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Sergio M. Pellis and Vivien C. Pellis

Restraint is thought to be essential to enable the reciprocity needed for play fighting to remain playful. Descriptions of playing in pigs suggest that they do not exhibit restraint. Analysis of videotaped sequences of play fighting in captive family groups of warty pigs was used to test three hypotheses about restraint and reciprocity. Hypothesis 1 asserts that the lack of restraint arises from neither participant handicapping its actions in favour of its opponent: this was supported. Hypothesis 2 asserts that the winner of a contest will show restraint by not prosecuting further attack: this was not supported. However, the winner did refrain from attacking if the loser signalled submission. Hypothesis 3 asserts that restraint by the winner will allow reciprocal attacks by the loser — this was supported. The dissociation of restraint and reciprocity evident in the pigs offers some new insights into the evolution of play fighting.

Merav Ben-David, Sergio M. Pellis and Vivien C. Pellis


The marbled polecat (Vormela peregusna syriaca), a small musteline, is represented in the fauna of Israel. Predatory behaviour of eleven marbled polecats, caught in the field, and held in captivity for a reproductive biology study, was observed during their routine feedings over the course of two years. Sequences of prey capture and killing were recorded on film or videotape. Killed prey were also retrieved for autopsy. Prey offered included Gallus gallus domesticus, Mus musculus, Acomys cahirinus, Microtus guentheri, Meriones tristrami, Cavia porcellus and Rattus norvegicus. All prey items were obtained as surplus from medical laboratories. In addition to using the typical musteline killing method of biting the nape of the neck, marbled polecats used a variety of killing methods suited to a range of prey types. Two variables determined the killing method used. 1) Size of prey: Small prey were killed by bites to the thorax, whereas large prey were bitten on the head or neck. 2) Prey defense behaviour: Fleeing prey were bitten dorsally, whereas defending prey were bitten ventrally, typically on the throat. This non-stereotypical predatory behaviour appears to correlate with the opportunistic feeding habits of marbled polecats in the wild. Evolutionary and ecological aspects of this relationship are discussed.

Stephanie M. Himmler, Brett T. Himmler, Vivien C. Pellis and Sergio M. Pellis

Studies on laboratory rats (Rattus norvegicus) have revealed that experience with social play in the juvenile period is important for the development of improved social skills, an improvement that appears to be mediated by the prefrontal cortex. But there is much variation in both the frequency with which play occurs and in the complexity of the actions performed among different strains of rats. Is all this variation adaptive in serving play’s critical developmental role? The integrative approach advocated by Tinbergen provides a framework with which to assess such variation. A review of what is known and the inclusion of some novel data suggest that irrespective of the form of the play, rats of all strains converge on the same key experiences, experiences that have been implicated in the development of social skills. The lessons learnt from rats may serve as a guide for broader cross-species comparisons.

Sergio M. Pellis, Melissa A. Blundell, Heather C. Bell, Vivien C. Pellis, Alan H. Krakauer and Gail L. Patricelli

Lekking male greater sage-grouse (Centrocercus urophasianus) compete with neighbours not only by strutting to attract females but also by directly challenging other males. These challenges include approaching another male and adopting an anti-parallel orientation at close quarters (‘facing past encounter’) and fighting, in which the birds strike one another with their wings. Facing past encounters and facing past encounters that led to fights in free-living sage-grouse were videotaped and analysed to test predictions arising from two sets of hypotheses to account for the features of such encounters. They could be used to assess or threaten opponents (index signal or threat signal hypotheses) or they may be the result of a stalemate in which one bird’s attempts to gain an vantage point for attack are neutralised by counter moves by the other bird (combat hypothesis). Frame-by-frame analyses of both facing past encounters and fights were used to extract data to test specific predictions arising from the three hypotheses. The results, overall, support the hypothesis that the facing past orientation arises from combat. However, the results also suggest that, once in the anti-parallel orientation, opportunities emerge for communication to take place.