EFFECTS OF BODY SIZE ON AGONISTIC ENCOUNTERS BETWEEN MALE JUMPING SPIDERS (ARANEAE, SALTICIDAE)

Agonistic encounters were staged between males of the jumping spider Zygoballus rufipes to study the relationship between body size (as estimated by weight) and encounter duration, intensity and outcome. Encounters terminated at one of five different levels of intensity. All encounters started at the lowest level of intensity, and most bilaterally escalated to higher levels. A small number of encounters were unilaterally escalated by the heavier male from the lowest level of intensity directly to the highest level. The associations between weight difference and encounter outcome, duration and maximum level of intensity were assessed using the likelihood ratio statistic. As weight difference decreased, more encounters than expected were won by the lighter male, more were of longer duration and more escalated bilaterally to higher levels of intensity before terminating. Within levels of weight difference, heavier males won more bilaterally-escalated encounters terminating at higher levels of intensity than expected by chance, but won fewer encounters terminating at lower levels of intensity. These results suggest that weight has a significant influence on agonistic encounter outcome, duration and maximum level of intensity, and that when weight difference is small, bilateral escalation to higher levels of intensity is necessary for males to accurately determine body size differences. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.