|Male marsh deer|
|Geographic range: Red=Former; Yellow=Present|
The marsh deer (Blastocerus dichotomus) is the largest deer species from South America reaching a length of 2 m (6.6 ft) and a height of 1.2 m (3.9 ft) at the rump. It is found in Argentina, Bolivia, Peru, Brazil, Uruguay and Paraguay. Formerly found through all of tropical South America today it is reduced to small isolated populations at marsh and lagoon zones in the basins of the rivers Paraná and Paraguay as in the Amazonian region of Peru where it is protected in Bahuaja-Sonene National Park. Current distribution of this species is east of the Andes, south of the Amazon rainforest, west of the Brazilian Atlantic rainforest and north of the Argentinian Pampa. Small population reside in the Amazon River basin but the largest population occur in the flood plains of the Paraguay, Guapore, Araguaia and Parana Rivers. There are some isolated populations The latter half of its scientific name refers to the forked antlers that they carry. They resemble Mule Deer or Blacktail Deer.
It is listed as a vulnerable species, according to CITES, appendix I.
The marsh deer lives only in marsh areas, pantanal and chaco, in which the level of water is less than 70 cm 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 claw, 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).
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 Negro River 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.
The natural predators of the marsh deer – the jaguar (locally called onça or yaguaraté) (Panthera onca) and the puma (Puma concolor)— have almost completely disappeared from its habitat. The former major threat was poaching for its antlers, but this is somewhat under control. Destruction of its habitat presents nowadays the major threat to marsh deer. The dam at Yacyretá 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. Contagious diseases from cattle are also a problem, though it has been shown that the deer is not affected by brucellosis.
- Duarte, J.M.B., Varela, D., Piovezan, U., Beccaceci, M.D. & Garcia, J.E. (2008). Blastocerus dichotomus. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 10 April 2009. Database entry includes a brief justification of why this species is of vulnerable.
- (Cabrera 1961; Thornback & Jenkins 1982; Tomas et al. 1997)
- (Tomas et al. 1997)
- (Tomas et al. 2001)
- (Tomas & Salis. 2000)
-  (2011).
-  (2011).
- (Tomas et al. 2001)
- (Duarte & Garcia, 1995)
- Tomas, W.M., and S.M. Salis. 2000. Diet of the marsh deer (Blastocerus dichotomus) in the Pantanal wetland, Brazil. Studies on Neotropical Fauna & Environment. 35:165–172.
- Tomas, W.M., S.M. Salis, M.P. Silva, and G.M. Mourao. 2001. Marsh deer (Blastocerus dichotomus) distribution as a function of floods in the Pantanal Wetland, Brazil. Studies on Neotropical Fauna & Environment. 56:9-13.
- Tomas, W.M., M. Beccaceci, and L. Pinder. 1997. Cervo-do-pantanal (Blastocerus dichotomus). Biologia e Conservacao de Cervideos Sul Americanos. 24-38.
- Cabrera, A. 1961. Catalogo de los mamiferos de America del Sur. Rev Mus Argentino Cien Nat Bernardino Rivadavia. 4:1-732.
- Thornback, J., and M. Jenkins. 1982. The IUCN mammal red data book, Part 1: Threatened mammalian taxa of the Americas and the Australian zoogeographic region (excluding Cetacea). IUCN. 516 pp.