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)
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)
Most sigmodontines have a promiscuous mating system. During mating, a copulatory plug forms and seals the female's reproductive tract, preventing subsequent males from successfully fertilizing the female's eggs. (Carleton and Musser, 1984)
Many rodents are prolific breeders and sigmodontines are no exception. They breed year round or seasonally, and during a year or season females often have two to three, or even six to seven litters. Ovulation is spontaneous, and females of many species have a postpartum estrus, becoming pregnant again just a few hours after giving birth. In some species, the embryos do not implant until the current litter is weaned; gestation after implantation occurs usually lasts 20 to 30 days. Some species can have as many as 13 young in a litter, although many have just three to five. The young are altricial and open their eyes anywhere from 1 to 11 days after birth. They are weaned as early as five and as late as 30 days. Female sigmodontines reach sexual maturity several weeks before males do. Some have been known to give birth at just four weeks of age. Other species mature much later, and do not reproduce until they are at least four months old. (Carleton and Musser, 1984; Nowak, 1999)
Sigmodontine females generally do not have any help in caring for their young. Most build nests out of plant material where they raise their babies. The young are altricial, and they nurse for 5 to 30 days. In a few species, the young remain with the mother for a few days after weaning is complete. (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)
Some sigmodontines are used in laboratory disease research. Others are trapped for their fur. (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)
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)
The earliest fossils of existing sigmodontine genera are from the late Miocene of North America. (Carleton and Musser, 1984)
Tanya Dewey (editor), Animal Diversity Web.
Allison Poor (author), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
uses sound to communicate
living in landscapes dominated by human agriculture.
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
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
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
active at dawn and dusk
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.
in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
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.
an animal that mainly eats leaves.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
an animal that mainly eats fruit
an animal that mainly eats seeds
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.
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
an animal that mainly eats fungus
specialized for swimming
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
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
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).
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
scrub forests develop in areas that experience dry seasons.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
living in residential areas on the outskirts of large cities or towns.
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
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.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
living in cities and large towns, landscapes dominated by human structures and activity.
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.
breeding takes place throughout the year
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