Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. “True rats” are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus. Many members of other rodent genera and families are also referred to as rats, and share many characteristics with true rats.
Rats are typically distinguished from mice by their size. Generally, when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, the name includes the term mouse. The muroid family is broad and complex, and the common terms rat and mouse are not taxonomically specific. Scientifically, the terms are not confined to members of the Rattus and Mus genera, for example, the pack rat and cotton mouse.
Share this Image On Your Site
NORWAY RAT (BROWN RAT)
The fur is coarse and usually brown or dark grey, while the underparts are lighter grey or brown. The brown rat is a rather large true murid and can weigh twice as much as a black rat and many times more than a house mouse. The length is commonly in the range of 20 to 25 cm (8 to 10 in), with the tail a further 18 to 25 cm (7 to 10 in), thus being roughly the same length as the body. Adult body weight averages 350 g (12 oz) in males and about 250 g (9 oz) in females. Exceptionally large individuals can reportedly reach 900 to 1,000 g (32 to 35 oz) but are not expected outside of domestic specimens. Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of other rodents, such as the coypu and muskrat. In fact it is common for breeding wild brown rats to weigh (sometimes considerably) less than 300 g (11 oz)
Similar to other rodents, brown rats may carry a number of pathogens, which can result in disease, including Weil’s disease, rat bite fever, cryptosporidiosis, viral hemorrhagic fever, Q fever and hantavirus pulmonary syndrome. In the United Kingdom, brown rats are an important reservoir for Coxiella burnetii, the bacterium that causes Q fever, with seroprevalence for the bacteria found to be as high as 53% in some wild populations.
Norway rats are primarily nocturnal. They usually become active about dusk, when they begin to seek food and water. Some individuals may be active during daylight hours when rat populations are high.
Rats have poor eyesight, relying more on their hearing and their excellent senses of smell, taste, and touch. They are considered color-blind. Therefore, for safety reasons, baits can be dyed distinctive colors without causing avoidance by rats, as long as the dye does not have an objectionable taste or odor.
Rats use their keen sense of smell to locate food items and to recognize other rats. Their sense of taste is excellent, and they can detect some contaminants in their food at levels as low as 0.5 parts per million.
Norway rats usually construct nests in below-ground burrows or at ground level. Nests may be lined with shredded paper, cloth, or other fibrous material. Litters of 6 to 12 young are born 21 to 23 days after conception. Newborn rats are hairless and their eyes are closed, but they grow rapidly. They can eat solid food at 2 1/2 to 3 weeks. They become completely independent at about 3 to 4 weeks and reach reproductive maturity at 3 months of age.
Females may come into heat every 4 or 5 days, and they may mate within a day or two after a litter is born. Breeding often peaks in spring and fall, with reproductive activity declining during the heat of summer and often stopping completely in winter, depending on habitat. These seasonal trends are most pronounced in more severe climates. The average female rat has 4 to 6 litters per year and may successfully wean 20 or more offspring annually.
This species can also serve as a reservoir for Toxoplasma gondii, the parasite that causes toxoplasmosis, though the disease usually spreads from rats to humans when domestic cats feed on infected brown rats. The parasite has a long history with the brown rat, and there are indications that the parasite has evolved to alter an infected rat’s perception to cat predation, making it more susceptible to predation and increasing the likelihood of transmission.
SHIP RAT (BLACK RAT or ROOF RAT)
Black rats are generalist omnivores. They are serious pests to farmers as they eat a wide range of agricultural crops.
Black rats are considered omnivores and eat a wide range of foods, including seeds, fruit, stems, leaves, fungi, and a variety of invertebrates and vertebrates. They are generalists, and thus not very specific in their food preferences, which is indicated by their tendency to feed on any meal provided for cows, swine, chickens, cats, and dogs. They are similar to the tree squirrel in their preference of fruits and nuts. They eat about 15 grams (0.53 oz) per day and drink about 15
As generalists, black rats express great flexibility in their foraging behavior. They are predatory animals and adapt to different micro-habitats. They often meet and forage together in close proximity within and between sexes. Rats tend to forage after sunset. If the food cannot be eaten quickly, they will search for a place to carry and
Black rats (or their ectoparasites) can carry a number of pathogens, of which bubonic plague (via the rat flea), typhus, Weil’s disease, toxoplasmosis
Rats serve as outstanding vectors for transmittance of diseases because they can carry bacteria and viruses in their systems. A number of bacterial diseases are common to rats, and these include Streptococcus
Adult bushy-tailed woodrat males usually weigh 300-600 g with an average of 405 g, and adult females usually weigh 250-350 g with an average of 270 g. These ranges are relatively large because this species occupies a large geographic range, and its body size is closely correlated with climate. Average males range in size from 310-470 mm with the average being 379 mm and average females range from 272 to 410 mm with the average being 356 mm.
A pack rat or packrat, also called a woodrat, can be any of the species in the rodent genus Neotoma. Pack rats have a rat-like appearance with long tails, large ears and large black eyes. Compared to deer mice, harvest mice and grasshopper mice, pack rats are noticeably larger and are usually somewhat larger than cotton rats.
Woodrats reach their greatest diversity in the deserts of the western United States and northern Mexico. Several species are also found in the deciduous forest of the east coast, juniper woodlands in the southwest, oak woodlands along the coastal western United States and in the Sonoran Desert, and in the forest and rocky habitats of the western United States and western Canada.
Pack rats are nest builders. They use plant material such as branches, twigs, sticks, and other available debris. Getting into everything from attics to car engines, stealing their ‘treasures’, damaging electrical wiring, and creating general noisy havoc can easily cause them to become a nuisance. A peculiar characteristic is that if they find something they want, they will drop what they are currently carrying, for example a piece of cactus, and “trade” it for the new item. They are particularly fond of shiny objects. These two traits have inspired an anecdote about a man finding his dime replaced by two nickels. They can also be quite vocal and boisterous.
Each species of pack rat is generally restricted to a given type of habitat within its range. Pack rats live anywhere from low, hot, dry deserts to cold, rocky slopes above timberline. Pack rats build complex houses or dens made of twigs, cactus joints, and other materials. These contain several nest chambers, food caches, and debris piles. Dens are often built in small caves or rocky crevices, but when close by human habitations, woodrats will opportunistically move into the attics and walls of houses. Some Neotoma species, such as the white-throated woodrat (N. albigula), use the bases of prickly pear or cholla cactus as the sites for their homes, using the cactus’ spines for protection from predators. Others, like the desert woodrat (N. lepida) will build dens around the base of a yucca or cactus, such as jumping and teddy-bear chollas. The largest species, Neotoma cinerea, has a bushy, almost squirrel-like tail. Bushy-tailed woodrats Neotoma cinerea occupy a range of habitats from boreal woodlands to deserts. They are cliff-dwellers and are often found on isolated, high-elevation exposed boulder areas under a variety of temperature and moisture conditions. They require adequate shelter inside the rocks, though they are occasionally found inhabiting abandoned buildings, as well.
Bushy-tailed woodrats feed primarily on green vegetation, twigs, and shoots. Mexican pack rats eat seeds, fruits, acorns, and cactus.
Bushy-tailed woodrats are active throughout the year. While primarily nocturnal, they can occasionally be seen during the day. They are usually solitary and very territorial.
These woodrats collect debris in natural crevices, and abandoned man-made structures when available, into large, quasistructures for which the archaeologists’ term ‘midden’ has been borrowed. Middens consist of plant material, feces, and other materials which are solidified with crystallized urine. Woodrat urine contains large amounts of dissolved calcium carbonate and calcium oxalates due to the high oxalate content of many of the succulent plants upon which these animals feed.
An important distinction to make is between middens and nests. Nests are the areas where the animal is often found and where the females raise their young. Nests are usually within the midden, but regional variations to this rule occur. When not contained within the midden, the nest is usually concealed in a rocky crevice behind a barricade of sticks.
In coniferous forests, the woodrat may build its house as high as 50 feet (15 m) up a tree.
Bushy-tailed woodrats do not hibernate. They build several food caches, which they use during the winter months.
The bushy-tailed woodrat engages in hind foot-drumming when alarmed. It will also drum when undisturbed, producing a slow, tapping sound.
This species has been know to transmit Arena virus: transmitted through the air where feces and urine are present. Hanta virus: airborne virus that can be fatal. Trichinosis, Bubonic plague, Typhoid, and Weil’s disease. Pack rats can also carry different bacteria like salmonella and parasites.
MARSH RICE RAT
The marsh rice rat is a medium-sized rodent that looks much like the common black and brown rats, but has greater differences in color between the upper- and underparts. The fur is thick and short. The upperparts are generally gray to grayish brown, with the head a bit lighter, and are sharply delimited from the underparts, which are off-white, as are the feet. There are small cheek pouches. The ears are about the same color as the upperparts, but there is a patch of light hairs in front of them. The tail is dark brown above and may be paler below. The guard hairs are long and have unpigmented, silvery tips. When rice rats swim, air is trapped in the fur, which increases buoyancy and reduces heat loss. As in most other oryzomyines, females have eight mammae.
The forefeet have four and the hindfeet five digits. On the forefeet, the ungual tufts (tufts of hair on the digits) are absent. The hindfeet are broad and have a short fifth digit. Many of the pads are reduced, as are the ungual tufts, but there are small interdigital webs. The Florida Keys form, argentatus, has even more reduced ungual tufts. Many of these traits are common adaptations to life in the water in oryzomyines.
A reddish-brown rat on soil with some debris
Marsh rice rats in much of Florida are more reddish than those elsewhere.
There is some geographic variation in fur color: western populations (texensis) are lighter than those from the east (nominate palustris), and Florida populations are generally more tawny or reddish than either, with those from southern Florida (coloratus) being brighter than those from the center of the state (natator). The Florida Keys form (argentatus) is silvery, and the two other Florida forms–planirostris and sanibeli–lack the reddish tones of mainland Florida populations and are instead grayish, resembling nominate palustris (planirostris), or brownish (sanibeli). In 1989, Humphrey and Setzer reviewed variation in color among Florida populations. They found argentatus to be substantially lighter and planirostris and sanibeli to be somewhat darker than mainland populations, and argentatus to have a less yellow fur, but found no significant differences in redness. There was also substantial variation within populations.
Total length is 226 to 305 mm (8.9 to 12.0 in), tail length 108 to 156 mm (4.3 to 6.1 in), hindfoot length 28 to 37 mm (1.1 to 1.5 in), and body mass 40 to 80 g (1.4 to 2.8 oz), with males slightly larger than females. The largest individuals occur in Florida and along the Gulf Coast east of the Mississippi River delta.
The stomach has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum) is covered by a glandular epithelium. The gall bladder is absent, a synapomorphy (shared-derived character) of Oryzomyini. The karyotype includes 56 chromosomes and a fundamental number of 60 chromosomal arms (2n = 56, FN = 60). The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but may be too variable among Oryzomys to be useful in differentiating species. X chromosome inactivation occurs in the marsh rice rat, even though the animal lacks LINE-1 retrotransposons, which have been suggested as components of the inactivation process. Mutants with fused or additional molars and with light fur have been recorded in laboratory colonies; the abnormal molars are apparently the result of a single autosomal recessive mutation. At about 50%, hematocrit (the proportion of red blood cells in the blood) is high in the marsh rice rat compared to other rodents; this may be an adaptation that enables the rice rat to increase oxygen capacity while swimming underwater
The marsh rice rat is classified as one of eight species in the genus Oryzomys, which is distributed from the eastern United States (marsh rice rat) into northwestern South America (O. gorgasi). Oryzomys previously included many other species, which were reclassified in various studies culminating in contributions by Marcelo Weksler and coworkers in 2006 that removed more than 40 species from the genus. All are placed in the tribe Oryzomyini (“rice rats”), a diverse assemblage of over 100 species, and on higher taxonomic levels in the subfamily Sigmodontinae of the family Cricetidae, along with hundreds of other species of mainly small rodents, most of which occur in South and Central America. In the United States, the marsh rice rat is the only oryzomyine rodent except for Oryzomys couesi in a small area of southern Texas; the only other sigmodontines present are several species of cotton rats (Sigmodon) in the southern half of the country.
The population density of the marsh rice rat usually does not reach 10 per ha (4 per acre). The weather may influence population dynamics; in the Everglades, densities may exceed 200 per ha (80 per acre) when flooding concentrates populations on small islands, In the Florida Keys, population density is less than 1 per ha (0.4 per acre). On Breton Island, Louisiana, perhaps an atypical habitat, home ranges in males average about 0.37 hectares (0.91 acres) and in females about 0.23 hectares (0.57 acres). A study in Florida found male home ranges to average 0.25 hectares (0.62 acres) and female 0.33 hectares (0.82 acres).
Population size is usually largest during the summer and declines during winter, although populations in Texas and Louisiana may be more seasonally stable. Animals also often lose weight during winter. Population size varies dramatically from year to year in southern Texas. In coastal Mississippi, a study found that storms probably do not cause the population to decline substantially and in Texas inundation of its habitat did not significantly influence population density. However, another study in Mississippi found that flooding did cause a marked decline in rice rat abundance.
In the northern part of its range, the species often occurs with the meadow vole (Microtus pennsylvanicus), but there is no evidence that they compete with each other. In the south, the hispid cotton rat (Sigmodon hispidus) and the rice rat regularly occur together; water levels are known to influence relative abundance of these two species in Florida. The cotton rat is mainly active during the day, which may help differentiate its niche from that of the rice rat
The marsh rice rat is the primary host of the Bayou virus (BAYV), the second most common agent of hantavirus infections in the United States. About 16% of animals are infected and the virus is most prevalent in old, heavy males. The virus may be transmitted among rice rats through bites inflicted during fights. It is also present in rice rat saliva and urine and human infections may occur because of contact with these excreta. Two related hantaviruses, Catacama virus and Playa de Oro virus, are known from Oryzomys couesi in Honduras and western Mexico, respectively. An arenavirus normally associated with woodrats (Neotoma) has also been found in Florida marsh rice rats. Antibodies against Borrelia burgdorferi, the bacterium that causes Lyme disease in the United States, have been found in marsh rice rats in Virginia, Maryland, North Carolina, and Tennessee.Another pathogenic bacterium, Bartonella, is known from Georgia marsh rice rats.