科学60秒：好奇害死鼠——它们为何会被猫尿吸引？
猫体内的寄生虫弓形虫会增强小鼠的好奇心，让它们更容易被猫逮到，从而延续自己的虫生。
寄生虫生活在另一种生物的身上或者体内，有些甚至能够改变宿主的行为，增加自己的传染率。以单细胞的弓形虫（Toxoplasma Gondii）为例，感染这种寄生虫的老鼠更容易被猫尿的气味吸引，而未感染的老鼠会聪明地避开，因此感染弓形虫增加了老鼠落入猫口中的几率。显然，这对老鼠来说不是什么好事，但对寄生虫来说可再好不过了，因为它需要一只小猫来走完自己的一生，并传播给更多的宿主。
不择手段，确实如此。但事实上，这一狡猾策略可能没有像最初看到的那样精准打击，一项研究发现，弓形虫并没有针对性地只解除了老鼠对猫的天然厌恶，而是在整体上减少了老鼠的焦虑，让它们更大胆更爱冒险，对包括猫在内的一切事物都更加好奇了。研究结果发表在《细胞通讯》（Cell Reports）期刊上。
“弓形虫操纵宿主行为的故事简直太神奇了。”研究的资深作者之一、日内瓦大学（University of Geneva）的生物学家伊万·罗德里格斯（Ivan Rodriguez）如此讲道。
“我们特别好奇，寄生虫究竟是如何针对性改变了老鼠对猫科捕食者的反应的神经回路的，这一点先前从未讲清楚。”
所以，罗德里格斯和同事着手确定影响猫科动物致命吸引力的分子机制。以下是他们的第一步：
“我们决定使用受感染的小鼠展开广泛的行为试验，综合了解寄生虫如何影响其宿主。很快，我们就观察到受感染的小鼠不怎么焦虑，更有探索精神，面对潜在的威胁，它们的行为反应也截然不同于对照组小鼠。”
例如，与未感染的小鼠相比，感染弓形虫的小鼠能更快地找到高架迷宫的出口；它们敢接触人类研究员的手；当研究员将一只麻醉了的大老鼠猛地丢进笼子里时……
Curiosity Killed the ... Mouse?
Parasites live on or even inside another organism. And some can even change the behavior of their host to boost the odds of their transmission. Take the single-celled Toxoplasma gondii. Mice infected with this bug appear to become attracted to the smell of cat pee, an odor that uninfected mice smartly avoid. The infection thus raises the chances that a mouse will wind up in a cat’s mouth. Obviously, bad news for the mouse—but good news for the parasite, which needs a kitty to complete its life cycle and spread to additional hosts.
Devious, indeed. But it turns out this cunning scheme may be less precisely targeted than it initially appears—because a new study finds that Toxoplasma doesn’t specifically eliminate a mouse’s natural aversion to cats. Rather the infection makes them generally less anxious and more adventurous—which makes them them curious about cats and pretty much everything else. Their work appears in the journal Cell Reports.
“The story about Toxoplasma gondii manipulating the behavior of its host is simply fascinating.”
Biologist Ivan Rodriguez of the University of Geneva, one of the study’s senior authors.
“It was particularly intriguing for us to understand how the parasite achieves a specific alteration of the neural circuits involved in the response toward feline predators—something that has never been elucidated.”
So Rodriguez and his colleagues set out to determine the molecular mechanisms that underlie this legendary feline fatal attraction. For their first step:
“We decided to perform a quite broad panel of behavioral assays with infected mice in order to get an overview of how the parasite affects its host. We quickly observed that infected mice were less anxious, were more explorative and reacted quite differently than control mice when confronted with potential threats.”
For example, mice infected with Toxoplasma were quicker to check out the far reaches of an elevated maze than their uninfected comrades. They interacted with the human investigators’ hands and were unperturbed when an anesthetized rat was plonked into the middle of their cage...