Today it is largely reduced to isolated populations 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.

They possess very large ears lined with white hairs, red-gold to tawny brown fur, blackish eyes and long dark legs.

[6] The hoof, which is large in relation to the body, has elastic interdigital membranes which are useful for swimming and walking on marshy surfaces.

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),[7] making it the largest species of living South American deer,[8] though it was exceeded in size by the extinct Antifer.

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 may use this to their advantage for breeding or finding mates because the densities of marsh deer are significantly higher on the Rio Negro marshland boundary during the dry season compared to the less dense, more distributed population during the flooded season.

In October 2018, Argentina established the Ciervo de los Pantanos National Park to help protect this species.