Scratch that itch: revisiting links between self-directed behaviour and parasitological, social and environmental factors in a free-ranging primate

Different hypotheses explain variation in the occurrence of self-directed behaviour such as scratching and self-grooming: a parasite hypothesis linked with ectoparasite load, an environmental hypothesis linked with seasonal conditions and a social hypothesis linked with social factors. These hypotheses are not mutually exclusive but are often considered separately. Here, we revisited these hypotheses together in female Japanese macaques ( Macaca fuscata fuscata ) of Kōjima islet, Japan. We input occurrences of scratching and self-grooming during focal observations in models combining parasitological (lice load), social (dominance rank, social grooming, aggression received and proximity), and environmental (rainfall, temperature and season) variables. Using an information-theory approach, we simultaneously compared the explanatory value of models against each other using variation in Akaike's information criterion and Akaike's weights. We found that evidence for models with lice load, with or without environmental–social parameters, was stronger than that for other models. In these models, scratching was positively associated with lice load and social grooming whereas self-grooming was negatively associated with lice load and positively associated with social grooming, dominance rank and number of female neighbours. This study indicates that the study animals scratch primarily because of an immune/stimulus itch, possibly triggered by ectoparasite bites/movements. It also confirms that self-grooming could act as a displacement activity in the case of social uncertainty. We advocate that biological hypotheses be more broadly considered even when investigating social processes, as one does not exclude the other.

Redirected aggression reduces the cost for victims in semi-provisioned free-ranging Japanese macaques (Macaca fuscata fuscata)

In many social species, the victim often attacks an uninvolved third individual soon after a conflict. This behaviour is called ‘redirected aggression’ or ‘redirection’, and its role(s) remain(s) controversial. We observed semi-provisioned free-ranging Japanese macaques at Iwatayama Monkey Park in Arashiyama, Kyoto, Japan, to test three hypotheses concerning the function of redirected aggression: Japanese macaques perform redirection to (1) indirectly retaliate against the aggressor, (2) reduce post-conflict stress, or (3) reduce post-conflict uncertainty. When we observed aggressive interactions, we recorded the behaviour of victims during the subsequent 10 min. Redirection occurred more frequently when the rank of the victim of the initial conflict was high, when the victim was an older monkey, and when conflicts occurred among kin. The results largely supported hypothesis 3. Victims received renewed aggression not only from the initial aggressor but also from bystanders more frequently within 1 min after the initial conflict than in the subsequent 9 min. Victims who performed redirection received less aggression from bystanders. Victims might have been able to avoid renewed aggression because they could change their state from victim to aggressor by performing redirection. This effect of redirection did not differ with the victim’s rank. However, the lower the victim’s rank, the higher the risk that they would receive retaliation from the target of the redirected aggression or the latter’s kin. Thus, redirection caused the same magnitude of benefit and a different magnitude of risk according to the victim’s rank. The victim may need to judge his/her own situation when making the decision as to whether to perform redirection.