Do Seabird Gut Sizes and Mean Retention Times Reflect Adaptation to Diet and Foraging Method?

Gastrointestinal transit times of fish, squid, and crustaceans of known in vitro digestibility were measured for five seabird species (a procellarii form, a sulid, and three species of penguin) to investigate the influence of (1) natural diet and (2) foraging method on mean retention time. Despite its relative indigestibility in vitro, squid is excreted faster than is fish by the four seabirds that breed at islands in the Southern Ocean, where squid is abundant. The Cape gannet excretes pilchard, its major natural prey item, more rapidly than it does squid. Mean retention times of these prey types were consistent with the ranking of gastric evacuation times within each seabird species. Mean retention time of digesta was found to be significantly correlated with intestine (small and large intestine plus rectum) length. Intestine length, volume, and planar surface area all scaled significantly with body mass. The slope of these three relationships differed between flying seabirds and penguins, significantly so in the case of intestine area. Greater gut surface area in penguins may be an adaptation enabling these birds to meet the relatively high energetic costs of swimming. Mean retention time was not directly correlated with body mass. Gut length appears to be the primary correlate of mean retention times, which are not invariably shorter in flying species than in penguins. Allometry provides a useful framework for consideration of the interaction between energy demands and digestive adaptations in seabirds.