Morphometric data on the intestines of the platypus Ornithorhynchus anatinus and the short-beaked echidna Tachyglossus aculeatus.

Abstract

ALTHOUGH the gross morphology of the monotreme gastrointestinal tract has been described (Griffiths 1978; Hume and Harrop 1980), morphometric data on the intestines of the two extant monotremes, the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus), are to date incomplete. Using an established method (Snipes and Kriete 1991, Snipes 1994, 1995, 1997) to measure the entire intestines of mammals, which does not rely on extrapolations from measured probes with the inherent danger of missing major surfaceenlargement structures such as folds and villi, major morphometric parameters were measured in both species. These included lengths of the small and large intestines, including the caecum, their respective basal or ground areas, and the factor of surface enlargement determined on histological specimens of each area. Multiplication of this factor with the basal area renders a total surface area for each intestinal segment. Additionally, volumes of each segment were calculated from lengths and basal areas. Table 1 gives measured values as well as percentages of totals for each segment. In order to compare these empirical data, coefficients were calculated according to a system proposed by Chivers and Hládlik (1980, 1984), i.e., coefficient of digestion (area of large intestine divided by the area of small intestine) and coefficient of volume (volume of large intestine divided by volume of small intestine). Obtained values can be set on a scale (Chivers and Hládik 1980) to categorise the dietary strategy of an animal (faunivore, intermediate feeder or nonruminant herbivore; see Table 1).