in an outdoor enclosure and watched them nose around. They added odors to some of the corners of the enclosure; sometimes the odor of rats, sometimes of rabbits, sometimes of cat urine. They found that healthy rats were deeply affected by the scent of a cat, becoming less curious. Parasite-infested rats showed no fear. They proposed that the shift in behavior was an adaptation of the parasite for getting into its final host–cats. (I included a description of this study in my book Parasite Rex.)

It was a remarkable result,