Despite being only a few millimetres long and living in what most humans would perceive as a featureless environment, desert ants (Cataglyphis fortis) have no problems finding their way back to the nest having meandered around on a foraging expedition. Ants use path integration to navigate: to get back home, they keep track of their travel direction, by using a celestial compass, and their travel distance. Until now, scientists were less clear about how ants measured distance, strongly suspecting that they use an internal pedometer. This takes into account an ant's stride length and stride frequency on the outward journey, telling them the distance of the return journey. Matthias Wittlinger and Harald Wolf from the University of Ulm, Germany, and their colleague Rü diger Wehner from the University of Zü rich, Switzerland, show that adjusting the length of ants' legs proves that they do use a pedometer to calculate distance(p. 198).

Wittlinger made a chance observation that clipping ants' legs shorter before they make their homeward journey causes them to start searching for the nest earlier, suggesting that they have an internal pedometer. He suspected that ants with short legs probably take shorter strides and should underestimate distance, while their longer legged peers should overestimate distance because they take longer strides.

To investigate further, the team trained ants to walk down a 10 m run from their nest to a feeder. After training, they caught ants at the feeder and clipped their legs to form stumps, or lengthened their legs by gluing on pig bristles so that they walked on stilts. Carrying tasty morsels provided for them by the experimenters, the ants were put into a test run immediately next to the training run. The team measured where the ants thought their nest was by noting where they started their nest searching behaviour. They found that ants on stilts overestimated the distance to the nest, while ants with stumps underestimated the distance. While this told the team that different leg lengths affected the ants' ability to find the nest again, they had to be sure that they were using information about their leg movements and not other information, such as travel time, to calculate their home journey.

To scrutinise the ants' leg movements in more detail, they filmed them walking to the nest, gave them stilts or stumps, and then filmed the return journey. They measured the ants' stride length, stride frequency and walking speed as they marched along on stilts or stumps, and found that at a given walking frequency, ants on stumps took shorter strides and ants on stilts took longer strides than normal ants. They also found that ants with longer legs strode at a slightly lower frequency, on average, than ants with shorter legs. And despite the experimenters' best efforts, the ants were very adept at walking stably on their short or long limbs without stumbling.

The team's results confirmed that changes in stride length and frequency,caused by manipulating the ants' legs, made them to go awry when searching for their nest, proving that ants rely on an internal pedometer to help them get around their desert home.

Wittlinger, M., Wehner, R. and Wolf, H. (
). The desert ant odometer: a stride integrator that accounts for stride length and walking speed.
J. Exp. Biol.