Editing Brown rat (section)

== Biology and behavior ==
[[File:The contemporary land mammals of Egypt (including Sinai) (1980) Fig. 85.png|thumb|left|Brown rat skull]]
The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders. Brown rats dig well, and often excavate extensive burrow systems. A 2007 study found brown rats to possess [[metacognition]], a mental ability previously only found in humans and some other primates,<ref>{{cite web |title=Rats Capable of Reflecting on Mental Processes |date=2007 |publisher=Science Daily |access-date=2 August 2007 |url=https://www.sciencedaily.com/releases/2007/03/070308121856.htm}}</ref> but further analysis suggested they may have been following simple operant conditioning principles.<ref>{{cite journal | last1 = Smith | first1 = J.D. | last2 = Beran | first2 = M.J. | last3 = Couchman | first3 = J.J. | last4=Coutinho | first4=M.V.C. |year=2008 |title = The Comparative Study of Metacognition: Sharper Paradigms, Safer Inferences |journal=Psychonomic Bulletin & Review |volume=15 |issue=4 |pages=679–691 |doi=10.3758/pbr.15.4.679 |pmid=18792496 |pmc=4607312 |url=http://pbr.psychonomic-journals.org/content/15/4/679.full.pdf+html | archive-url = https://archive.today/20130917210618/http://pbr.psychonomic-journals.org/content/15/4/679.full.pdf+html | archive-date = 2013-09-17}}</ref>

=== Communication ===
Brown rats are capable of producing ultrasonic vocalizations. As pups, young rats use different types of ultrasonic cries to elicit and direct maternal search behavior,<ref>{{cite journal |doi=10.1037/0735-7036.108.3.298 | pmid = 7924260 | last1 = Brunelli | first1 = S.A.| last2 = Shair | first2 = H.N.| year = 1994 | last3 = Hofer | first3 = M.A. | title = Hypothermic vocalizations of rat pups (Rattus norvegicus) elicit and direct maternal search behavior |journal = [[Journal of Comparative Psychology]] | volume = 108 | issue = 3| pages = 298–303}}</ref> as well as to regulate their mother's movements in the nest.<ref>{{cite journal | doi = 10.1016/0163-1047(92)90363-9 | pmid = 1456932 |last1=White, N. | last2 = Adox, R. |name-list-style=amp |year=1992 |last3=Reddy, A. |last4 = Barfield, R. | title = Regulation of rat maternal behavior by broadband pup vocalizations |journal = [[Behavioral and Neural Biology]] | volume = 58 |issue = 2| pages = 131–137}}</ref> Although pups produce ultrasounds around any other rats at the age of 7 days, by 14 days old they significantly reduce ultrasound production around male rats as a defensive response.<ref>{{cite journal |doi=10.1037/0735-7036.106.1.69 | last1 = Takahashi | first1 = L.K. |year = 1992 | title = Developmental expression of defensive responses during exposure to conspecific adults in preweanling rats (''Rattus norvegicus'') |url=https://zenodo.org/record/1231486| journal = Journal of Comparative Psychology | volume = 106 | issue = 1| pages = 69–77 | pmid = 1313347}}</ref> Adult rats will emit ultrasonic vocalizations in response to predators or perceived danger;<ref>{{cite journal | doi = 10.1007/s10519-004-0858-3 | pmid = 15674535 | last1 = Brudzynski, S.M. | year = 2005 |title=Principles of Rat Communication: Quantitative Parameters of Ultrasonic Calls in Rats |url=http://www.cursoecologia.ufba.br/Arquivos/Comunicacao_Animal/rat_communication.pdf | journal = [[Behavior Genetics (journal)|Behavior Genetics]] | volume = 35 | issue = 1 | pages = 85–92 | citeseerx = 10.1.1.472.2300 | s2cid = 15888375 | access-date = 1 November 2017 | archive-url = https://web.archive.org/web/20170922005839/http://www.cursoecologia.ufba.br/Arquivos/Comunicacao_Animal/rat_communication.pdf | archive-date = 22 September 2017 }}</ref> the frequency and duration of such cries depends on the sex and reproductive status of the rat.<ref>{{cite journal | doi = 10.1037/0735-7036.106.3.270 | last1 = Blanchard | first1 = R.J. | last2 = Agullana | first2 = R. |name-list-style=amp | year = 1992 | last3=McGee |first3=L. |last4=Weiss | first4 = S. | last5 = Blanchard | first5 = D.C. | title = Sex differences in the incidence and sonographic characteristics of antipredator ultrasonic cries in the laboratory rat (''Rattus norvegicus'') |journal=Journal of Comparative Psychology | volume = 106 | issue = 3| pages = 270–277 | pmid = 1395496}}</ref><ref>{{cite journal | doi = 10.1037/0735-7036.107.4.373 | last1 = Haney | first1 = M. | last2 = Miczek |first2=K.A. | year = 1993 | title = Ultrasounds during agonistic interactions between female rats (''Rattus norvegicus'') | journal=Journal of Comparative Psychology |volume=107 |issue=4| pages=373–379 |pmid = 8112049}}</ref> The female rat also emit ultrasonic vocalizations during mating.<ref>{{cite journal | doi = 10.1016/S0003-3472(85)80002-6 | last1=Thomas |first1=D.A. | last2=Barfield | first2 = R.J. |name-list-style=amp |year=1985 |title=Ultrasonic vocalization of the female rat (''Rattus norvegicus'') during mating |journal=[[Animal Behaviour (journal)|Animal Behaviour]] |volume=33 | issue=3| pages=720–725 |s2cid=53193219}}</ref>

Rats may also emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, before receiving [[morphine]], or mating, and when tickled. The vocalization, described as a distinct "chirping", has been likened to [[laughter]], and is interpreted as an expectation of something rewarding.<ref>{{cite web|url=http://www.sciencenews.org/articles/20010728/fob9.asp|archive-url=https://web.archive.org/web/20040505115335/http://sciencenews.org/articles/20010728/fob9.asp|archive-date=2004-05-05|title=Search Content|work=Science News}}</ref> Like most rat vocalizations, the chirping is too high in pitch for humans to hear without special equipment. [[Bat detector]]s are often used by pet owners for this purpose.

In research studies, the chirping is associated with positive emotional feelings, and social bonding occurs with the tickler, resulting in the rats becoming conditioned to seek the tickling. However, as the rats age, the tendency to chirp appears to decline.<ref>{{cite journal |doi=10.1016/S0031-9384(03)00159-8 |last1=Panksepp |pmid= 12954448 |first1=J. |last2=Burgdorf |first2 = J. |name-list-style=amp |title="Laughing" rats and the evolutionary antecedents of human joy? |year=2003 |pages=533–47 |issue= 3 |volume=79 |journal=Physiology & Behavior |s2cid =14063615 |url=http://www.psych.umn.edu/courses/fall06/macdonalda/psy4960/Readings/PankseppRatLaugh_P&B03.pdf}}</ref>

Brown rats also produce communicative noises capable of being heard by humans. The most commonly heard in domestic rats is bruxing, or teeth-grinding, which is most usually triggered by happiness, but can also be 'self-comforting' in stressful situations, such as a visit to the vet. The noise is best described as either a quick clicking or 'burring' sound, varying from animal to animal. Vigorous bruxing can be accompanied by boggling, where the eyes of the rat rapidly bulge and retract due to movement of the lower jaw muscles behind the eye socket.<ref>{{cite web|url=https://petpartners.org/wp-content/uploads/2015/07/Rat-Behavior-Packet.pdf | title = Rat Behaviour Packet }}</ref>

In addition, they commonly squeak along a range of tones from high, abrupt pain squeaks to soft, persistent 'singing' sounds during confrontations.

=== Diet ===
[[Image:RatEatingSunflowerseads.jpg|thumb|right|A brown rat eating sunflower seeds]]
The brown rat is a true omnivore and consumes almost anything, but cereals form a substantial part of its diet. The most-liked foods of brown rats include [[scrambled eggs]], raw carrots, and cooked corn kernels. The least-liked foods are raw beets, peaches and raw celery.<ref>{{cite journal |last=Schein |first=M.W. |author2=Orgain, H. |title=A Preliminary Analysis of Garbage as Food for the Norway Rat |journal=The American Journal of Tropical Medicine and Hygiene |volume=2 |issue=6 |pages=1117–1130 |pmid=13104820 |date=1953 |doi=10.4269/ajtmh.1953.2.1117}}</ref>
Foraging behavior is often population-specific, and varies by environment and food source.<ref name="Fragaszy, Dorothy Munkenbeck 2003 pp. 165">{{cite book |last1=Fragaszy |first1= D.M. |first2= S. |last2=Perry |year=2003 |title=The Biology of Traditions: Models and Evidence |url=https://archive.org/details/biologytradition00frag |url-access=limited |publisher=Cambridge University Press |page=165 |isbn=978-0-521-81597-0 }}</ref> Brown rats living near a hatchery in [[West Virginia]] catch fingerling fish.<ref>{{cite journal |last1=Cottam |first1=C. |author-link=Clarence Cottam|year=1948 |last2=Stickel |first2=W.H. |last3=Stickel |first3=L.F. |last4=Coleman |first4=R.H. | last5=Mickey |first5=A.B. |last6=Schellbach |first6=L. |last7=Schorger |first7=A.W. |last8=Negus |first8=N.C. |last9=Polderboer |first9=E.B. |title=Aquatic habits of the Norway rat |jstor=1375396 |journal=[[Journal of Mammalogy]] |volume=29 |issue=3 |page=299 |doi=10.1093/jmammal/29.3.299}}</ref> 
Some colonies along the banks of the [[Po River]] in Italy dive for [[mollusks]],<ref>{{cite journal |last1=Gandolfi |first1=G. |last2=Parisi |first2=V. |year=1972 |title=Predazione su Unio Pictorum L. da parte del ratto, ''Rattus norvegicus'' (Berkenhout) |journal=Acta Naturalia |volume=8 |pages=1–27}}</ref><ref>{{cite journal |last1=Parisi |first1=V. |last2=Gandolfi | first2 = G. |year=1974 |title=Further aspects of the predation by rats on various mollusc species | journal =Bollettino di Zoologia |volume=41 |issue=2 |pages=87–106 |doi=10.1080/11250007409430096 |doi-access=free}}</ref> a practice demonstrating social learning among members of this species.<ref>{{cite journal |doi=10.1037/h0077678 |last1=Galef |first1=Jr. |last2=Bennett |first2=G. |year=1980 |title=Diving for Food: Analysis of a Possible Case of Social Learning in Wild Rats (''Rattus norvegicus'') |url=http://www.sociallearning.info/storage/pdf/comp%20psych%20943%20416-425.pdf |journal=[[Journal of Comparative and Physiological Psychology]] |volume=94 |issue=3 |pages=416–425}}</ref> Rats on the island of [[Norderoog]] in the [[North Sea]] stalk and kill sparrows and ducks.<ref>{{cite journal |last1=Steiniger |first1=F. |year=1950 |title=Beitrage zur Sociologie und sonstigen Biologie der Wanderratte | journal=Zeitschrift für Tierpsychologie |volume=7 |issue=3 |pages=356–79 |doi=10.1111/j.1439-0310.1950.tb01630.x|bibcode=1950Ethol...7..356S }}</ref>

Also preyed upon by brown rats are chicks, mice and small lizards. Examination of a wild brown rat stomachs in Germany revealed 4,000 food items, most of which were plants, although studies have shown that brown rats prefer meat when given the option. In metropolitan areas, they survive mainly on discarded human food and anything else that can be eaten without negative consequences.<ref name=ADW>{{cite web |url=https://animaldiversity.org/accounts/Rattus_norvegicus/ |title=Rattus norvegicus (Brown rat) |website=[[Animal Diversity Web]]}}</ref>

=== Reproduction and life cycle ===
[[Image:Neonatal rats huddle.jpg|thumb|right|Newborn brown rats]]

The brown rat can breed throughout the year if conditions are suitable, with a female producing up to five litters a year. The [[gestation]] period is only 21 days, and litters can number up to 14, although seven is common. They reach sexual maturity in about five weeks. Under ideal conditions (for the rat), this means that the population of females could increase by a factor of three and a half (half a litter of 7) in 8 weeks (5 weeks for sexual maturity and 3 weeks of gestation), corresponding to a population growing by a factor of 10 in just 15 weeks. As a result, the population can grow from 2 to 15,000 in a year.<ref name=":7">{{cite journal|title=Menopause-causing bait is curbing rat populations in New York|journal=New Scientist|date= 2017 |url=https://www.newscientist.com/article/mg23431254-600-menopausecausing-bait-is-curbing-rat-populations-in-new-york/}}</ref> The [[maximum life span]] is three years, although most barely manage one. A yearly mortality rate of 95% is estimated, with predators and interspecies conflict as major causes.

When lactating, female rats display a 24-hour rhythm of maternal behavior, and will usually spend more time attending to smaller litters than large ones.<ref>{{cite journal | doi = 10.1016/S0003-3472(69)80019-9 |last1=Grota |first1=L.J. |last2=Ader |first2=R. |name-list-style=amp |year=1969 |title=Continuous recording of maternal behavior in ''Rattus norvegicus'' |journal=Animal Behaviour |volume=17 |issue=4 |pages=722–729}}</ref>

Brown rats live in large, hierarchical groups, either in burrows or subsurface places, such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.<ref>{{Cite web|url=http://envirotroldfw.com/rodents.html|title=Envirotrol Pest Management Systems - Rodent Control|website=envirotroldfw.com|access-date=2019-10-24|archive-url=https://web.archive.org/web/20191024233916/http://envirotroldfw.com/rodents.html|archive-date=24 October 2019}}</ref>

The female is capable of becoming pregnant immediately after giving birth, and can nurse one [[Litter (animal)|litter]] while pregnant with another. She is able to produce and raise two healthy litters of normal size and weight without significantly changing her own food intake. However, when food is restricted, she can extend pregnancy by over two weeks, and give birth to litters of normal number and weight.<ref name=":0">{{Cite journal |last1=Woodside, B. |last2=Wilson, R. |last3=Chee, P. |last4=Leon, M. |name-list-style=amp |year=1981|title=Resource partitioning during reproduction in the Norway rat|journal= Science|volume=211|issue=4477|pages=76–77 |doi=10.1126/science.7444451 |pmid=7444451 |bibcode=1981Sci...211...76W}}</ref>

=== Mating behaviors ===
Males can ejaculate multiple times in a row, and this increases the likelihood of pregnancy as well as decreases the number of stillborns.<ref name=":1">{{Cite journal |last1=Davis|first1=H.N. |last2=Connor|first2=J.R. |name-list-style=amp |year=1980 |title=Brief Report: Male modulation of female reproductive physiology in Norway rats: effects of mating during postpartum estrus |journal=Behavioral and Neural Biology|volume=29 |issue=1 |pages=128–131 |doi=10.1016/s0163-1047(80)92582-0|pmid=7387582}}</ref> Multiple ejaculation also means that males can mate with multiple females, and they exhibit more ejaculatory series when there are several oestrous females present.<ref name=":5" /> Males also copulate at shorter intervals than females.<ref name=":5">{{Cite journal|last1=McClintock|first1=M.K. |last2=Anisko|first2=J.J. |name-list-style=amp |year=1982 |title=Group mating among Norway rats I. Sex differences in the pattern and neuroendocrine consequences of copulation|journal=Animal Behaviour|volume=30 |issue=2|pages=398–409|doi=10.1016/s0003-3472(82)80051-1|s2cid=53201526}}</ref> In group mating, females often switch partners.<ref name=":2">{{Cite journal |last1=McClintock|first1=M.K. |last2=Anisko|first2=J.J. |last3=Adler|first3=N.T. |name-list-style=amp |year=1982 |title=Group mating among Norway rats II. The social dynamics of copulation: Competition, cooperation, and mate choice|journal=Animal Behaviour |volume=30 |issue=2 |pages=410–425 |doi=10.1016/s0003-3472(82)80052-3 |s2cid=53145394}}</ref>

Dominant males have higher mating success and also provide females with more ejaculate, and females are more likely to use the sperm of dominant males for fertilization.<ref name=":2" />

In mating, female rats show a clear mating preference for unknown males versus males that they have already mated with (also known as the [[Coolidge effect]]), and will often resume copulatory behavior when introduced to a novel sexual partner.<ref name=":3">{{Cite journal|last1=Ventura-Aquino|first1=E. |name-list-style=amp |last2=Banos-Araujo|first2=J. |last3=Fernandez-Guasti|first3=A. |last4=Parades|first4=R.G. |year=2016|title=An unknown male increases sexual incentive motivation and partner preference: Further evidence for the Coolidge effect in female rats |journal=Physiology & Behavior|volume=158|pages=54–59 |doi=10.1016/j.physbeh.2016.02.026 |pmid=26902417 |s2cid=7695527}}</ref>

Females also prefer to mate with males who have not experienced [[social stress]] during adolescence, and can determine which males were stressed even without any observed difference in sexual performance of males experiencing stress during adolescence and not.<ref name=":4">{{Cite journal |last1=McCormick|first1=C.M. |last2=Cameron|first2=C.M. |name-list-style=amp |last3=Thompson|first3=M.A. |last4=Cumming|first4=M.J. |last5=Hodges|first5=T.E. |last6=Langett|first6=M. |year=2017|title=The sexual preference of female rats is influenced by males' adolescent social stress history and social status|journal=Hormones and Behavior |volume=89 |pages=30–37|doi=10.1016/j.yhbeh.2016.12.001|pmid=27956227|s2cid=4228451}}</ref>

=== Social behavior ===
Rats commonly [[Social grooming|groom each other]] and sleep together.<ref>{{cite web|url=http://members.madasafish.com/~cj_whitehound/Rats_Nest/Norway_Rats/Social_behaviour.htm|title=Social behaviour of fancy rat|work=madasafish.com}}</ref> Rats are said to establish an order of hierarchy, so one rat will be dominant over another one.<ref>{{cite web|url=http://www.petwebsite.com/rats/rat_behavior.htm|title=Pet Rats: behaviour of the rat|access-date=22 August 2008|archive-url=https://web.archive.org/web/20170623110834/http://www.petwebsite.com/rats/rat_behavior.htm|archive-date=23 June 2017}}</ref> Groups of rats tend to "play fight", which can involve any combination of jumping, chasing, tumbling, and "boxing". Play fighting involves rats going for each other's necks, while serious fighting involves strikes at the others' back ends.<ref name="behavior">{{cite web|url=http://www.ratbehavior.org/norway_rat_ethogram.htm#PlayFighting|title=Norway Rat Behavior Repertoire|work=ratbehavior.org}}</ref> If living space becomes limited, rats may turn to aggressive behavior, which may result in the death of some animals, reducing the burden over the living space.

Rats, like most mammals, also form family groups of a mother and her young.<ref name="Barnett, S. 1975 pp. 52-115">Barnett, S. (1975). The Rat: a study in behavior (pp. 52–115). Chicago, MI: The University of Chicago Press.</ref> This applies to both groups of males and females. However, rats are territorial animals, meaning that they usually act aggressively towards or scared of strange rats. Rats will fluff up their hair, hiss, squeal, and move their tails around when defending their territory.<ref name="National Fancy Rat Society">{{cite web|title=Why Rats Need Company|url=http://www.nfrs.org/company.html#q6|work=National Fancy Rat Society|access-date=11 December 2014|archive-url=https://web.archive.org/web/20161124222202/http://www.nfrs.org/company.html#q6|archive-date=24 November 2016}}</ref> Rats will chase each other, groom each other, sleep in group nests, wrestle with each other, have dominance squabbles, communicate, and play in various other ways with each other.<ref name="National Fancy Rat Society" /> Huddling is an additional important part of rat socialization. Huddling, an extreme form of herding and like chattering or "bruxing" is often used to communicate that they are feeling threatened and not to come near. The common rat has been more successful at inhabiting and building communities on 6 continents and are the only species to have occupied more land than humans.

During the wintry months, rats will huddle into piles – usually cheek-to-cheek – to control humidity and keep the air warm as a heat-conserving function. Just like elderly rats are commonly groomed and nursed by their companions, nestling rats especially depend on heat from their mother, since they cannot regulate their own temperature. Other forms of interaction include: crawling under, which is literally the act of crawling underneath one another (this is common when the rat is feeling ill and helps them breathe); walking over to find a space next to their closest friend, also explained in the name; allo-grooming, so-called to distinguish it from self-grooming; and nosing, where a rat gently pushes with its nose at another rat near the neck.<ref name="Barnett, S. 1975 pp. 52-115"/>

=== Burrowing ===
Rats are known to burrow extensively, both in the wild and in captivity, if given access to a suitable [[Substrate (biology)|substrate]].<ref name="Boice">{{cite journal | doi = 10.1037/h0077338 | last1 = Boice | first1 = R. | year = 1977 | title = Burrows of Wild and Albino Rats: Effects of Domestication, Outdoor Raising, Age, Experience, and Maternal State | journal = Journal of Comparative and Physiological Psychology | volume = 91 | issue = 3| pages = 649–661 | pmid = 559696 }}</ref> Rats generally begin a new burrow adjacent to an object or structure, as this provides a sturdy "roof" for the section of the burrow nearest to the ground's surface.<ref name="Calhoun">Calhoun, J. B. (1962) Bethesda, MD: U.S. Department of Health, Education, and Welfare.</ref> Burrows usually develop to eventually include multiple levels of tunnels, as well as a secondary entrance.<ref name="Boice" /> Older male rats will generally not burrow, while young males and females will burrow vigorously.<ref name="Boice" /><ref>{{cite journal | doi = 10.2307/1379585 |last1=Price |first1=A. O. |year=1977 | title = Burrowing in Wild and Domestic Norway Rats | jstor=1379585| journal = Journal of Mammalogy | volume = 58 | issue = 2| pages = 239–240}}</ref>

Burrows provide rats with shelter and food storage, as well as safe, thermo-regulated nest sites.<ref name="Boice" /> Rats use their burrows to escape from perceived threats in the surrounding environment; for example, rats will retreat to their burrows following a sudden, loud noise or while fleeing an intruder.<ref>{{cite journal | doi = 10.1016/0376-6357(94)90034-5 | pmid = 24897414 | last1 = Kitaoka | first1 = A. | year = 1994 | title = Defensive aspects of burrowing behavior in rats (Rattus norvegicus): A descriptive and correlational study | journal = Behavioural Processes | volume = 31 | issue = 1 | pages = 13–28 | s2cid = 23501112}}</ref> Burrowing can therefore be described as a "pre-encounter defensive behavior", as opposed to a "post-encounter defensive behavior", such as flight, freezing, or avoidance of a threatening stimulus.