Polar bears (Ursus maritimus) give birth to relatively few offspring compared to other mammals. A female will have an average of 2-4 litters in their lifetime, each consisting of 1-3 cubs. Meanwhile, a lemming can have 192 offspring during its lifetime1 (source: BBC Discover Wildlife). To understand this, we need to learn more about their reproductive system. So, how do polar bears reproduce?
Polar bears are polygynous and will reproduce with many partners. Males and females will mate for a week during spring until the egg is fertilized. However, the egg will not enter the ovaries until later in the year when the females have entered a maternity den.
Keep reading for more information about each stage of a polar bear’s reproductive cycle and answers to some popular questions about mating behavior.
Reproductive Cycle of Polar Bears
The reproductive cycle of polar bears can be broken down into 4 key stages:
- Spring – Mating
- Fall – Denning
- Winter – Birth
- Spring – Emerging from the den
In the rest of this article, we’ll take a look at each of these stages in more detail, or you can watch the video version here:
Polar bears are polygynous and often described as promiscuous3 (source: Canadian Journal of Zoology, E. Zeyl, et al, Vol 87, No. 12, 2009). It is rare that they will mate with the same bear in different years. Male polar bears are not paternal and only stay with the female for a few days after mating.
One of the adaptations that help polar bears survive is an excellent sense of smell which they use for hunting prey as well as tracking females who are potential mating partners. As males are looking to mate every year but females only mate every 3 years when their young have grown up, competition is high among males.
There is a lot of fighting between males during mating season, often resulting in wounding, scarring, and breakage of canine teeth4 (source: Canadian Journal of Zoology, M.A. Ramsay and I. Stirling, Vol 64, No. 10, 1986).
The larger males have an advantage in these battles which explains how male polar bears have evolved to be much larger than females.
A male might follow a female for many miles and, according to Jon Aars, a researcher from the Norwegian Polar Institute, they might follow them for many miles and even attempt to herd them to isolated areas away from other males5 (source: Live Science).
Female polar bears often mate with several males in the same year which can result in giving birth to offspring from different fathers at the same time6 (source: Canadian Journal of Zoology, E.Z. Fiskebeck, et al, Vol 87, No. 12, 2009).
Bears have a reproductive strategy called embryonic diapause (also known as delayed implantation) where the fertilized egg will not immediately enter the uterus but remains dormant until later in the year.
The purpose of this is to give females the chance to build up food stores before implantation ready for the maternity den. It also allows newborn cubs maximum time to develop when they emerge from the den before the next winter.
Pregnant females who have built up enough fat reserves will enter a maternity den where they will remain until they have given birth and the cubs have grown enough to leave the den.
The maternity den is a small cave that they dig into snowdrifts on the sea ice or raised peat soil alongside lakes and rivers. In a survey of polar bear dens in the Alaskan Beauford Sea, just under two-thirds were found to be on the sea ice with the remainder inland7 (source: Polar Bears: A Complete Guide to Their Biology and Behavior, A. E. Derocher, 2012).
Although polar bears do not hibernate, whilst in the maternity den, they will enter a ‘hibernation-like’ state where they will not eat, drink, urinate, or defecate for the duration of their time in the den8 (source: Polar Bears: A Complete Guide to Their Biology and Behavior, A. E. Derocher, 2012).
After entering the den in fall, they will give birth around 60 days later in winter, usually between mid-November and January9 (source: Marine Ecology Progress Series, A. Derocher, et al, 2011).
The average gestation period (the time between conception and birth) of a polar bear is about 8 months. Females will give birth to between 1-3 cubs. The cubs will be about 30cm (12 inches) long and weigh between 454-680g10 (source: The Polar Bear, A. Hemstock, 1999).
Larger litter sizes will usually result in smaller cubs. In a triplet, there tends to be one very tiny cub and two larger cubs. The tiny cub usually won’t survive until adulthood11 (source: Polar Bears: A Complete Guide to Their Biology and Behavior, A. E. Derocher, 2012).
When they are born, they will only have a thin covering of fur and a limited capacity to keep themselves warm so they must remain in the den for several months12 (source: Marine Ecology Progress Series, A. Derocher, et al, 2011).
Emerging from the Den (Spring)
The mother will remain in her den until the cubs are have learned to walk and are ready to leave, this could be a total of 5-7 months from initially building it. Emergence from the den usually occurs between February and May14 (source: Marine Ecology Progress Series, A. Derocher, et al, 2011).
On average, the size of polar bear cubs leaving the den will weigh around 10kg15 (source: Arctic Animals and Their Adaptations to Life on the Edge, A. S. Blix, 2005), although other estimates put it anywhere between 3kg (6.6lbs) and 24.5kg (54 lbs).
They will stay in the area around the den for between 1-4 weeks depending upon the progress of the cubs16 (source: Polar Bears: A Complete Guide to Their Biology and Behavior, A. E. Derocher, 2012). After this, they will start to move across the sea ice where the mother will teach them all the skills to survive, including hunting for food.
They will be vulnerable to hypothermia if they attempt to swim after leaving the den so the mother will attempt to find routes across ice rather than the sea17 (source: Marine Ecology Progress Series, A. Derocher, et al, 2011).
How Many Cubs Do Polar Bears have?
Polar bears reach the adult stage of their lifecycle when they become sexually mature. This varies between regions and studies but most scientists suggest that males reach maturity between ages 5-10 and females between ages 4-6.
A 2009 study in the Barents sea area (north of Norway) suggests that females had 2-4 litters in their lifetime, each consisting of 1-3 cubs and that males father offspring every 3.9 years on average, although both of these were based on relatively small sample sizes.
This level of productivity is fairly common among bears, for example, pandas give birth to around 7 offspring during their lives. However, this is still minuscule compared to other mammals such as Norway lemmings that can produce 192 offspring and European rabbits that can have up to 320 offspring in their lives 18(source: BBC Discover Wildlife).
Do Polar Bears Lay Eggs or Give Birth?
Polar bears are viviparous animals which means they give birth, rather than laying an egg. The cubs develop inside the mother for around 60 days before they give birth.
Do Polar Bears Have Periods?
No, polar bears are induced ovulators, this means that they do not have periods which are associated with cyclical ovulation. In polar bears, the act of mating causes the egg to be released. This can take several attempts so bears will mate for many days before successful fertilization.
What Age Do Polar Bears Stop Having Babies?
Male polar bears tend to stop having cubs at age 19 and females stop having cubs at age 2019 (source: San Diego Zoo Wildlife Alliance Library), although the lifespan of a polar bear isn’t much longer than this anyway. Females will mother between 2-4 litters in their lifetime.