Diet predicts intestine length in Lake Tanganyika’s cichlid fishes

Summary 1. Among vertebrates, herbivores have longer digestive tracts than animals at higher trophic levels, a pattern thought to reflect a trade‐off between digestive efficiency and tissue maintenance costs. However, phylogenetic influences on this pattern have rarely been considered. Taxa that have undergone diversification accompanied by dietary shifts provide a powerful opportunity to examine the relationship between diet and intestine length while accounting for phylogeny. 2. In this paper we assess the relationship between diet and intestine length in the cichlid fishes of Lake Tanganyika, which are renowned for their diversity of species and trophic strategies. 3. First, we test the effect of trophic position on intestine length across 32 species, while controlling for phylogeny. Trophic position was inferred from nitrogen stable isotopes, which provide a temporally integrated, quantitative perspective on the complex diets of tropical fish. Second, we examine patterns of intraspecific variation in intestine length of an algivorous cichlid ( Tropheus brichardi ) along a natural spatial gradient in algal nitrogen content. 4. Trophic position explains 51% of size‐standardized variation in intestine length after accounting for phylogeny. Accounting for phylogeny does not substantially alter the relationship between trophic position and intestine length, despite the existence of phylogenetic signal in both traits. Thus, diet is a strong predictor of variation at the interspecific level. 5. There is a striking inverse relationship between intestine length and algal nutrient content among populations of T. brichardi , suggesting substantial plasticity in response to food quality, and thus a strong dietary influence on patterns of intraspecific variation. 6. Diet is a strong predictor of intestine length at both intra‐ and interspecific scales, indicating that fish adjust their phenotype to balance nutritional needs against energetic costs. Furthermore, functional explanations for trophic diversification of cichlid fishes in the African Great Lakes have long focused on jaw structures, but our results indicate that intestinal plasticity in response to diet quality may also be an important mechanism for accommodating trophic shifts during evolutionary radiations.