How cooperation and colony size affect prey size use among sympatric social spider species

Background/Question/Methods

Social spiders live in temporary or permanent colonies in which individuals cooperatively raise young, repair webbing and capture prey. Previous studies have shown that where their ranges overlap, different species target insect prey of different sizes. This study aims to determine whether and to what extent this difference in prey size preference is due to differences among species in their degree of cooperation. Colonies from 3 different species were presented with prey of varying sizes and data was recorded on the number of spiders that attacked the prey and the number of spiders that emerged from their refuges but declined to attack (i.e., freeloaders). We evaluated the degree of cooperation for each species using two non-mutually exclusive metrics of cooperation. 

Results/Conclusions

Two of the three species had previously shown a positive correlation between the size of the colony and the average size of prey captured, while the third species showed no such correlation. From this we predicted that these species with the positive correlation would be more cooperative than the other species. Our results indicate that all species recruit a similar number of attackers for the hunt, but that the species without the correlation tend to have a higher proportion of freeloaders, meaning that they are arguably less cooperative as a group. However, there is a tight correlation for all species between the number of spiders that attack the prey and the total population of the colony, so the species that build large colonies are able to draw from a larger pool of potential attackers and subdue the largest prey items.  It appears then that differences among species in prey size use are not only due to differences in cooperation, but are also greatly determined by the size of the colonies.