Ontogenetic shifts in the costs of living in groups: Focal observations ofa pholcid spider (Holocnemus pluchei)

Holocnemus pluchei spiders (Family Pholcidae) facultatively live in groups: sometimes they live alone and sometimes they share webs. In the field grou ps vary in size and composition and include spiders of all ages and either sex. Group membership is flexible and individuals move frequently among gro ups. To understand group formation and maintenance it is necessary to under stand the costs of group membership. We used focal animal sampling to inves tigate the cost of group living for spiders of different ages across a rang e of group sizes. Both spider age and group size affected the costs incurre d by group-living spiders. There was no variation among groups of different sizes in the percentage of time focal small or large spiders spent in cost ly behaviors (moving, web maintenance, bouncing or interactions with conspe cifics), but medium-sized spiders spent more time engaged in costly behavio rs with increasing group size. Medium and large spiders also had more inter actions with greater numbers of different conspecifics when they were in gr oups larger than three, whereas small spiders interacted rarely with conspe cifics regardless of group size. These results suggest that there are signi ficant ontogenetic shifts in the costs of group living in H. pluchei.