A modest proposal: displacement activities as an indicator of emotions in primates

Displacement activities are behaviour patterns (mostly body care activities) characterized by their apparent irrelevance to the situation in which they appear. Scratching, autogrooming, yawning and body shaking are among the most commonly reported displacement activities in non-human primates. A review of the primate literature indicates that displacement activities tend to occur in situations of psychosocial stress and that their frequency of occurrence is affected by anxiogenic and anxiolytic drugs. In the light of this evidence, it is suggested that displacement activities can be used as indicators of emotional states arising in a variety of primate social interactions. Methodological problems associated with such a use are discussed. The hypothesis that displacement activities may also have a communicative function in non-human primates is not supported adequately by available data. According to the usual definitions, displacement activities are behaviour patterns exhibited by an animal that are 'apparently irrelevant' to its ongoing activity (see Tinbergen 1952; Zeigler 1964). For example, in several sexual or agonistic contexts, passerine birds can clean their bills or feathers, eat or drink, or commit themselves to other activities that apparently do not have any obvious contextual relationship (Hinde 1953; Morris 1954; Rowell 1961). Similar examples have been reported in other animal groups such as arthropods, fish and mammals (e.g. Tinbergen & van Iersel 1947; Sevenster 1961; Duncan & WoodGush 1972; Hansen & Drake af Hagetsrum 1984; Roper 1984). Displacement activities are thought to occur in conflict situations (i.e. when two incompatible motivational tendencies are elicited simultaneously) or in situations in which an animal is prevented from attaining its goal (frustration; Tinbergen 1952; McFarland 1966). Different hypotheses concerning the emergence of displacement activities in these situations have been advanced. Some authors have taken the view that ?Present address: Yerkes Primate Research Center, Emory University, 2409 Taylor Lane, Lawrenceville, GA 30243, U.S.A, tension builds up during conflict, and that this is in some way released by the performance of displacement activity (e.g. Tinbergen 1952). Others have suggested that the conflicting tendencies inhibit each other, allowing a third and irrelevant tendency to gain expression, so giving rise to displacement activity (e.g. van Iersel & Bol 1958). Another possibility is that conflict and thwarting situations are characterized by increased levels of arousal and that in this condition activities emerge 'which are prepotent in the animal's repertoire' (Bindra 1959). In recent years displacement activities have been rather neglected as a topic of study. This may have resulted in part from a difficulty in recognizing these behaviour patterns. In fact, some of the features initially suggested as characterizing displacement activities (e.g. the absence of evident external stimulation, and morphological differences from the same behaviour patterns in their normal context) proved not to be entirely reliable. For example, it became apparent that (1) displacement activities can be strongly influenced by their normal causal factors (e.g. displacement grooming by state of the plumage: van Iersel & Bol 1958; displacement feeding by degree of prior food deprivation: McFarland 1965) and (2) under 0003-3472/92/110967+13 $08.00/0 9 1992 The Association for the Study of Animal Behaviour