Rats, Mice, and Relatives IV: South American Rats and Mice (Sigmodontinae)
Rats, mice, and relatives IV
South American rats and mice (Sigmodontinae)
Class Mammalia
Order Rodentia
Suborder Sciurognathi
Family Muridae
Subfamily Sigmodontinae
Thumbnail description
Small- to medium-sized mice and rats; in general brownish or blackish upper pelage and grayish or whitish underparts; tail usually thinly haired; molars with two rows of cusps
Size
Head and body length: 2.4–11.4 in (61–290 mm); tail length 1–6.3 in (30–160 mm); weight: 0.4–18 oz (12–510 g).
Number of genera, species
71 genera; 373 species
Habitat
All habitats (e.g., dry deserts, wet tropical forests, wetlands, savannas, steppes, temperate woodlands, high altitude grasslands, salt flats, scrublands) found in their distribution range up to 17,000 ft (5,200 m) above sea level
Conservation status
Extinct: 6 species; Critically Endangered: 8 species; Endangered: 24 species; Vulnerable: 23 species; Lower Risk/Near Threatened: 36 species; Data Deficient: 3 species
Distribution
Endemic to the New World; living forms range from Tierra del Fuego, the southernmost part of South America, up to southern United States; also distributed on some islands adjacent to the continent as well as in the oceanic Galápagos archipelago
Evolution and systematics
Over the last several decades, the contents and limits of the subfamily Sigmodontinae have been a center of much debate. However, as of 2001, an agreement has been reached in defining Sigmodontinae to only encompass the predominantly South American complex-penised mice and rats, leaving out of the subfamily those predominantly North American simple-penised mice and rats (e.g., the wood rats Neotoma, the grasshopper mice Onychomys, and deer mice Peromyscus). Now defined, the subfamily Sigmodontinae includes 71 living genera and 373 species. Remarkably, these numbers keep growing as new genera and species are being discovered and identified from both field and museum based work. New sigmodontines are being discovered even in areas that are presumably well known from a mammalogical point of view. For example, Oxymycterus josei was described in 2002 for the first time from areas near the southern coast of Uruguay. These areas have been frequently visited by mammalogists since the early nineteenth century. Sigmodontines include several familiar rats and mice, such as the cotton rats (Sigmodon), and the rice rats (Oryzomys), but the subfamily also encompasses an enormously diverse collection of other rodents practically unknown to most people.
Traditionally, sigmodontine genera have been arranged into different groups, some have been formalized as tribes in zoological classifications. Depending on the relative weight that is given to different trenchant characters, the number and content of sigmodontine groups vary from author to author. In the early 1990s an almost universal adoption of a phylogenetic approach to delimit these groups caused major reconsiderations on the identity of the groups as well as on their limits and contents. These studies have promoted: the recognition of a previously unnoted group (the abrothricines, mostly confined to the central and south Andes); the subsuming of some main groups within others (e.g., scapteromyines into akodontines); the corroboration of the distinction of some groups (e.g., reithrodontines, wiedomyines); and called into question the nature of at least one group (the thomasomyines) and the composition of most groups. As of 2003, nine main sigmodontine groups are recognized (Smith and Patton, 1999).
Diversity is not evenly distributed among them. The akodontines, oryzomyines, and phyllotines account for more than half of extant genera. The abrotrichines, the ichthyomyines, the reithrodontines, the sigmodontines, the thomasomyines, and the wiedomyines are groups of one to five extant genera each. In addition, there are several extant genera whose phylogenetic relationships are not clear; these genera cannot be assigned with certainty to any monophyletic group less inclusive than the subfamily. In formal classifications these genera (e.g., Abrawayaomys, Delomys, Irenomys, Juliomys, Phaenomys, Punomys, Rhagomys, Wilfredomys) are generally considered as incertae sedis (of uncertain taxonomic position). Phylogenetic relationships among sigmodontine main groups or tribes are mostly unclear. The only point that appears well corroborated is that the Sigmodontini, composed only of one extant genera (Sigmodon), is the sister group of all remaining sigmodontines.
Although extensive, past sigmodontine diversity is not as great as the present (Pardiñas et al. in press). The oldest South American known sigmodontine comes from the latest Miocene sediments of the Argentinean province of La Pampa. Sigmodontines are rare in early Pliocene beds and become more abundant in late Pleistocene sediments. While, more than a half of extant genera are also recorded as fossils (many from Holocene beds), only seven known genera have become extinct during geologic times. In addition, three genera presumably became extinct in historic times. The sigmodontine nature of some North American fossils, such as Prosigmodon, Bensonomys, and Symmetrodontomys, is questionable and deserves further scrutiny.
The understanding of sigmodontine evolutionary history has proved to be a complex task. All students agree on the fact that in spite of most sigmodontines being endemic to South America, the immediate ancestor of the group did not originate in South America. In other words, at some point in history sigmodontines or their immediate ancestor invaded South America. The basis for this agreement lies on the fact that no potential ancestor has yet been discovered in South America. The fossil record indicates that this invasion was as late as Late Miocene. As of 2003, two main aspects of sigmodontine historical biogeography remain unresolved. These are: 1) whether the geographic origin of the stock that gave rise to the sigmodontines was in the Old World or in Central-North America; and 2) the geographic placement of the basal sigmodontine radiation; in other words, how many sigmodontine lines invaded South America? Evidence at hand tends to indicate that the sigmodontine ancestor inhabited Central-North America. However, the numbers of sigmodontine lines that invaded South America remains to be solved. Therefore, it is not clear if North American sigmodontine are direct descendents of the first sigmodontines or if they represent secondary invasions to North America of lines first differentiated in South America.
Physical characteristics
Although most sigmodontines are typical mice and rats, the group exhibits several deviations from this morphotype.
Examples of these departures are the shrew mouse Blarinomys, the mole mouse Geoxus, the chinchilla mouse Chinchillula, and the rabbit rat Reithrodon. Sigmodontines are small. They range from about 0.4 to about 14.1 oz (12 to about 400 g). The largest living form, Kunsia tomentosus, does not surpass 11.8 in (300 mm) of head and body length. External ears vary from almost absent or much reduced (e.g., the aquatic rat Anotomys and the long-clawed mole mouse Geoxus) to moderately large (e.g., the long-eared mice of the genus Phyllotis) or large (e.g., the rabbit rats of the genus Reithrodon). Climbing, arboreal and scansorial species tend to have tails longer than head and body length; tail approximately equal to combined head and body is characteristic of the more generalized terrestrial sigmodontines; while the tail of fossorial species is usually less than half of head and body length. The tail is usually thinly haired, but in some sigmodontines ends in a penciled tip (e.g., Andalgalomys). Most sigmodontines have a brownish or blackish upper pelage and grayish or whitish underparts. However, Chinchillula is noticeable in this regard; its upper parts are buffy or grayish with black lines, the underparts are white, and the white hips and white rump banded with black. Sigmodontine fur may be velvety, soft, woolly, long, thick, harsh, composed of underfur and guard hairs. Abrawayaomys, Neacomys, and Scolomys have some hairs modified into spines. Sigmodontine feet also show variation. In general, feet are relatively small, but those of fossorial forms like the giant rat Kunsia are robust and extremely long clawed; those of swimmers like the marsh rat Lundomys are large and webbed, and those of climbers (e.g., Rhipidomys) are broad and strongly grasping. Finally, the hindfoot of the arboreal mouse Rhagomys is remarkable among sigmodontine because the fifth toe is long, and the big toe presents a nail instead of a claw.
As in all rodents, one upper and one lower incisor (sometimes grooved) are found on each side of the jaw, and canines and premolars are never present. After an empty space, called diastema, following the incisor, three cheek teeth, or molars, occur on each side. The total number of teeth is 16. The exception to this pattern is the ichthyomyine Neusticomys oyapocki that has two molars per upper and lower jaw. There is a large variation in molar size, morphology, and number of roots. For example, Reig in 1977 nominated more than 30 cusps, cuspules, styles, lophs, folds, and islands, in an idealized sigmodontine molar. Molar crown height seems to be related to diet. In general, sigmodontines that feed on animals, seeds, fruits, or fungus have low-crowned molars (brachydont) that retain complex occlusal surface. High-crowned molars (hypsodont), which extend far above the gumline, are usually present in those sigmodontines, as Andinomys and Reithrodon, which feed on abrasive vegetation (e.g., grass). Usually, the hypsodont condition is associated with a simplification of the occlusal surface, connection of lophs on each tooth side to form lamina, planation of the crown, and an increase in the number of tooth roots.
With a few well recognized exceptions, sigmodontines have complex penises. A complex penis has two lateral horns on the cartilagenous distal baculum, making it appear as trident shaped.
Distribution
Sigmodontines are a New World group; currently they range from Tierra del Fuego, the southernmost part of South America, north to the southern United States. They are also distributed on some islands adjacent to the continent, as well as in some oceanic islands as the Galápagos archipelago. However, most of extant sigmodontine diversity is confined to the South American continent. In fact, 61 living genera are endemic to South America and the neighboring islands such as Trinidad. Three genera: Oligoryzomys, Oryzomys, and Sigmodon
inhabit areas of the three Americas. Seven genera are distributed in Central and South America, although three of these, Ichthyomys, Oecomys, and Rhipidomys, have only Panama as their extra South American part of the range. The water mouse Rheomys is the only mainland genus not present in South America. In addition, the living genus Nesoryzomys is endemic to the Galápagos Islands. Finally, three genera that presumably became extinct in historic time: Megalomys, Megaoryzomys, and Noronhomys were endemic, respectively, to the Lesser Antillas, and the oceanic Galápagos and Fernando de Noronha Islands. However, this last statement may change since an undescribed fossil from mainland Argentina may be assignable to Noronhomys (Pardiñas, pers. com.).
The past distribution of the subfamily as a whole roughly matches current distribution. Lack of fossil records from some areas (e.g., most of the Amazon basin or the highland Puna) is probably due to the lack of adequate sediment beds and/or insufficient exploratory work. However, it is worth noting that the distributions of several genera, such as Bibimys, Kunsia, and Pseudoryzomys, have markedly shifted in a few thousand or even hundred years (Pardiñas, 1999).
It has to be emphasized that much still remains to be learned about sigmodontine distribution, as is shown by the results of field work conducted in the late twentieth and early twenty-first centuries, which prompted extensions of the sigmodontine known distribution. For example, the genus Rhagomys, previously known from the Atlantic coastal Brazilian state of Rio de Janeiro, has been reported from the eastern slopes of the Peruvian Andes (Luna and Patterson, 2003). A less spectacular example, although still remarkable was reported by Emmons in 1999, who collected in eastern Bolivia specimens of Juscelinomys, a genus previously considered to be restricted to the Brazilian Federal District.
Habitat
Sigmodontines occupy almost all ecosystems found through their distributional range, including dry deserts, wet tropical forests, wetlands, savannas, steppes, temperate woodlands, high altitude grasslands, salt flats, and scrublands. Some species, such as Nectomys squamipes, are semiaquatic inhabitants of forests, and others, like Scapteromys tumidus, are semi-aquatic inhabitants of grasslands. Notiomys edwardsii and Kunsia tomentosus are largely fossorial species that live in arid steppes and savanna forest, respectively. Thomasomys aureus and Irenomys tarsalis are arboreal species that live in cloud forest and temperate forest, respectively.
Some sigmodontines inhabit more than one habitat type. For example, the rabbit rat Reithrodon auritus is found, among others, in steppes, bunchgrass prairies, dense grasslands, beech forests, and sparse shrub lands. Similarly, Azara's field mouse Akodon azarae lives in several types of open environments, including dry and humid Chaco, pampas grasslands, and agrosystems. Meanwhile, other sigmodontines appear restricted to specific habitat types. For instance, the poorly known Punomys lemminus occurs only in the treeless Puna of Peru between 14,600 and 17,000 ft (4,450–5,200 m) of elevation.
While some sigmodontines appear to be highly sensitive to habitat destruction (e.g., Anotomys leander and Pearsonomys annectens), others appear to adapt well to human disturbed habitats. The latter is the case of the field mouse Akodon azarae, which successfully invaded agroecosystems. Similarly, the forest mouse Akodon montensis is collected in abundance in secondary grown forests of Argentina, Brazil, and Paraguay. In addition, some sigmodontines are commensal with humans. For example, Akodon reigi has been collected inside rural houses in Uruguay.
Microhabitat selection has been studied in relatively few sigmodontines. It was documented for three sympatric sigmodontines from the Patagonian steppes. Abrothrix longipilis
prefers the steppe's bushy patches, Eligmodontia morgani prefers the bunchgrass patches, and Abrothrix olivaceus evenly distributes in both microenvironments. Similarly, in the humid forest where it lives, the water rat Nectomys squamipes, prefers places close (< 10 m) to water courses with tree ferns and exposed tree roots.
Bilenca and Kravetz (1999) studied in agroecosystems of central Argentina the seasonal changes in microhabitat use by the field mouse Akodon azarae and the vesper mouse Calomys laucha. They showed that the structure of this sigmodontine community is highly influenced by seasonal changes in habitat structure and rodent abundance. In summer, both species were equally distributed between the mature crop fields and their surrounding weedy borders. In the crop fields both species preferred covered microhabitats and at the borders they did not select microhabitats. In contrast, in winter there was sharp habitat segregation. The vesper mouse was numerically dominant in post-harvest crop fields and the field mouse was more abundant in borders. In addition, there were clear differences in microhabitat selection at the borders, A. azarae occupying the more covered microhabitats and C. laucha the less covered ones.
Field ecology studies are scarce in sigmodontine literature; therefore, basic information is lacking for most species. A field study of the water rat Nectomys squamipes shows that its home range covers 0.5–3.9 acres (0.2–1.6 ha). Similarly, Gentille and collaborators (1997) showed that the size of the home range of the forest mouse Akodon cursor is seasonally constant, and that on average covers 0.91 and 0.47 acres (0.37 and 0.19 ha). for males and females respectively. Females of A. cursor are territorial while males are not. With males having a larger territory, they have contact with several females, while females would defend food and nesting resources. Interestingly, the size of the home range of A. cursor is neither affected by individual body size nor by population densities. Therefore, the overlap of different individuals would increase at higher population densities. Similarly, males of Abrothrix olivaceus, Akodon azarae, and Necromys lasiurus have larger home ranges than females. On the other hand, males and females of Eligmodontia morgani have home ranges of similar size.
Behavior
Field and laboratory observations of sigmodontine behavior are scarce, and limited to a small number of species. Therefore, no generalizations can be made.
Not much is known about sigmodontine social behavior; most reports are of anecdotic fashion. Wiedomys pyrrhorhinos may be gregarious, since eight adults and 13 young were found in one termite nest. Similarly, Darwin reported six specimens of Calomys laucha were discovered in one burrow. The social and gregarious habits of Phyllotis sublimis were described by Pearson (1951).
From the patterns of trapping, it can be inferred that sigmodontines are mostly nocturnal. For example, the water rat Nectomys squamipes is primarily active just after sunset. Similarly, the rice rat Oryzomys intermedius remains sheltered during the day and becomes active at night. Several species, such as the long-nosed mouse Oxymycterus nasutus and the long-clawed mole mouse Geoxus valdivianus, are active both diurnally and nocturnally. Finally, other sigmodontines, such as the long-nosed mouse Auliscomys boliviensis, are primarily diurnal.
Sigmodontines use nests to shelter and to raise the young. Nectomys squamipes and Oryzomys intermedius use dry leaves and grasses to build nests that are egg-shaped, of about 6 in long and 4 in wide (15 and 10 cm), and lack an obvious entrance. Nests are usually placed at the end of tunnels of 4.0–12 in (10–30 cm) long inside or under fallen logs. Less commonly,
some nests are located between rocks or exposed tree roots. N. squamipes nests closer to water courses than O. intermedius. Other species, such as Thomasomys aureus, build their nests in trees. At the same time, other sigmodontines take advantage of bird nests. This is the case of the red-nosed tree rat Wilfredomys oenax that in Uruguay uses abandoned nests of both the firewood-gatherer (Anumbius annumbi) and the golden-winged cacique (Cacicus chrysopterus) as diurnal shelter.
There are limited data on sigmodontine dispersal. Disperser individuals of Akodon azarae are in general smaller than those that do not disperse, but they neither differ with respect to sex nor with reproductive condition. The highest dispersal rate occurs in autumn. The water rat Nectomys squamipes only makes short movements and exhibits limited migration.
Several sigmodontines including, among others, the rice rats Oryzomys couesi and O. palustris, the water rats of the genera Amphinectomys, Lundomys, Holochilus, and Nectomys, the marsh rats of the genus Scapteromys, and the ichthyomyines are excellent swimmers and divers. By swimming and diving these sigmodontines are able to escape predators and to exploit surface and underwater resources. Other species, such as Sigmodon hispidus, are less skilled swimmers, swimming only on the water surface (Cook et al., 2001).
Grooming behavior has been studied in Nectomys squamipes. The water rat licks most reachable body parts, and grooms its face by circular movements of the forepaws.
Little is known on sigmodontine communication. Highpitched vocalizations have been reported, mostly in anecdotic terms, for a few sigmodontines species, including Scapteromys aquaticus, S. tumidus, and Nectomys squamipes. Dominance status in the hispid cotton rat Sigmodon hispidus is communicated by urinary and fecal odors.
Feeding ecology and diet
Most of the information available on sigmodontine diet comes from analysis of stomach contents (in which, usually a small number of individuals were studied) and not from field observation or experimental studies. As expected from such a diverse group, sigmodontines show a large range of diets. Sigmodontines are omnivorous (e.g., Zygodontomys brevicauda), grassivorous (e.g., Reithrodon typicus), primarily frugivorous (e.g., Thomasomys aureus), primarily granivorous (e.g., Eligmodontia typus), and primarily animalivorous (e.g., Oxymycterus nasutus, Rheomys mexicanus); still others consume large amounts of fungus (e.g., Chelemys macronyx).
Mice and rats of the ichthyomyine tribe are among the most distinctive sigmodontines with regard to their diet because they feed on aquatic organisms (Voss, 1988). In much of the scientific literature ichthyomyines are referred to as the fish-eating sigmodontines. However, though ichthyomyines feed on fishes, they represent a relatively uncommon item of the ichthyomyine diet. Ichthyomyines primarily feed on arthropods, of which the vast majority is part of the stream benthic fauna, including crabs, amphipods, and several insect orders (larvae, ninphae, and adults). Other less common ichthyomyine food items are tadpoles and salamanders. Ichthyomys exhibits feeding plasticity: in the lowlands, crabs are an important food item, while in the highlands, where crabs are absent, it may be largely insectivorous.
After entering the water Ichthyomys pittieri uses its whiskers to explore the substrate, searching for, and identifying live prey. The rat attacks its prey when they move after being touched. Small prey (up to 0.8 in long; 20 cm) are seized with the forepaws and are usually eaten immediately, while sitting on its hindquarters in the shallow water. Larger prey are pinned to the bottom with forepaws, bitten repeatedly until moribund, and then carried out of the water where they are consumed.
The feeding behavior of the omnivorous water rat Nectomys squamipes was studied by observations of captive animals (Ernest and Mares, 1986). The rat picks up immobile items (e.g., leaves) with the incisors, then sits back on its haunches and holds the food in the forepaws, to bite and chew the food. Mobile items (e.g., cockroaches, Blattidae) are caught on the ground by jumping on it with the forepaws. Flying insects are caught close to the ground and eaten head first, and entirely. The water rat takes mobile prey in the water (e.g., tadpoles) with the forepaws. After the prey is secured, it is eaten like the immobile items.
Combining analyses of stomach contents and cafeteria tests, Castellarini and collaborators (1998) established that the vesper mouse Calomys venustus is omnivorous, and that it shows a tendency to folivory in spring and autumn and to granivory in summer. C. venustus does not consume the leaves that are abundant in the habitat, or consume them in low proportions, rather, it shows a high preference for seed consumption under conditions of equal food availability.
Reproductive biology
Information on sigmodontine reproduction comes primarily from observations recorded during specimen collections (e.g., external measurements, testes position, presence of vaginal closure membranes, and number of embryos). From these observations, inferences on the part of the year where species are reproductively active, number of estrous cycles per year, and litter size, are made. There are species that seem to breed throughout the whole year; for example Zygodontomys brevicauda reproduce continuously, even in habitats with marked seasonality (e.g., dry and wet season). Litter size ranges from one to 11, averaging 4.5. Similarly, Sigmodon hispidus also breeds, at least in the warmer parts of its distribution, during the whole year. It has one to 15 young per litter.
Other sigmodontine species restrict reproduction to one or two specific parts of the year (seasonally active). Reithrodon auritus concentrates breeding in spring and early summer. Litter sizes vary from one to eight, with a mean of 4.5 embryos. Sigmodontines are in general short-lived, and in several species at least one sex reaches sexual maturity in the same season of birth. This is the case of Reithrodon auritus females. Eligmodontia typus rarely live longer than 9 months; it reaches sexual maturity at approximately 45 days (Pearson et al., 1987).
Information on the patterns of sigmodontine copulatory behavior is sparse. Calomys, however, is a relatively well known genus in this regard. There is information on three of the 11 species of the genus. They display intravaginal penile thrusting and multiple ejaculations, in combination with single or multiple pre-ejaculatory intromissions. Locks are either rare or absent. In addition, C. musculinus, a non-monogamous species, undergoes a stereotyped precopulatory behavior composed mostly of agonistic behaviors in which the female and the male play the aggressive and submissive roles, respectively (Laconi and Castro-Vasquez, 1998).
A remarkable aspect of sigmodontine population dynamics is the massive population irruptions or outbreaks recorded in several parts of South America since the sixteenth century. These explosive increases in sigmodontine population abundance or density during a relatively short period have been called ratadas. Starting in 1522, 63 ratadas are well-documented in literature; these have occurred in Argentina, Brazil, Chile, and Peru. The outbreak estimated densities of some sigmodontines are as follows: Oryzomys xantheolus (range = 250–259 individuals per 2.5 acres [1 ha]), Abrothrix olivaceus (52–237), Phyllotis darwini (30–225), Oligoryzomys longicaudatus (46–154), and Akodon azarae (115). As ratadas involve one to four species simultaneously, the total combined sigmodontine density may be even higher. Naturalists noted that massive flowering and subsequent massed seeding of bamboos preceded several rodent irruptions, so sigmodontine outbreaks became associated with bamboo blooming. Later, it was shown that several ratadas are associated with rainfall peaks, some of which are caused by El Niño Southern Oscillation disturbances. Both types of ratadas occur with approximately equal frequency and appear geographically located. Brazil, southern Chile, and southern Argentina are affected mainly by bamboo-associated rodent outbreaks, while Peru, northern Chile, and northern Argentina are affected by rainfall-associated outbreaks. Functionally, ratadas are the result of a population closely tracking changes in the environment. However, as it was shown in studies of the leaf-eared mouse Phyllotis darwini, a species that undergoes ratadas in semi-arid Chile, intrinsic factors such as density-dependence variables and competitors or predators also play a key role in shaping the demographic dynamics of these species. This evidence underscores the interplay of different phenomena regulating reproduction and demography in natural populations.
Finally, it is interesting to note that in nine species of Akodon (azarae, boliviensis, kofordi, mollis, montensis, puer, subfuscus, torques, and varius) there are females with a XY pair of sexual chromosomes (and not XX as is typical in mammals; XY females also occur in the arvicolines Dycrostonix torquatus, Microtus cabrearae, and Myopus schisticolor). The prevalence in wild populations of these heterogametic females ranges from 30 to 60%. These females are fertile. XY sex reversed females are assumed to occur due to a deficient expression of the Y chromosome linked gene Sry, resulting in the developing of ovaries instead of testes.
Conservation status
In April 2003 the IUCN Red List of Threatened Species listed 54 sigmodontine species. Three sigmodontines, Nesoryzomys darwini, N. indefessus, and Oryzomys nelsoni, are considered to be extinct; four other species, Nectomys parvipes, Oryzomys gorgasi, Rhagomys rufescens, and Sigmodontomys aphrastus, are listed as Critically Endangered. Ten sigmodontines species are considered Endangered, 13 Vulnerable, one species Near Threatened, and 19 to be at Lower Risk. Data are deficient to evaluate the status of four species. For the sole fact that the understanding of species limits of most sigmodontine genera is far from being settled, these figures are subject to change. For instance, instead of subsuming Oryzomys galapagoensis under O. bauri, Dowler et al. (2000) recognize both forms as distinct Galápagos endemic species. This classificatory scheme implies that another sigmodontine species, O. galapagoensis that has not been collected since 1835, should be regarded as extinct.
One of the major sigmodontine threats is habitat destruction due to land conversion for urbanization and agricultural and logging expansion. Another important threat is the introduction of exotic fauna. For instance, the most likely cause of the extinction of the Galápagos endemic Nesoryzomys darwini and N. indefessus is the introduction of black rats (Rattus rattus), either through the introduction of pathogens to which native species were susceptible, or through direct competition. Similarly, Megaroyzomys, another Galápagos endemic, became extinct presumably in historic times, through predation by introduced dogs, cats, pigs, and Rattus. Hunting pressure may be considered as a minor threat affecting sigmodontine conservation. Mann (1945) suggested that in some Chilean areas extensive trapping may have seriously reduced the populations of Chinchillula.
Significance to humans
The importance of sigmodontines to humans cannot be overstated. Some species cause much damage to agricultural lands and stored foods. For example, in Formosa Province, Argentina, the water rat Holochilus chacarius is reported to damage banana, mandioca, and sugar cane plantations. Similarly, in the Mexican state of Veracruz, the cotton rat Sigmodon hispidus and rice rat Oryzomys couesi are plagues on sugar cane plantations of such important magnitude that eradication plans are being designed. It is estimated that a single individual of Calomys laucha or Akodon azarae, which consume stored foods in farm and village structures, can eat 2.2 to 4.4 lb (1–2 kg) of food per year.
Other sigmodontines are the reservoirs of the etiological agents of a number of human diseases. In South America, all hantaviruses known to cause Hantavirus Pulmonary Syndrome are associated with species of sigmodontine rodents, including different long-tailed mice of the genus Oligoryzomys, the field mice Necromys benefactus and Akodon azarae, the vesper mouse Calomys laucha, and the cotton rat Sigmodon alstoni. In addition, the vertebrate reservoir of the Machupo virus, the etiological agent of the Bolivian hemorrhagic fever, is a non described species of vesper mouse (Calomys). Similarly, the reservoir of the Junin and Guanarito viruses, which cause the Argentine and Venezuelan hemorrhagic fevers, are, respectively, the vesper mouse Calomys musculinus and the cane mouse Zygodontomys brevicauda. Finally, it has been shown that the forest mouse Akodon montensis, the long-tailed mouse Oligoryzomys nigripes, the subterranean mouse Thaptomys nigrita, and the rice rat Oryzomys russatus are reservoirs of Borrelia-like spirochetes which cause a Lyme disease simile.
On the other hand, many sigmodontines species are beneficial to man by the role played in ecosystems. Some are keystone species in maintaining the vigor of ecosystems, through their role in dispersing seeds (e.g., Oryzomys intermedius) and/or being prey of many medium- and large-sized vertebrates (e.g., owls, snakes, and other carnivores). Other species are directly exploited by man. For example, the chinchilla mouse Chinchillula is trapped for its thick, soft, and silky fur. Trimmings and robes are made of these skins. Remarkably, one of these robes may contain more than 150 skins. Zooarcheological work indicates that during the Holocene period, sigmodontines did not represent a food item for the indigenous peoples that lived in the areas that are now known as the Argentinean Patagonia and Pampas. However, present Mbyá Guaraníes from Misiones, Argentina hunt and eat sigmodontines. Finally, some species play an increasing role as "domestic animals" used in medical research; this is the case of Calomys laucha and C. musculinus.
Species accounts
List of Species
Rabbit ratKemp's grass mouse
Marsh rice rat
Chilean tree mouse
Brazilian shrew mouse
Andean mouse
Water mouse
Rio de Janeiro rice rat
Hispid cotton rat
Rabbit rat
Reithrodon auritus
taxonomy
Reithrodon auritus (Fischer, 1814), Buenos Aires Province, Argentina. Tribe Reithrodontini.
other common names
Spanish: Rata conejo.
physical characteristics
Total length 8.3–10.6 in (212–269 mm), tail length 3.3–4.0 in (84–103 mm), ear length 0.6–1.1 in (15–29 mm). Weight 0.7–4.1 oz (20.5–116 g). Pelage color varies from dark brown to pale buffy gray dorsally, and from white to brownish ochreous ventrally. Eyes and ears are large. It has four pairs of mammary glands. Hind legs are very long. Upper incisors have two frontal grooves. Molars are markedly high.
distribution
Patagonian region of Argentina, from Tierra del Fuego and southern Chile to 36ºS. North of 36ºS is restricted to a few high-altitude (> 6,562 ft; > 2,000 m) localities in central and northern Argentina. There is an unconfirmed record from Malvinas Islands.
habitat
Most common in steppes and prairies. However, the rabbit rat inhabits a wide range of environments, including beech forests, sparse shrub lands, bunchgrass prairies, dense grasslands, cultivated fields, overgrazed pastures, sandy coasts, and stony hills.
behavior
Active both diurnally and nocturnally year-round. Activity begins in the evening and lasts until the early hours of the morning. Reithrodon excavates tunnels of 1.6–2.8 in (4–7 cm) in diameter. It also uses tunnels made by other animal such as those of tuco-tucos (Ctenomys).
feeding ecology and diet
The diet consists only of grasses.
reproductive biology
Females reach reproductive maturity at about two months of age. Males reach maturity later than females. Specimens reproductively active have been collected in spring and summer (September to March). Most gravid females were found in spring. Litter size ranges from one to eight. Juveniles were found in spring and in higher numbers during summer and autumn.
conservation status
Not threatened. Populations are protected in several Argentinean and Chilean national parks and reserves.
significance to humans
In the Argentinean Province of Buenos Aires the rabbit rat is considered a pest because it eats large amounts of grass.
Kemp's grass mouse
Deltamys kempi
taxonomy
Deltamys kempi Thomas, 1917, Isla Ella, Delta of the Parana River, Argentina. Tribe Akodontini.
other common names
Spanish: Ratón aterciopelado.
physical characteristics
Measurements of the holotype. Total length 6.9 in (174 mm), tail length 3.3 in (85 mm), ear length 0.5 in (13 mm), length of foot without claws 0.9 in (22 mm). Eyes are small and barely visible, ears are narrow. The soft pelage is blackish brown dorsally and dull brownish gray ventrally. Tail is faintly bicolored.
distribution
Small area of southeastern South America, from northeast Buenos Aires Province and southern Entre Ríos Province in Argentina, throughout south, central east, and northeast Uruguay, to the Atlantic coast of the Rio Grande do Sul State in Brazil.
habitat
Marshy areas, especially edges of wetlands, flooded grasslands, and places with reeds and straws. In Uruguay also inhabits, although less abundantly, some woodlands. In
Argentina it has also been collected in tall grass coastal areas and gallery forest.
behavior
Probably fossorial. It rests under grasses and leaves.
feeding ecology and diet
Mostly carnivorous. Insect remains have been recorded in stomachs of Argentinean and Uruguayan specimens. In addition, seeds and remains of green plants have been found in stomachs of specimens collected in Uruguay.
reproductive biology
Little is known on reproductive aspects of Kemp's grass mouse. Litter size may be three since that numbers of young were found in one nest in Argentina and another in Uruguay.
conservation status
In Argentina it is considered rare and at lower risk. Uruguayan populations are considered not threatened. The status of Brazilian populations is not known.
significance to humans
None known.
Marsh rice rat
Oryzomys palustris
taxonomy
Oryzomys palustris (Harlan, 1837), near Salem, New Jersey, United States. Tribe Oryzomyini.
other common names
Spanish: Rata arrocera, coludo.
physical characteristics
Total length 8.9–12.0 in (225–305 mm), tail length 3.9–6.9 in (100–175 mm), hindfoot length 1.1–1.5 in (28–37 mm). Weight 1.4–3.0 oz (40–85 g). Generalized rat-like appearance. Dorsal coloration range varies from grayish brown to gray. The venter is much lighter. The underfur is water repellent. Eight pairs of mammary glands. The forefoot and hindfoot have four and five toes, respectively. Tail is scaly, long, and nearly naked.
distribution
Endemic to the United States, ranging from southeastern Pennsylvania and southern New Jersey to the tip of Florida and westwards to eastern Texas. There are records from southern Kentucky and Illinois, southwestern Missouri, and southeastern Oklahoma.
habitat
Common in wetlands, including marshes at the Atlantic and Gulf coasts. It also inhabits swamps, meadows, and prairies.
behavior
Semiaquatic, being good swimmers and divesr. Primarily nocturnal. High-pitched squeaks were associated with agonistic behavior upon females by males in captivity. It constructs spherical grassy nests to sleep in during the day.
feeding ecology and diet
Carnivorous, but also feeds on seeds, succulent plant parts, and fungus. Food items may vary seasonally upon resource availability.
Animal items vary, but insects, fiddler crabs, snails are the most common; other animal items are fishes, clams, eggs, and young of long-billed marsh wrens.
reproductive biology
Breeding may occur throughout the year, although it may be low during hot summer months. Gestation period lasts between 21 and 28 days. Different average litter size has been reported, 3.6, 4.8, and 5. Newborn weigh around 0.1 oz (3.5 g). Weaning occurs at age 11–20 days. Sexual maturity is reached between 40 and 45 days of age. Life expectancy is generally less than one year.
conservation status
Not threatened.
significance to humans
Reservoir of the Bayou hantavirus, a pathogenic virus associated with human pulmonary syndrome.
Chilean tree mouse
Irenomys tarsalis
taxonomy
Irenomys tarsalis (Philippi, 1900), Fundo San Juan, Valdivia Province, Chile. Formerly placed in the phyllotine tribe.
other common names
Spanish: Ratón arbóreo, laucha arbórea.
physical characteristics
Total length 10.6–12.8 in (270–326 mm), tail length 3.5–7.7 in (90–196 mm), hindfoot length 1.1–1.3 in (28–32 mm), ear length 0.8–1.0 in (20–25 mm). Weight 1.1–2.4 oz (30–67 g). Dorsal coloration is grayish cinnamon. The venter is washed with pinkish cinnamon buff. Ears are brownish black. Tail is blackish brown, and it may be paler on the ventral side; it ends in a penciled tip. Upper incisors are grooved.
distribution
Although vernacularly known as the Chilean tree mouse, Irenomys tarsalis is not endemic to Chile, it also has populations in Argentina. In Chile it ranges from Chillán (VII Región) south to Puerto Ibañez (XI Región), including Chiloe Island and the Guaitecas Islands. In Argentina distributes in along the western side of Neuquen, Río Negro, and Chubut Provinces.
habitat
Generally confined to humid, temperate forested habitats, especially bamboo stands or shrubby areas, although it is also present in the interface between forest and steppe.
behavior
Nocturnal. It appears docile when removed from traps. After release it often climbs bamboo or trees, although it may run across the forest floor.
feeding ecology and diet
Granivorous and frugivorous, although also eats green vegetation, fungi, and seeds.
reproductive biology
Breeding occurs in spring but may extend into summer. Birth takes place in summer. Litter size ranges from three to six.
conservation status
Not threatened.
significance to humans
None known.
Brazilian shrew mouse
Blarinomys breviceps
taxonomy
Blarinomys breviceps (Winge, 1887), Rio das Velhas, Minas Gerais State, Brazil. Tribe Akodontini.
other common names
Portuguese: Rato do Mato.
physical characteristics
Total length 5.1–6.3 in (129–161 mm), tail length 1.2–2.0 in (30–52 mm), hindfoot length 0.6–0.8 in (16–21 mm), ear length 0.3–0.4 in (8–10 mm). Shrewlike; superficially resembles the North American insectivore genus Blarina, and hence its scientific generic name. Pelage is dark gray. Head is short and conical. Eyes extremely reduced. Claws well developed.
distribution
First known from the Brazilian states of Bahia, Espiritu Santo, Minas Gerais, and Rio de Janeiro States; it has also been recovered from owl pellets in the Argentinean province of Misiones.
habitat
Montane forest.
behavior
Primarily fossorial.
feeding ecology and diet
Carnivorous. Stomachs of two specimens contained arthropods of six different orders, mostly insects (90%) but also arachnids.
reproductive biology
Little is known. Males with scrotal testes were collected in January and February. Pregnant females were trapped in September, January, and February. Observed number of embryos has been one or three.
conservation status
Listed as Lower Risk/Near Threatened.
significance to humans
None known.
Andean mouse
Andinomys edax
taxonomy
Andinomys edax Thomas, 1902, El Cabrado, Potosi Department, Bolivia. Formerly placed in the phyllotine tribe.
other common names
Spanish: Rata andina.
physical characteristics
Total length 10.1–12.4 in (257–315 mm), tail length 4.2–6.3 in (107–160 mm), hindfoot length 1.2–1.3 in (30–32 mm), ear length 0.9–1.1 in (22–27 mm). Heavy-bodied. Pelage soft, lax, upperparts and sides drab, underparts gray. The tail is thinly haired and sharply bicolored. Feet are well developed.
distribution
High-altitude regions of northwestern Argentina, Bolivia, northern Chile, and southern Peru.
habitat
It lives in the puna in dense vegetation near water courses and bushy thickets.
behavior
Nocturnal; terrestrial, but may also be scansorial. It nests inside round holes carpeted with fine straw.
feeding ecology and diet
Feeds on green herbs.
reproductive biology
Breeds at the end of the dry season. A female collected by O. Pearson on December 18 had three well-developed embryos.
conservation status
Not threatened.
significance to humans
None known.
Water mouse
Chibchanomys orcesi
taxonomy
Chibchanomys orcesi Jenkins and Barnett, 1997, Las Cajas, Azuay Province, Ecuador. Tribe Ichthyomyini.
other common names
Spanish: Rata acuática.
physical characteristics
Total length 8.3–9.0 in (211–229 mm), tail length 4.3–4.8 in (108–122 mm), hindfoot length 0.8–0.9 in (19–24 mm), ear length 0.4–0.6 in (9–14 mm). Weight 1.2–1.4 oz (35–40 g). Pelage is soft, dense, and woolly, dark brownish gray dorsally, and light gray ventrally. Ears and eyes are reduced. The tail is longer than head and body.
distribution
Known only from three localities in the Las Cajas Plateau, Ecuador.
habitat
High altitude páramo (10,170–13,100 ft; 3,100–4,000 m), close to fast-flowing streams.
behavior
Semiaquatic.
feeding ecology and diet
Primarily an aquatic insectivore, but also eats fish.
reproductive biology
Nothing is known. It appears to be a scarce páramo inhabitant.
conservation status
Not known, but Barnett (1997) noted that the Las Cajas Plateau, the only place from where C. orcesi is known, may become severely impacted by human activities in the near future.
significance to humans
None known.
Rio de Janeiro rice rat
Phaenomys ferrugineus
taxonomy
Phaenomys ferrugineus (Thomas, 1984), Rio de Janeiro, Brazil. Formerly placed in the Thomasomyine tribe.
other common names
Portuguese: Rato do mato ferrugíneo.
physical characteristics
Measurements of the holotype: total length 13.2 in (335 mm), tail length 7.4 in (187 mm), hindfoot length 1.3 in (34 mm), ear length 0.7 in (17 mm). Pelage is dorsally rust-colored, cheeks and sides are lighter, and the underparts are whitish. Ears are small and well furred. The tail is unicolored, dark gray.
distribution
Known only from three localities in the Brazilian states of Rio de Janeiro and San Pablo.
habitat
One specimen collected in 1998 was trapped in herbaceous vegetation in an area that is considerably altered for agriculture and pasture. The collection site is in a valley surrounded by forested hills that have been selectively logged.
behavior
Nothing is known.
feeding ecology and diet
Nothing is known.
reproductive biology
Nothing is known.
conservation status
Listed as Critically Endangered by the IUCN.
significance to humans
None known.
Hispid cotton rat
Sigmodon hispidus
taxonomy
Sigmodon hispidus Say and Ord, 1825, St. Johns River, Florida, United States. Tribe Sigmodontini.
other common names
Spanish: Rata de campo; rata algodonera.
physical characteristics
Total length 8.8–14.4 in (224–365 mm), tail length 3.2–6.5 in (81–166 mm), hindfoot length 1.1–1.6 in (28–41 mm), ear length 0.6–0.9 in (16–24 mm). Weight 3.5–8.0 oz (100–225 g). Pelage is grizzled with blackish or dark brownish hairs. Underparts are pale to dark grayish. Tail is dark. Female possesses five pairs of mammae; however, females with six and four pairs have also been found.
distribution
Inhabits the three Americas; from southeast United States (south Nebraska, central Virginia, southeast Arizona, peninsular Florida), through interior and east Mexico through Central America, to northern Colombia and Venezuela.
habitat
Usually an inhabitant of grass dominated landscapes.
behavior
Both diurnal and nocturnal, and is able to swim.
feeding ecology and diet
Feeds primarily on grasses. There are data showing that it selects food items and combines them into a nutritious diet. It does not hoard food.
reproductive biology
Breeding is through the year; however, differences exist in relation to latitude. Gestation lasts approximately 27 days. Litter size varies from one to 15, with animals from northern populations having larger litters. Neonates are well developed at birth; their eyes open within 36 hours after birth and are weaned in 10–15 days. Males reach reproduction maturity in two or three months; females reach it earlier, even in 10 days and in an average of 30–40 days. Females normally produce several litters per year.
conservation status
The subspecies S. h. eremicus and S. h. insulicola are considered at Lower Risk/Near Threatened by the IUCN.
significance to humans
Sigmodon hispidus has been used as a biomonitor to assess environmental contamination. Sigmodon hispidus is the reservoir of two strains of hantavirus called Black Creek Canal virus and Muleshoe viruses; these are pathogenic viruses associated with human pulmonary syndrome.
| Common name / Scientific name/Other common names | Physical characteristics | Habitat and behavior | Distribution | Diet | Conservation status |
| Allen's woodrat Hodomys alleni | Long, narrow head. Upperparts are reddish brown to dusky brown. Under-parts are white to buff. Tail from dusky to white. Head and body length 14.5–17.5 in (36.8–44.5 cm), tail length 6.2–8.1 in (15.8–20.6 cm). | Dry wooded slopes, tropical deciduous forest, rocky outcrops to dense scrub. Little known of reproductive habits. Females produce one to two offspring. | Southernmost Sinaloa to Oaxaca; interior Mexico along basin of Rio Balsas to central Puebla. | Coco-oil seeds along with other types of seeds, crabs. | Lower Risk/Near Threatened |
| White-throated woodrat Neotoma albigula German: WÏstenratte | Coloration is brownish gray, underside is white to gray. Tail is brownish gray on top, lighter on bottom. Feet are white. Head and body length 12.9 in (32.8 cm), weight 7.6 oz (215 g). | Desert habitats, where they build nests in rocky areas, under shrubs, small trees, or cacti. Breeding season from January to August, two or more litters per season. Nocturnal, solitary, and territorial. | Extreme southeastern California to southern Colorado to western Texas, United States, south to northeastern Michoacan and western Hidalgo, Mexico. | Mainly cacti. | Not threatened |
| Bushy-tailed woodrat Neotoma cinerea German: Buschschwanzratte | Coloration varies across range, from buff to darker. White coloration around the feet. Average male weight 14.3 oz (405 g), female weight 9.5 oz (270 g) | A variety of habitats, from boreal woodlands to deserts. Litter size is usually three offspring. Build constructs called middens, do not hibernate. | Southeast Yukon and westernmost North-west Territories, south through British Columbia and western Alberta, Canada, to northwestern United States, as far south as northern New Mexico and Arizona and east to western Dakotas. | Consists mainly of vegetable matter, such as woody plants, and arthropods. | Not threatened |
| Eastern woodrat Neotoma floridana | Coloration is brown around base of neck to dark brown and black on rump. Nasal area is pink. Body is small, compact. | Wooded marshes, grass-lands, and coastal plains. Litter size ranges from two to seven offspring. Breeding season varies with geographic location. Nocturnal and solitary, except during breeding season. | South-central and eastern United States from east-central Colorado to eastern Texas, eastwards along Appalachians to western Connecticut, and along Gulf-Coast states to southern North Carolina and central Florida. | Leaves, bark, fruits, and seeds. | Not threatened |
| Dusky-footed woodrat Neotoma fuscipes | Coloration is cinnamon with tints of buff and pink. Ears are thin, large, rounded, and broad. Claws are short. Head and body length 15.2–17.4 in (38.5–44.3 cm), weight 8.1–10.6 oz (230–300 g). | On hillsides, valleys, and close to water. Avoid open grassland and open oak woods with little underbrush. Nocturnal, uses branches for travel. | Western Oregon through western and central California, United States, to northern Baja California, Mexico. | Seventy-two different types of plants. | Not threatened |
| Golden mouse Ochrotomys nuttalli | Coloration of fur is gold, underparts are white, tail is cream. Cheeks contain thick folds of enamel. Head and body length 2–4.5 in (5.1–11.5 cm), tail length 2–3.8 in (5–9.7 cm). | Thick woodlands, swampy areas, among vines, and within small trees and shrubs. Reproduces all year; nocturnal and solitary. | Southeastern Missouri across to southern Virginia, south to eastern Texas, the Gulf Coast, and central Florida. | Mainly seeds. | Not threatened |
| Southern grasshopper mouse Onychomys torridus | Fine, dense fur, gray or pinkish cinnamon in color. Underside of tail is white. Head and body length 3.5–5.1 in (9–13 cm), tail length 1.2–2.4 in (3–6 cm). | Found within burrow systems in the ground. Most reproductive activity between late spring and summer. Extremely aggressive, nocturnal, good climbers. | Central California, southern Nevada, and extreme southwestern Utah, United States, south to northern Baja California, western Sonora, and northern-most Sinaloa, Mexico. | Seeds, plants, and vegetables. | Not threatened |
| Big-eared climbing rat Ototylomys phyllotis | Coloration is gray and brown on dorsal side, white and gray on the ventral side. Hands and feet are pale. Tail is long, hairless, covered with scares, from dark gray/brown to a paler color on ventral surface. Eyes and ears are large and hairless. Head and body length 3.7–7.5 in (9.5–19 cm), tail length 3.9–7.5 in (10–19 cm). | Tropical forests, both dry and wet, with abundant rocks or rocky ledges. Nocturnal and arboreal. | Central Costa Rica north to Yucatán Peninsula, southern Tabasco, and northern Chiapas, Mexico; isolated record from north-central Guerrero, Mexico. | Fruits and leaves. | Not threatened |
| Common name / Scientific name/Other common names | Physical characteristics | Habitat and behavior | Distribution | Diet | Conservation status |
| Texas mouse Peromyscus attwateri | Coloration of dorsal side is brown with darker and blackish marks mixed in. Underside is lighter. Head and body length 7.8 in (19.8 cm), weight 0.9–1.2 oz (25–35 g). | Rocky areas including cliffs and limestone outcrops with woody vegetation. Breeding season from late September to winter. Four offspring per litter. | Edwards Plateau of north-central Texas, north through eastern Oklahoma, to south-eastern Kansas, south-western Missouri, and northwestern Arkansas, United States. | Seeds, fruits, flowers, nuts, and other plant products. | Not threatened |
| California mouse Peromyscus californicus | Coloration is yellowish brown or gray mixed with black dorsal coloring. Under-parts are white. Fulvous throat patch and lateral line are present. Head and body length 8.7–11.2 in (22–28.5 cm), tail length 4.6–6.1 in (11.7–15.6 cm), weight 1.2–1.9 oz (33.2–54.4 g). | Dense chaparral and broad-sclerophyll woodland. Nocturnal, poor burrower, breeding occurs year-round. | Central and southern California, United States, excluding San Joaquin Valley, to northwestern Baja California Norte, Mexico. | Fruits, seeds, and flowers of shrubs. | Not threatened |
Resources
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Guillermo D'Elía, PhD