Comparative Natural History of Small Mammals from Tropical Forests in Western Mexico

Abstract

testes abdominal or scrotal) were recorded. A few individuals of all species except Reithrodontomys fulvescens and Sigmodon mascotensis were kept in the laboratory to study diet and reproduction. Eleven species of small mammals (Baiomys musculus, Liomys pictus, Marmosa canescens, Nyctomys sumichrasti, Oryzomys couesi, Oryzomys melanotis, Osgoodomys banderanus, Peromyscus perfulvus, Reithrodontomys fulvescens, Sigmodon mascotensis, and Xenomys nelsoni) were captured. One species, Sciurus colliaei, was seen but not captured, and two additional species, Hodomys alieni and Pappogeomys bullen, previously recorded in the area (Ceballos and Miranda, 1986) were not seen. Of the mammals known from the Chamela Biological Station, three genera (23%) and nine species (64%) are endemic to Mexico. The number of species per habitat ranged from one to nine. The most abundant species overall was the spiny pocket mouse (Liomys pictus). In the deciduous and arroyo forests (the most intensively sampled habitats) it accounted for 95% (302) and 71% (251) of all individuals captured, respectively. All species except Sigmodon mascotensis and Sciurus colliaei were nocturnal, and most (71%) were herbivorous. Marmosa canescens.-The grayish mouse opossum is endemic to Mexico (Hall, 1981). It is solitary and semiarboreal (six of 11 captures were on the ground). It was present in all forest habitats and croplands, but not in grassland. Nests are' made of dry leaves and are lined with grasses and plant fibers. The nests (n = 25) were found in cavities in trees and shrubs, such as Caesalpinia eriostachys, Jacquinia pungens, Opuntia excelsa, and Prosopis juliflora, at heights from 70 to 500 cm; only one was found on the ground beneath a pile of brush. Ten nests were found in abandoned nests of the white-bellied wren (Uropsila leucogastra-L. Marquez, pers. comm.). Estimated densities of M. canescens in both deciduous and arroyo forests ranged from 0.4 to 4.5 individuals per ha. Females with young were found from terrestrial; no specimens were captured in traps set in trees. Nests (n = 30) were underground. The sex ratio did not differ significantly from 1:1 (N = 592, X2 = 0.6756, d.f. = 1, P > 0.05). Males with scrotal testes and lactating or pregnant females were found throughout the year, but reproduction was concentrated during the dry season (Ceballos, 1989). Number of young in three litters was three, three, and five. Newborn young were pink, hairless, blind, and had a mass of approximately 2 g. During the first 38 days, total length and mass increased an average of 3.9 mm/day and 0.56 g/day, respectively (n = 5 young). In the laboratory, weaning occurred at 23 days of age. The cheek pouches of approximately 200 individuals of L. pictus from the deciduous and arroyo forests contained 1,152 seeds, representing 26 families and 54 species of native plants. Seeds varied in size (greatest length) from 1 to 18 mm, and the number of seeds in the cheek pouches of an individual ranged from one to 87. The plant families most frequently represented were Leguminosae, Euphorbiaceae, and Convolvu-laceae. The most abundant species were Phaseolus microcarpus (vine; 243 seeds = 21%), Cyclantera mul-tifoliolata (vine; 237 seeds = 20.5%), Ipomoea sp. (vines; 353 seeds = 30.6%), and Recchia mexicana (tree; 42 seeds = 3.6%). Several of the species found in the pouches (e.g., Caesalpinia eriostachys) are known to contain toxic compounds and were rejected in feeding trials in the laboratory. Animals forced to eat these seeds decreased in mass and died in 3-5 days. On seven occasions small phoretic moths (Psilopsaltis sp.) were found on the dorsum of spiny pocket mice collected in the deciduous forest. One moth was on each mouse, except on two occasions, when two moths were on the same individual. Females of P. santarosae are phoretic on L. salvini in Costa Rica (Davis et aI., 1986). Nyctomys sumichrasti.-Vesper rats appear to be strictly arboreal. Only five individuals were caught, all in trees, at heights from 0.7 to 7 m above ground, and all in arroyo forest habitat. At the Chamela Biological Station, Collett et al. (1975) collected eight specimens in trees at heights from 1 to 3 m. Nests (n = 4) were similar to those of squirrels (Sciurus), constructed of leaves and plant fibers in the upper forks of trees. Reproduction in N. sumichrasti probably occurs throughout the year (Fleming, 1970; Genoways and Jones, 1972). Two females maintained in the laboratory each had a litter with two offspring. Young were pink, hairless, blind, and had a mass of approximately 4 g. During the first 5 weeks, offspring were attached continuously to their mothers' teats; this behavior is similar to that of other arboreal and semiarboreal small mammals, such as the Magdalena rat (Xenomys nelsoni) and probably represents a specialization for ar-boreality. Eyes opened at 18 days, weaning occurred at approximately 25 days, and adult body mass was attained after 8 weeks. In the wild, N. sumichrasti ate the hard fruits of the false evergreen needle bush