The total level of mercury detected in the sediment and in the tissues of molluscs from the Bento Gomes basin, although low, have shown that the mercury used in the gold mining activities in the Poconé wetlands has contaminated those aquatic habitats in Pantanal. From 69 sediment samples analyzed, 26 % (N = 18) have shown levels ranging from 0.01 to 0.25µg.g-1 of mercury (moist weight). Mercury levels analyzed in 54 samples of mollusc tissues (Ampullaria scalaris Orbigny, 1835; A. canaliculata Lamarck, 1819 and Marisa planogyra Pilsbry, 1933) have shown that 30% (N = 16) were contaminated with levels ranging from 0.02 to 1.16µg.g-1 moist weight. This study shows that the mercury used in digs for gold mining and released into the environment has reached the habitats of Pantanal spread from the sediment into the molluscs living in the region.

(1) The Pantanal is the central portion of a sunken flat plain bordering the upper Paraguay river, with an area of 140,000 km². Seasonal floods begin in January and end in April. From May to October the land dries out and campos (grassland formations) and scattered pools appear. (2) Typical capybara habitat in the Pantanal is composed of three components: the water, a patch of forest, and a grass field for foraging. Highest densities of capybaras (14 individuals/km²) were found during the rainy season when the available space for capybaras was reduced due to the flooding. Areas without ponds or creeks presented low densities (from 0.38 to 0.84 capybaras/km²). Group sizes ranged from 2 to 49 individuals (x = 9.48). (3) Reproduction occurred year-round and young of different ages were seen throughout the year. However, the principal recruitment of young to the population was observed in July-August. While the females took turns in caring for young of different ages, probably the offspring of mothers who are sisters in the same social groups, the males competed intensively with each other for access to breeding females. The male's reproductive success appears to be limited to the number of breeding females that males have access to in the social group.

The pantanal is one of the world's richest freshwater wetlands. The pantanal is located in the flood plain of the headwaters of the Paraguai river, covering 140,000km². The habitats of the Pantanal are present in a complex mosaic. The major habitat types are: pockets of forests, called capão or cordilheira, seasonally flooded grasslands or campos, and permanent or temporary lagoons, called baías. The Pantanal harbors both rich and abundant mammal fauna. A survey was conducted at Fazenda Nhumirim, a research station run by the Centro de Pesquisa Agropecuária do Pantanal (CPAP) in Corumbá, a research branch of EMBRAPA. The Fazenda covers an area of 4,310 ha in the sub-region of Nhecolândia, appoximately 150km east of Corumbá, Mato Grosso do Sul. The area receives an average annual rainfall of 1,022mm, and has a mean monthly temperature that varies between 29.1ºC (January) and 22.0ºC (June). Four routes were followed between two times during ten days of each month. The survey routes were covered on horseback by between two and four observers. Each route was followed and equal number of times in the morning and in the afternoon, to eliminate bias related to the activity patterns of mammals. In order to evaluate relative abundance of nocturnal species, we also conducted a number of nocturnal censuses. These censuses were done by car; a high intensity searchlight was used to spot the animals. We also conducted a trap-mark-recapture survey of the small mammals at Fazenda Nhumirim. After the census period, we continued to collect observations on the occurrence of mammals at the Fazenda, noting wherever possible the kind of habitat in which the animals were observed. The survey identified a diversity of mammals at Fazenda Nhumirim: six orders, 14 families, 19 genera, and 20 species. Nasua nasua was the most frequently observed species during the diurnal census, accounting for 61.5 percent of all observations. Dusicyon (formely Cerdocyon) thous was the most frequently observed species on the nocturnal census (39.13%). The small mammal community of Fazenda Nhumirim is composed of seven species: one marsupial, four cricetine rodents and two echimyid rodents.