Ontogenetic Shifts in the Costs of Living in Groups: Focal Observations of a Pholcid Spider (Holocnemus pluchei)

Publication Date

January 2000

Journal or Book Title

The American Midland Naturalist


Holocnemus pluchei spiders (Family Pholcidae) facultatively live in groups:sometimes they live alone and sometimes they share webs. In the field groups vary in sizeand composition and include spiders of all ages and either sex. Group membership is flexibleand individuals move frequently among groups. To understand group formation and maintenanceit is necessary to understand the costs of group membership. We used focal animalsampling to investigate the cost of group living for spiders of different ages across a rangeof group sizes. Both spider age and group size affected the costs incurred by group-livingspiders. There was no variation among groups of different sizes in the percentage of timefocal small or large spiders spent in costly behaviors (moving, web maintenance, bouncingor interactions with conspecifics), but medium-sized spiders spent more time engaged incostly behaviors with increasing group size. Medium and large spiders also had more interactionswith greater numbers of different conspecifics when they were in groups larger thanthree, whereas small spiders interacted rarely with conspecifics regardless of group size. Theseresults suggest that there are significant ontogenetic shifts in the costs of group living in H.pluchei.