M. edwardsi has an oval, sooty-colored patch around each eye. The hands and feet are plentifully sprinkled with fine-textured short hairs. The whiskers are fine, long, and dark. The tail is so poorly haired that in some specimens it appears to be almost naked. It has been suggested that the structure of the feet permits the paws to spread more than those of most terrestrial rats, as an adaptation for traveling over swampy ground.
Females usually have four or six mammae.
These rodents seem to be mainly nocturnal and usually inhabit areas of dense vegetation near water. It apparently eats animal and vegetable matter in nearly equal proportions, feeding on fallen fruits, seeds, nuts, roots, insects, slugs, snails, and crabs. Pregnant females were taken from November to July, which is mainly a dry period, and peak reproduction occurred in January. Females could produce a litter every one or two months during this season.