Blastocerus dichotomus (Illiger, 1815)

Common Name & Appearance Comparison

Blastocerus dichotomus, commonly known as the marsh deer, resembles the North American mule deer or blacktail deer in appearance.

Facial & Body Fur Features

This species has very large ears lined with white hairs, fur that ranges from red-gold to tawny brown, blackish eyes, and long dark legs. Their coat becomes darker during the winter, and they have white markings on their hips and around their eyes.

Limb & Muzzle Coloration

The lower portion of the legs below the tarsal bone and the muzzle are both black.

Tail Coloration

The upper side of the tail is a paler reddish shade than the rest of the body, while the underside of the tail is black.

Head & Body Dimensions

Marsh deer have a head-and-body length between 153 and 200 cm (5.02 to 6.56 ft), with an additional tail length of 12 to 16 cm (4.7 to 6.3 in). Their shoulder height ranges from 100 to 127 cm (3.28 to 4.17 ft).

Hoof Structure

Their hooves are large relative to their body size, and feature elastic interdigital membranes that help with swimming and walking on marshy surfaces.

Antler Characteristics

Only male marsh deer grow antlers, which are branched and can reach up to 60 centimetres (24 in) in length.

Weight Range

Most adults weigh between 80 and 125 kg (176 to 276 lb), though occasional large males can reach up to 150 kg (330 lb).

Size Status Among South American Deer

This makes the marsh deer the largest living deer species native to South America, though the extinct genus Antifer was larger.

Current Geographic Range

The marsh deer is found across Argentina, Bolivia, Peru, Brazil, Uruguay, and Paraguay.

Historical Distribution

Historically, it occupied most of tropical and subtropical South America, with a range that extended east of the Andes, south from the Amazon rainforest, west of the Brazilian Atlantic rainforest, and north of the Argentinian Pampa.

Current Population Distribution

Today, most of its population has been reduced to isolated groups in marsh and lagoon zones of the Paraná, Paraguay, Araguaia, and Guaporé river basins. Small populations also exist in the southern Amazon, including in Peru’s protected Bahuaja-Sonene National Park.

Conservation Status

This species is classified as Vulnerable by the IUCN and is protected under CITES Appendix I.

Habitat Requirements

Marsh deer live exclusively in marsh habitats, most notably the Pantanal and Chaco, where water depth is less than 70 cm (28 in) deep.

Swimming Ability

They are strong, swift swimmers.

Habitat Benefits

The dense vegetation of their marsh habitats protects them from predators and provides them with food.

Seasonal Migration Pattern

Marsh deer follow a small seasonal migration pattern, moving with changing water levels between the dry and flood seasons. This movement lets them access new food sources uncovered by receding water during the dry season.

Population Density in Pantanal Ponds

Observations of freshwater ponds in Brazil’s Pantanal Wetland found low densities of marsh deer, indicating these small ponds cannot support large populations of the species.

Social Structure

Marsh deer are typically solitary, or live in groups of fewer than six individuals that include only one adult male.

Predators

Their main predators are jaguars and pumas.

Breeding Rut Timing

The breeding rut for marsh deer usually falls during the dry season, though the timing can vary between individuals.

Rut Timing Advantage

Rutting during the dry season is advantageous because marsh deer densities are much higher on the Rio Negro marshland boundary in the dry season, compared to the lower, more dispersed population densities seen during the flooded season, making it easier to find mates.

Gestation Period

Gestation lasts approximately 271 days.

Offspring Birth Characteristics

Females usually give birth to a single offspring at a time, though twins occur occasionally. Offspring are typically born between October and November.

Infant Development

Infant marsh deer are whitish in color, and develop their adult coat pattern and coloration after one year.