This has got to be the smartest protozoan on earth. Toxoplasma gondii, a parasite that infects hundreds of millions of people (an estimated 50 million in the U.S.), has a genome that's only 80,000 units long compared to humans' 3 billion, and certainly no room for a brain in an organism smaller than the dot over this i. Yet in its unstoppable drive to survive, it manages an astonishingly clever trick: it erases rats' and mice's innate fear of cats.
Why would Toxoplasma want to do that? Because although it lives in rodents' brains, it can reproduce only in the intestine of a cat. To perpetuate itself, it therefore has to induce a cat to eat a rodent. Hardly a challenge "but getting the rodents to sit still for such a thing is. In 2000, scientists at Oxford University found that Toxoplasma manages the feat by making rodents lose their fear of cats, regarding them as no more a threat than, say, bunny rabbits.
Now scientists have figured out how the protozoan manages this. Scientists at Stanford University report that Toxoplasma-infected mice and rats don't lose their fear of everything, nor do they become less anxious overall. They are just as neurotic about strolling through an open field (you never know when a predator might swoop down from the skies or in from the sidelines) or eating unfamiliar food. But they lose their fear of bobcat urine, the smell of which tells them they might be on the dinner menu. The reason, the Stanford team finds, is that Toxoplasma cysts make a beeline for a particular brain circuit centered on the amygdala, the structure that coordinates fear responses; the cysts were twice as dense there as in other brain regions. Rodent brains have a brain circuit dedicated to feeling afraid of the odor of predators, and this seems to be what the Toxoplasma cysts disrupt. The loss of fear "is remarkably specific," the scientists report in Proceedings of the National Academy of Sciences. The discovery underlines how exquisitely adapted even the lowliest protozoan is for survival.