New communal roosting tradition established through experimental translocation in a Neotropical harvestman

Abstract

harvestmen Opiliones pheromone scent marking translocation 'Tradition' has been invoked to explain instances in which animals aggregate repeatedly in the same locations for no apparent reason, but alternative explanations, such as cryptic habitat selection, are difficult to rule out. Distinguishing among these hypotheses requires field experiments. We studied a species of harvestman (Prionostemma sp.) that forages solitarily at night and forms roosting aggregations in spiny palms by dawn. Aggregations have formed repeatedly in the same sites for over a decade, and yet the aggregation sites do not appear to differ from unused sites in tree characteristics or microclimate. Previous research suggested that the harvestmen find aggregation sites by detecting chemicals left behind by conspecifics. If so, it should be possible to establish new aggregation sites simply by moving harvestmen to new locations. We carried out such an experiment and, as predicted, release sites attracted harvestmen at much higher rates than did matched control sites. Most individuals that came to release sites were new recruits, and one site continued to attract several harvestmen for at least 7 days, suggesting that a new roosting tradition was established. We also revisited the question of whether any other factors, besides past usage, predict site use. Among sites used by the harvestmen, the size of the aggregations was strongly predicted by microclimate and spiny palm characteristics. Together these findings support the hypothesis that attraction to conspecific cues is the primary mechanism responsible for the long-term stability of aggregation sites, while also suggesting that site characteristics play a previously unrecognized role. Understanding how site traditions develop may have direct applications in conservation.

Communal roosting sites are potential ecological traps: experimental evidence in a Neotropical harvestman

“…Roost sites on the edges of clearings or in treefall gaps experience higher daytime air temperatures than do roost sites with intact overstory forest canopies. Spiny palms used as aggregation sites do not differ in forest canopy cover from a random sample of unused spiny palms , but among used sites, the size of the aggregations correlates positively with canopy cover and negatively with mean air temperature (Teng et al 2012), which is consistent with the hypothesis that sites with intact overstory canopies are more suitable. Prionostemma enter roost sites before dawn and may have no way of sensing whether conditions at a site will be survivable later in the day, but these animals are quite capable of moving during daylight hours (Smith et al 2012), and would probably leave an exposed roost before succumbing to desiccation.…”

“…Spiny palms that are used by the harvestmen do not appear to differ from those that are not used, either in the characteristics of the palms themselves or in microclimate Teng et al 2012). Nevertheless, some spiny palms have been used for over 10 years, while others of the same species have remained unused (Teng et al 2012). While the locations of the communal roosts are very stable over time, individual harvestmen are not site faithful .…”

“…Roosting aggregations usually form on the trunks or fronds of spiny palm trees (Bactris, Astrocaryum) in the forest understory. Spiny palms that are used by the harvestmen do not appear to differ from those that are not used, either in the characteristics of the palms themselves or in microclimate Teng et al 2012). Nevertheless, some spiny palms have been used for over 10 years, while others of the same species have remained unused (Teng et al 2012).…”

“…While the locations of the communal roosts are very stable over time, individual harvestmen are not site faithful . The reason the roosting aggregations form repeatedly in the same locations appears to be that the harvestmen preferentially settle in sites marked with conspecific scent Teng et al 2012).…”

“…Second, traditional roosts may continue to be used long after they become unsuitable (Rahaingodrahety et al 2008;Teng et al 2012). Protection of roosting habitat is recognized as a conservation priority for many species (Aguirre et al 2003;Alonso-Mejia et al 1997;Barclay and Brigham 2001;Brooke et al 2000;Cardiff et al 2012;Curtis and Machado 2007;Dellasala et al 1998;Dennis 2004;Fleming et al 2013;Harms and Eberhard 2003;Kessler et al 2013;Lambertucci et al 2008;McGeoch and Samways 1991;Rahaingodrahety et al 2008;Smith and Racey 2005), but when traditional roosting sites are degraded, the availability of alternative sites is only relevant if the animals will use them (Donazar and Feijoo 2002;Rahaingodrahety et al 2008).…”