Animal behaviour: A stilted story

　　IF THERE were a Royal Society for the Prevention of Cruelty to Ants, Matthias Wittlinger of the University of Ulm, in Germany, would probably be top of its hate list. The reason is that Dr Wittlinger and his colleagues have, as they report in this week's Science, been chopping the feet off ants. And not only that. They have been making other ants walk around on stilts.
　　Saharan desert ants of the genus Cataglyphis have to travel long distances to discover food in their impoverished, sandy environment. How they find their way home once they have done so is a mystery. Ants in more temperate climates often lay down chemical trails, but Cataglyphis, apparently, does not. Like honeybees and ancient mariners, they can navigate by the sun, so they know the general direction in which to travel. But, also like ancient mariners (who knew their latitude, but not their longitude), such solar reckoning cannot tell them when to stop.
Dr Wittlinger, therefore, decided to investigate a century-old hypothesis that desert ants have internal pedometers-in other words, they count their steps out, and they count them back. When one total matches the other, they are home. To test this idea he trained his ants to walk from their nests to a feeding station through a ten-metre-long channel. When they had picked up the food, he caught them and made them return through a different channel, which also led to the nest. When they made this return journey, they began their characteristic nest-searching behaviour, quartering the ground in detail looking for the entrance, after travelling about ten metres.
　　Once the ants had mastered this trick, the experiment proper began. Some ants, when they arrived at the feeding station, had the ends of their legs amputated, to shorten their stride length. Others were fitted with stilts in the form of pig-bristles glued to their feet. Both lots were then returned to the feeding station, to make the journey home.
　　As predicted, the ants on stilts, whose stride-length meant their internal pedometers had not clicked enough times, walked blithely past their nests, and were left stranded almost five metres on the far side before they started looking for the hole. Meanwhile, the poor stumped ants travelled only about six metres before they started their search. 
　　The story, however, has a happy ending. Having proved his point, Dr Wittlinger returned both stumped and stilted ants to the nest and gave them a few days to recover. Then he let them out for another run. Now that they could re-count their outbound journeys, they were able to calculate the journey home correctly. Ants may not be very bright, but it seems they have a head for figures.

　　参考译文：

　　动物行为：踩高跷的故事

　　假如有一个皇家防止残害蚂蚁协会的话，德国乌尔姆大学的马提亚•威特林格可能会首当其冲地成为其憎恨对象。这是因为威特林格博士和他的同事们在本周《科学》杂志上报道说他们砍去了一些蚂蚁的脚。这还不算，他们还让别的蚂蚁踩着高跷走路。
　　撒哈拉沙漠的箭蚁不得不在贫瘠的沙质环境中长途跋涉寻找食物。它们找完食物后如何返回蚁巢却一直是个谜。温和气候环境下的蚂蚁往往会在路途中留下化学记号，可箭蚁显然不会。它们跟蜜蜂和古代海员一样，可以通过日照导航，从而知道出行的大致方向。但是也跟古代海员一样（他们只会辨认纬度不会辨认经度），这种根据太阳测算位置的方法无法告知它们该在何时结束行程。
所以，威特林格博士决定验证一下这个世纪猜想--沙漠蚂蚁体内含有步数计（里程表），换言之，它们在出行时算好步数，然后再一步步数着返回。等到两者总数吻合时，就"到家"了。为此，威特林格博士对其养殖的蚂蚁进行了训练，让它们经过一条十米长的沟从蚁巢走到一个饲喂点。当它们获取食物后，就把它们捉住，让它们通过另一条通往蚁巢的沟返回。返回时，它们特有的觅巢行为就开始了，即在行走了大约10米远后，它们会在地上仔细地刻下指向蚁巢入口的标记。
　　等到蚂蚁学会了这一窍门，实验才真正开始。有的蚂蚁抵达饲喂点后，它们的腿部末端被切除，以缩短其步长。其他的蚂蚁则在其脚部粘上猪鬃，做成高跷。然后，两群蚂蚁均被放还至饲喂点，让其返回蚁巢。
　　不出所料，踩高跷的蚂蚁由于其步长造成体内步数计无法运作足够次数，因此竟似闲庭信步一般走过了蚁巢，结果在离蚁巢近5米远的地方愣住了，然后才开始继续寻找洞穴。与此同时，遭"截肢"的那些可怜的蚂蚁只走了大约6米远就开始寻找洞穴。
　　不过，这个故事的结尾还是令人高兴的。威特林格博士证实其猜想后，又把"截肢"蚂蚁和踩高跷蚂蚁都放回蚁巢，给它们几天时间来康复。然后，他把它们放出去开始又一轮试验。这一次它们可以重新计算出行步数，因此也就可以准确地计算出返程的步履了。蚂蚁也许并不太聪明，但似乎天生就是数字高手。