Rats, Bats, and Xenarthrans

Britannica Educational Publishing, John P Rafferty

• Publisher: Britannica Educational Publishing
• Year: 2010
• eBook ISBN: 9781615304097
• Topic: Biological Sciences
• Subtopic: Zoology
• Index: Biological Sciences

While we may often find them nestled in the corners of our attics or traversing our yards, rodents and bats are more frequently found in nature and originate in a large variety of shapes and sizes. Together they make up over half of the planet's mammal species. While there are only 29 species of xenarthrans—an ancient line of mammals comprised of sloths, anteaters, and armadillos—they are significant players in various ecosystems, as well. This colorful volume considers the features, behaviors, and major species of these three intriguing categories of mammals.

CHAPTER 1
RODENTS

There are more than 2,050 living species of rodents (order Rodentia)—mammals characterized by upper and lower pairs of ever-growing rootless incisor teeth. Rodents are the largest group of mammals, constituting almost half the class Mammalia’s approximately 4,660 species. They are indigenous to every land area except Antarctica, New Zealand, and a few Arctic and other oceanic islands, although some species have been introduced even to those places through their association with humans. This huge order of animals encompasses 27 separate families, including not only the “true” rats and mice (family Muridae) but also such diverse groups as porcupines, beavers, squirrels, marmots, pocket gophers, and chinchillas.

GENERAL FEATURES

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All rodents possess constantly growing rootless incisors that have a hard enamel layer on the front of each tooth and softer dentine behind. The differential wear from gnawing creates perpetually sharp chisel edges. Rodents’ absence of other incisors and canine teeth results in a gap, or diastema, between incisors and cheekteeth, which number from 22 (5 on each side of the upper and lower jaws) to 4, may be rooted or rootless and ever-growing, and may be low- or high-crowned. The nature of the jaw articulation ensures that incisors do not meet when food is chewed and that upper and lower cheekteeth (premolars and molars) do not make contact while the animal gnaws. Powerful and intricately divided masseter muscles, attached to jaw and skull in different arrangements, provide most of the power for chewing and gnawing.

The range in body size between the mouse (18 grams [0.64 ounce], body 12 cm [4.7 inches] long) and the marmot (3,000 grams [6.6 pounds], body 50 cm [19.7 inches] long) spans the majority of living rodents, but the extremes are remarkable. One of the smallest is Delany’s swamp mouse (Delanymys brooksi), associated with bamboo in the marshes and mountain forests in Africa. It weighs 5 to 7 grams (0.18 to 0.25 ounce), and the body is 5 to 6 cm (2 to 2.4 inches) long. The largest rodent is the capybara (Hydrochoerus hydrochaeris) of Central and South America, which weighs 35 to 66 kg (77 to 146 pounds) and stands 50 to 60 cm (20 to 24 inches) at the shoulder, with a body 100 to 135 cm (39.4 to 53.2 inches) long. Some extinct species were even larger, attaining the size of a black bear or small rhinoceros. The largest rodent ever recorded, Josephoartigasia monesi, lived some two to four million years ago, during the Pleistocene and Pliocene epochs. By some estimates it grew to a length of about 3 metres (10 feet) and weighed nearly 1,000 kg (about 2,200 pounds).

IMPORTANCE TO HUMANS

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Rodents have lived on the planet for at least 56 million years and modern humans for less than one million, but the consequences of their interactions during that short overlap of evolutionary time have been profound. For rodents, early humans were just another predator to avoid, but with Homo sapiens’ transition from nomadic hunting and gathering to sedentary agricultural practices, humans became a reliable source of shelter and food for those species having the innate genetic and behavioral abilities to adapt to man-made habitats. The impact of these species upon human populations ranges from inconvenient to deadly. Crops are damaged before harvest; stored food is contaminated by rodent waste; water-impounding structures leak from burrowing; and objects are damaged by gnawing. Certain species are reservoirs for diseases, including plague, murine typhus, scrub typhus, tularemia, rat-bite fever, Rocky Mountain spotted fever, and Lassa fever. Only a few species are serious pests or vectors of disease, but it is these rodents that are most closely associated with people.

Various other rodents are beneficial, providing a source of food through hunting and husbandry, apparel derived from their fur, test animals for biomedical and genetic research (especially mice and rats), pleasure as household pets, and insight on mammalian biology and evolutionary history.

NATURAL HISTORY

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Rodents may be diurnal, nocturnal, or sometimes active part of the day and night. Although some species are herbivorous, the diets of most include both vegetable and animal matter. When in search of meat, there are what might be called opportunistic generalists, as well as others that are specialized predators—not only of arthropods but sometimes of vertebrates. Food is either eaten where gathered or carried to burrows and stored (the latter option being favored by the pocket mouse, African pouched rat, and hamster). Species living in arid habitats and on oceanic islands are able to obtain their water requirements from their food. A wide variety of shelters are used or constructed, ranging from tree holes, rock crevices, or simple burrows, to hidden nests on the forest floor, leaf and stick structures in tree crowns, mounds of cut vegetation built in aquatic environments, or complex networks of tunnels and galleries. Rodents may be active all year or enter periods of dormancy or deep hibernation. Breeding time and frequency, length of gestation, and litter size vary widely, but two of the most prolific species are both associated with humans. The brown rat (Rattus norvegicus) can give birth to litters of up to 22 offspring, and the house mouse (Mus musculus) can produce up to 14 litters annually, the average birth resulting in 5 to 6 offspring, with as many as 12 per birth occasionally occurring, as well. Rodent population size may remain stable or fluctuate, and some species, most notably lemmings, migrate when groups become excessively large.

MURIDAE

Muridae is the largest extant rodent family—indeed the largest of all mammalian families—encompassing more than 1,383 species of the “true” mice and rats. Two-thirds of all rodent species and genera belong to family Muridae. The members of this family are often collectively called murids, or muroid rodents.

The 300 genera of muroid rodents are classified within 18 subfamilies, but more than 200 of them (and nearly 1,000 species) belong to only two subfamilies—Sigmodontinae (New World rats and mice) and Murinae (Old World rats and mice). Two other subfamilies (Arvicolinae and Gerbillinae) include approximately 250 additional species, with the remaining 14 subfamilies accommodating various other genera, some of which consist of a single species.

Not all specialists agree on the number of subfamilies or that all of these should be included within Muridae. For instance, some assemblages, such as blind mole rats and bamboo rats, are very distinctive and have been treated in the past as separate families. The Malabar spiny tree mouse was originally described as a kind of dormouse (Myoxidae), but was reclassified as a murid similar to blind tree mice. Many subfamilies, including hamsters, were formerly considered as part of a family separate from Muridae, but these groups are now most often viewed as muroid subfamilies. Inclusion of these subfamilies emphasizes their closer evolutionary relationships to one another than to any other group of rodents, but such affinity could also be expressed by recognizing each as a separate family and then bringing them together within a larger category, the superfamily Muroidea. This would be satisfactory if each group could be clearly demonstrated to have a common ancestor (i.e., to be monophyletic). Some groups are known to be monophyletic (hamsters, voles, African pouched rats, gerbils, Old World rats and mice, African spiny mice, platacanthomyines, zokors, blind mole rats, and bamboo rats). Other groups, however, cannot be classified with certainty and may or may not be a hodgepodge of unrelated genera and species (New World rats and mice, dendromurines, and Malagasy rats and mice). Also unresolved are the affinities of subfamilies containing only one genus (mouselike hamsters, the maned rat).

Pending better resolution of the relationships between these problem groups, some specialists prefer to retain them as subfamilies within Muridae, but others still separate them as families under the umbrella of Muroidea. Fossil evidence may support the single-family arrangement because clearly diagnosable groups of living species, such as mole rats and bamboo rats, lose their distinction when their lineages are traced far back in time. Whether recognized as the family Muridae or the superfamily Muroidea, the living members of these 18 groups show an impressive range of variation in body form, locomotion, and ecology.

FORM AND FUNCTION

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The body form of tree squirrels may be the model for the earliest, and presumably generalized, rodents (genus Paramys). With their ability to adhere to bark with their claws, squirrels adeptly scamper up tree trunks, run along branches, and leap to adjacent trees; but they are equally agile on the ground, and some are capable swimmers. Burrowers are also represented in the form of long-tailed ground squirrels.

The widely recognizable eastern gray squirrel (Sciurus carolinensis). Hope Lourie Killcoyne

The specialized body forms of other kinds of rodents tie them closer to particular locomotor patterns and ecologies. Some strictly arboreal species have a prehensile tail; others glide from tree to tree supported by fur-covered membranes between appendages. Highly specialized fossorial (burrowing) rodents, including blind mole rats, blesmols, and ground squirrels, are cylindrical and furry with protruding, strong incisors, small eyes and ears, and large forefeet bearing powerful digging claws. Semiaquatic rodents such as beavers, muskrats, nutrias, and water rats possess specialized traits allowing them to forage in aquatic habitats yet den in ground burrows. Terrestrial leaping species, such as kangaroo rats, jumping mice, gerbils, and jerboas, have short forelimbs, long and powerful hind limbs and feet, and a long tail used for balance. Body forms of some rodents converge on those in nonrodent orders, resembling shrews, moles, hares, pikas, pigs, or small forest deer. There is also convergence between distantly related groups of rodents in particular body forms and associated natural histories.

Regardless of body form, though, all rodents share the same basic tools that, as mammologists Louise Emmons and Francois Feer noted, “can be used to cut, pry, slice, gouge, dig, stab, or delicately hold like a pair of tweezers; they can cut grass, open nuts, kill animal prey, dig tunnels, and fell large trees.”

EVOLUTION

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As documented by fossils, the evolutionary history of rodents extends back 56 million years to the Late Paleocene Epoch in North America. Those species, however, are considered to have originated in Eurasia, so the origin of the order Rodentia is certainly older. However, lack of fossil evidence prior to the Late Paleocene makes the understanding of evolutionary relationships between rodents above the familial level a continuing quest. Specialists agree with the definitions of most families, but they historically have disagreed, and still do, about the arrangement of families into larger groups—namely, suborders. Past classifications either have omitted suborders altogether and grouped the families into superfamilies or have grouped the families into 2, 7, 11, or 16 suborders. Some specialists recognize just two suborders, Sciurognathi and Hystricognathi, which were proposed in 1899 and were based on conformation of the lower jaw. But any arrangement is simply a hypothesis of relationships between rodent families that is continually being tested by discovery of new fossils, reanalyses of data from conventional sources, and new analyses of data from many different, unrelated sources.

CHAPTER 2
RATS AND SIMILAR FORMS

The term rat is generally and indiscriminately applied to numerous members of several rodent families having bodies longer than about 12 cm, or about 5 inches. (Smaller thin-tailed rodents are just as often indiscriminately referred to as mice.) In scientific usage, rat applies to any of 56 thin-tailed, medium-sized rodent species in the genus Rattus native to continental Asia and the adjacent islands of Southeast Asia eastward to the Australia-New Guinea region. A few species have spread far beyond their native range in close association with people. The brown rat, Rattus norvegicus (also called the Norway rat), and the house rat, R. rattus (also called the black rat, ship rat, or roof rat), live virtually everywhere that human populations have settled; the house rat is predominant in warmer climates, and the brown rat dominates in temperate regions, especially urban areas. Most likely originating in Asia, the brown rat reached Europe in the mid-1500s and North America around 1750. The house rat most likely originated in India.

Norway rat (Rattus norvegicus). John H. Gerard

Brown and house rats exploit human food resources, eating and contaminating stored grains and killing poultry. They have been responsible for the depletion or extinction of native species of small mammals, birds, and reptiles, especially on oceanic islands. Both the brown and house rat have been implicated in the spread of 40 diseases among humans, including bubonic plague, food poisoning, schistosomiasis, murine typhus, tularemia, and leptospirosis. On the other hand, the brown rat has been used in laboratories worldwide for medical, genetic, and basic biological research aimed at maintaining and improving human health. Rats are also kept as pets.

GENERAL FEATURES

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Rats are generally slender with a pointed head, large eyes, and prominent, thinly furred ears. They have moderately long legs and long, sharp claws. The bald soles of their narrow hind feet possess fleshy pads of variable size, depending on species. The brown rat has a larger body than the house rat, and its tail is shorter relative to the body. The brown rat also has thicker fur and 12 pairs of mammae (mammary glands) instead of 10. Tail length among rats ranges from shorter than body length to appreciably longer. The tail appears smooth and bald but is actually covered with very short, fine hairs. In a few species, these hairs become longer toward the tip, which gives the tail a slightly tufted appearance. As with any large group of rodents, body size varies within the genus. Most species are about the size of Hoffman’s rat (R. hoffmanni), native to the Indonesian islands of Sulawesi (Celebes) and weighing 95 to 240 grams (3.4 to 8.5 ounces), with a body length of 17 to 21 cm (6.7 to 8.3 inches) and a tail about as long. One of the smaller species is Osgood’s rat (R. osgoodi) of southern Vietnam, with a body 12 to 17 cm (4.7 to 6.7 inches) long and a somewhat shorter tail. At the larger extreme is the Sulawesian white-tailed rat (R. xanthurus), measuring 19 to 27 cm (about 7.5 to 10.6 inches) long with a tail of 26 to 34 cm (10.2 to 13.4 inches).

Like Hoffman’s rat, most species have a moderately short, soft, and dense coat. In some species the coat may be thicker and longer, somewhat woolly, or long and coarse; in others, such as the Sulawesian white-tailed rat and the Sikkim rat (R. remotus) of India, long and slender guard hairs resembling whiskers extend 4 to 6 cm (1.6 to 2.4 inches) beyond the coat on the back and rump. Very few Rattus species have spiny fur. Hoffman’s rat also exhibits the basic colour pattern seen in the genus—upperparts of brownish yellow peppered with black to dark brown and speckled with buff and underparts from silvery gray to dark gray, sometimes suffused with buff tones. Tail, ears, and feet are dark brown. As with fur texture, colour is variable. The Sikkim rat has brownish upperparts and a pure white underside; the Himalayan field rat (R. nitidus) has a brown back, gray underparts, and feet of pearly white. Others have very dark fur, such as the Mentawai rat (R. lugens) native to islands off the west coast of Sumatra. It has brownish black upperparts and a grayish black belly. Although the tail is uniformly gray to dark brown...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: Rodents
• Chapter 2: Rats and Similar Forms
• Chapter 3: Mice and Similar Forms
• Chapter 4: Familiar Rodents
• Chapter 5: Exotic Rodents
• Chapter 6: Bats
• Chapter 7: Xenarthrans
• Glossary
• Bibliography
• Index