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Play of domestic lambs (Ovis aries) was recorded from birth to l0 weeks during two lambing seasons. A higher proportion of males than females mounted and butted; females were more likely than males to gambol. No differences were evident between the behaviour of females from same-sex twins and females from mixed-sex twins. The ontogeny of play followed a bimodal course, with little play in the first week of life, peaks of play during weeks 2 to 3 and 8 to 9, and a trough of activity between the peaks. Concurrently circulating hormones probably do not contribute to the sex differences or the development of play in lambs. The ontogeny of play may reflect ancestral adaptations to natural selection. This paper describes certain juvenile activities ('play') of domestic sheep (Ovis aries) from birth to l0 weeks, in order to enlarge the comparattve base for discussions of sex differences in behavi- oural ontogeny. The most extensive relevant studies on non-human mammals are those on rhesus macaques (Phoenix et al. 1968; Goy & Phoenix l97l). Data on sex differences in play are available for several other species including domestic dog (Beckoff 1974), black bear (Henry & Herrero 1974), Stellar sea lion (Gentry 1974), horse (Schoen et al. 1976) and several primates (Poirier & Smith 1974). Interpretation of the data from some studies is limited by the lack of identification of individual animals. In such cases, apparent sex differences in the total frequency of behaviour may stem from different proportions of response by each sex or frotn different frequencies of response by similar pro- portions. Despite systematic observation of many aspects of the social behaviour of domestic sheep (e.g. Scott 1945; Banks 1964), feral sheep (Grubb & Jewell 1966; Grubb 1974), and some of their wild congeners (Geist 1971), few quan- titative data are available on the juvenile activities of lambs that are not mother-orientated. With a reputation for vigorous play, domestic lambs appeared advantageous to observe.

Aggression in Xenopus is expressed behaviourally by the approach, the push and the nip which represent increasing intensities of aggression in the order given. There is an inverse relation between intensity of the aggression and frequency of occurrence. A category designated as “unclassified” represents contacts in which no aggression is visible, and the total frequency of such contacts is as great or greater than all other types combined. Heavier frogs display aggressive behaviour more frequently than do lighter ones and more aggressive frogs eat more than less aggressive animals. Individuals eat least when isolated, more when placed with one other frog and most when placed with eleven others. Differences between the isolated and paired situations are not significant, but total individual consumption in groups is significantly greater than either the isolated or paired conditions.