A cataglyphis ant with legs lengthened by stilts.Credit: Harald Wolf
When humans want to count their steps they use pedometers, but, according to new findings published in the June 29th issue of Science, ants need no such device. German scientists discovered that ants have an internal step-counting mechanism, which they use to gauge the distance they've traveled.
Navigation is a universal problem for animals, said Harald Wolf, professor of neurobiology at the University of Ulm, Germany, and lead author of the study.
"Whether you are an ant or you are a hamster, you have to measure direction and distance in some way," he said.
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Ants, bees and other insects are known to use the position of the sun and the pattern of polarized light as a "celestial compass" to guide their travels. Bees are also able to gauge distance by measuring how fast images move across their eye, but this mechanism does not work for land-bound creatures. In the past, behavioral scientists have proposed that ants might determine distance by measuring the energy required for locomotion, the time of their journey or the number of steps they take.
Along with Rüdiger Wehner, professor of zoology at the University of Zurich, Wolf's team trained two groups of ants to locate a feeding site 10 m from their nest. After the ants had learned the way, the team removed them from the feeding site. They then clipped the legs of one group, and extended the legs of the other group with glued-on boar bristles, which served as stilts.
"I remember people were really laughing when we proposed this," said Wolf, "but the ants were very co-operative."
The two groups were then released in another location from which they tried to make a run for home, stopping where they thought their nest should be. If the animals were using a step-counting mechanism, the change in stride-length for their return journey would throw off their calculations. Indeed, the stilt-leg ants overshot the distance, stopping after 15.3 m, while the amputees stopped after just 5.75 m.
However, when the ants began their search for food from home base, both groups had no trouble judging the correct return distance, despite their altered limbs.
The step-counting hypothesis was first proposed about a century ago, but it was never tested directly until Matthias Wittlinger, a graduate student in Wolf's lab, had a somewhat macabre Eureka moment: One evening, he clipped off the lower leg segments of a few Sahara desert ants studied in the lab. In the wild, these segments often dry and break off, without interfering with the insects' abilities to walk long distances. But Wittlinger noted that the procedure resulted in a shorter than normal stride.
"That gave us the idea that there might be a step-counter," said Wolf. "Of course, ants don't actually count."
A more accurate description is a "stride integrator," a neural mechanism that might keep a tally of motor excitation caused by each stride and then converts this activity to a measure of distance.
"It's a very nice experiment," said Thomas Collett, a neurobiology professor at Sussex University.
In mammals, Collett notes, navigation is more complicated because it is linked to visual landmarks, but insects provide a simplified model.
"With ants, you can do the sorts of experiments you can't do with hamsters and birds," said Wolf.
What's more, he added, understanding the ant walking mechanism may help researchers design better navigation systems for walking robots.