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 Puno-mys 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 sig-modontines from the Patagonian steppes. Abrothrix longipilis

A deer mouse (Peromyscus maniculatus) mother and five-day-old litter. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)
A northern grasshopper mouse (Onychomys leucogaster) eating a little pocket mouse (Perognathus longimembris) that it killed. It is the only carnivorous North American rodent. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

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 Elig-modontia morgani have home ranges of similar size.

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