How do animals learn if rangeland plants are toxic or nutritious?

Over the past 15 years, Dr. Frederick Provenza and his associates at Utah State University have studied how animals use certain physiological and behavioral processes to learn whether rangeland forages are toxic or nutritious. This article summarizes key concepts presented in recent articles (e.g., Provenza et al. 1992; Provenza 1995, 1996, 1997) about how learning plays a major role in the development of dietary choices made by rangeland animals. Animals continually sample and evaluate the nutritional value of forages using their senses of taste, smell, and sight. Postingestive feedback adjusts a forage's hedonic value (i.e., preference and palatability) commensurate with its utility to the animal (i.e., animal age, morphology, and physiology) enabling survival when the animal's foraging environment and nutritional needs are constantly changing. Plant species that cause positive hedonic shifts are usually highly correlated with nutritional well-being, while plant species that cause negative hedonic shifts are typically highly correlated with nutrient deficiencies and toxicosis. Hence, what makes a forage taste 'good or bad' (and thus, sought or avoided) is not taste per se, but rather nutritional benefits or deficits received from forage ingestion, which are sensed by animals through feedback and linked with a forage's taste. Animals use their senses (smell and sight) to seek foods that cause positive feedback (i.e., nutritional well-being) and avoid foods that cause negative feedback (i.e., nutrient deficiencies and toxicosis), and accordingly possess a high degree of 'nutritional wisdom.' This process occasionally breaks down when animals fail to properly link the feedback of a particular food with its taste, smell, or sight, or when their physiological means for binding, metabolizing, or detoxifying toxic compounds is exceeded.