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140 Order Rodentia Gerardo Ceballos The apparently fragile and harmless kitty was abandoned one afternoon on Estanque Island, an island near Angel de la Guarda Island . . . In the dead of night, alone and starving, a sudden movement caught his attention; he instinctively jumped and caught an endemic mouse (Peromyscus guardia guardia) with his claws! The following months the cat managed to survive by eating these mice, until finally in the summer of 1997 the last specimen of this subspecies died. — G. Ceballos, 1999 W ith more than 2,200 living species, rodents are by far the largest order of mammals; 42% of the total species of mammals belong to the order Rodentia (Wilson and Reeder, 2005). They are found around the world, except on some very isolated islands and in the extreme polar regions. They have a wide variety of forms. There are, however, two distinctive features of the order: the continuous growth of the incisors and the lack of canines. The continuous growth of the incisors implies a constant wearing down to prevent excessive growth (Nowak,1999b).In Mexico,this order is represented by 8 families and 243 species, which include squirrels (Sciuridae), beavers (Castoridae), gophers (Geomyidae), kangaroo rats (Heteromyidae), rats and mice (Muridae), acuchis (Dasyproctidae), agoutis (Agoutidae), and porcupines (Erethizontidae ; List et al., 1999). Some groups, such as the agoutis, are diurnal, but most, such as the flying squirrels, are nocturnal. The majority of species are terrestrial,but some groups specialize in other types of habitats,such as squirrels that live in trees, gophers that live underground, and beavers that are semi-aquatic. Some species, such as prairie dogs, form aggregations of thousands of individuals, and their colonies can occupy thousands of hectares (Ceballos et al.,1993).Flying squirrels (Glaucomys volans) are gregarious during winter in order to withstand cold temperatures (Nowak, 1999b). Reproduction may be restricted to a defined period of the year or may occur throughout the year. The number of offspring is low in the larger or more specialized species and very high in generalist species. The offspring in most of the rats and mice are generally precocious, but those of some species are altricial (Eisenberg ,1981).Most species are herbivores,and their diet is based on seeds,bark,fruits,leaves,bulbs,and foliage. They usually supplement their diet with insects, other invertebrates, and even small vertebrates. With these eating habits they play an important role in all ecosystems, acting as dispersers and predators of seeds and seedlings, hence affecting the structure and function of plant communities (Ceballos and Galindo, 1994; Eisenberg, 1981). In general, most species do not face conservation problems because they have survived through changing conditions and have high birth rates.Specialized species or those with restricted distributions , however, are very susceptible to extinction because of anthropogenic activities. 141 Family Sciuridae Gerardo Ceballos and Don E. Wilson The family Sciuridae includes prairie dogs, marmots, and both terrestrial and flying squirrels. It has a wide distribution throughout the world, with the exception of Australia, Madagascar, part of South America, and remote islands. It consists of 60 genera and 278 species (Wilson and Reeder,2005).In Mexico,it is represented by 11 genera and 36 species, of which 13 are endemic. The genera Ammospermophilus, Callospermophilus , Cynomys, Glaucomys, Ictidomys, Otospermophilus, Tamias, Tamiasciurus, and Xerospermophilus are limited to arid and temperate areas. Prairie dogs (Cynomys mexicanus and C. ludovicianus),flying squirrels (Glaucomys volans),and some squirrels such as Tamiasciurus mearnsi are at risk of extinction from destruction and fragmentation of their habitat. Subfamily Sciurinae The subfamily Sciurinae contains three genera of tree squirrels, chickarees, and flying squirrels. Thirty-six species are known from Mexico. Ammospermophilus harrisii (Audubon and Bachman, 1854) Harris’s antelope squirrel Reyna A. Castillo SUBSPECIES IN MEXICO Ammospermophilus harrisii harrisii (Audubon and Bachman, 1854) Ammospermophilus harrisii saxicola (Mearns, 1896) DESCRIPTION: Ammospermophilus harrisii is a medium-sized squirrel. There is geographic variation in body size,with larger individuals in populations of the northeast (Best et al., 1990a). The tail is medium to short (25% to 33% of the total length), and the ears are short and wide. The eyes are moderate in size and are surrounded by a white circle. The body is thin; the head is small and delicate; the neck and limbs are long. On the forefeet, the thumb is reduced to a small tubercle with a dull claw, and the second toe is the longest.On the hind feet,the middle digit is the longest.The paws are very hairy...


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