Doctoral Dissertations

Date of Award

8-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Life Sciences

Major Professor

Susan E. Riechert

Committee Members

Gordon Burghardt, Arthur Echternacht, John Gittleman, Charles Pless

Abstract

Spiders in the Anelosimus studiosus species group with the largest geographic distribution are quasisocial, showing true cooperative behavior. Furthermore, the geographic range of quasisocial behavior in this group is correlated with annual precipitation while temperature seems to be of secondary importance. Anelosimus studiosus shows behavior exhibited by the most advanced social spiders, fulfilling the three classical conditions of spider sociality: interattraction, tolerance and cooperation. Interattraction in A. studiosus was found as a tendency to aggregate, based on both lab tests and field observations. Tolerance for conspecifics was directly observed in both field and lab colonies. Cooperation, like tolerance, was found under natural and manipulated conditions. Cooperation in these animals was found to include prey capture, brood care, and, since the spiders inhabit a single group nest, web maintenance. By exhibiting skewed sex ratios and limited dispersal as well, A. studiosus fits the mold of a cooperative species. The cooperative habits of A. studiosus have allowed this animal to exploit a niche not available to solitary temperate spiders. A stochastic, individual-based computer model based on A. studiosus was used to test four hypotheses pertaining to group foraging: (1) energetic intake and fitness are vii positively correlated, (2) relatedness has no effect on optimal and equilibrium group sizes, (3) history (initial group size) has no effect on optimal and equilibrium group sizes, and (4) groups are more easily invaded at high food availability and by related individuals. Results show that (1) energetic and reproductive fitnesses differ markedly with group size, (2) both relatedness and history influence significantly different measures of fitness, and (3) groups are more easily invaded when small and at high food availability, but relatedness had no significant bearing on results. Insect prey levels and predation have been identified as factors effecting group size in the social spider Anelosimus studiosus. Colonies under conditions that changed for the worse decreased the group size through emigration, though these spiders did not leave the vicinity of the parent nest. Colonies will maintain themselves under conditions where per capita nest volume is below that found in solitary A. studiosus webs built in low insect density conditions.

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