Comparing cooperative hunting capabilities of social and sub-social spiders

Background/Question/Methods

Despite its potential costs, cooperation can be beneficial when it allows a species to exploit a resource that would otherwise be unavailable to a solitary organism. Anelosimus is a genus of cobweb spiders whose member species show varying degrees of social behaviour. “Social” species live in large permanent colonies containing multiple adult females and their offspring, while “sub-social” species live in single-family groups during the early portion of their life and solitarily as adults. Though these social types are usually allopatric due to prey size availability and dispersal costs in different environments, there are several locations where their ranges overlap, allowing for a direct comparison of their cooperative behaviours. This study compares the prey-capture abilities in four species with varying levels of sociality, aiming to (a) determine how effective each species is at capturing prey of different sizes, and (b) quantify cooperation in terms of the number of spiders recruited to hunt prey. Prey of various sizes was propelled into dozens of webs of each species and data was recorded on the number of spiders that attacked the prey and whether or not the spiders were successful in subduing it. 

Results/Conclusions

As expected, social species recruited a larger number of individuals to subdue the prey than sub-social species, and they were better at capturing the largest prey items. Since social species have a larger range of prey sizes available to them it was predicted that the sub-social species might overcome this niche overlap by being more proficient at capturing smaller prey. We found, however, that social and sub-social species were equally successful at capturing small prey. The difference is that the sub-social species achieved this with fewer individuals, suggesting that the average hunting capability of a single sub-social individual is greater than that of a social one. This makes sense given that the sub-social spiders experience a solitary phase to their lifecycle, during which they must be self-sufficient.