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

Abstract

G. 2004. Experience influences diet mixing by herbivores: implications for plant biochemical diversity. Á/ Oikos 107: 100 Á/109.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...

Self-medication and homeostatic behaviour in herbivores: learning about the benefits of nature’s pharmacy

“…Subsequently, when these foods were offered in a choice, lambs with experience ate significantly more of the foods containing the PSMs, up to 40% of their diet, even when safe and nutritious foods (alfalfa, barley) were available ad libitum. These differences in food preferences and intake persisted during trials a year later (Villalba et al, 2004).…”

“…Lambs in a group (experienced) were forced to consume foods with tannins, terpenes and oxalates. These foods are complementary as the combined intake of the three foods is comparable with the intake of food without PSMs (Villalba et al, 2004). A second group of lambs (naïve) was not forced to consume the PSM-containing foods.…”

“…Thus, PSMs that affect different organs or detoxification pathways are likely to be less toxic as a diluted mixture than as a larger dose of one PSM (Freeland and Janzen, 1974). Indeed, sheep eat more when offered choices of foods with various PSMs that affect different detoxification mechanisms, and thus are complementary Villalba et al, 2004).…”

“…However, not only do herbivores manifest partial preferences (Westoby, 1978), they also ingest substantial amounts of PSMs (Provenza, 1995 andVillalba et al, 2004), likely due in part to their health and medicinal effects. If we view nutrients from another angle, they can also enhance resilience and resistance of the host to disease (Coop andKyriazakis, 1999 and.…”

Preferences of sheep, when supplemented, for forages in a Mediterranean rangeland management system

“…Grazing preferences by Bedouin sheep may vary from previously reported studies because almost all of the nutrients sheep consume are from barley grain. Preferences are not constant and can be affected by experience and availability of other feeds (Provenza 1996;Villalba et al 2004). For example, sheep fed barley grain showed a greater preference for high protein foods compared with controls (Villalba and Provenza 1999).…”