Editing Brown rat (section)

=== 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>