Evolution of Communication - Volume 2, Issue 1, 1998

The production of facial expressions is important for social communication, and has been described for many primate species. The perception of facial expressions, however, has received considerably less attention. This study reports the results of two experiments that investigated the ability of chimpanzees to discriminate five of their species-typical facial expressions: bared-teeth display, hoot face, relaxed-lip face, relaxed open mouth face, and scream face. Unfamiliar con-specifics' facial expressions were presented as black and white, digitized images using a computerized joystick-testing apparatus and matching-to-sample paradigm. Subjects were first presented with a sample facial expression, and were then required to choose between two comparison, one of which showed the same expression as the sample. The first task demonstrated that all expressions except the relaxed-lip face were discriminated from a neutral face significantly above chance, several on the first day. The second task showed impaired discrimination when the sample and nonmatching comparison expressions shared similar features, supporting the hypothesis that facial expressions were categorized using distinctive features. The overall correlation between the number of shared features and performance was only weakly negative, suggesting that additional discrimination strategies may be involved.

Crying is melodramatic in the sense that crying babies seem to respond to a great variety of distressing situations with behaviors, such as gasping, choking, and panting that would be appropriate to a very specific respiratory emergency. In this paper we develop models to explore whether extortion or deception is the more plausible origin of the melodrama in a baby's cry. According to these models, deception seems a more plausible origin than extortion because extortion requires the incoherent assumption that nature can select against the genetic interests of an organism. By comparison, the assumptions required to rationalize a deception explanation — that the parent share in the benefits given to its offspring — seem relatively harmless and consistent with contemporary sociobiological theory.

Two domestic dogs (Canis familiaris) participated in a series of studies in which they communicated with a human about the location of hidden food. In the first study both dogs were able to follow human pointing reliably to one of several locations where food was hidden, both in front of them and behind them. They also showed some skills at following human gaze direction in this same task, when both head and eyes indicated the same location. They did not follow eye direction when it conflicted with head direction. A second study clearly ruled out a low-level visual tracking explanation for at least one of the subjects. In a third study one of the two dogs was able to lead a naive human to one of three locations containing food consistently, mainly by barking and orienting its body to the food. The subject did not behave differently, however, when the human turned his back or covered his eyes; he continued to orient to the food and bark under all conditions. In a fourth study in which more clearly visual signals were involved, both subjects strongly preferred to drop a retrieved object at the front of, rather than at the back of, the human — even when the human turned his back so that subjects had to bring the object around his body upon return. The knowledge of human pointing and gaze direction displayed by these two domestic dogs is in many ways comparable to that displayed in experimental studies by nonhuman primates.