Fig. 2.

Fractional absorption (bioavailability) in intact animals of inert, nonactively transported, water-soluble carbohydrates, a measure of paracellular absorption. (A) For all individuals in the 10 focal species (indicated by * in Table 1), absorption of l-rhamnose was measured with a standard protocol. In the comparison of birds and nonflying mammals by ANCOVA, fractional absorption declined with increasing log₁₀body mass (F_1,6 = 19.5) and differed significantly between the two taxa (F_1,6 = 9.5), although the difference diminished with increasing size (interaction F_1,6 = 7.3). Values of l-rhamnose fractional absorption in two species of bats were as high, or higher, than the values of the avian species, and above those for nonflying mammals. (B) The broader survey includes additional measures from the literature (Table 1) of fractional absorption of l-rhamnose or similarly sized water-soluble carbohydrate probes, l- glucose (MM = 180) and mannitol (MM = 182). Each point represents the fractional absorption of a probe by a species. In five of the 29 cases, the value shown is the mean of more than one study. In the comparison of birds, bats, and nonflying mammals by ANCOVA, fractional absorption differed significantly according to taxa (F_2,23 = 11.9), although the difference diminished with increasing size (interaction F_2,23 = 5.46). None of the nonflying mammals exhibited fractional absorption of these carbohydrates as high as that in small birds or bats.