Rattus norvegicus

659.

Brown Rat

Rattus norvegicus

French: Rat brun / German: \Wanderratte / Spanish: Rata parda

Other common names: Common Rat, Domestic Rat, Fancy Rat, Lab Rat, Norway Rat, Sewer Rat

Taxonomy. Mus norvegicus Berkenhout, 1769,

“Great Britain.”

Rattus norvegicus is currently in the R. nor- vegicus species group, along with R. nitidus and R. pyctoris, based on genetic and morphological data. There is less genetic and morphological variation in R.norvegicus than R. rattus, which was recently broken up into multiple species. Rattus norvegicus originated in south-western Siberia, northern China, and Japan and spread throughout Asia and Europe as early as the 16" century, although this is unconfirmed and the actual date when it became commensal with humansis uncertain. Since Western Civilization began colonizing the New World and Australia, R. norvegicus followed, spreading in similar ways as R. rattus, stowing away on ships. Subfossils of Brown Rats can be found all over the world and are indicators of when and where some occurred or became extinct. Genetic studies are needed to identify possible subspecies and to determine the way in which it spread across the world. Subspecific taxonomy requires further investigation.

Distribution. Originally native to N China (possibly Mongolia), SW Siberia in Russia, and Japan, and now introduced worldwide, except Antarctica (modern introductions not shaded on the map).

Descriptive notes. Global: head-body 160-290 mm, tail 122-250 mm, ear 15-26 mm, hindfoot 30-50 mm; weight 195-540 g (known to reach 1 kg in rare cases in cities with lots of available resources). China: head—body 205-260 mm, tail 190-250 mm, ear 19-26 mm, hindfoot 38-50 mm; weight 230-500 g. Russia: head—body 140-248 mm,tail 100-220 mm, ear 17-22 mm, hindfoot, 27-44 mm. South-east Asia: head-body 160-265 mm,tail 170-250 mm, hindfoot 35-50 mm; weight 200-500 g. Australia: head—body 180-255 mm,tail 150-210 mm, ear 16-20 mm, hindfoot 35-45 mm; weight 280-500 g. Europe: head-body 190-290 mm,tail 170-230 mm, ear 19-22 mm, hindfoot 38-45 mm; weight 270-520 g. North America: head-body 170-260 mm, tail 150-220 mm, ear 15-20 mm, hindfoot 37-44 mm; weight 200-540 g. Central America: head-body 186-240 mm, tail 122-215 mm, ear 15-20 mm, hindfoot 37-44 mm; weight 195-485 g. The Brown Rat is a typical, variable, robustly built, large species of Rattus —more robust and larger than the Roof Rat (R. rattus). Pelage is long, shaggy, and coarse, and snoutis broad and blunt. Dorsum is dirty grayish brown to dark brown. Dorsal pelage becomes somewhat lighter along sides, shading into ventral pelage. Albinos are very common, more common than in the Roof Rat, and melanistic individuals are also fairly common. Venter is white to grayish white, with pure white hairs mixed in near axillary region in some individuals, and not sharply demarcated from dorsal pelage. Feet and digits are generally white on top, with a pearly luster, and pink below. Feet are broad; hindfeet are longer; and claws are fairly short but sharp and unpigmented. Ears are short and hairy (shorter and hairier than in the Roof Rat), usually same color as dorsum;vibrissae are long and stiff. Tail is ¢.80-97% of head-body length, lightly haired, and indistinctly bicolored, brown dorsally and paler brown ventrally. Skull is robust and broad, broader and less smooth than in the Roof Rat, with more convex structure and angular ridges on top of braincase. There are 5-6 pairs of mammae: 2-3 pairs axillary and three inguinal. Chromosomal complementis 2n = 42, FN = 62-64.

Habitat. Historically, moist primary forests and fields and now typically disturbed habitats of all kinds, including almost every major city and agricultural and arable land. Throughout its human-expanded distribution, the Brown Rat is more common in temperate and colder habitats and less common in tropical and warm habitats, usually stayingnear water. In cities, it generally stays in moist, wet areas that are dark and provide secluded nesting areas (e.g. sewers, walls in buildings, and underground burrows).

Food and Feeding. Brown Rats are very opportunistic omnivores and eat a wide variety of fruits, seeds, vegetation, carrion, birds, small mammals, invertebrates, and human trash. Presence of humans generally does not disturb their feeding habits, probably due to their commensal nature. They consume many more animal foods than Roof Rats, and they even actively hunt shorebirds in some places. In urban environments, they usually consume readily available fresh foods. Small crumb-like foods are usually eaten immediately, but larger foods are taken to secluded areas to avoid predators; when small and large food items are available, they choose larger items and occasionally attempt to take multiple items at once. Hoarding and caching are rare, although some individuals eat food more regularly at specific locations in their home range.

Breeding. Reproduction occurs year-round in all regions and is constant in good conditions. They can have up to 5 litters/year, although there are peaks in summer and autumn. Estrous cycles can be in synch within groups, allowing large numbers of young to be added to the group all at once and increasing chance of surviving to weaning from 28% to 80%. Gestation is c.3 weeks, and litters usually have 3-7 young. Eyes open at c.15 days old; young are able to eat solid food at c.3 weeks old, are weaned at c.1 month old, and reach sexual maturity as early as ¢.3 months old. After birth, there is usually a pause before a female becomes pregnant again. Younger females have less stable reproductive rates; older females breed at a more uniform rate. Brown Rats generally only live c.1 year in the wild, with 90-95% mortality yearly; captive individuals can live as long as four years.

Activity patterns. Brown Rats are primarily nocturnal, and most activity tends to occur before midnight, after which very few rats are seen. They can occasionally be found during the day when densities are high; otherwise,they usually sleep in their nests. For the most part, foraging occurs shortly before or after sunset. Younger Brown Rats generally forage earlier in the day to avoid larger and older rats, which puts them at a higherrisk of predation. Bad weather will generally discourage rats from doing anything outside their nests/shelters. Females forage in many short bouts, except when they are pregnant or lactating when they eat more and forage longer to maintain energy requirements for raising young; males forage less frequently but for longer periods at a time. The Brown Rat is mostly terrestrial, rarely climbing trees unless necessary; they are also excellent swimmers, which provides an advantage in sewersystems. It does not usually compete with the Roof Rat becauseit is generally more terrestrial than arboreal and scansorial Roof Rats, but at high densities in cities and rural areas, Brown Rats will outcompete Roof Rats and even steal their nests.

Movements, Home range and Social organization. Brown Rats nest communally with many individuals in a single large nest or with one individual in a small nest. A recent study found that when they are deprived of food, they socialize and share rather than fight over food. Cooperative behavior is a result of their hierarchal social structure in large nests with more dominant individuals. Nevertheless, fighting is common, and there can be a lack of cooperative behavior among members of a group, which leads to most individuals ignoring each other except for when mating or sleeping as a group in the nest. Dominance plays an important role in population regulation because relative dominance in a group hierarchy determines access to resources and mates. Larger individuals tend to be more dominant because they can overpower other individuals. Density is highest during late summer and early autumn. Brown Rats are cautious yet inquisitive. Exploration is common, but they can be neophobic toward food and other objects; individuals will cautiously approach and smell or touch an unfamiliar object and then dart away quickly. Barriers to rat movement cause home ranges of urban rats to be small compared with those in rural areas, but many other factors influence home range size, which is extremely variable. Dispersal from nests is generally very short, with individuals usually staying close to their natal nests. In cases of extreme environmental pressure, Brown Rats will move further away from their original nest sites and can make new nests. They make many sounds, including various chirps and squeaks (up to ten different sounds) and some ultrasonic sounds.

Status and Conservation. Classified as Least Concern on The IUCN Red List. Brown Rats probably spread from their native eastern Asia by stowing away in traded goods being sent to Europe and other parts ofAsia. Exact time when Brown Rats became commensal is unknown, but the 16" century seems to be when they entered and became established in Europe. They are found throughout much of North and South America, Australia, and New Zealand but are less common in Africa. Although the Brown Rat is considered a major pest throughout most of the world,it is also a very important model organism in research of all kinds and has been widely domesticated for the pet trade. The Brown Rat and its parasites are vectors of many viral, bacterial, and protozoan diseases. Bacterial infections include streptobacillosis, spirochetosis, Streptococcus pneumoniae infection, Enterococcus infection, pseudomoniasis, Clostridium piliforme infection, Corynebacterium kutscher: infection, and Salmonella infection; viral DNA diseases and infections include parvoviruses, rat cytomegalovirus, poxviruses, adenovirus, and papovavirus; and viral RNA diseases and infections include coronaviruses, paramyxoviruses, rotavirus and reovirus, and picornaviruses. Some common diseases that are rarely found in populations of Brown Rat include Cryptosporidium, Pasteurella, Listeria, Yersinia, Coxiella, Bartonella (including two species recently named from European populations of Brown Rat, Bartonella rattimassiliensis and B. phoceensis), and hantavirus, which are all very common in other species of rodents and particularly the Roof Rat (especially Yersinia pestis, the cause of bubonic plague). The Brown Rat is also regarded as an agricultural pest because it regularly eats commercial grains in rural regions. It is also known to cause damage in households and businesses and regularly consume human trash. Eradication programs have been attempted in many places, specifically Alberta (Canada), Rat Island (Alaska), New Zealand, and Faroe Islands. New Zealand is attempting to extirpate the Brown Rat because to its negative impact on many of ground-nesting birds. Brown Rats have extirpated numerous island endemic species of mammals, birds, and reptiles, especially because they eat eggs and can even hunt smaller prey (Roof Rats aremore prevalently on tropical islands than Brown Rats because they are better adapted to tropical habitats).

Bibliography. Aplin, Brown et al. (2003), Aplin, Chesser & ten Have (2003), Aulagnier et al. (2008), Balakirev & Rozhnov (2012), Barnett (1956, 1963), Barnett & Spencer (1951, 1953), Brown & Simpson (1981), Calhoun (1963), Cavia et al. (2009), Davis (1949, 1951), Davis & Christian (1958), Farahat et al. (2014), Feng & Himsworth (2014), Francis (2008), Gardner-Santana et al. (2009), Glass et al. (1988), Gundi et al. (2004), Inglis et al. (1996), Kawahara et al. (2004), Kingdon (2015), Menkhorst & Knight (2010), Musser & Carleton (2005), Reid (2006, 2009), Robins et al. (2007), Ruedas, A.R. (2016), Silver (1927), Smith & Yan Xie (2008), Steiniger (1950), Suckow et al. (2006), Tate (1936), Webster & Macdonald (1995), Yong (1969).