Life in the Namib desert is harsh. With high temperatures reaching up to 71°C, venturing out of the nest can be risky. Rüdiger Wehner and Martin Müller from the University of Zürich, Switzerland, explain that foraging Namibian desert ants find their burrows quickly to avoid the lethal heat, but how do they locate their burrow homes in such a featureless landscape? According to Wehner and Müller, roaming ants keep track of their outbound route and return by taking the most direct path between their final destination and the nest. However, returning foragers don't always target the burrow as precisely as they might like: ‘Because of the inevitable accumulation of path integration errors, homing accuracy rapidly decreases as foraging distance increases,’ say Wehner and Müller. Instead, returning foragers have to rely on local landmarks to help them pinpoint their goal in the final approach. Curious to find out how the ants use landmarks to locate the nest, Wehner and Müller recorded the insects' homing times with and without guiding landmarks (p. 4174).

According to Wehner and Müller, ambient air temperature has a dramatic effect on the insect's running speed so an ant could take anything from 0.9 to 1.6 s to cover the last 40 cm to the nest, depending on the heat. However, when they measured the time it took returning ants to cover the distance without landmarks to refer to, the ants searched for an average of 23.5 s before finding the entrance and some took several minutes. Then, when the nest entrance was 15 cm from a granite rock landmark, the ants' accuracy improved significantly, reducing their search time to an average of 3.3 s. Having a local landmark to steer by significantly reduced the ants' homing time.

Curious to find out how the insects use local landmarks to locate home, Wehner and Müller positioned two 15 cm high cylinders 40 cm from a nest separated by an angular distance of 90 deg. Sure enough, with the benefit of the landmarks to steer by, the ants' homing time decreased significantly, but how were they using the landmarks to improve their performance?

Wehner and Müller divided the 40 cm radius area around the ants' nest into four sectors and monitored the length of time it took ants in each sector to identify the nest entrance. Ants that ran head on to the nest (with the cylinders behind the nest) had the most difficult time, with one taking over a minute to find home. However, the ants that scurried in between one of the cylinders and the nest usually found the burrow in under 10 s. Plotting the way that the images of the cylinders moved over the insects' retinas as they scampered through each of the sectors, the duo found that the amount of movement of the image across the running ant's eye – visual flow – varied dramatically from sector to sector. Instead of focusing on discrete landmarks to guide them, the ants rely on a completely panoramic view and they use visual flow across the eye as they run to locate their burrows and return home speedily.

Piloting in desert ants: pinpointing the goal by discrete landmarks
J. Exp. Biol.