Nectomys squamipesSouth American water rat

Geographic Range

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. (Ernest and Mares, 1986; Ernest, 1986)


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. (Bergallo and Magnusson, 1999; Ernest and Mares, 1986; Ernest, 1986)

  • Range elevation
    10 to 2200 m
    32.81 to 7217.85 ft

Physical Description

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. (Ernest and Mares, 1986; Ernest, 1986; Hershkovitz, 1944)

  • Range mass
    160 to 420 g
    5.64 to 14.80 oz
  • Average mass
    216 g
    7.61 oz
  • Range length
    162 to 254 mm
    6.38 to 10.00 in


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. (Bergallo and Magnusson, 1999; Bergallo and Magnusson, 2002; D'Andrea, et al., 1996; Ernest and Mares, 1986; Gentile, et al., 2000)

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. (Bergallo and Magnusson, 1999; D'Andrea, et al., 1996; Ernest and Mares, 1986; Gentile, et al., 2000; Mares and Ernest, 1995)

  • 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
  • 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

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. (D'Andrea, et al., 1996)


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. (Gorbunova, et al., 2008)

  • Average lifespan
    Status: wild
    12 months
  • Typical lifespan
    Status: captivity
    2 to 5 years
  • Average lifespan
    Status: captivity
    3 years


South American water rats are solitary, nocturnal and semi-aquatic. Females are highly territorial, and although males have less defined home ranges than do females, they often migrate towards female territories during mating season. By rapidly alternating movements of the fore paws underwater, South American water rats are able to detect submerged objects. When foraging in the water, once and object is discovered, it grasps the object, raises it out of the water determines whether or not the object is edible. Water rats are able to catch prey on land by pouncing with both forepaws. When terrestrial prey is captured, South American water rats secure it in their forepaws. South American water rats tend to consume prey under vegetative cover and are highly adept climbers. (Briani, et al., 2001; Ernest and Mares, 1986)

South American water rats build nests from dry leaves and grasses that are woven together, similar those built by many birds. Typical nests are 15 cm long and 10 cm wide and have an egg-like shape. Nests are often built inside fallen trees, alongside streams, and among tree ferns, palms, and other dense vegetation. Less frequently, nests are found between rocks and throughout exposed root systems. (Briani, et al., 2001; Ernest, 1986)

  • Average territory size
    2200 m^2

Home Range

Mean home range size for South American water rats is 2,200 m^2. Males expand their home range during breeding season, in order to increase potential mating opportunities. (Ernest and Mares, 1986)

Communication and Perception

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. (Ernest and Mares, 1986; Hershkovitz, 1944)

Food Habits

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. (Bergallo and Magnusson, 1999; Ernest and Mares, 1986)

  • Animal Foods
  • amphibians
  • fish
  • insects
  • terrestrial non-insect arthropods
  • aquatic crustaceans
  • other marine invertebrates
  • Plant Foods
  • leaves
  • fruit


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.

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

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. (Ernest and Mares, 1986; Ernest, 1986; Gentile, et al., 2000; Olmos, 1991)

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. (Ernest and Mares, 1986; Ernest, 1986; Gentile, et al., 2000; Olmos, 1991)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

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. (D'Andrea, et al., 2000; Ernest and Mares, 1986; Savioli, et al., 1997)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Other than the potential bite, there are no known adverse effects of Nectomys squamipes on humans. (D'Andrea, et al., 1996)

  • Negative Impacts
  • injures humans
    • bites or stings

Conservation Status

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. (Bergallo and Magnusson, 1999)

Other Comments

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. (Ernest, 1986)


Alexandra Okihiro (author), Indiana University-Purdue University Fort Wayne, Mark Jordan (editor), Indiana University-Purdue University Fort Wayne, John Berini (editor), Animal Diversity Web Staff.



living in the southern part of the New World. In other words, Central and South America.

World Map


uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females


an animal that mainly eats leaves.


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


an animal that mainly eats fruit


An animal that eats mainly plants or parts of plants.


An animal that eats mainly insects or spiders.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


an animal that mainly eats all kinds of things, including plants and animals


an animal that mainly eats fish


having more than one female as a mate at one time


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sound above the range of human hearing for either navigation or communication or both


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year


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Bergallo, H., W. Magnusson. 2002. Effects of weather and food availability on the condition and growth of two species of rodent in Southeastern Brazil. Mammalia - International Journal of the Systematics, Biology and Ecology of Mammals, 66/1: 17-32. Accessed March 31, 2011 at

Briani, D., E. Vieira, M. Vieira. 2001. Nests and nesting sites of Brazilian forest rodents (Nectomys squamipes and Oryzomys intermedius) as revealed by a spool-and-line device. Acta Theriologica, 46/3: 331-334. Accessed March 03, 2011 at

D'Andrea, P., L. Maroja, R. Gentile, R. Cerqueira, A. Maldonado Jr., L. Rey. 2000. The parasitism of Schistosoma mansoni (Digenea-Trematoda) in a naturally infected population of water rats, Nectomys squamipes (Rodentia-Sigmodontinae) in Brazil. Parasitology, 120/6: 573-582.

D'Andrea, P., C. Horta, R. Cerqueira, L. Rey. 1996. Breeding of the water rat (Nectomys squamipes) in the laboratory. Laboratory Animals, 30: 369-376. Accessed March 03, 2011 at

Ernest, K. 1986. Nectomys squamipes. Mammalian Species, 265: 1-5. Accessed March 03, 2011 at

Ernest, K., M. Mares. 1986. Ecology of Nectomys squamipes, the neotropical water rat, in central Brazil:home range, habitat selection, reproduction and behaviour. Journal of Zoology, 210/4: 599-612.

Gentile, R., P. D'Andrea, R. Cerqueira, L. Maroja. 2000. Population dynamics and reproduction of marsupials and rodents in a Brazilian rural area: a five-year study. Studies on Neotropical Fauna and Environment, 35: 1-9.

Gorbunova, V., M. Bozzella, A. Seluanov. 2008. Rodents for comparative aging studies: from mice to beavers. The Official Journal of the American Aging Association, 30: 111-119. Accessed May 13, 2011 at

Hershkovitz, P. 1944. A systemic review of the neotropical water rats of the genus Nectomys (Cricetinae). Miscellaneous Publications, Museum of Zoology, University of Michigan, 58: 1-109. Accessed March 31, 2011 at

Mares, M., K. Ernest. 1995. Population and community ecology of small mammals in a gallery forest of central Brazil. Journal of Mammalogy, 76/3: 750-768. Accessed March 03, 2011 at

Mares, M., H. Genoways. 1981. Mammalian biology in South America. The Pymatuning Symposia in Ecology: 120-160.

Olmos, F. 1991. Observations on the Behavior and Population Dynamics of some Brazilian Atlantic Forest Rodents. Mammalia, 55/4: 555-566.

Savioli, L., E. Renganathan, A. Montresor, A. Davis, K. Behbehani. 1997. Control of schistosomiasis - a global picture. Parasitology Today, 13/11: 444-448.