A large bird’s digestive tract has an opposite intersexual pattern than body size

Abstract The appropriate structure of the digestive tract is crucial for individual adaptation to ecological conditions. In birds, the length of the small intestine, responsible for food absorption, is generally believed to be positively correlated with body size. In this study, we investigated the variation in small intestine length in the White Stork ( Ciconia ciconia ), a monomorphic species without visible sexual dimorphism, but characterized by differing parental efforts, which can be reflected by the small intestine lengths between the sexes. Then, we examined the relationship between small intestine length, body size and condition within the sexes. Our findings show that male White Storks have significantly shorter small intestines than females, despite having larger body sizes than the latter. Furthermore, we found a strong relationship between body condition and small intestine length, but it was of a different nature in the two sexes. Males exhibited a previously unreported phenomenon, whereby improving body condition was associated with shortening small intestines, whereas females exhibited the opposite pattern. Moreover, in contrast to females, increasing body size in males resulted in significant small intestine shortening. These novel findings shed light on the anatomical adaptations of the digestive tract in birds.