Do sheep (Ovis aries) categorize plant species according to botanical family?
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The ability of grazing herbivores to assign food types to categories by relying on certain relevant criteria could considerably reduce cognitive demand and increase their foraging efficiency when selecting among many different plant items. Grasses and legumes differ functionally in vegetation communities as well as in nutritive value. We aimed to determine whether sheep can generalize an aversion they learnt for a grass or a legume species to another species of the same functional type and consequently whether botanical family is a potential level of categorization. Over four successive weeks, 12 lambs were conditioned against either a freshly cut grass (tall fescue—Festuca arundinacea, N = 6) or legume species (sainfoin—Onobrychis viciifolia, N = 6) using a negative post-ingestive stimulus (lithium chloride) on day 1. Preference of all lambs between another grass (cocksfoot—Dactylis glomerata) and another legume (alfalfa—Medicago sativa) was assessed on day 3 by measuring their relative consumptions. Preference for alfalfa progressively became lower for lambs that were conditioned against sainfoin than against tall fescue, indicating that lambs generalized the aversion between species along some perceptual gradient and classed the considered grasses and legumes in distinct categories. Beyond this original result, the question now is to identify which specific plant characteristics or functional traits the animals rely on in order to form categories.
KeywordsCategorization Diet selection Grass Legume Functional type Sheep
We thank all the staff at the INRA-UR1213 experimental farm, particularly Pascal Payard for his technical assistance during tests and for animal care. Thanks also to Laurent Beaudonnat for help with plant species collection and to Aline Le Morvan and Pierre Capitan for analysing the sward samples.
The animals were handled by specialized personnel who applied animal care and welfare in accordance with European Union Directive No. 609/1986 under agreement number B63 345.17. The main ethical issue associated with the work was the conditioning procedure, associating handling with the administration of lithium chloride, which causes nausea and gastrointestinal malaise. Handling may have contributed to the aversiveness of the programme. However, in protocols where animals are positively conditioned, handling does not prevent the preferences from developing (Villalba and Provenza 2000b; Favreau et al. 2010b). Consequently, we can consider that the malaise caused by LiCl was the main aversive factor that changed the value of the plant species during conditionings. In order to minimize the discomfort to the level necessary to get an answer, we applied the following procedure. First, the administration rates were low (70 mg/kg) and far lower than those classically used in ruminant conditionings (100 to 200 mg/kg: Villalba et al. 2002; Pfister et al. 2007). Second, the animals did not receive the full dose if their intake during conditionings was low, and a low intake did not affect their basal diet. After conditionings, the lambs did not show any signs of distress. The most sensitive indicator would have been a decrease in basal diet consumption, and monitoring of intake revealed no such decrease.
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