The influence of landmarks on the systematic search behaviour of the desert isopod Hemilepistus reaumuri

Summary

Each family of the desert isopod Hemilepistus reaumuri uses its faeces to build an embankment around its burrow entrance in the form of a ring with an inner radius of 5–10 cm and an outer radius of 8–15 cm. Although the embankment like the burrow entrance, is detected only by contact with the antennae, it facilitates the return to the burrow after an excursion. When a desert isopod returns from foraging it reaches the burrow entrance itself (diameter 12 mm) only in ca. 12% of the observed cases by “dead reckoning”; the embankment, however, is reached six times more frequently. After detecting this landmark an isopod needs a further 12% of the pathlength of the preceding excursion to locate the burrow entrance. In contrast, it travels five times further to reach its burrow from the same distance (10 cm) if the landmark is missing. The landmark is even more important when the isopod, because of a large error in orientation, can reach its burrow only by searching. Both the characteristics of the search path of H. reaumuri and the success of its search behaviour correspond well to the respective features of a theoretical procedure, which best solves the search problem of the desert isopods assuming that a landmark is available for orientation. The optimal search procedure was calculated by using information about the probability density that an error in orientation has a given size, and the cumulative probability that the landmark is detected if the isopod searches in its region with a given intensity. The coupling of the systematic search strategy of H. reaumuri with landmark orientation shortens the average time needed for homing by one-third. It helps and isopod to return to its burrow after foraging even if all the orientation mechanisms normally used for this task have broken down.