Scratching around stress: hierarchy and reconciliation make the difference in prosimians

The scratching–stress linkage has been demonstrated in monkeys and apes but never in strepsirrhines, either in the wild or in captivity. We analysed data collected on a 14-animal group of Eulemur fulvus in the Berenty forest (South Madagascar, March–July 2008). We applied a protocol (same weather conditions, time slot, social/activity context, forest quadrat, and subgroup formation) involving four conditions, under which we recorded the scratching response: predation attempt, reconciled conflict, non-reconciled conflict, and control. We found that the scratching–stress linkage remains valid in strepsirrhines. Scratching increased after predatory attacks by the hawk Polyboroides radiatus and intra-group aggressions and decreased after reconciliation, probably buffering post-conflict stress. Scratching negatively correlated with the linear hierarchy, but only in the absence of stressful events. Compared to aggressions, predation attempts induced a greater increase in scratching, with dominants showing the highest differential increase. Thus, scratching is sensitive to different kinds of homeostasis perturbation (predation/aggression) and does not simply provide all-or-nothing information. Following a theoretical framework based on previous cortisol analyses, we showed that scratching and hormonal data converge in indicating that the stress profile of a species is shaped by its social network features