331. Allen’s Cotton Rat Sigmodon alleni French: Sigmodon dAllen / German: Allen-Baumwoliratte / Spanish: Rata de algodén de Allen Other common names: Brown Cotton Rat Taxonomy. Sigmodon allen: Bailey, 1902, “San Sebastian, Mascota, Jalisco, Mexico.” Sigmodon allen: was formerly included in S. planifrons but is now considered a full species; guerrenensis, and volcani, appear to be proper synonyms of S. alleni, but alignment of Oaxacan populations described as macdougalli and macrodon, requires additional study and clarification. Validation of subspecies awaits a thorough review. Monotypic. Distribution. W Mexico, along Pacific lowlands, foothills, and higher elevations of the Sierra Madre Occidental from S Sinaloa SE to Isthmus of Tehuantepec in E Oaxaca. Descriptive notes. Head—body 139-152 mm, tail 88-127 mm, ear 20-22 mm, hindfoot 27-32 mm; maximum weight 178 g for males and 180 g for non-pregnant females. Sexes are size monomorphic. Allen’s Cotton Rat is medium-sized; dorsum is brown; belly is white, with dark gray underfur; forefeet and hindfeet are yellowish brown; and tail is bicolored, black on top and brown below. It differs from other members of the fulviventer species group (fulviventer, ochrognathus, and leucotis) byits rich brownish back, strongly opisthodont upper incisors, flattened skull in lateral view, slight development of capsular projections for upperincisors, and paraoccipital processes (viewed from below) slightly hooked rather than straight or curved. It differs from members of the hispidus species group (hispidus, arizonae, and mascotensis) by narrow tail scales (0-5 mm in width), very hairy tail, strongly opisthodont upper incisors, short basioccipital relative to breadth, slightly hooked rather than straight paraoccipital processes, and moderately deep, not shallow, palatal pits. Chromosomal complement is 2n = 52, FNa = 64. Habitat. Mixed grass-herb-shrub habitats in tropical deciduous forest on Pacific coastal lowlands and mesic boreal-tropical forest ecotone on western slopes of Sierra Madre Occidental, Sierra Madre Sur, and where these mountain chains join at western end of Trans-Mexican Volcanic Belt, from sea level to elevations of ¢.3050 m. Food and Feeding. Allen’s Cotton Rat eats plants (stems, leaves, and seeds), insects, lizards, and bird eggs. Breeding. Gestation is ¢.27 days. Young are extremely precocial and are weaned within seven days. Females are fertile at ¢.78 days of age. Activity patterns. Allen’s Cotton Rats have been trapped day and night. Activity peaks during the 24hour period are unknown, but likely. Movements, Home range and Social organization. Runways are evident in thick grassy areas but indistinct in brushy habitats with little ground cover; grass nests are constructed in various places, including rotten logs, beneath volcanic rocks, under piles of brush, and in grass-shrub clumps. Average density is low at 0-023-0-52 ind/ha in cloud forests and agricultural lands. Status and Conservation. Classified as Vulnerable on The IUCN Red List. Populations have declined 30% in the past decade, caused by increasing habitat loss across its known distribution, which is expected to continue in the coming decade. Bibliography. Allen (1906), Bailey (1902), Baker (1969), Baker & Womochel (1966), Ceballos (2014a), Goodwin (1969), de Grammont & Cuarén (2008a), Jiménez (1971, 1972), Musser & Carleton (2005), Ramirez & Chavez (2014a), Shump & Baker (1978a), Webb & Baker (1962), Zimmerman (1970).

Published as part of Don E. Wilson, Russell A. Mittermeier & Thomas E. Lacher, Jr, 2017, Cricetidae, pp. 204-535 in Handbook of the Mammals of the World – Volume 7 Rodents II, Barcelona :Lynx Edicions on page 403, DOI: 10.5281/zenodo.6707142