Part 2

Rodents are mainly herbivorous mammals, mostly of small size, having a furry, sometimes a spiny, integument, clawed digits, and usually plantigrade feet. The most important distinguishing character of the order is its dentition. This is marked by the absence of canine teeth and the presence of strongly developed incisors growing from permanent pulps. The incisors are never more than two in the lower jaw and usually but two in the upper. They are elongated, curved, chisel-like in shape, and continue to grow throughout the life of the animal. Only the front of these teeth is covered with enamel, a provision which keeps them sharp by the more rapid wearing away of the softer dentine in the body of the tooth, as the upper and lower pairs meet in gnawing. Between the incisors and the cheek, or molar, teeth of rodents there is a wide, vacant space, marking the entire absence of canines.

The most extensive family of rodents is the _Muridæ_, a name which applies to rats and mice in the widest sense of those terms. It is difficult to characterize the family, since its members differ widely. However, most of them are rat-like in form and light and active in movements. None of the family have premolars; and, except in a single genus (_Hydromys_), the number of molars is three. Oldfield Thomas, the eminent English zoologist, includes in this family no less than 77 genera, or almost half the total of 159 which he ascribes to the whole order _Rodentia_.[A] He further subdivides the _Muridæ_ into a dozen subfamilies, of which the _Murinæ_ and the _Sigmodontinæ_ are the most extensive. The name _Cricetinæ_ is now generally used instead of _Sigmodontinæ_, though not always with the same limitations.

Footnote A:

Proc. Zool. Soc., pp. 1012–1028, 1896.

The _Murinæ_ comprise only Old World rats and mice, while the _Cricetinæ_ are, in the main, American forms. In the _Murinæ_ the cusps, or tubercles, of the unworn upper molars are arranged triserially, or in three longitudinal rows; in the _Cricetinæ_ they are arranged biserially, one row on the outer and one on the inner margin (fig. 1). The wearing away of these cusps leaves characteristic curved lines of hard enamel surrounding areas of dentine. When the cusps are in pairs the worn pattern looks somewhat like the Greek letter sigma (Σ), whence the name sigmodont, often applied to native American rats and mice.

[Illustration:

Fig. 1a. Fig. 1b.

FIG. 1a.—Upper molars of the brown rat (_Mus_): tubercles in three rows.

FIG. 1b.—Upper molars of the rice rat (_Oryzomys_): tubercles in two rows. ]

The _Murinæ_ are the true rats and mice, typified by the genus _Mus_, which contains by far the largest number of species. Trouessart, in his Catalogus Mammalium, enumerates 260 species of _Mus_ described before 1905. Since that date a number of new forms have been described.

The genus _Mus_ is characterized by narrow, ungrooved incisors; three small, rooted molars; soft fur mixed with hairs, sometimes with spines; a rudimentary pollex having a short nail instead of a claw; a long tail bearing rings of overlapping scales and often naked or nearly so. The ears are rather large, the eyes bright and prominent, and the muzzle somewhat pointed. The members of the genus are natives of the Old World, throughout which, with the exception of Madagascar, they are quite generally distributed. Nearly seven-eighths of the whole number of species are commonly called rats.

The distinction between rats and mice is arbitrary and based on size. Exclusive of the tail, rats may be said to vary in length from 4½ to 10 inches or more, while mice measure from 2 to 4 inches. With few exceptions, rats have six well-defined footpads (plantar tubercles), the last on the hind foot being elongated in shape; the last hind-foot pad of mice is usually circular (fig. 2).

Of the many species of _Mus_ only three or four have developed the ability to adapt themselves to such a variety of conditions as to become cosmopolitan. Four have found lodgment in America: The common house mouse (_Mus musculus_); the old English black rat (_Mus rattus_); the Egyptian, or roof, rat (_Mus alexandrinus_); and the brown rat (_Mus norvegicus_), known also as the gray rat, barn rat, wharf rat, sewer rat, and Norway rat. The black rat and the roof rat differ from each other chiefly in color. Indeed some zoologists regard them as races of the same species, and the trinomial _Mus rattus alexandrinus_ for the roof rat is now in use among zoologists.

[Illustration:

FIG. 2a.—Right hind foot of brown rat, showing long sixth foot pad.

FIG. 2b.—Right hind foot of house mouse, showing round sixth foot pad. ]

DISTRIBUTION OF THE GENUS MUS IN AMERICA.

The common house mouse (_M. musculus_) found its way to America soon after the first settlement by Europeans. It now inhabits all settled parts of North and South America, as well as nearly the entire Old World; but in very cold regions it does not always survive the winters, and is therefore comparatively scarce or local. It almost always reaches a new settlement sooner than the rat.

The black rat (_M. rattus_) has been known in Europe since the twelfth century. It was carried to South and Middle America about three and a half centuries ago (1554). The time of its arrival in the English colonies of North America is not known with certainty, but it was well established in the settled parts by the beginning of the eighteenth century. Soon after the arrival of the brown rat, the black species began to decrease in numbers, and has become extinct in most localities. At present it is not uncommon in some parts of the South, and still occurs in scattered colonies in Canada and some of the States east of the Mississippi, and also on some of the coastal islands. It is occasional in many of our seaports, being apparently brought from the Far East in merchandise. Except in a few ports like San Francisco, where new arrivals are probably rather frequent, these introduced individuals are often destroyed before they multiply. The history of the black rat in Europe and its disappearance before the brown rat is an exact parallel to its history here, and the animal is now comparatively rare north of the Alps, except in the Channel Islands.

The Biological Survey has specimens of the black rat from Massachusetts, New Hampshire, Georgia, Florida, Alabama, California, and Washington, and also from Mexico, Honduras, Nicaragua, and Hawaii. There are authentic records of its recent occurrence in Newfoundland, Quebec, Nova Scotia, New York, North Carolina, Tennessee, West Virginia, and Mississippi. In parts of South and Middle America it is abundant.

The roof or Alexandrian rat (_M. alexandrinus_) is similar to the black rat in form and general habits, though not in color. Little is known of its history, but it is thought to be a native of Egypt, where it is still abundant. It has established itself in many parts of the world, mainly in warm climates, and is common near the coast in the southern parts of the United States.

The Biological Survey has specimens of the roof rat from North Carolina, Georgia, Florida, Alabama, Mississippi, Texas, Arizona, and California. In the last-named State it is abundant in the Sacramento Valley. It is known also from Dismal Swamp, Virginia, and from Cuba, the Bermudas, Trinidad, San Domingo, Costa Rica, Nicaragua, Mexico, and Hawaii. Also, it inhabits many parts of South America, where in places it is the dominant species.

The most destructive of the rat family is the brown rat (_M. norvegicus_). In most parts of the United States it is the common rat about houses and barns in the country and about markets, wharves, and warehouses in cities. It is larger and more robust than either the black or the roof rat, and differs from both in habits. It is more of a burrower, and lives in excavations which it makes under buildings and in loose soil along hedges and river banks. This habit, combined with its greater strength and ferocity has enabled it to supplant the other species in temperate latitudes; but in the warmer parts of America and the Old World it has not been able to drive out the others. The house mouse everywhere holds its own against the brown rat by its ability to escape into retreats too small for the rat to follow.

The brown rat inhabits most of the thickly populated parts of America. North of Panama it occurs generally except in the arid interior, from the Isthmus to the Yukon Valley and southern Greenland. In the Great Basin it is practically unknown, and in New Mexico and Arizona it is confined chiefly to towns along the railroads. The Biological Survey is without records of its presence in Nevada, Utah, Wyoming, Idaho, and the greater part of Montana. The reason for its absence in that region is not understood, but its ability to withstand extreme cold is proved by the fact that it flourished in latitude 78° 37′ north on board Doctor Kane’s ship _Advance_, in the Second Grinnell Expedition, during the two winters when that vessel was icebound. It has also adapted itself to the continuous low temperatures of cold-storage warehouses, in which it appears to breed freely.

HISTORY OF THE BROWN RAT.

We know little of the history of this species. Greek and Roman writers make no mention of rats of any kind, but possibly knew the animals and included them in their frequent references to mice. Pallas, in 1778, described the brown rat under the name _Mus decumanus_, and this was generally used until it was found that Erxleben had called it _M. norvegicus_ in 1777. Previously, the common name Norway rat had often been used for this species.

The brown rat is generally supposed to be of Asiatic origin. Various modern writers have asserted that it came originally from Persia or India; but W. T. Blanford states that the species is at present unknown in Persia, and that in India the black rat is the generally distributed species, while the brown rat occurs only along the coast and the navigable rivers.[B] This implies that the latter species is a comparatively recent immigrant into India.

Footnote B:

Fauna of British India. Mammals, p. 409, 1891.

As regards the arrival of the brown rat in Europe, two facts are known. The species reached England from some eastern port about 1728 or 1729, and according to Pallas, a little earlier, 1727, crossed the Russian frontier from Asia and soon spread over the greater part of that country.[C] This statement, taken in connection with that of Blanford, makes it highly probable that before this migration the Asiatic home of the species was north, rather than south, of the high mountains of northern India. This view, which has been adopted by several naturalists, is further strengthened by the fact that the animal flourishes better in temperate than in tropical climates.

Footnote C:

Zoographica, Rosso-Asiatica, vol. 1. p. 165, 1831.

Possibly earlier and unrecorded westward migrations of the brown rat took place. A few years ago Professor Waile, the archeologist, while making excavations at Cherchell on the coast of Algeria, dug up the skull of a rat, which he stated was contemporary with the Roman occupation of the country under the Cæsars. The skull had but one molar, much worn, but the cranial bones were intact, and French zoologists pronounced the remains as undoubtedly those of the “surmulot,” or brown rat.[D] This shows that we have little more than conjectures for the early history of this species.

Footnote D:

Comptes Rendus des Séances de L’Académie des Sciences, Paris, vol. 116, p. 1031, 1893.

The brown rat is said to have first appeared in Paris in 1750. It was brought to the United States, probably from England, about the beginning of the Revolution, 1775. According to Audubon, it was unknown on the Pacific coast of the United States in 1851; but Dr. J. S. Newberry thought it must have arrived at San Diego, Monterey, and San Francisco at a much earlier date.[E] Doctor Cooper recorded its arrival at Fort Steilacoom, Washington, as occurring about 1855.

Footnote E:

Pac. R. R. Reports, Vol. 6, Zoological Report, pt. 2, p. 60, 1857.

GENERAL DESCRIPTION.

The brown rat differs considerably from the black rat and the roof rat. It is larger, has a shorter head, a more obtuse muzzle, smaller ears, and a relatively shorter and stouter tail. The general color is grayish-brown above and whitish below. The over hairs of the upper parts have black tips. The tail is usually shorter than the head and body combined. The average measurements of adult specimens of the brown rat in the Biological Survey collections are as follows: Total length, 415 millimeters (16.3 inches); tail, 192 millimeters (7.1 inches); hind foot, 43 millimeters (1.7 inches). This species sometimes attains a total length of 19 to 20 inches, and has been known to weigh 24 to 28 ounces and even more. The average weight of an adult brown rat is considerably less than a pound.

The black rat is less robust than the brown rat. It has a longer head, a sharper muzzle, and larger and broader ears (fig. 3). The tail is longer than the head and body combined. The fur is of a sooty, or plumbeous black, color, paler on the underparts. It is much softer and denser than that of the brown rat, and the mixture of very dark and lighter over hairs gives it a peculiar shining appearance. The average measurements of 20 apparently adult specimens in the collection of the Biological Survey are as follows: Total length, 379 millimeters (14.9 inches); tail, 207.4 millimeters (8.1 inches); hind foot, 35.8 millimeters (1.4 inches).

The roof rat in general resembles the black rat, except as to color and texture of fur. Above it does not greatly differ in color from the brown rat, but its underparts are usually more yellowish. The fur is commonly shorter and harsher in texture than that of the black rat, but this difference might not always be apparent in specimens of the two forms from the same latitude. The average measurements of 50 adult specimens of the roof rat in the collections of the Biological Survey are as follows: Total length, 393.3 millimeters (15.5 inches); tail, 212.8 millimeters (8.4 inches); hind foot, 36.2 millimeters (1.4 inches).

[Illustration:

FIG. 3A, FIG. 3B.—Ears of brown rat and black rat, showing relative size. ]

Both albinism and melanism are frequent among rats, and pied forms also are common. It has been claimed that all the white rats (albinos) of the bird stores are _Mus rattus_, but albinism is by no means confined to this species. Doctor Hatai found that all the colonies of white rats maintained at the neurological laboratories of Chicago University and the Wistar Institute of Anatomy, Philadelphia, were of the _M. norvegicus_ species.[F] The same is true of all the albino rats in the collections of the National Museum and the Biological Survey. These collections contain also several spotted rats (gray and white) and sooty-black specimens indistinguishable in color from _M. rattus_, all being undoubtedly of the _M. norvegicus_ species.

Footnote F:

Biological Bulletin, vol. 12, pp. 266–273, March, 1907.

KEY TO THE SPECIES OF MUS IN AMERICA.

Size small. Total length of adult less than 200 millimeters _Mus musculus_

Size large. Total length of adult exceeding 300 millimeters.

Ears moderate, when laid forward barely or not reaching eye; tail shorter than (rarely equal to) the length of head and body, darker above than below; color of body normally gray-brown above, white below; hind foot 38–46 millimeters _Mus norvegicus_

Ears larger, when laid forward reaching at least to middle of eye; tail longer than head and body, dusky all around; hind foot 33–37 millimeters.

Color grayish-brown above, white or yellowish _M. rattus white below. alexandrinus_

Color blue-black above, slaty below _M. rattus_

HABITS OF RATS.

BREEDING HABITS.

Both climate and food supply affect the rate of multiplication of most rodents. The rat probably increases more rapidly in a temperate and equable climate than in one of great variability. Extremes of heat and cold retard multiplication, decreasing both the number of litters in a year and the number of young at a time. In northern latitudes, apparently, more or less interruption of breeding occurs in the winter months.

Where the country is well settled the food supply of rats is not likely to be deficient; and when the animals have access to stores of grain, the young mature very quickly and probably reproduce earlier than when grain is absent.

The brown rat is more prolific than either the roof rat or the black rat. The female brown rat has usually 12 mammæ—3 pairs of pectoral and 3 pairs of inguinal—although these numbers are not constant, one or more teats frequently being undeveloped. The black rat and the roof rat have only 10 mammæ—2 pairs of pectoral and 3 pairs of inguinal—with but little tendency to vary. Records of actual observations on the number of young confirm the deductions that might be drawn from the above facts. At Bombay, India, during the recent investigations made by the India Plague Commission, 12,000 rats were trapped and examined. The average number of embryos found in pregnant brown rats was 8.1; the highest number, 14. The average for the black rat was 5.2; the largest number, 9.[G]

Footnote G:

Etiology and Epidemiology of Plague, p. 9, Calcutta, 1908.

In temperate latitudes the average number of young produced by the brown rat is undoubtedly greater. Instances of very large litters observed in England are recorded in The Field (London). In two instances 22 and 23 young, respectively, were found in a single nest, though no evidence is offered that these were the progeny of a single female; but in two other cases 17 and 19 embryos were found in gravid females. A dealer in feedstuffs in Washington, D. C., relates that he found 19 young rats in a single nest in his store. Within the past few months the writer has examined four pregnant brown rats taken in traps. The numbers of embryos they contained were 10, 11, 11, and 13, respectively. While we have not enough data for definite conclusions, we may safely state that the average litter for this latitude is not less than 10.

Frank T. Buckland, in Curiosities of Natural History, relates that a white rat which he kept in captivity gave birth to 11 young when only eight weeks old. As gestation in rats occupies three weeks, this animal must have bred when only five weeks old.

The number of times rats breed in a year is not definitely known, and probably varies considerably with local conditions. Kolazy makes the almost incredible statement that two female white rats, kept by him in confinement and well fed, within thirteen months gave birth to 26 litters of young, numbering 180 in all. One of them produced young regularly at intervals of 25 days.[H]

Footnote H:

Verh. Zool. Bot. Gesel. Wien., pp. 731–734, 1871.

The writer recently kept two young female brown rats with a male in a large open cage for several months. One of the females gave birth to young on April 15; the other on April 17. The number in these litters was not observed, as some were devoured soon after birth, and all within three days, presumably by the male rat. On May 23 both females gave birth to young, 24 in number, all in one nest.

The known facts concerning the breeding of the brown rat may be briefly summarized as follows: The animals breed from three to five times a year, each time bringing forth from 6 to 19 young. After a gestation period of twenty-one days, the females give birth to their young in nests built in underground burrows or under floors, stacks, lumber, woodpiles, or other shelter. The young are blind and naked when born, but grow rapidly, and young females are capable of breeding when less than three months old.

Early spring and summer are the periods of greatest reproductive activity among rats. Young, however, are to be found every month of the year.

The above statements apply in the main also to the black and the roof rat, but the number of young in a litter is somewhat smaller. The newly born young of the black rat have not the bright pink color of those of the brown and the roof rat, but are bluish, especially on the upper parts. Black-and-white spotted rats are at first bluish-and-red spotted, the red areas representing the white of the adults.

ABUNDANCE OF RATS.

From the foregoing account of the breeding habits of rats, the great difficulty of ridding cities or large areas of the animals may be readily understood. Ordinarily, they breed more rapidly than they are destroyed. Although few are seen in daytime, at night they fairly swarm along river fronts and wharves, as well as in sewers, stables, warehouses, markets, and other places where food is abundant. Their real numbers may sometimes be discovered when any such harbor is demolished.

An ordinary farm sometimes supports an astounding number of rats. In 1901, an estate of 2,000 acres near Chichester, England, was badly infested with the pests. They were systematically destroyed by traps, poisons, and ferrets, under the supervision of the proprietor. In this way 31,981 were killed, while it was estimated that tenants at the thrashing had destroyed fully 5,000 more. Even then the property was by no means free from rats.[I]

Footnote I:

The Field (London), vol. 100, p. 545, 1902.

During a plague of rats on the island of Jamaica in 1833, the number of rats killed on a single plantation in a year was 38,000. The injury to sugar cane on the island caused by the animals was at that time estimated at half a million dollars a year.[J]

Footnote J:

New England Farmer, vol. 12, p. 315, 1834.

The report of the Indian Famine Commission presented to the English Parliament in 1881 affords one of the best illustrations of the number of rats that may infest a country. An extraordinary number of the animals at that time inhabited the southern Deccan and Mahratta districts of India. The autumn crop of 1878 and the spring crop of 1879 were both below the average, and a large portion of each was destroyed by rats. The resulting scarcity of food led to the payment of rewards for the destruction of the pests, and over 12,000,000 were killed.[K]

Footnote K:

British Medical Journal, Sept. 16, 1905, p. 623.

MIGRATIONS AND INVASIONS.

Migrations of rats have often been recorded. The brown rat is known in Europe quite generally as the migratory rat. The Germans call it the Wanderratte. Pallas narrates that in the autumn of 1727 this species arrived from the east at Astrakhan, southeastern Russia, in such great numbers and so suddenly that nothing could be done to oppose them. They crossed the Volga in immense troops. The cause of this general migration was attributed to an earthquake; but since similar movements of the same species often occur without earthquakes, it is probable that only the food supply of the animals was involved in the migration which first brought the brown rat to Europe.

A seasonal movement of rats from houses and barns to the open fields takes place in spring when green and succulent plant food is ready for them. The return movement takes place in the autumn. This seasonal migration is noticeable even in large cities.