Why be social? The costs and benefits of sociality in spiders

Methods

Summary

To determine modes of transmission of the various parasites, I will map the population structures of the social spider and their associated parasites using genetic markers obtained with RAD sequencing. If host and parasite have a similar degree of relatedness across space, then I have evidence of vertical transmission. Here, the parasites move with the social spiders when a daughter colony buds off a parent colony. If host and parasite have different degrees of relatedness, that would suggest horizontal transmission. In this case, parasites move among neighboring spider colonies.

I will perform an exclusion experiment to assess the parasites’ effect on fitness. Colonies will be assigned to one of three treatments: (1) no parasites, (2) normal level of parasites, and (3) higher than normal level of parasites. I will surround colonies in all three treatments with a fine netting to prevent anything from getting in or out. Because netting will exclude prey as well parasites, I will introduce an appropriate amount of food for the size of the colony based on the results of my energetics research in the summer of 2016. At this time, I will also collect a sample of spiders from each colony and assess the condition of the adult females. One month into the trials, I will begin collecting and dissecting every new egg sac to count the number of eggs and parasite larva within. Additionally, at every visit to the nest I will note the presence of any new cocoon webs built by parasitized spiders.

Challenges

Social spider colonies follows patterns of boom and bust. The nest is a three dimensional silk structure (think of a basket made of silk), with support and prey capture lines attaching the basket to vegetation above. The colony is three dimensional, but the prey capture surface is only two dimensional. This means that when nests grow too large, they can't get enough prey and are susceptible to colony extinction. Because of these scaling properties, colonies can quickly overshoot a sustainable size and go extinct in a short period of time. This makes longer term studies tricky because sample sizes will only shrink during the study period.