The marsh deer lives only in marsh areas, notably the Pantanal and Chaco, in which the level of water is less than 70 cm (28 in) deep. They are swift swimmers. The marshes with their high vegetation density protect them from predators and provide them with food. These deer also have a small migratory pattern, they follow the water levels between the dry season and flooding season. With the fluctuation in water levels, they are able to find new food sources that the water uncovers during the dry season. Some freshwater ponds on the Pantanal Wetland, Brazil reported low densities of individuals dictating that those ponds are not able to support large populations of marsh deer.
Since marsh deer live near aquatic habitats, they eat a majority of their diet in aquatic plants. A study was conducted and they found 40 different species of plants in which they ate. The main food component was Graminae which took up 22% of their diet, Pontederiaceae took up 12%, Leguminosae was about 11%, and the rest was filled in with Nymphaeaceae, Alismataceae, Marantaceae, Onagraceae, and Cyperaceae. They also enjoy eating aquatic flowers and shrubs that grow in the swamps and the floating mats. They can be best classed as a grazer-browser for food. Their diet also changes between the dry season and the flood season.
They possess very large ears lined with white hairs, reddish brown colored body and long dark legs. The hair turns darker during winter. There are also white marks on the hips and around the eyes. The legs are black below the tarsal as is the muzzle. The tail is of a paler reddish tone than the rest of the body on its upper part and black on the under part. The head-and-body length is 153 to 200 cm (5.02 to 6.56 ft), while the tail adds a further 12–16 cm (4.7–6.3 in). The height at the shoulder can range from 100 to 127 cm (3.28 to 4.17 ft).
The hoof, which is large in relation to the body, has elastic interdigital membranes which are useful for swimming and walking on marshy surfaces. Only the males possess antlers which are ramified and reach a length of 60 cm (23 inches). An adult typically grows to a weight of 80 to 125 kg (176 to 276 lb), although an occasional big male can weigh up to 150 kg (330 lb). They are solitary animals or living in groups with less than 6 individuals with only an adult male. Their main predators are the jaguar and the puma.
Usually the rutting season coincides with the dry season but can change from animal to animal. They may use this to their advantage for breeding or finding mates because the densities of marsh deer are significantly higher on the Rio Negro marshland boundary during the dry season compared to the less dense, more distributed population during the flooded season. Gestation lasts approximately 271 days. The offspring (normally one per female, though occasionally twins are born) are born between October and November. The infant deer are whitish which becomes more adult-like after a year.