The marsh deer occurs from savanna patches along the southern margins of Amazonian Peru and Brazil south through northeastern Argentina. While formerly known in Uruguay as well, it is probably now extinct there. The major distributional area is defined by the Paraguay and Parana river basins. Pleistocene fossil deposits indicate that the marsh deer once occurred through northeastern Brazil as well (Magalhães et al. 1992, Pinder and Grosse 1991, Redford and Eisenberg 1992, Whitehead 1972).

Biogeographic Regions: neotropical (Native )

Blastocerus dichotomus is the largest South American deer, recognizable in part by its large multitined antlers of eight to ten points when mature. Head-body length is usually just under two meters, with shoulder height from 1.0 to 1.2 meters. The pelage is reddish brown in the summer, turning a darker brown in the winter. The tail is reddish orange, bushy, and 10 to 15 cm in length. Marsh deer have large feet with an elastic membrane between the hooves, which may help to keep them from sinking in the mud of their preferred marshy habitat. The legs are black below the carpal/tarsal joints, and there is a black band on the muzzle. Marsh deer have white eye rings and borders of the ears (Mares et al. 1989, Pinder and Grosse 1991, Redford and Eisenberg 1992, Whitehead 1972).

Range mass: 89 to 125 kg.

Average mass: 109 kg.

Sexual Dimorphism: ornamentation

• Terrestrial
• Freshwater

Marsh deer prefer marshy, swampy ground with standing water and dense vegetation. They also utilize flooded savannas during the wet season, but stay close to dense stands of reeds or similar vegetation near permanent water during the dry season. Surrounding mountainous terrain may also be favorable, but this may be an artifact of human hunting pressure as access is probably most difficult in mountainous areas (Mares et al. 1989, Whitehead 1972, Redford and Eisenberg 1992).

Terrestrial Biomes: savanna or grassland

Aquatic Biomes: rivers and streams

Marsh deer are herbivorous with diets comparable to other species of deer, although marsh deer consume mainly aquatic and riparian vegetation. Stomach analyses found that water lily and other leaves, grass, and browse were consumed. One study found that grass comprises about 50% of their diet and legumes 31% (Redford and Eisenberg 1992).

Fawns are born singly, with mature coloration (no spots). Males do not shed their antlers at any particular time of the year, and may retain them almost two years. Rut usually occurs, however, in October and November, but the breeding season may not be fixed, and males do not seem to be particularly aggressive to each other. Newborn fawns are reported from May to September, as well as from September to November. The gestation period may be as long as a year (Whitehead 1972, Pinder 1996, Redford and Eisenberg 1992, Mares et al. 1989).

Average birth mass: 4200 g.

Average gestation period: 270 days.

Average number of offspring: 1.

Marsh deer have suffered from uncontrolled hunting and habitat destruction, resulting in small, greatly-fragmented populations. Current hydraulic projects such as the proposed Hidrovia project on the Paraguay and Parana rivers threaten much of what remains of the habitat. One area, threatened with inundation in 1996 by a planned hydroelectric plant on the Parana River, contained 950 individuals making it the second largest population in Brazil. Marsh deer populations have also been reduced by cattle diseases, to which they are quite susceptible. Once common in Argentina, only a few hundred individuals may remain. Blastocerus dichotomus has been apparently extirpated in Uruguay (Mares et al. 1989, Pinder 1996, Quintana et al. 1992, Redford and Eisenberg 1992, Roig 1991, Whitehead 1972).

CITES: appendix i

IUCN Red List of Threatened Species: vulnerable

No negative impacts of B. dichotomus on human activities are reported in the literature. Wild ungulates sometimes compete with livestock for forage, and can serve as disease reservoirs, but the marsh deer's preference for wetland habitat may limit contact with some types of domestic livestock. Roxo and Gaspirini (1996) tested 116 marsh deer in Brazil and found that these deer do not harbor brucellosis in the São Paulo State region. Current numbers of deer are so low that negative impacts are very unlikely.

Marsh deer are valuable game animals, hunted for meat and sport (Roig 1991).