It"s one of our common beliefs that mice are afraid of cats. Scientists have long known that even if a mouse has never seen a cat before, it is still able to detect chemical signals released from it and run away in fear. This has always been thought to be something that is hard-wired into a mouse"s brain. But recently Wendy Ingram, a graduate student at the University of California, Berkeley, has challenged this common belief. She has found a way to "cure" mice of their inborn fear of cats by infecting them with a parasite, reported the science journal Nature.
The parasite, called Toxoplasma gondii, might sound unfamiliar to you, but the shocking fact is that up to one-third of people around the world are infected with it. This parasite can cause different diseases among humans, especially pregnant women—it is linked to the blindness and death of unborn babies.
However, the parasite"s effects on mice are unique. Ingram and her team measured how mice reacted to a cat"s urine (尿) before and after it was infected by the parasite. They noted that normal mice stayed far away from the urine while mice that were infected with the parasite walked freely around the test area.
But that"s not all. The parasite was found to be more powerful than was originally thought—even after researchers cured the mice of the infection, they no longer reacted with fear to a cat"s smell, which indicated that the infection has caused a permanent change in mice"s brains.
Why does a parasite change a mouse"s brain instead of making it sick like it does to humans The answer lies in evolution. Toxoplasma gondii can only reproduce inside a cat. So the parasite had to develop a way of tricking the mice into getting eaten more easily—thus helping itself go inside a cat—by taking away mice"s sense of alarm. The underlined part "hard-wired" in Paragraph 1 probably means （）.