The role of olfactory cues in the discrimination of agemates by lambs SE ´ VERINE LIGOUT, MATTHIEU KELLER & RICHARD H. PORTER Laboratoire de Comportement, INRA/CNRS UMR 6175, Universite ´ de Tours (Received 17 October 2003; initial acceptance 28 November 2003; final acceptance 10 January 2004; published online 25 August 2004; MS. number: 7884) We assessed the effect of olfactory cues on agemate discrimination between lambs, Ovis aries. In experiment 1, a local anaesthetic was sprayed into the nostrils of 2–3-week-old lambs to inhibit olfaction. To verify whether the treated animals were anosmic, they were tested for their responses to a food source tainted with the odour of dog faeces, a scent that is strongly avoided by intact lambs. In a simultaneous- choice test, lambs that were categorized as anosmic, i.e. that fed in the presence of dog faeces odour, responded preferentially to a familiar penmate over an alien agemate, indicating that olfaction is not essential for social discrimination. We then examined intact lambs’ responses to pairs of agemates that were anaesthetized and partially hidden, thereby eliminating vocal and salient visual characteristics of the stimulus lambs, but allowing access to their odours. During the choice tests, subject lambs responded more positively to their familiar twin than to an unfamiliar, unrelated lamb, but they did not discriminate between a familiar, unrelated penmate and a strange stimulus lamb. Thus, olfactory cues appeared to be a sufficient basis for twin recognition in this context. We conclude that discrimination of a twin but failure to discriminate between familiar and unfamiliar unrelated lambs probably reflects differential familiarity between twins and unrelated penmates, but perceptible similarities between the odour phenotypes of twins may also facilitate the discrimination. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Terrestrial mammals rely greatly on olfactory cues to dis- tinguish between individual conspecifics (reviews in Brown 1979; Halpin 1980, 1986; Hepper 1991; Sommerville & Broom 1998). In domestic sheep, Ovis aries, the critical influence of olfaction on the establishment of the ewe’s bond with her newborn offspring has been particularly well documented (Lindsay & Fletcher 1968; Baldwin & Shillito 1974; Morgan et al. 1975; Poindron 1976; Poindron et al. 1993; Le ´vy et al. 1995). Lambs’ individually characteristic olfactory signatures are believed to emanate from the wool or skin, rather than the amniotic fluid, and there is no clear evidence that they are acquired postnatally from the mother (Alexander & Stevens 1981, 1983; Le ´vy et al. 1991; Porter et al. 1994). Experiments with twin lambs indicate that monozygotic twins have more similar odours than do dizygotic ones (Porter et al. 1991; Romeyer et al. 1993). This result suggests that lambs’ recognizable olfactory signatures are mediated, at least partially, by genotypic factors. Both observations of free-ranging animals and con- trolled experiments show that lambs are capable of discriminating between individual conspecifics and mem- bers of different social categories, including others of their own breed (Arnold & Pahl 1974). Aside from recognition of the mother, which develops within several hours of birth (Nowak et al. 1987, 1990), discriminative interac- tions with individual peers also occur. For example, twins orient preferentially, and keep close, to one another (Shillito Walser et al. 1981; Walser et al. 1983; Shillito Walser & Williams 1986; Nowak 1990; Ligout & Porter 2004). Furthermore, when lambs are separated from their mother, the presence of their twin has a greater calming effect than does contact with an alien agemate (Porter et al. 1995, 1997; Ligout & Porter 2003). When lambs are reared together in small groups, they also become familiar with and develop preferential relationships with their unrelated penmates, especially when they are reared artificially in the absence of their mother (Porter et al. 2001; Ligout et al. 2002; Ligout & Porter 2004). Correspondence: S. Ligout, Laboratoire de Comportement, INRA/CNRS UMR 6175, Universite ´ de Tours 37380 Nouzilly, France (email: ligout@ tours.inra.fr). 785 0003–3472/04/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2004, 68, 785–792 doi:10.1016/j.anbehav.2004.01.008