Subfamily Sigmodontinae
South American rats and mice

Sigmodontinae is the second-largest subfamily of muroid rodents, with 377 species and 74 genera in eight tribes. Members of this group, the New World rats and mice, display a vast array of habits and physical characteristics that is surpassed in scope only by the Murinae, the Old World rats and mice. (Musser and Carleton, 2005)

Sigmodontines range from Tierra del Fuego north throughout South America, Central America, and Mexico, and into the United States as far north as Nebraska and New Jersey. They are also found on the Galapagos Islands. (Nowak, 1999)

Sigmodontines live in a wide range of habitat types, including grasslands, deserts, wet and dry forests, scrub forests, savannahs, steppes, agricultural areas, marshes, swamps, streams, sandy coastlines, barren highlands, alpine meadows, and human habitations. They live at elevations from sea level to over 5,500 meters. (Nowak, 1999)

Traditionally, sigmodontines were classified as part of a broadly-defined Cricetinae, which included New World muroid rodents as well as Palearctic hamsters (Miller and Gidley 1918, Ellerman 1941, Simpson 1945). Alternatively, the New World group was separated and given the subfamily name Hesperomyinae (Tullberg 1899, Chaline et al. 1977). Hooper and Musser (1964) broke the New World group down even further, recognizing that North American taxa and South American taxa had different penis morphologies. The North American, or "neotomine-peromyscine" group, later became formally known as the subfamily Neotominae, whereas the South American group became the Sigmodontinae (Reig 1980). Still, some authors were reluctant to recognize two subfamilies based on penis morphology alone, and thus retained neotomines within a broadly-defined Sigmodontinae (Carleton and Musser 1984). Molecular data supports the separation of sigmodontines and neotomines into different subfamilies, and also supports the monophyly of each group (Michaux et al. 2001, Jansa and Weksler 2004, Steppan et al. 2004). Steppan et al. (2004) date the divergence between the North American Neotominae and the mostly South American tylomyine-sigmodontine clade at 16.8 to 18.1 million years ago. (Carleton and Musser, 1984; Chaline, Mein, and Petter, 1977; Ellerman, 1941; Hooper and Musser, 1964; Jansa and Weksler, 2004; Michaux, Reyes, and Catzeflis, 2001; Miller and Gidley, 1918; Reig, 1980; Simpson, 1945; Steppan, Adkins, and Anderson, 2004; Tullberg, 1899)

Sigmodontines are small to medium-large muroid rodents--head and body length ranges from 62 to 360 mm, tail length ranges from 30 to 330 mm, and they weigh 7 to 455 grams. They are extremely diverse in body form, resembling mice, rats, voles, moles, gerbils, gophers, and shrews. They have short to long fur, ranging from soft to coarse and including spiny forms. Fur colors include many different shades of brown, gray, reddish, and yellow, with the pelage generally paler, even white, on the underparts. Some populations are polymorphic in fur color or pattern. Sigmodontine tails are naked to well-furred, and some have tufted tips. Ears can be very short and nearly hidden in the fur, to very long--almost 1/3 the length of the head and body. Most sigmodontines have feet adapted for cursorial locomotion, but some have specializations for digging (such as long, heavy foreclaws) or swimming (such as webbed hind feet). Many male sigmodontines have prominant ventral sebaceous glands, but they usually lack rump, hip, and flank glands.

The sigmodontine dental formula is 1/1, 0/0, 0/0, 3/3 = 16, except for one species, Neusticomys oyapocki, which has the formula 1/1, 0/0, 0/0, 2/2 = 12. The incisors are usually orthodont or opisthodont, and the molars are rooted and have a biserial cusp arrangement (in contrast to the triserial cusp arrangement of most murines). Each molar has a longitudinal enamel crest (mure or murid). The molars range from brachydont to hypsodont, and the third molars are usually smaller than the second molars. Sigmodontine skulls generally have flat pterygoid fossae, and small to medium-sized auditory bullae. In addition, the mastoid bullae are not hypertrophied, and an accessory tympanum is always present. The malleus is of parallel construction. All other sigmodontine skull characteristics vary widely. A skeletal characteristic shared by most sigmodontines is the presence of a prominant neural spine on the second thoracic vertebra. Finally, sigmodontines have one- or two-chambered stomachs, and the tongue bears a single circumvallate papilla. (Carleton and Musser, 1984; Nowak, 1999)

Sigmodontines have short lives. Most do not make it past their first birthday. In captivity, some species have lived as long as five years. (Nowak, 1999)

Sigmodontines are adapted for a wide range of lifestyles. Most are terrestrial, but there are also arboreal, fossorial, and semiaquatic species. Most are active year round, but those in cooler climates may enter torpor during cold spells. They are nocturnal, diurnal, crepuscular, or active at any time of the night and day. Most sigmodontines use some type of shelter in the form of burrows that they dig themselves; nests placed under rocks or logs or above ground in grasses, shrubs, or trees; crevices between rocks; or tree hollows. They also use burrows dug by other animals and abandoned birds' nests. Some species maintain runways through the grass or moss where they go about their daily business. Social habits in this group range from solitary to gregarious. Some species are territorial and aggressive, and others are colonial and share shelters with several conspecifics. Some seem to establish dominance hierarchies. (Carleton and Musser, 1984; Nowak, 1999)

Sigmodontines perceive their environment using vision, hearing, touch, smell, and taste. Hearing and olfaction may be especially important, as auditory and chemical cues are often used for communication. Sigmodontines make a variety of squeaking sounds in social contexts, and they can detect and produce ultrasounds. Territorial males use their urine and feces to scent-mark their domains. (Carleton and Musser, 1984; Nowak, 1999)

These rodents are herbivorous, omnivorous, or carnivorous. Foods consumed by the group as a whole include: grasses, seeds, fruit, berries, fungi, lichen, insects, crustaceans, other arthropods, mollusks, worms, small fish, tadpoles, and bird eggs. (Carleton and Musser, 1984; Nowak, 1999)

Sigmodontines are preyed upon by a variety of other animals, including hawks, owls, snakes, and carnivorous mammals. The neutral-colored coats of sigmodontines may help them blend in with their background. Most species are vigilant and agile, helping them to avoid predation. Semiaquatic species avoid predation by quickly diving into the water when threatened. One sigmodontine species, Auliscomys boliviensis, avoids predation by associating with viscachas (Lagidium) and dashing for cover when the viscachas give alarm calls. (Nowak, 1999)

Sigmodontines are primary and higher-level consumers, and they are food for a wide range of other animals. Some species are commensal with humans, depending on human food stores or agriculture to survive. Others take advantage of burrows made by other animals, such as armadillos (Dasypus novemcinctus) and tuco-tucos Ctenomys, or nests made by birds. Some, as mentioned in the previous section, rely on other animals to help them avoid predation. Finally, sigmodontines may be important dispersers of mycorrhizal fungi (Mangan and Adler 2000). (Mangan and Adler, 2000; Nowak, 1999)

Several sigmodontine species are considered household or agricultural pests. They raid buildings, gnawing on and destroying household goods and food stores, and they damage crops. Some also carry diseases such as haemorrhagic fever. (Nowak, 1999)

Some sigmodontines are used in laboratory disease research. Others are trapped for their fur. (Nowak, 1999)

There are currently 18 lower risk, 1 near threatened, 13 vulnerable, 11 endangered (aquatic rats, Anotomys leander, 4 Neusicomys species, Cleber's arboreal rice rats, Oecomys cleberi, St. Vincent pygmy rice rats, Oligoryzomys victus, Rushi's rats, Abrawayaomys ruschii, Rio de Janeiro arboreal rats, Phaenomys ferrugineus, and both Scolomys species), and 4 critically endangered (small-footed bristly mice, Nectomys rattus, Gorgas' rice rats, Oryzomys gorgasi, Harris's rice water rats, Sigmodontomys aphrastus, and Brazilian arboreal mice, Rhagomys rufescens) sigmodontine species on the IUCN's Red List of Threatened Species. In addition, three species are lacking sufficient data to be assessed, and five species have gone extinct recently (both Megalomys species, Darwin's Galapagos mice, Nesoryzomys darwini, indefatigable Galapagos mice, Nesoryzomys indefessus, and Nelson's rice rats, Oryzomys nelsoni). Sigmodontines with restricted ranges are vulnerable to habitat loss and destruction, and those that dwell on islands are especially vulnerable to predation or competition by invasive species, such as rats, cats, and mongooses. (IUCN, 2004; Nowak, 1999)