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 sig-modontines 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 com-

A white-footed mouse (Peromyscus leucopus) in Ohio, USA. (Photo by Gary Meszaros/Photo Researchers, Inc. Reproduced by permission.)

monly, some nests are located between rocks or exposed tree roots. N. squamipes nests closer to water courses than O. in-termedius. 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. High-pitched 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.

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