Can solitary spiders (Araneae) cooperate in prey capture?

1. Cooperation in prey capture has never been reported for a solitary spider. Here, we conducted a laboratory experiment to determine if group hunting occurs in the ant-eating Zodarion spiders and if it yields higher individual benefit than solitary prey capture. 2. Spiderlings of the second instar were kept in groups of one, three and six individuals. Their foraging behaviour was studied with two ant species as prey: a large Messor and a small Tapinoma. 3. Groups of three and six spiders had better capture success than a single individual. The paralysis time of Messor was reduced as the number of spiders in a group increased, while that of Tapinoma ants was similar for all group sizes. The feeding time on Messor increased with the number of spiders in a group; all six individuals could feed simultaneously. For Tapinoma ants, six spiders fed for a significantly shorter time than the group of three spiders due to more intraspecific interactions. Only one to two spiders fed on Tapinoma at the same time. For spiders feeding on Messor the individual gain was on average 1.5 times greater than that when feeding on Tapinoma. The per capita gain decreased with group size but was not shared equally between the group members: the catchers gained about twice as much as the intruders. The catchers, however, experienced higher capture risk than intruders. 4. Our results show that solitary foraging is more beneficial when Zodarion spiders hunt small ants, while the strategy of group foraging is more beneficial when hunting large ants. The observed foraging behaviour of immature Zodarion spiders seems to be an example of mutual parasitism in which catchers are parasitized by intruders. Such interaction has a character of cooperation on a long-term scale.