The Marsh deer is the largest deer species in South America. They have very large ears lined with white hairs, red-gold to tawny brown fur, blackish eyes and long dark legs. Their coat turns darker during winter and there are also white marks on the hips and around the eyes. Marsh deer have large hooves with elastic interdigital membranes which are useful for swimming and walking on marshy surfaces. Only the males possess antlers which are ramified and can grow up to 60 cm (23 in).
Marsh deer live in South America. They are found in Argentina, Bolivia, Peru, Brazil, and Paraguay. They occur at marsh and lagoon zones in the Paraná, Paraguay, Araguaia, and Guapore river basins. Small populations also occur in the southern Amazon, including Peru where protected in Bahuaja-Sonene National Park. Marsh deer live only in marsh areas, notably the Pantanal and Chaco, in which the level of water is less than 70 cm (28 in) deep.
Marsh deer are generally solitary animals; however, they may form small groups of less than 6 deer with only an adult male when they gather around water sources in dry periods. Males have larger home ranges which overlap with those of several females. Marsh deer are usually active at dawn and dusk, although they may also feed during the day and at night. 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, Marsh deer are able to find new food sources that the water uncovers during the dry season.
Marsh deer are herbivores and a majority of their diet consists of aquatic plants. They also enjoy eating aquatic flowers and shrubs that grow in the swamps and the floating mats. Their diet also changes between the dry season and the flood season.
Little is known about the mating system in Marsh deer. Their rutting season usually coincides with the dry season but can change from animal to animal. The gestation lasts approximately 271 days after which 1 fawn or occasionally twins are born between October and November. At birth, fawns are whitish in color and become more adult-like after a year. They are weaned at around 5 months but may stay with their mother for over a year after birth. Young Marsh deer become reproductively mature between 1 and 2 years of age.
The major threats to Marsh deer at present are hunting and the destruction of their habitat for agriculture, tree plantations, and dams. The Yacyretá Dam altered an area in which several hundred individuals lived and the draining of marshes for farmland and cattle threaten hundreds of hectares every year in Argentina and Brazil. Marsh deer also suffer from competition with domestic livestock and pollution of waterways cause by gold mining (Pantanal region).
According to the IUCN Red List, the total population size of the Marsh deer is unknown. However, there are estimated populations of the species in the following areas: Brazil - 41,000 individuals; Argentina - 2,000 individuals; Bolivia (north of Madidi National Park) - 700 individuals. Currently, this species is classified as Vulnerable (Vu) on the IUCN Red List and its numbers today are decreasing.