Social aggregation in the terrestrial isopod Armadillidium vulgare

Abstract

Social aggregative behaviour may have evolved in marine isopods to aid hunting, and help the animals protect themselves from predation. This social behaviour could have assisted some isopod species to make the evolutionary transition from marine to terrestrial environments, as aggregative huddling of isopods on land reduces desiccation when out of water. To determine if the common pill-bug Armadillidium vulgare demonstrates social behaviour, groups of animals were filmed for 60 min in an arena that contained a binary choice of two shelters. This experiment was directly repeated with the same specimens three more times in the same day and this series of four experimental runs was replicated a total of 10 times with a different group of animals for each of the 10 series. In most cases, the majority of specimens chose one of the two identical shelters at random, and congregated under this winning shelter. The amount of animals that congregated under the winning shelter, however, significantly decreased over the four consecutive runs, whereas the amount of animals that congregated under the losing shelter significantly increased with prior experience. These data indicate that A. vulgare is a social animal but this social behaviour can decrease with exposure to the novel environment. This attenuation in social behaviour may serve to decrease the drive to join the larger of the two groups in an area where a danger from predation has not been encountered and allow experienced animals to explore new terrain.