Sigmodontinae

Tree adapted primarily from Dickerman (1992) and Steppan (1995).

Introduction

The sigmodontines comprise one of the greatest recent radiations of mammals. There are currently more than 300 species occupying a wide variety of niches: from hopping mice to semi-aquatic swimmers that hunt fish and crustaceans. Other habitats and behaviors include arboreal and burrowing species, grass eating specialists, insectivores, and omnivores. They can be found from sea level to over 5000 meters (16,400 ft.) high in the Andes. Estimates for the age of this group range from 4.5 to 15 million years. There is a strong consensus that the lineage originated in North America, but there is controversy over whether their greatest diversification occurred in North or South America. If the former, then a remarkable pattern of numerous invasions must have taken place. If the later, then they might constitute a large scale adaptive radiation. In either case, they figure prominantly in the Great American Faunal Interchange. The centers of diversity are now in South America, but many species can be found in Central America and two or three species occur as far north as southern USA. Some of the more commonly known species include the cotton rats (Sigmodon), rice rats (Oryzomys), marsh rats (Holochilus), South American field mice (Akodon), leaf-eared mice (Phyllotis), and vesper mice (Calomys). Although the majority of species avoid human contact, a few species have become agricultural pests. Others are vector for human diseases, such as Argentine hemoragic fever, which is carried in some Calomys. Because of their abundance in the wild and museum collections, sigmodontines have have figured prominantly in ecological and multivariate evolutionary studies.

Characteristics

Sigmodontines are stereotypic mice and rats, but as might be expected in such a large group, there are many variations on the theme. Like all murids, the dental formula is 1/1 0/0 0/0 3/3 (one pair of incisors and three pairs of molars, both upper and lower). The skeleton is relatively generalized. Sigmodontines all possess complex penises, with a few well defined exceptions. A complex penis has two lateral horns on the cartilagenous distal baculum, making it appear trident shaped (Fig. 3). The predominantly North American Neotominae (often subsumed within Sigmodontinae) lack these lateral horns and are characterzied as simple-penised forms. Although still subject to debate, most systematists assume that the complex penis is ancestral. Few other characters define this diverse array of species, and there are few diagnostic dental features that can be used to definitively assign fossils, which are frequently only represented by teeth or lower jaws. Possible synapomorphies for the subfamily from Steppan (1995) are: a complex penis, the entoglossal process of the hyoid is absent, a dual articulation of the first rib with the seventh cervical and first thoracic vertebrae, the fifth metatarsal does not extend posterior to the cuboid/calcaneum articulation, the entepicondylar foramen of the humerus is absent while the supertrochlear foramen is present, and a gap is present between the trochlear process and articular facet of the calcanuem.

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Figure 2. Generalized diagram of a sigmodontine molar, from Reig (1986).

Discussion of Phylogenetic Relationships

The above tree summarizes the available phylogenies, drawing mostly from nuclear DNA studies of Steppan et al. (2004 and unpubl. data), Jansa and Weksler (2004), D'Elia (2003), Weksler (2003), DNA hybridization study of Dickerman (1992), mitochondrial DNA phylogenies of Engel et al. (1998) and Smith and Patton (1999), and some from the morphological cladistic analysis of Steppan (1995). Earlier studies of Karyotypes (Gerdner and Patton, 1976), phallic morphology (Hooper and Musser, 1964), and synthetic studies (Hershkovitz, 1962; Reig, 1980; 1986) have provided a foudation for the later studies.

Sigmodontini is placed at the base of the sigmodontines based primarily on a variety of molecular studies (immunological, mtDNA sequence, DNA hybridization). Morphological data tend to place Sigmodontini with Phyllotini or closely related to it, a result that is probably due to convergence on simplified dentitions and the long branch leading to Sigmodon, the sole member of Sigmodontini.

The Ichthyomyini, the fish-eating rats, are also placed as an early branch, although there is less direct evidence to support the arrangment. Voss' (1988) supposition that Ichthyomyini is not closely related to any other sigmodontine group is supported by DNA data (Dickerman, 1992; Jansa and Weksler 2004; Steppan et al. unpubl. data). Steppan (1995) placed them in the tetralophodont tribal group (lacking a complete mesoloph and thus with four rows of cusps on the molars rather than the plesiomorphic five rows). Support from morphological characters was weak and supported primarily by the simplification of the molars, a commonly repeated trend in rodents.

The Thomasomyine group, conventionally treated as a tribe or subsumed within Oryzomyini, appears to be paraphyletic and incudes several basal lineages once the Oryzomyini sensu stricto are defined and removed.

Akodontini conventionally included the abrothicine group but not the oxymycterines or scapteromyine. Work by Smith and Patton (1999) using the mitochondrial gene cytochrome b and by D'Elia (2003) and Steppan et al. (unpubl. data) using multiple nuclear and mitochondrial genes however support the arrangment shown above.

The Phyllotini had been taxonomically stable for a long time and morphologists were largely confident about its close relationship to the Akodontini. However, mtDNA phylogenies do not support monophyly (Engel et al., 1998; Smith and Patton, 1999) and in particular Smith and Patton (1999) found that it included several genera of no clear affinities. The nuclear data sets confirm and extend those findings, hence the proliferation of "unique lineages" (Smith and Patton, 1999) on the tree with no tribal affiliation.

Geographic Distribution

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Figure 3. Although several species of sigmodontines can be found as far north as the central United States, the large majority of species are restricted to South America or the southern portion of Central America.

Other Names for Sigmodontinae

• Neotropical mice and rats

References

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