A stereological comparison of villous and microvillous surfaces in small intestines of frugivorous and entomophagous bats: Species, inter-individual and craniocaudal differences

The extents of functional surfaces (villi, microvilli) have been estimated at different longitudinal sites, and in the entire small intestine, for three species of bats belonging to two feeding groups: insect- and fruit-eaters. In all species, surface areas and other structural quantities tended to be greatest at more cranial sites and to decline caudally. The entomophagous bat (Miniopterus inflatus) had a mean body mass (coefficient of variation) of 8.9 g (5 %) and a mean intestinal length of 20 cm (6 %). The surface area of the basic intestinal tube (primary mucosa) was 9.1 cm² (10 %) but this was amplified to 48 cm² (13%) by villi and to 0.13 m² (20 %) by microvilli. The total number of microvilli per intestine was 4x10¹¹ (20 %). The average microvillus had a diameter of 89 nm (10 %), a length of 1.1 μm (22 %) and a membrane surface area of 0.32 μm² (31%). In two species of fruit bats (Epomophorus wahlbergi and Lisonycteris angolensis), body masses were greater and intestines longer, the values being 76.0 g (18 %) and 76.9 g (4%), and 73 cm (16%) and 72 cm (7%), respectively. Surface areas were also greater, amounting to 76 cm² (26 %) and 45 cm² (8 %) for the primary mucosa, 547 cm² (29 %) and 314 cm² (16 %) for villi and 2.7 m² (23 %) and 1.5 m² (18 %) for microvilli. An increase in the number of microvilli, 33x10¹¹ (19 %) and 15x10¹¹ (24 %) per intestine, contributed to the more extensive surface area but there were concomitant changes in the dimensions of microvilli. Mean diameters were 94 nm (8 %) and 111 nm (4 %), and mean lengths were 2.8 μm (12%) and 2.9 μm (10%), respectively. Thus, an increase in the surface area of the average microvillus to 0.83 μm² (12 %) and 1.02 μm² (11%) also contributed to the greater total surface area of microvilli. The lifestyle-related differences in total microvillous surface areas persisted when structural quantities were normalised for the differences in body masses. The values for total microvillous surface area were 148 cm² g^-1 (20 %) in the entomophagous bat, 355 cm² g^-1 (20%) in E. wahlbergi and 192 cm² g^-1 (17%) in L. angolensis. This was true despite the fact that the insect-eater possessed a greater length of intestine per unit of body mass: 22 mm g^-1 (8 %) versus 9-10 m mg^-1 (9-10 %) for the fruit-eaters.