A comparative morphological and morphometric study of the chiropteran intestine
The structural characteristics of the intestinal tract of three species of bats namely the fruit bat Epomophorus wahlbergi and the entomophagous bats Miniopterus inflatus and Rhinolophus hildebrandti have been examined macroscopically, with the light microscope and with the scanning and transmission electron microscopes. Stereological techniques and formulae have been modified and utilized to estimate the surface characteristics of the intestinal mucosa on two species of bats. In all the bats examined, neither a caecum nor an appendix was observed and the intestine was a narrow tube of almost uniform diameter save for the rectum whose diameter was greater than that of the rest of the intestine. A colon was only observed in the fruit bat but could only be distinguished from the rest of the intestine from its characteristic mucosal surface after opening the intestine. The villi in the cranial 20 % of the intestine of the fruit bat branched and interconnected while those occurring in the rest of the intestine were generally finger-like discrete projections. In the insectivorous bats, the villi were ridge-like and were transversely oriented, the only deviation from this pattern being observed in a small posterior part of the intestine of the horseshoe bat. In the latter, the villi anastomosed profusely forming shallow hexagonal compartments semblant of the reticular cells of the ruminant stomach. In the two insectivorous species of bats, a short segment of the mucosa immediately posterior to the pylorus had numerous hexagonal and cylindrical pits which were thought to be either involved in enzyme secretion, absorption of nutrients or both. The ultrastructural picture of the intestinal mucosa in the frugivorous bat showed remarkable cellular and pericellular modifications that were absent in the entomophagous bats. The enteric epithelium was made of tall columnar cells between which were large intercellular spaces. These columnar cells had large intracellular vacuoles and sent long and tortuous cytoplasmic projections (pseudopodia) into the intercellular spaces. Adjacent cells were joined by means of desmosomes formed between two apposing cytoplasmic processes. The role of the cytoplasmic processes besides structural reinforcement, was thought to be pinocytosis. In the insectivorous bats, intercellular spaces were narrow and epithelial cells were devoid of vacuoles and no specializations were observed on the lateral membranes of the absorptive cells. In both groups of bats, the epithelial cells had numerous mitochondria distributed over the apical and basal sides of the centrally located nucleus. Macroscopic morphometric comparisons showed that the intestine of the frugivorous bat is longer than that of the insectivorous bat but when intestinal length is normalized with body mass, the insectivore had a longer intestine. The microvillous dimensions (mean length, diameter and surface) in the fruit-eating bat and insectivorous bat showed no significant trends but such trends were significant for segmental microvillous numbers, microvillous amplification factors and segmental surface areas with these values being highest in the proximal intestinal segments and lowest in the posterior segments of the intestine. In the fruit bat, the average values for microvillous height, diameter and surface were 2.87 J..Lm0,.097 J..Lamnd 0.8739 J..Lmr2espectively with an uncorrected total microvillous surface area of 2.50 X 1012 J..Lm2T.he mean microvillous height, diameter, and surface area for the insectivorous bat were 1.09 J..Lm0,.088 J..Lamnd 0.3069 J..Lm2 respectively with an uncorrected absolute intestinal surface area of 1.32 X 1011J..LmT2h.e microvillous packing density and the absolute number of microvilli in the fruit bat were 58 J..Lma-n2d 3.24 x 1012 respectively. In the insectivorous bat the microvillous packing density was 88 J..Lmw-i2th an absolute number of 3.90 X 1011• This study indicates that the chiropteran intestine is structurally and hence functionally better adapted for absorption than that of the land-based mammals for which similar studies have been conducted. The qualitative and quantitative results of this study indicate that the frugivorous bat has a superior intestine than the entomophagous bat. This may in part be explained in terms of the differences in their energetic demands of flight and the differences in the types of diet on which these bats thrive. Although an attempt has been made to explain the adaptive characteristics of the chiropteran intestine and the observed species differences, conclusive explanations of these peculiarities must not only await broader morphometric studies on a wider range of species, but also detailed observations of their ecological, physiological and flight biomechanical characteristics.