Scratching around stress: hierarchy and reconciliation make the difference in wild brown lemurs (Eulemur fulvus)

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.