Body size and gastrointestinal morphology of nutria (Myocastor coypus) reared on an extensive or intensive feeding regime.

Although plasticity of growth rates is mainly associated with ectotherm species, it does occur in endotherms as well, but has not been documented systematically for many species. We compared the effect of 2 common types of feeding systems, differing in energetic value, on body size and gastrointestinal tract morphology in nutrias (Myocastor coypus). A total of 30 extensively (E) fed and 20 intensively (I) fed animals were used in the study. We noted significant effects of age, sex, and feeding regime on body weight and length, with 1-yr-old females attaining 3.7 ± 0.4 kg and 33.4 ± 1.5 cm on E and 4.9 ± 0.3 kg and 36.1 ± 2.3 cm on I. A significant treatment-sex interaction indicated that treatment had a greater effect on the length growth in males (1-yr-old males attaining 4.0 ± 0.2 kg and 34.7 ± 1.2 cm on E and 5.4 ± 0.4 kg and 41.0 ± 1.4 cm on I). The differences matched individual literature reports of free-ranging or intensively fed nutrias. The majority of gastrointestinal tract measurement results were only related to body weight, without additional effect of the diet regime, except for a higher small intestinal tissue weight on I (79 ± 14 g vs. 61 ± 7 g on E). In contrast, the wet content weight of the stomach, caecum, and the total gastrotinestinal tract was higher on E (196 ± 34 g vs. 164 ± 51 g on I). Overall, we observed strong influence of dietary regime on body development but not on digestive anatomy, indicating a distinct phenotypic flexibility in growth rates in nutrias.