ANIMAL BEHAVIOUR, 1998, 56, 337–345 Article No. 980786 A rule of thumb in mammalian herbivores? M AGNUS AUGNER, FREDERICK D. PROVENZA & JUAN J. VILLALBA Department of Rangeland Resources, Utah State University (Received 19 May 1997; initial acceptance 24 June 1997; final acceptance 12 December 1997; MS. number: 5549R) ABSTRACT In two experiments on appetitive learning we conditioned lambs, Ovis aries, to particular concentrations of a flavour by mixing the flavour with an energy-rich food that complemented their energy-poor diet. The lambs were subsequently offered energy-rich food with five different concentrations of the flavour (the concentration to which they were conditioned, two higher concentrations, and two lower concen- trations). At these tests, the lambs consistently preferred the weaker flavours. This finding stands in contrast to earlier results on generalization gradients. In a third experiment, similarly designed to the other two, we tested for effects of a strong flavour on the behaviour of lambs when they were offered a novel nutritious food. Half of the lambs were offered unadulterated wheat, and the others strongly flavoured wheat. We found that the flavour in itself was initially aversive. We propose that the lambs’ avoidance of foods with strong flavours may be an expression of a rule of thumb of the type ‘given a choice, avoid food with strong flavours’. Such a rule could be part of a risk-averse foraging strategy displayed by mammalian herbivores, and which could be of particular importance when they encounter unfamiliar foods.  1998 The Association for the Study of Animal Behaviour The use of information provided by plants is fundamental to the whole foraging process of herbivores. The plant community provides herbivores with large amounts of (potential) information, in the form of olfactory, gusta- tory, visual and tactile stimuli. Th ese stimuli can be perceived both before and during feeding. Taken together, there are probably no two plants whose ‘infor- mation signatures’ are identical. This large amount of information potentially constitutes a problem for gener- alist herbivores, and they must be selective as to what fraction of the potential information they evaluate. Con- sequently, the ability to learn to recognize foods and the ability to generalize over cues are potentially important traits. It has been proposed that familiarity is the major determinant of animals’ responses to foods and flavours (e.g. Kalat 1974 ; Rozin & Schulkin 1990 ). Another information-related problem for generalist herbivores is that they may encounter plants with unfamiliar informa- tion signatures in new environments as well as in familiar surroundings. When such a novel plant is encountered the herbivore can either (1) ignore it, (2) sample it to some extent, or (3) eat from it in an uninhibited fashion. In this decision, the herbivore may rely on earlier experi- ences made by itself or by accompanying conspecifics. For instance, a plant with a familiar odour may be more readily accepted and sampled than one with an unfamil- iar odour (e.g. Provenza 1996 ). However, when a plant and the sensory stimuli associated with it are truly novel the animal has no previous experience yet still has to make a decision. Mammalian herbivores generalize in a qualitative fashion over cues (sensory stimuli) provided by unpalat- able foods (e.g. Launchbaugh & Provenza 1993 ). How- ever, it is not known if they generalize quantitatively, that is, there are no studies of generalization gradients of food-associated cues in mammalian herbivores. A gener- alization gradient is defined as the intensity of the behav- ioural response of an animal as a function of the stimulus, when the stimulus varies (quantitatively) in one- dimensional space, for example, in brightness or in flavour concentration (Spence 1937 ). The intensity of the response is generally strongest to stimuli that are close to the stimulus to which the animal is conditioned; as the stimuli diverge, the responses decrease in intensity. A conditioned generalization gradient could be seen as a learnt behaviour that gives rise to a kind of rule of thumb: ‘react the strongest to familiar stimuli’. Mammalian herbivores often show a high degree of neophobia when first encountering an unfamiliar food (e.g. Provenza et al. 1995 ; Provenza 1996 ). Furthermore, a familiar food that is normally readily eaten is sampled Correspondence and present address: M. Augner, Department of Theoretical Ecology, Lund University, Ecology Building, S-223 62 Lund, Sweden (email: Magnus.Augner@teorekol.lu.se). F. D. Provenza and J. J. Villalba are at the Department of Rangeland Resources, Utah State University, Logan UT 84322-5230, U.S.A. 0003–3472/98/080337+09 $30.00/0  1998 The Association for the Study of Animal Behaviour 337