Experience influences diet mixing by herbivores: implications for plant biochemical diversity

We often assume the intrinsic value of a food or habitat is similar for individuals of a species and above a certain threshold density more profitable foods should always be preferred over less profitable foods. Nevertheless, individual herbivores differ in their preferences for foods due in part to experience, and experience in variable environments is variable. In this report, we show that how sheep learned about their foraging environment was crucial to the development of their dietary habits, and that experience with foods that contained plant secondary metabolites (PSM) markedly enhanced their use of PSM-containing foods, even when familiar, nutritious alternatives were available ad libitum. Lambs who learned to eat foods that contained either tannins, terpenes, or oxalates ate more when they could select two of the foods offered simultaneously (tannins-terpenes, tannins-oxalates, or terpenes-oxalates) than when they were offered only one food. Lambs offered foods containing all three toxins ate more than lambs offered two of the toxins, and their intake was comparable to lambs offered the food that contained no toxins. Experience and the availability of nutritious alternatives both influenced food choice when the preferences of lambs with 3 months' experience mixing tannin, terpenes, and oxalates were compared with lambs naive to the toxin-containing foods. During these studies, all lambs were offered five foods, two of them familiar to all of the lambs (ground alfalfa and a 50:50 mix of ground alfalfa:ground barley) and three of them familiar only to experienced lambs (a ground ration containing either tannins, terpenes, or oxalates). Half of the lambs were offered the familiar foods ad libitum, while half of the lambs were offered only 200 g of each familiar food daily. Throughout the study, naive lambs ate much less of the foods with toxins if they had ad libitum as opposed to restricted access to the nutritious alternatives (66 vs 549 g d(-1)). Experienced lambs also ate less of the foods with toxins if they had ad libitum, as opposed to restricted, access to the nutritious alternatives (809 vs 1497 g d(-1)). In both cases, however, lambs with experience ate remarkably more than naive lambs of the foods containing the toxins, whether access to the alfalfa-barley alternatives was ad libitum (811 vs 71 g d(-1)) or restricted (1509 vs 607 g d(-1)). These differences in food preferences and intake persisted during trials 8 months later. Plant communities offer a diverse matrix of biochemicals to herbivores, which may produce an array of interactions not accounted for by the traditional approach of studying nutrients and plant secondary metabolites (PSM) in isolation. How herbivores experience nutrient-PSM interactions may influence defoliation patterns and the potential for plant survival within plant communities. Thus, learning to mix foods that differ in kinds and concentrations of nutrients and PSM can enhance diet breadth and promote more uniform use of all plants in a community, which can influence the structure and function of ecosystems. Conversely, lack of experience learning to eat a variety of foods can diminish diet breadth and result in less uniform use of plants in a community.