Social diffusion of modified louse egg-handling techniques during grooming in free-ranging Japanese macaques

Changes in louse egg-handling techniques during grooming were studied among a free-ranging troop of Japanese macaques, Macaca fuscata, from 1990 to 1997 at Jigokudani Monkey Park, the Shiga Heights, Nagano prefecture, Japan, where the skills were distributed based on matrilineages. One female altered her louse egg-handling technique from a one-hair combing technique, in which macaques comb a single hair with an egg without first loosening the cement of the egg, to a forefinger nail loosening technique, in which macaques use the nail of the forefinger to loosen an egg before combing. Therefore, louse egg-handling skills were not fixed, but could change. This female's new technique was later adopted by her sister, daughters and granddaughter. Not all elements of the technique were acquired at the same time, however, and individuals varied in when they acquired them. These shifts suggest that the mechanism of acquiring louse egg-handling techniques is information transfer of the partial functions and partial structure of the techniques together with individual learning. One female's initial technique (skin pinching), against which groomees reacted, was replaced by the forefinger nail loosening technique even though the techniques, once mastered, were equally efficient and the new technique was initially less efficient. The social interaction between grooming partners may thus impede the diffusion of the harmful behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.