Editing Polar bear (section)

===Reproduction and development===
[[File:Встреча 2.jpg|thumb|right|Courting male approaching female]]
Polar bear mating takes place on the sea ice and during spring, mostly between March and May.<ref name="DeMaster1981"/><ref name=Ramsay1986>{{cite journal|last1=Ramsay|first1=M. A.|last2=Stirling|first2=I.|year=1986|title=On the mating system of polar bears|journal=Canadian Journal of Zoology|volume=64|issue=10|pages=2142–2151|doi=10.1139/z86-329|bibcode=1986CaJZ...64.2142R }}</ref><ref name=Derocher2010/>{{sfn|Stirling|2011|p=105}} Males search for females in [[estrus]] and often travel in twisting paths which reduces the chances of them encountering other males while still allowing them to find females. The movements of females remain linear and they travel more widely.<ref>{{cite journal|last1=Laidre|first1=K. L.|last2=Born|first2=E. W.|last3=Gurarie|first3=E.|last4=Wiig|first4=Ø|last5=Dietz|first5=R.|last6=Stern|first6=H.|year=2013|title=Females roam while males patrol: divergence in breeding season movements of pack-ice polar bears (''Ursus maritimus'')|journal=Proceedings of the Royal Society B: Biological Sciences|volume=280|issue=1752 |page=20122371|doi=10.1098/rspb.2012.2371|pmid=23222446 |pmc=3574305 }}</ref> The mating system can be labelled as female-defence [[Polygyny in animals|polygyny]], [[serial monogamy]] or [[Promiscuity#Other animals|promiscuity]].<ref name=Derocher2010>{{cite journal|last1=Derocher|first1=A. E.|last2=Anderson|first2=M.|last3=Wiig|first3=Ø|last4=Aars|first4=J.|year=2010|title=Sexual dimorphism and the mating ecology of polar bears (Ursus maritimus) at Svalbard (''Ursus maritimus'') at Svalbard|journal=Behavioral Ecology and Sociobiology|volume=64|issue=6|pages=939–946|doi=10.1007/s00265-010-0909-0|s2cid=36614970 }}</ref><ref name=Zey2009/>

Upon finding a female, a male will try to isolate and guard her. Courtship can be somewhat aggressive, and a male will pursue a female if she tries to run away. It can take days for the male to mate with the female which [[Induced ovulation (animals)|induces ovulation]]. After their first copulation, the couple bond. Undisturbed polar bear pairings typically last around two weeks during which they will sleep together and mate multiple times.{{sfn|Derocher|2012|pp=141, 145–147}} Competition for mates can be intense and this has led to [[sexual selection]] for bigger males. Polar bear males often have scars from fighting.<ref name=Ramsay1986/><ref name=Derocher2010/> A male and female that have already bonded will flee together when another male arrives.{{sfn|Derocher|2012|pp=145–147}} A female mates with multiple males in a season and a single litter can have more than one father.<ref name=Zey2009>{{cite journal|last1=Zeyl|first1=E.|last2=Aars|first2=J.|last3=Ehrich|first3=D.|last4=Bachmann|first4=L.|last5=Wiig|first5=Ø|year=2009|title=The mating system of polar bears: a genetic approach|journal=Canadian Journal of Zoology|volume=87|issue=12|pages=1195–1209|doi=10.1139/Z09-107|bibcode=2009CaJZ...87.1195Z }}</ref> 
[[File:Polar bear cubs in the snow.jpg|thumb|left|Polar bear cubs]]
When the mating season ends, the female will build up more fat reserves to sustain both herself and her young. Sometime between August and October, the female constructs and enters a [[maternity den]] for winter. Depending on the area, maternity dens can be found in sea ice just off the coastline or further inland and may be dug underneath snow, earth or a combination of both.{{sfn|Stirling|2011|pp=112, 115, 120}} The inside of these shelters can be around {{cvt|1.5|m}} wide with a ceiling height of {{cvt|1.2|m}} while the entrance may be {{cvt|2.1|m}} long and {{cvt|1.2|m}} wide. The temperature of a den can be much higher than the outside.{{sfn|Ellis|2009|p=85}} Females hibernate and give birth to their cubs in the dens.{{sfn|Derocher|2012|pp=28, 155}} Hibernating bears [[fasting|fast]] and internally recycle bodily waste. Polar bears experience [[delayed implantation]] and the fertilized embryo does not start development until the fall, between mid-September and mid-October.{{sfn|Stirling|2011|p=124}} With delayed implantation, [[gestation]] in the species lasts seven to nine months but actual pregnancy is only two months.{{sfn|Derocher|2012|p=171}}

Mother polar bears typically give birth to two cubs per litter. As with other bear species, newborn polar bears are tiny and [[Precociality and altriciality|altricial]].{{sfn|Stirling|2011|pp=124–125, 131}} The newborns have woolly hair and pink skin, with a weight of around {{cvt|600|g}}.<ref name="DeMaster1981"/>{{sfn|Derocher|2012|p=10}} Their eyes remain closed for a month.{{sfn|Ellis|2009|p=84}} The mother's fatty milk fuels their growth, and the cubs are kept warm both by the mother's body heat and the den. The mother emerges from the den between late February and early April, and her cubs are well-developed and capable of walking with her.{{sfn|Stirling|2011|pp=126–127}} At this time they weigh {{cvt|10|–|15|kg}}.<ref name="DeMaster1981"/> A polar bear family stays near the den for roughly two weeks; during this time the cubs will move and play around while the mother mostly rests. They eventually head out on the sea ice.{{sfn|Stirling|2011|p=128}}
[[File:Cub polar bear is nursing 2.OGG|thumb|right|Mother nursing her young]]
Cubs under a year old stay close to their mother. When she hunts, they stay still and watch until she calls them back.{{sfn|Derocher|2012|pp=173, 184}} Observing and imitating the mother helps the cubs hone their hunting skills.{{sfn|Stirling|2011|p=186}} After their first year they become more independent and explore. At around two years old, they are capable of hunting on their own.{{sfn|Derocher|2012|p=184}} The young suckle their mother as she is lying on her side or sitting on her rump.{{sfn|Stirling|2011|p=128}} A [[lactating]] female cannot conceive and give birth,{{sfn|Derocher|2012|p=181}} and cubs are weaned between two and two-and-a-half years.<ref name="DeMaster1981"/> She may simply leave her weaned young or they may be chased away by a courting male.{{sfn|Derocher|2012|p=184}} Polar bears reach sexual maturity at around four years for females and six years for males.{{sfn|Stirling|2011|pp=128–129}} Females reach their adult size at 4 or 5 years of age while males are fully grown at twice that age.{{sfn|Derocher|2012|p=185}}