Many species roost communally but the proximate causes and ultimate functions of this widespread behavior remain poorly understood. We studied the communal roosts of two undescribed species of harvestmen in the genus Prionostemma Pocock 1903 at a Caribbean rainforest site in southeastern Nicaragua. The species are quite similar in gross morphology but differ in body coloration, male genitalia, and roosting behavior. One species roosts primarily on spiny palms while the other species, which is darker in coloration, roosts inside buttress root cavities. In a mark-recapture study, the cavity-roosting species had higher levels of individual site fidelity than found previously in the spiny palm-roosting species, perhaps because suitable cavities are scarcer than spiny palms. The tree cavity aggregations were strongly male-biased, which our review of the literature suggests is unusual for harvestman roosts. The overall sex ratio of the spiny palm aggregations was 1∶1, but some roost sites were strongly male biased while others were strongly female biased. Removing all harvestmen from 10 spiny palm roost sites shifted the overall sex ratio toward males on subsequent days, but the sites with skewed sex ratios remained skewed in the same directions despite complete turnover in roost membership. These results are discussed in relation to mechanisms of roost formation and possible sex differences in vagility, microhabitat preferences and sensitivity to disturbance. Both species also occur at La Selva Biological Station in Costa Rica but neither forms roosting aggregations in spiny palms or tree cavities there. A possible explanation for the geographic variation is that roosting patterns change over time through cultural drift.
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Vol. 42 • No. 3