The name Nectomys is derived from the greek words “swimming” and “mouse”. The specific epithet squamipes refers to scale adaptations present on the soles of the hind feet. Pleistocene fossils of Nectomys can be found in a cave in Minas Gerais, Brazil.
South American water rats use their forepaws to forage for aquatic prey. When potential prey items are detected, they raise the object to their nose and use olfaction to determine if it is edible. When on land, they use olfaction and sight to detect terrestrial prey and subdue it with their forepaws or by pouncing. Males chatter when they are in distress, or when female are present. Females respond by producing a low volume, high frequency sound. Although there is little evidence suggesting communication between conspecifics via pheromones, most rodents communicate mating status and demarcate territorial boundaries with scent markings.
Communication Channels: visual ; acoustic
Perception Channels: visual ; tactile ; acoustic ; ultrasound ; chemical
The IUCN Red List of Threatened Species classifies South American water rats as a species of least concern. However, current population trends are unknown and habitat alteration and destruction are becoming an increasing concern for a number of rodent species throughout South America. For example, the Atlantic forest of southeastern Brazil is now approximately 5% of its original size, which affects minimum of 57 species of resident rodents.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
IUCN Red List of Threatened Species: least concern
Other than the potential bite, there are no known adverse effects of Nectomys squamipes on humans.
Negative Impacts: injures humans (bites or stings)
South American water rats are important in biomedical research concerning Schistosoma mansoni and the pathogens it transmits. In addition, because South American water rats are omnivorous and consume a number of different insect species, they may help control insect pest populations.
Positive Impacts: research and education
South American water rats are sympatric with several other species of rodent, including many species of rice rats and water rats. Species that are commonly found in areas inhabited by South American water rats include black-footed pygmy rice rats, striped Atlantic Forest rats, cursor grass mice, Ihering's akodonts, hispid hocicudos and Atlantic Forest climbing mice. Because water rats are primarily omnivorous and consume prey both in water and on land, they help mediate the flow of energetic resources between the terrestrial and aquatic food webs. In addition, water-borne disease and/or water contamination via metals, nitrates, and phosphates, have the ability to be transferred into terrestrial food webs using water rats as an intermediate.
South American water rats are host to several ecto- and endoparasites. Common ectoparasites include ticks, mites, fleas, and sucking lice. Endoparasites include several species of trematode and nemotode. South American water rats also fall victim to Schistosoma mansoni, a species of trematode thought to transmit a number of pathogens known to also infect humans.
Commensal/Parasitic Species:
South American water rats are primarily omnivorous. Their diet includes fruits, leaves, arthropods, including insects such as beetles and cockroaches, and small vertebrates such as tadpoles, frogs and fish. The abundance of food peaks immediately after the wet season, and neonates born during this time have a tendency to grow larger than those born during other times, likely due to variations in the abundance of food.
Animal Foods: amphibians; fish; insects; terrestrial non-insect arthropods; aquatic crustaceans; other marine invertebrates
Plant Foods: leaves; fruit
Primary Diet: carnivore (Piscivore , Insectivore , Eats non-insect arthropods); herbivore (Folivore , Frugivore ); omnivore
South American water rats are found in moist forested areas in north and central South America. Their geographic range extends from the northern coast of South America to southern Brazil (encompassing the surrounding area of Misiones, Argentina) and from the Atlantic coast of Brazil to the eastern slope of the central region of the Andes mountain range.
Biogeographic Regions: neotropical (Native )
South American water rats are habitat specialists and can be found either in or within 2 m of tropical forest streams. Population densities are significantly higher in wet, moist forested areas and swamps than in drier habitats, and they are occasionally found in caves as well. They can be found across a broad range of elevations, from 10 m above sea level to 2200 m.
Range elevation: 10 to 2200 m.
Habitat Regions: tropical
Terrestrial Biomes: forest ; rainforest
Wetlands: marsh ; swamp
Other Habitat Features: riparian ; caves
There is no information available regarding the average lifespan of South American water rats. Many species in the same family (i.e., Cricetidae) live approximately 12 months in the wild and from 2 to 5 years in captivity, with an average life expectancy of 3 years.
Average lifespan
Status: wild: 12 months.
Typical lifespan
Status: captivity: 2 to 5 years.
Average lifespan
Status: captivity: 3 years.
Members of the Nectomys genus can be distinguished from other Sigmondontinae by the presence of partially webbed hindfeet and relatively large body size that averages 216 g. Nectomys also possess relatively primitive and unspecialized cheek teeth compared to rodents in the same sub-family. The defining characteristic of South American water rats are their long, sparsely haired hindfeet, which range in length from 43.5 mm to 54.9 mm. Webbing exists between all toes, but is reduced between the hallux and second digit, and between the fourth and fifth digits. Five to six tubercles are also observed on the soles of the hindfeet. South American water rats are the largest and most aquatically adapted of all Nectomys species. Head and body length varies between 162 mm to 254 mm. Their pelage is long and is either buffy or tawny mixed with dark brown on the dorsum. Ventral pelage is paler in color and is either gray or white. The tail is roughly 75% the length of the head and body, and is brown, lacking bi-coloration. Hairs along the tail are longer and stiffer on the ventral surface, forming a keel that is useful for aquatic navigation. Tail length varies between 165 mm and 250 mm. South American water rats have 16 teeth (dental formula = 1/1, 0/0, 0/0, 3/3). The incisors lack curvature, and the molars are hyposodont with low buccal apices. The second and third molars are longer than they are wide. Both male and female offspring gain adult pelage around 14 days after birth. Eighteen subspecies have been described. Sexual dimorphism has not been described in this species.
Range mass: 160 to 420 g.
Average mass: 216 g.
Range length: 162 to 254 mm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Due to their nocturnal nature, risk of predation for South American water rats is highest during night. Barn owls are their only documented predator. They are adept climbers and swimmers, which likely reduces risk of predation, and their coloration probably helps camouflage them from potential predators.
Known Predators:
Anti-predator Adaptations: cryptic
South American water rats are polygynous, and males often breed with several different females. During mating season, males increase their home range size to to increase potential mating opportunities. Although some males are sexually active throughout the year, pregnant females are only found during the early part of the rainy season (August to November), suggesting seasonal breeding. Survival of neonates is highest when food resources (e.g., arthropods and fruits) are most abundant, which typically occurs during the rainy season. Water rats reproduce opportunistically in favorable (i.e., wet) conditions, and as a result, local populations can increase rapidly. Water rats bred in captivity reproduce year-round, and agonistic behavior between males and females is uncommon.
Mating System: polygynous
Gestation for South American water rats bred in captivity lasts for about 30 days, and there is no variation in gestation length between non-lactating and lactating females. At birth, males have an average body mass of 11.1 g and females have an average body mass of 10.3 g. In captivity, litters range in size from 1 to 6 offspring, with an average of 4.1 offspring per litter. Weaning occurs between 20 and 25 days after parturition for captive-bred offspring. Captive females become sexually mature by 75.1 days after birth and at an average weight of 140.3 g, and individuals can mate as soon as 80 days after birth. In the wild, litters range in size from 2 to 7 offspring with an average of 5 offspring per litter. Sexual maturity in wild females occurs at an average age of 44.4 days and in wild males at an average age of 37.8 days.
Breeding interval: South American water rats breed once a month under optimal (i.e., wet) conditions.
Breeding season: South American water rats breed during the wet season, from August to November.
Range number of offspring: 2 to 7 .
Average number of offspring: 5 .
Average gestation period: 30 days.
Range weaning age: 20 to 25 days.
Range age at sexual or reproductive maturity (female): 31 to 51 days.
Average age at sexual or reproductive maturity (female): 44.4 days.
Range age at sexual or reproductive maturity (male): 27.7 to 45.5 days.
Average age at sexual or reproductive maturity (male): 37.8 days.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
In South American water rats, females provide the majority of parental care to neonates, as males are almost always absent. Females nurse their offspring, which are naked, blind and unable to care for themselves until weaning, about 20 to 25 days after parturition.
Parental Investment: female parental care ; pre-weaning/fledging (Provisioning: Female)
Nectomys squamipes, also known as the Atlantic Forest nectomys,[2] South American water rat,[1] or scaly-footed water rat,[3] is a semiaquatic insectivorous rodent species. It is from Argentina, Brazil, and Paraguay — found primarily near forest rivers and streams in the Atlantic Forest ecoregion.[1]
Nectomys squamipes, or the South American water rat can be distinguished from other Sigmodontinae based on their size and morphology of their hindfeet. They are the largest of the family (average of 216g) and have long hind legs with partially webbed feet to aid in swimming and have completely scaled heels. Nectomys is derived from Greek words meaning "mouse" and "swimming". One unique morphological characteristic is the distinct primitive cuspidate pattern of their cheekteeth, which have hypsodont molars, teeth crowned earlier in life and the outer fold of the upper molars tend to be more isolated as enamel islands. The Nectomys squamipe tends to have relatively longer, less hairy ears and a glossy, long pelage. This pelage is made up of wool hairs and guard hairs and is darker dorsally and tends to be lighter grey on its sides and ventrally. The tail is much less covered in hair, and length is around 75% of the body length.
N. squamipes have a larger skull that occupies well-developed supraorbital ridges and lambdoidal crests, giving the mammal great attachments for jaw muscles and a strong bite. This strong bite is shown useful with its large, strong incisors that are greatly curved.
The South American water rat inhabits elevations from sea level to around 2,000 m, in areas from the northern coast of South America to Brazil, Argentina and the western slope of the Andes. Almost all of these mammals are found a small (2 m) distance from tropical forest streams and rivers. They tend to prefer the moist habitats in forested areas such as swamps or marshes, and are sometimes found inhabiting caves
The South American water rat has mainly one predator, the barn owl (Tyto alba). They are most vulnerable at night when the barn owls are hunting and the nocturnal rat is out of its nest. Nectomys squamipes are very good swimmers and climbers, which act as short-term defenses against predation. Their darkened dorsal pelage may also act as camouflage.
Nectomys squamipes is an omnivorous mammal with a diet ranging from fruits and leaves to insects and small fish. The South American water rat tends to feed more often after the end of the wet season, when food is in abundance. Many newborn water rats will grow bigger during this time period, because they have access to more food than those born during drier periods.
The South American water rat is a polygynous mammal in which males will increase their home range in attempt to mate with many females to produce the most offspring possible. Although males tend to be sexually active all year long, seasonal breeding patterns are suggested since females have a higher pregnancy rate during the wet season, which is August to November in South America. This will produce offspring that have an abundant food source when born, resulting in greater growth and larger populations.
Gestation periods last around 30 days and young have an average body length between 10-11 grams, with litters as small as 1 offspring and as large as 7. Males are generally absent when raising the altricial young, so the female is responsible for feeding and caring for their young.
Nectomys squamipes is a solitary mammal that is active during the night. The males have a much less defined home range and tend to roam, while the females are very territorial.
When swimming and searching for food, the South American water rat is able to detect objects under water and lift them above the surface in order to inspect to see if the object is edible or not using olfaction and sight. When hunting on land it is able to secure prey but pouncing and grasping with its forepaws.
Nests are generally built that are similar to those of birds and consists of dry twigs and leaves woven together. They are built inside dead trees alongside streams and under dense vegetation for protection.
Nectomys squamipes is commonly studied as a reservoir for the organism Schistosoma mansoni, which causes the disease Schistosomiasis. This disease is caused by parasitic worms found in freshwater, which may have been consumed by the South American water rat. Studies focus in endemic areas of Brazil that have large populations of N. squamipes, in attempt to find a correlation between the water rat and disease in residents of endemic areas. A great correlation between species presence and disease has been shown.
Nectomys squamipes, also known as the Atlantic Forest nectomys, South American water rat, or scaly-footed water rat, is a semiaquatic insectivorous rodent species. It is from Argentina, Brazil, and Paraguay — found primarily near forest rivers and streams in the Atlantic Forest ecoregion.
