Comparative analysis of the gastrointestnal traits and routes of nutrient absorption in bats and rodents: evidence for convergent adapation with birds?

Abstract

The capacity for sustained flight evolved independently in three lineages of vertebrates, and is regarded as a classic example of convergent evolution. Among birds and bats, despite the evolution of different functional patterns to meet the demands of the flight, there are numerous similarities in several features associated with the flight that unambiguously corroborate the idea of convergent evolution. From the viewpoint of morphological and physiological characteristics associated with flight, birds, when compared to non-flying mammals, display a reduction in the size of the small intestine. The hypothesis to explain this finding is that the size of the digestive tract and, therefore, the load it carries should be minimized, because the costs of the flight increase with the load and also because the ability of take-of and maneuverability vary inversely with the body mass. If there is convergence in the functional solutions associated with the flight between birds and bats, then we might expect a reduction in the dimensions of the small intestine in bats, compared to non-flying mammals, with the same magnitude that was observed in a comparison between birds and non-flying mammals. In this context, one of the goals of this project is to test this hypothesis by analyzing and comparing, in an ecological and evolutionary context, the morphology of the small intestine of several species of bats and rodents with similar body mass.If birds and bats actually have a small intestine proportionally smaller than those observed for non-flying mammals, one interesting question is how they can absorb enough energy to meet the demands of flight. In this context, studies on absorption routes of nutrients showed that birds make greater use of passive routes, when compared with non-flying mammals. The increased use this route, which is independent of feeding frequency and, hence, enables a more rapid and efficient absorption of nutrients, was considered as an adaptive response to compensate for the decrease in the size of the small intestine. Studies of bats suggest the same pattern, but the limited number of experiments (two species) does not allow a firm conclusion about the existence of convergence with birds Thus, the second objective of the project is to quantify and compare the routes of uptake in species of bats and rodents with different dietary habits, aiming to evaluate the existence of convergence response wit birds. (AU)