COMPETITIVE MATE FORAGING IN THE BOWL AND DOILY SPIDER (FRONTINELLA PYRAMITELA)

STEVEN NELS AUSTAD, Purdue University

Abstract

Because male bowl and doily spiders rarely if ever eat, their only important resource is female eggs. This resource and males' exploitation of it were measured and manipulated in order to critically examine two optimal foraging models as well as a game theoretical model of combat behavior. By x-ray sterilization of males, and double matings using both normal and sterile males, I determined that the first male to mate will fertilize essentially all a female's eggs, even if a second male replaces him immediately after copulation. This first male sperm priority, though rare in insects, is common and perhaps prevalent in spiders. A standard foraging model was adapted to address the problem of optimal mate choice. The model predicts that males should mate with adult females and guard penultimate females until they mature. Males do this, but in addition they guard antepenultimates in contradiction to the model's predictions. A second foraging model considers optimal male mating duration. In contradistinction to this model's predictions males mate for an equal duration in habitats of high and low female density and throughout the breeding season in spite of major changes in female density and reproductive state. I quantitatively examined whether a modified model which assumes that although males cannot track short-term changes in these female parameters, they still optimize their mating duration according to a long-term averaging of the parameters. However males mate longer than even the modified model predicts. I argue that all of these foraging models err in neglecting two significant variables with which male spiders must contend: (1) a higher predation risk when travelling between webs, and (2) the existence of competing males in the environment. Males who meet a female web fight for access to her. A game theoretical model predicts that stronger males will win these fights when the female is equally valuable to both combatants; that the duration of these contests will be inversely correlated with the combatant's difference in strength; and that when male strength is equal, the combatant to whom the female is worth more will win. These predictions are borne out in staged encounters in which relative male strength and female value are manipulated.

Degree

Ph.D.

Subject Area

Ecology

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