Marsh Deer
The marsh deer is the largest deer in South America, and everything about it is built for life in the water. Adults stand up to about 127 cm at the shoulder and usually weigh 80 to 125 kg, while an occasional big male reaches 150 kg. Their hooves are wide and joined by a stretchy membrane that spreads under their weight to grip soft mud and keep them from sinking. They wade through flooded grasslands and swamps across the Pantanal, the Iberá wetlands, the Paraná Delta and the floodplains of Bolivia, Paraguay and southeastern Peru, feeding on grasses, aquatic herbs and the stems of emergent plants. They are also strong swimmers, crossing rivers with ease. Populations have dropped sharply because of the draining of wetlands, dam construction, disease spread by cattle and poaching, and the species has already vanished from Uruguay. The IUCN lists it as Vulnerable, with numbers still decreasing.
Marsh deer, guasu puku or cervo-do-pantanal: its names
The marsh deer goes by many names across the five countries where it lives. In Argentina, Bolivia, Paraguay and Peru it is the ciervo de los pantanos, Spanish for marsh deer. In Brazil it is the cervo-do-pantanal, the deer of the Pantanal, the great wetland that holds most of the species. In Guaraní, spoken widely in Paraguay and northeastern Argentina, it is guasu puku, which means long deer or tall deer, a fitting name for an animal that towers over the smaller deer of the region. Its scientific name, Blastocerus dichotomus, refers to its forked antlers, which split again and again into pairs of points. The marsh deer is the only member of its genus. It should not be confused with the swamp deer, the usual English name for the barasingha of India and Nepal, a different species on another continent that happens to share a taste for wet grasslands. When people search for how many swamp deer are left, they are almost always asking about that Asian deer, not this South American one.
Where do marsh deer live?
Across central South America, the marsh deer is tied closely to wetland landscapes. Its range covers the Pantanal of Brazil and Bolivia, the Iberá wetlands of northeastern Argentina, the Paraná Delta near Buenos Aires and the floodplains along the Paraguay and Paraná rivers. Smaller populations persist in the Llanos de Moxos of Bolivia, in Paraguay and in the Pampas del Heath of southeastern Peru, while the species has disappeared from Uruguay. The species depends on flooded grasslands, swamp edges, and riverine marshes where water levels rise and fall with the seasons. It avoids dense forest and dry upland terrain, rarely straying far from standing water. Seasonal flooding shapes where individuals move and feed, and large connected wetland systems are essential for sustaining healthy populations. Habitat fragmentation has pushed many groups into isolated patches, reducing their ability to find mates and fresh feeding grounds.
How big is a marsh deer? Size, antlers and appearance
The marsh deer is the largest deer in South America. Adults stand roughly 1 to 1.27 meters at the shoulder, and most weigh between 80 and 125 kilograms, with an occasional big male reaching 150. Males are clearly heavier than females, often by 30 to 50 percent. The coat is a warm reddish brown to tawny, turning darker in winter, while the lower legs are black, as if the deer had just walked out of the mud. The muzzle is black too, and the large, rounded ears are lined with white hair. Only males grow antlers, and they are among the most impressive of any South American deer: each side usually carries four or five points that split in pairs, and a full set can be around 60 centimeters long. Unlike most deer, marsh deer do not shed their antlers on a fixed schedule; a set can be kept for well over a year, and shedding happens in any month, although it peaks around September. The hooves are long and spread wide, with a membrane between the toes that works like a snowshoe on soft mud and floating vegetation.
What do marsh deer eat?
Food for the marsh deer comes almost entirely from the wetland itself. It grazes on grasses and sedges growing along flooded margins, browses the stems and leaves of emergent aquatic plants, and pulls up submerged vegetation from shallow water. Aquatic herbs form a particularly important part of its diet during the wet season, when rising water levels push terrestrial grasses out of reach. The deer uses its long legs to wade into flooded areas and reach plants that smaller grazers cannot access. Feeding tends to be most active in the early morning and late afternoon, with animals resting in shaded cover during the hottest hours. Diet shifts somewhat with the season as plant availability changes, but the species remains dependent on wetland vegetation throughout the year.
Built for the swamp: its adaptations
Few large mammals are as well suited to wetland life as the marsh deer. Its hooves are notably wide and are joined by a stretchy membrane between the toes, which spreads under the animal's weight to distribute pressure across soft mud and prevent sinking. Long legs keep the body well above the waterline during wading, and the deer is a capable swimmer, able to cross rivers and flooded channels with little effort. A coarse, reddish brown coat provides insulation and some protection against biting insects. Males grow large, branching antlers of around 60 cm, which they replace roughly every year or so. The senses of smell and hearing are acute, helping the deer detect predators such as pumas and jaguars in dense wetland vegetation where visibility is limited.
Breeding and fawns
Marsh deer can breed at almost any time of year, and there is no single rutting season as in deer of colder climates. After a pregnancy of around 260 days, roughly eight and a half months, the female gives birth to a single fawn, usually in a patch of taller vegetation on slightly higher, drier ground where the newborn can stay hidden. One of the most surprising facts about the species is that its fawns are born without spots. Most deer fawns, from white tailed deer to red deer, wear a spotted coat as camouflage, but the young marsh deer is plain, born with a pale, whitish coat that slowly changes to the reddish brown of the adults during its first year. The mother keeps the fawn concealed for its first weeks and returns to nurse it, and the young deer is weaned at around five months. Marsh deer are usually seen alone or in very small groups, often a female with her latest fawn, and adults do not form the large herds seen in other deer. Because each female produces only one fawn per pregnancy, losses to hunting, dogs or disease take a long time to make up.
How many marsh deer are left?
The IUCN lists the marsh deer as Vulnerable, and its numbers are still decreasing. There is no recent count for the whole species, only regional surveys from different years. The Pantanal of Brazil holds by far the largest population: an aerial survey in 1991 and 1992 estimated around 44,000 animals there. Along the upper Paraná River in Brazil, around Ilha Grande National Park, a later aerial count estimated about 1,080 deer. Argentina's largest population lives in the Iberá wetlands of Corrientes, and the second is in the Paraná Delta, where hunting and changing land use caused a sharp fall in the late twentieth century. Elsewhere the deer survives in small, isolated groups, and it is already extinct in Uruguay. Wetland drainage for farming and cattle has destroyed much of its habitat, and dams such as Itaipú, Porto Primavera and Yacyretá have flooded floodplains and altered the natural rise and fall of the rivers. Cattle spread diseases such as foot and mouth disease and brucellosis, while poaching, attacks by dogs and road collisions add more losses. Argentina created Ciervo de los Pantanos National Park in Buenos Aires province in 2018, several provinces protect the species as a natural monument, and breeding programs in Brazil and Paraguay have returned deer to the wild, including a reintroduction at Yacyretá in 2025.
Ecological role
Wetland ecosystems benefit from the marsh deer in several ways. As a large grazer, it trims and opens dense stands of aquatic vegetation, which allows light to reach the water surface and supports a greater variety of plant species beneath. Seeds pass through its digestive system intact and are deposited across wide areas as the deer moves between feeding grounds, contributing to plant dispersal across the wetland. Its trails through flooded vegetation also open pathways that smaller animals use to move through the dense marsh. As prey, the marsh deer is an important food source for large predators including pumas and, in some regions, jaguars. The species is woven into the food web and vegetation structure of every wetland system it inhabits.
5 Curiosities about the Marsh Deer
Here are five things about the marsh deer that tend to surprise people: • It holds the title of the largest deer in South America, with most adults weighing 80 to 125 kg and the biggest males reaching 150 kg. • Its toes are joined by a stretchy membrane, a feature shared with very few other deer species in the world. • Males grow impressive antlers with multiple branches that can reach over 60 cm across, and they can keep one set for more than a year. • Its fawns are born without spots, unlike those of almost every other deer, with a pale coat that turns reddish brown during the first year. • Domestic cattle are among its biggest indirect threats, as livestock diseases can spread to wild populations and have caused local mass deaths.
