Diet Mixing: Nonadditive Interactions of Diet Items in an Omnivorous Freshwater Turtle

Diet selection and optimization may be affected by nonadditive interactions (or associative effects) among ingested diet items. Associative effects occur when one diet item affects the digestion (either positively or negatively) of another diet item. Feeding trials were conducted with an opportunistic omnivore, the yellow—bellied slider turtle (Trachemys scripta scripta) on three diets: duckweed, insect larvae, and duckweed/larvae mix. Diet has a significant effect on digestibilities and intake, but not on transit time. There was a significant associative effect in the duckweed/larvae diet. For all diet components except lipids, the digestive efficiencies measured in the duckweed/larvae diet were significantly greater than those predicted from the digestive efficiencies for duckweed and larvae diets when fed alone. The presence of larvae in the digesta significantly improved the digestive processing of duckweed, probably by supporting greater numbers or diversity of cellulolytic microbes. A given mass of duckweed provided °70% more energy and 20% more nitrogen to the turtle when ingested with insect larvae than when ingested alone. Associative effects should be incorporated in optimal foraging models and studies of diet selection because the value of a diet item can vary with the foods with which it is ingested. The assigned value must include not only the direct nutritional gain from the item, but also the indirect nutritional gain (or loss) through positive (or negative) effects on digestion of food items with which it is ingested.