A game theoretical interpretation of male combat in the bowl and doily spider (Frontinella pyramitela)

Abstract

Male bowl and doily spiders (Frontinella pyramitela: Linyphiidae) fight over access to adult females. The probability of disablement or fatal injury in these contests is a linear function of their duration. Thus the fights fit closely the assumptions of a version of the game theory ‘war of attrition’. Because the details of sperm competition are known, female value (expected number of eggs fertilized) to either a copulating male or an intruder can be quantified for any stage of copulation. Using this information, the predictions of the war of attrition model were critically examined by experimental manipulation of fighting ability and female value for both web residents and intruders. When female value was equal for both combatants, the ability to win fights was correlated with body size and the duration of contests was inversely correlated with size difference. The winning percentage of males of specific size differences varied directly with female value. The treatments with the highest percentage of injuries and fatalities were those in which a smaller resident was defending a female of great value to him. In two treatments in which larger web residents were contesting for females of low value, the predictions of the model were seemingly not supported. However, a quantitative analysis of the treatments shows that these apparently anomalous results are in accord with the model.