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LIFE STAGES - Editorial Comment

Editorial Comment

(Editorial Overview Text Replicated on Overall Species page - Ursus maritimus - Polar bear)

BREEDING SEASON: The mating season is spring, from March to June.

OESTRUS/OVULATION: Polar bears appear to be induced ovulators, with several copulations over a period of days needed for ovulation.

GESTATION/PREGNANCY: Polar bears have delayed implantation. The total gestation length is 195 - 265 days including the period before implantation. The active gestation period lasts only about 60 days.

PARTURITION/BIRTH: In the wild, females enter the maternity den up to three months before parturition. In zoos, females may show nesting behaviour in the two to three weeks before birth, and are uncomfortable (lying down then standing up again) immediately before birth. They give birth quite rapidly, standing or sitting, and immediately lick the cub. If twins are born there may be about 37 to 273 minutes between births. Births occur in November to January.

NEONATAL/DEVELOPMENT: Polar bear cubs are altricial; at birth they are covered with fine white hair, but are blind, the eyes closed, and they lack subcutaneous fat. Data from zoo-born cubs indicates that the eyes open at 24 - 42 days, teeth first erupt by 35 - 50 days and they start to walk at 46 - 60 days. They start to accompany the dam by about 90 days and also can take some solid food by this age; this is an important time for learning. In the wild, cubs emerge from the den and follow their mother from late March/early April, weighing about 10 - 15 kg. They are weaned at about 24 - 28 months. In southern areas cubs may be able to hunt well enough to be self sufficient from 1.5 years but in northern areas where the ice is thicker this may not be possible until they reach 2.5 years. Cubs usually remain with their mother for 2.5 years. In the 1970s and early 1980s, as many as 60% of cubs in the Western Hudson Bay area were independent as yearlings (18-20 months old) in some years, but this has declined and now weaning at 1.5 rather than 2.5 years is uncommon anywhere.

LITTER SIZE:  Litter size is one to three cubs, very rarely four; two may be usual and is common in captivity. Reported average litter sizes in the wild range from 1.58 - 2.0. The largest, heaviest females, eight to 15 years old, produce the largest litters.

TIME BETWEEN LITTERS / LITTERS PER YEAR: Polar bears breed every two to four years, usually every three years. Previously, breeding often occurred every two years in the lower Hudson Bay area, but this has changed recently.

LACTATION / MILK PRODUCTION:  Cubs are suckled to about 24 - 28 months. Milk production peaks while cubs are still in the maternity den, or soon after they emerge, then declines over a prolonged period.

SEXUAL MATURITY: Females may be seen pairing at 3.5 years in some areas, with first cubs born when the female is four, but usually the first mating is at four to seven years, with the first litter born when the female is five to eight. Males may be sexually mature in terms of sperm production by three years of age, but are unlikely to compete successfully for a female, and therefore to actually breed, before six years old.

MALE SEASONAL VARIATION: Male polar bears show yearly regression and recrudescence of the testes. Spermatogenesis occurs from February to May or possibly June; testes are largest during the breeding season (e.g. in May), but smaller by October. There are also seasonal variations in hormone levels such as testosterone, LH and prolactin, all of which are high in April and low in October.

LONGEVITY / MORTALITY:  Maximum longevity has been suggested at 20 - 25 years, or 25 - 30 years, and 45 years has been reported for a zoo bear. Annual pre-weaning mortality of cubs is about 10 - 30%; annual mortality rates for adults are 8 to 16 % and for younger animals may be 20%. A critical time is when the cubs first become independent. It is common for at least one member of large litters (three cubs) not to survive to weaning. Mortality is due to starvation, intraspecific predation and accidents. Starvation is the most important cause of death, particularly for inexperienced and physically disabled bears when seal numbers drop suddenly.

(References are available in detailed literature reports below)

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Breeding Season

Source Information

SUMMARY: The mating season is spring, from March to June.
  • Bears have been observed forming pairs in Alaska in March to May in Alaska and in Spitzbergen 8 March to 20 June. (D244)
  • March to June. (B147)
  • April and May. (B285.w4)
  • April to May. (B180.w4)
  • April to May, just before the period of greatest food availability. (J371.38.w2)
  • Spring. (B336.51.w51)
  • March and April. (B288.w11)
  • In zoos of the Soviet Union, mating were observed 25th January to 13th June, with mating for as short as one day up to as long as 43 days (average 11.2 days, n =104). (P74.1989.w2)
  • Late March to early May. (B406.37.w37)
  • Pairs have been seen together from as early as 8th March to as late as 20th June. (B490.27.w27)

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Oestrus / Ovulation

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SUMMARY: Polar bears appear to be induced ovulators, with several copulations over a period of days needed for ovulation.
  • Oestrus lasts three days. (B147)
  • Induced ovulators; several matings over a period of days are required for ovulation. Pairs remain together for one to two weeks. One male may displace another male, in which case the female will mate with more than one male. (B285.w4)
  • It is thought that polar bears are induced ovulators. (B406.37.w37)

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Mating / Gestation / Pregnancy

Source Information

SUMMARY: Polar bears have delayed implantation. The total gestation length is 195 - 265 days including the period before implantation. The active gestation period lasts only about 60 days.
  • Delayed implantation. (D244)
  • Total gestation from conception to parturition 195 - 265 days. (D244)
  • Total 195 to 265 days including period before implantation. (B147)
  • About eight months including the period of delayed implantation. (B285.w4)
  • Implantation occurs in mid-September to mid-October, depending on latitude. (B285.w4)
  • Six to eight months. (B336.51.w51)
  • Possibly eight to nine months. (B144)
  • Eight months. (B288.w11)
  • 240 days. (B288.w11)
  • Implantation occurs in autumn. (J46.214.w2)
  • The period of post-implantation development is about 60 days, with reactivation of the corpora lutea and implantation probably occurring in late September to early November. (J82.37.w1)
  • Implantation has been reported to occur in November, about the time of entry into the den in the Beaufort Sea. (B490.27.w27)

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Parturition / Birth

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SUMMARY: In the wild, females enter the maternity den up to three months before parturition. In zoos, females may show nesting behaviour in the two to three weeks before birth, and are uncomfortable (lying down then standing up again) immediately before birth. They give birth quite rapidly, standing or sitting, and immediately lick the cub. If twins are born there may be about 37 to 273 minutes between births. Births occur in November to January. 
  • In the wild, females enter a maternity den in mid-October to early November and give birth in December/early January. (B406.37.w37)
  • In the wild, females enter dens up to three months prior to parturition. (V.w100)

Parturition:

  • At Denver Zoological Gardens, females showed increased nesting behaviour for two to three weeks before parturition. Just before parturition they appeared uncomfortable, with repeated lying down and immediately standing up again, and licking the vulva and feet. Birth was rapid, with the bear usually lying down, sometimes sitting and licking the vulva. Cubs were quickly licked and within a few minutes could be heard making their first noises. Twins were born 37 to 273 minutes apart (mean 124 +/- 105 minutes). (J23.39.w1)

Seasonality:

  • Births occur December to January, in the den. (D244)
  • Births occur in the winter den two to three months after implantation. (B285.w4)
  • Born in winter den, November to January. (B180.w4)
  • November to January. (B336.51.w51)
  • November to December. (B288.w11)
  • 18th November and 22nd November for two singleton litters at Edinburgh Zoo, Scotland, UK. (B407.w5)
  • In Zoos of the Soviet Union, 10th October to 31st December. (P74.1989.w2)
  • December to early January. (B406.37.w37)
  • The timing of births may vary between different parts of the polar bear's range. Different studies have indicated births as early as 30th November or as late as the end of December or even later. (B490.27.w27)
    • In the Canadian Arctic, births probably mainly occur before 15th December. (J332.75.w2)
    • In the Beaufort Sea area, females entered land dens 8th October to 24th November; those denning on sea-ice entered their dens 17th October to 13th December, and probably did not give birth immediately. (B490.27.w27, J40.58.w1)
    • A captive female in Barrow, Alaska gave birth 27th December. (B490.27.w27)
    • Implantation has been reported to occur in November, about the time of entry into the den in the Beaufort Sea. (B490.27.w27)
    • In the western Hudson Bay, cubs are born mid-November to mid-December. (J343.50.w1)

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Neonatal Development

Source Information

SUMMARY: Polar bear cubs are altricial; at birth they are covered with fine white hair, but are blind, the eyes closed, and they lack subcutaneous fat. Data from zoo-born cubs indicates that the eyes open at 24 - 42 days, teeth first erupt by 35 - 50 days and they start to walk at 46 - 60 days. They start to accompany the dam by about 90 days and also can take some solid food by this age; this is an important time for learning. In the wild, cubs emerge from the den and follow their mother from late March/early April, weighing about 10 - 15 kg. They are weaned at about 24 - 28 months. In southern areas cubs may be able to hunt well enough to be self sufficient from 1.5 years but in northern areas where the ice is thicker this may not be possible until they reach 2.5 years. Cubs usually remain with their mother for 2.5 years. In the 1970s and early 1980s, as many as 60% of cubs in the Western Hudson Bay area were independent as yearlings (18-20 months old) in some years, but this has declined and now weaning at 1.5 rather than 2.5 years is uncommon anywhere.

Birth: 

  • Altricial; blind at birth; hair is present but newborn cubs are blind. (D244)
  • Blind but with a good covering of short white fur. (B147)
  • Very fine hair covering at birth. (B285.w4)
  • Covered with fine white hair at birth; the eyes are closed. (B288.w11)
  • At birth cubs had pink skin, but some black pigment was visible on the nose and foot pads at birth, and these areas continued to get darker over the following days, with the skin at the lips, muzzle and round the eyes darkening by about 22 days. At 5.5 and 5.6 months, a cub's skin was grey (back and part of the abdomen), with noticeable darkening over the month, and grey colouration advancing in the inguinal area. Cubs had a fine, sparse coat of white hair at birth. At about two weeks, the natal coat started to be replaced by undercoat and longer guard hairs. At 25 days, fine fuzzy hair was growing on the bottoms of their feet. At 34 days, eyelashes and muzzle whiskers were visible. The tongue was noted to change from pink to pink-and-black and later to black by about eight months. (J23.39.w1)
  • Blind at birth, lightly furred, with no subcutaneous fat, and helpless. (B490.27.w27)

Eyes and ears:

  • Hand-reared cub: the cubs eyes were opening 33 days and orientating at 47 days; there was obvious olfaction by 50 days and hearing by 68 days. (J23.4.w1)
  • For two hand-reared cubs: eyes opened 26 days, first upper incisor visible in one cub at 39 days, lower incisors and canines at 50 days; in a second cub the upper canines appeared at 48 days and the lower canines at 52 days. The cubs were walking, unsteadily, at 68 days. (J23.14.w3)
  • Eyes open: 24 - 42 days. (D251.8.w8)
  • From zoo data: Eyes open: 26 - 40 days. (D247.6.w6)
  • Eyes open at about 30 days. (B406.37.w37)
  • At Denver Zoological Gardens, cubs' eyes opened at 30 - 42 days (mean 35.1 +/- 3.6 day); it was noted that usually a small gap between the eyelids was visible one or two days before full opening. (J23.39.w1)
  • A hand-reared cub from Prague Zoo opened its eyes at 33 days but did not show any signs of visual orientation until it was 47 days old. The ears opened on the 26th day, but hearing was "imperfect" until the 68th day, then improved. (J46.117.w1)

Thermoregulation:

  • Cubs have hair at birth, but would not be able to survive outside the den in their first three months. (B406.37.w37)
  • Hand-reared cubs at Denver Zoological Gardens were observed to have poor thermoregulation in their first two weeks, preferring the incubator at 29-31 C. At about two weeks old, coinciding with the development of more hair, thermoregulation appeared to improve and cubs preferred an incubator temperature of 18 C or lower. (J23.39.w1)

Growth rate/weight gain:

  • Weight increases from 0.6 kg at birth to 10-15 kg by the time of emergence from the den (late March/early April). (D244)

Tooth development:

  • For two hand-reared cubs: first upper incisor visible in one cub at 39 days, lower incisors and canines at 50 days; in a second cub the upper canines appeared at 48 days and the lower canines at 52 days. (J23.14.w3)
  • At Denver Zoological Gardens, the first teeth were noted on inspection of cubs at eight weeks of age. It was noted that deciduoud teeth would turn dark immediately by being replaced by permanent teeth, and that the permanent tooth would erupt directly behind the deciduous tooth it was replacing. (J23.39.w1)
  • From zoo data: First tooth: 35 - 50 days. (D247.6.w6)
  • Incisors erupt at 36 - 53 days. (D251.8.w8)
  • Canines erupt at 46 - 53 days. (D251.8.w8)

Feeding, exploration and dispersal:

  • Cubs emerge from the den and follow their mother from late March/early April. (B285.w4)
  • Hand-reared cub: able to balance on hind legs at 81 days. (J23.4.w1)
  • For two hand-reared cubs: the cubs were walking, unsteadily, at 68 days. (J23.14.w3)
  • From zoo data: Attempt to walk from: 45 - 60 days. (D247.6.w6)
  • A hand-reared cub from Prague Zoo started walking at 47 days "but was not independent until the 70th day." (J46.117.w1)
  • Cubs first stand at 60-82 days. (D251.8.w8)
  • From zoo data: First accompany dam: 80 - 90 days. (D247.6.w6)
    • A female at Edinburgh Zoo first brought her cubs out when they were just over two months old (two singleton cubs, in different years). (B407.w5)
  • A study of one cub by video monitoring in the zoo's den showed that during the first month the cub nursed continuously, in full contact with her mother (the cub's body weight on the mother); by two months of age some of the time was spent resting alone while the mother was out of the den and partial contact (cub's weight on the substrate, but with a body part contacting the dam) exceeded full contact by the age of 10 weeks. By 15 weeks, full contact had ceased, and 90% of the time the cub was in partial contact; out of contact time was rarely more than 20%. Gradually, nursing became less frequent, active behaviours (locomotion, play) increased and the cub began to spend some time out of the den. (J54.22.w2)
  • Observation of cubs at Denver Zoological Gardens indicates that cubs show musculoskeletal development and movement coordination starting with the head and neck and progressing caudally. First, they raise the head and neck, then, in succession over a period of several weeks, prop themselves on their elbows, reach a dog-sitting position (up on their forefeet), finally raising up on the hind legs also. Initially when learning to walk they fall over often. Mother-reared cubs were certainly walking by 60-70 days. They often rolled onto their backs and vocalised, but their mothers would leave them to struggle, for several minutes, righting themselves, rather than assisting. A cub allowed access, with its mother, to a pool at three months, swam for 15 minutes then shivered uncontrollably once it left the pool, stopping after it had nursed and slept on its mother's feet. The initial method of propulsion is a dog-paddle using all four legs, and without any diving. Swimming in the adult mode (with the fore feet for propulsion, hind legs trailing, and short dives, develops only after several weeks of swimming. (J23.39.w1)
  • Cubs emerge from the den and follow their mother from late March/early April; they weight about 10 - 12 kg (22 to 26 lb) at the time of emergence. (B285.w4)
  • In Svalbard, for the period 1972-1980, females (and their cubs) started to emerge from maternity dens in the first week of March and most dens were empty by mid-April. (J40.49.w3)
  • When cubs first emerge from the den the family remains near the den and cubs still spend much time in the den. It is thought that this is an important learning period. After a few weeks, the family travels to the coast or the sea ice; this may be only 25 km (15 miles) or longer up to e.g. 230 km (144 miles). (B406.37.w37)
  • From zoo data: First intake of solid food: 80 - 90 days. (D247.6.w6)
  • Cubs are weaned at about 24 - 28 months. (D244)
  • Cubs up to 2.5 years of age do nearly no hunting during winter and early spring. (D244)
  • Two-year-olds are significantly more likely to hunt successfully than are yearlings, although their hunts have similar durations. (D244)
  • A study of wild polar bears found that cubs of 1.5 years of age hunted, but only near their mother - within about 0.5 km. Generally their still hunts do not last as long as those of older bear, but some are exceptions and have mean still-hunt durations similar to those of adults; one orphaned cub of one and a third years showed an investigative pattern similar to that of adults. Older cubs (2.5 years) hunted more independently from their mother, at distances typically of 1-2 km from her. At this age, some were still carrying out only short still-hunts while others had longer hunts. Two cubs of this age each caught a seal. (J30.52.w3)
  • Cubs usually remain with their mother for 2.5 years. (B406.37.w37)
  • Cubs are usually independent from their mothers at about 2.5 years old. (J332.86.w1)
  • A study in western Hudson Bay during the summer ice-free period, when there are severe nutritional restrictions on polar bears, noted that milk intake of cubs-of-the-year was 7.8 MJ/day, while yearlings consumed only 1.5 MJ/day. However, the proportional loss of weight was the same for both age groups over this period, indicating that cubs are more dependent on milk than are yearlings, and are less able to survive nutritional restrictions. (J30.72.w1)
    • Females with cubs-of-the year lost on average 38 kg (20% body weight) over the ice-free period, compared with 21 kg (11%) loss for females with yearlings and 19 kg loss (8%) for lone females. (J46.214.w2)
  • In the Hudson Bay area, cubs often become self-sufficient (able to hunt effectively) from 1.5 years old, while in most of the Canadian Arctic they only become self sufficient at 2.5 years. This may be related to ice thickness, with ice being thicker further north, so that bears need to be larger before they can break it effectively. Bears have a higher survival rate if they stay with their mother for at least two years than if they separate earlier. (B406.37.w37)
    • In the Hudson Bay area, studies in the 1970s and early 1980s found that cubs were often weaned at 1.3 years, and a similar weaning age was noted in an early study in the Svalbard area. However, later studies in the Hudson Bay found cubs not weaned until they were 2.3 years old. (B490.27.w27)
    • In western Hudson Bay, prior to 1980 81% of yearlings were independent from their mother in autumn. In 1980-1992, only 34% were independent at that age; after 1986, most cubs remained with their mother for longer. (J30.73.w2)
    • In the Western Hudson bay there has been a long-term change between 1981 and 1999, with a reduced proportion of cubs independent as yearlings. In 1982, about 60% of cubs were independent as yearlings at 18-20 months, but since 1991 this has declined to only 15-20% independent at this age since 1991 [data to 1998]. (J343.52.w3)
  • Weaning at 1.5 years is now uncommon anywhere in the polar bear's range. [2007] (V.w100)

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Litter Size

Source Information

SUMMARY: Litter size is one to three cubs, very rarely four; two may be usual and is common in captivity. Reported average litter sizes in the wild range from 1.58 - 2.0. The largest, heaviest females, eight to 15 years old, produce the largest litters.
  • Average in the wild 1.58 - 1.87; in captivity commonly two. (D244)
  • There are geographical variations in average litter size. (D244)
  • One to four, average about two. (B147)
  • About two thirds of litters are of two cubs, next most common are litters of a single cub, while triplets are found usually in less than 5% of litters but in some populations up to 12% of litters are triplets. (B285.w4)
  • One to four, usually two. (B180.w4)
  • The largest, heaviest females, age eight to 15 years, produce the largest and heaviest litters; younger and older females are more likely to produce a single cub. (B285.w4)
  • Usually one to two; one to four reported. (B336.51.w51)
  • Usually two, may be one to three. (B144)
  • Mean litter size varies with area; usually it is about 1.6 - 1.8, but it may reach 2.0 in the lower Hudson Bay. (B406.37.w37)
  • A study in the Hudson Bay area, Canada, 1980-1985, found that litters of two cubs were predominant (nearly 70% of litters), with about 20% of litters having a single cub. Females with litters of three or (one case) four cubs were 16.3 years old (mean), older than the average age of 13.0 years for females with twins. (J46.214.w2)
  • Larger, heavier females tend to produce larger litters. A study of 27 females in the western Hudson Bay area, 1980-1992, found that five of the six females which were heaviest in autumn before denning produced triplets. (J46.234.w1)
  • In Svalbard, for counts over the period 1973 to 1983 in different areas, counts of females included one with four cubs-of-the-year (COY), three with three COY, 98 with two, and 27 with one, giving an average litter size of 2.27. The data might be biased by the observed triplets and litter of four cubs from Edgeoya and Barentsoya in 1983; triplets are rare and if the triplets and litter of four cubs are ignored, the data gives an average litter size of 1.78. However, a possible bias for some cubs not to be seen, if a cub is still in the den while its mother is outside and counted, was acknowledged also. It was suggested that the actual average probably was close to 2.0 at the time of den emergence. For females with yearlings, 13 were accompanied by one yearling and seven by two yearlings. (J40.49.w3)
  • Litter size of polar bears varies between areas, but this is not correlated with latitude. Litter size may be affected by ecosystem productivity and population demography. Females in their mid teens, with heavier body weights, are more likely to produce large litters than are younger females of lower body weight: if harvesting affects population age structure, it is likely also to affect average litter size; density-dependent population regulation may also play a role, as the proportion of triplet litters may relate to population density relative to the carrying capacity of the area. Observed litter size varies depending on the date of sampling, due to mortality of cubs after they leave the maternity den; cub mortality varies between years. (J344.22.w3)

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Time between Litters/ Litters per year

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SUMMARY: Polar bears breed every two to four years, usually every three years. Previously, breeding often occurred every two years in the lower Hudson Bay area, but this has changed recently.
  • Mean time between litters 3.6 years (western Beaufort Sea data). (D244)
  • Range of two to four years, median and mode three years, for eight known reproductive cycles based on capture-recapture data. (D244)
  • Two to four years. (B147)
  • Three years. (B285.w4)
  • Two years. (B180.w4)
  • In most areas polar bears breed only every three years or at longer intervals; in the lower Hudson Bay area breeding at intervals of two years is common. (B406.37.w37)
  • In the 1970s and early 1980s, studies in Hudson Bay indicated that a two-year interval between litters, with cubs weaned at 1.3 years of age, was common. Later studies found breeding every three years to be more common. Females which lose their cubs may then breed. (B490.27.w27)
  • The interbirth interval is usually three years, if cubs are successfully raised to weaning. (J30.64.w1)
  • A study in the Hudson Bay area, Canada, 1980-1985, found that the mean time between successive litters was 2.9 years, with 40% having a two-year interbirth interval. Rate of litter production tended to increase with age. (J46.214.w2)
  • Polar bears generally have a three-year interbirth interval. In Western Hudson Bay, a two-year interval, with litters weaned at 1.3 years, was common, but more recently a three-year interval has been noted, with few yearlings independent at 18-20 months. (J343.52.w3)
  • In western Hudson Bay, the interbirth interval has increased from an average of 2.1 years to 2.9 years. (J30.73.w2)
  • Litters at intervals of two years are now uncommon anywhere in the polar bear's range. (V.w100)
  • Females which lose their cubs before or during the breeding season may come into oestrus and breed. (J30.64.w1)

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Lactation / Milk Production

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SUMMARY: Cubs are suckled to about 24 - 28 months. Milk production peaks while cubs are still in the maternity den, or soon after they emerge, then declines over a prolonged period.
  • Cubs are suckled to about 24 - 28 months, but weaning as late as three years has been reported. (D244)
  • After two and a half years. (B144)
  • Analysis of milk samples found that for mothers with cubs-of-the-year, sampled while on land, fat content declined as follows: 35.8% when emerging from the dens (cubs three months old), 27.5% by that autumn (cubs 10 months old), and 20.6% by the time the cubs were 22 months old; a single sample collected from a mother with offspring of 34 months old contained 16.8% fat. For females samples while on sea ice, there was very little difference in fat content whether the cubs were four, 16 or 28 months old (about 33%). Protein content increased with time for both fasting bears on land and feeding bears on sea ice, from 10.5% for females with three-month-old cubs, to 12.1% (10-month-old) and 13.2 % (22 months). Total solids varied between 21.6 to 51.0%. For bears on land, the gross energy content of the milk decreased from 16.9 kJ/g (for females with 3-month-old cubs) to 9.7 kJ/g (offspring at 34 months old). The proportion of females lactating declined through the autumn, both for females with cubs and those with yearlings. It was suggested that secretary activity of the mammary glands may be reduced as fat stores are depleted, and that females may produce milk of lower quality prior to ceasing lactation. When polar bears lack access to food, those with older cubs may cease lactating earlier in the autumn. Changes in the composition of the milk during the period of fasting may conserve maternal energy stores, ensuring survival. Milk energy transfer from females to yearlings is much less than transfer from mothers to cubs-of-the-year (1.5 MJ/day versus 7.5 MJ/day). (J30.71.w1)
  • Peak lactation is thought to occur while polar bears are still in the maternity den, or soon after they emerge, followed by a prolonged period of declining lactation. Carbohydrate levels declined with time: 4.7% at three months, 3.6% at four months, 1.8% at 10 months, 1.5% at 16 months, 2.1% at 22 months, 1.3% at 28 months, 2.3% at 34 months. (J30.72.w1)
  • A study in western Hudson Bay compared milk production of females with cubs-of-the-year (eight months old) versus those with yearlings (20 months old) during the summer ice-free period, when there are severe nutritional restrictions on polar bears. Females with cubs produced 10.9 MJ per day, significantly more than the 2.6 MJ per day produced by those with yearlings. (J30.72.w1)
  • Suckling of yearlings during the summer apparently is not essential, since during a study at Hudson Bay, 1980-1985, survival of yearlings which were found without their mothers in the summer (when they were 1.5 years old) was not different from those found with their mothers at this age. (J46.214.w2)
  • Polar bear milk has a higher fat content than that of other bears (bears in general have higher fat and protein contents of milk than do most other carnivores). Some females may continue suckling their cubs to two years old, but others stop allowing suckling after the cubs are about a year old. Even when cubs continue suckling, milk is a less important nutrient source in the second year. Milk provided to cubs-of-the-year contains on average 31.2 +/- 1.6%, while that given to yearlings contains 18.3 +/- 2.4% fat. (B490.27.w27)
  • A milk sample from a wild female polar bear with a 16 to 17-month-old cub was described as creamy white with a strong fishy odour and a consistency similar to sweetened condensed milk. The sample contained 44.1% total solids, including 31.1% fat, 10.2% protein, 0.49% lactose, 1.17% ash. The fat was high in butyric, palmitoleic and oleic fatty acids. (J30.41.w1)
    • Milk lipids in carnivores are primarily from the uptake of circulating fatty acids; they mainly reflect dietary lipid intake. (J464.27.w1)
  • A milk sample from a wild female polar bear with cubs of 16-17 months of age contained 46.7% solids, including 32% fat, 12.6% protein, 0.6% lactose, 1.30 % ash. Another sample from a bear with cubs of similar age contained 44.1% solids, including 31.1% fat, 10.2% protein, 0.49% lactose, 1.17% ash. A milk sample from a wild female polar bear with cubs of 28-29 months of age contained 43.9 % solids, including 31% fat, 10.3% protein, 1.1% lactose, 0.97% ash. For two samples tested, more than 75% of the fatty acids were palmitic (16:0), palmitoleic (16:1), stearic (18:0) and oleic (18:1) fatty acids; there was little butyric acid in one sample and none detected in the other. For the first sample, with 1.3% ash, the ash was composed of calcium 23%, phosphorus 15.4%, potassium 11.0%, sodium 7.7%, magnesium 1.4% and iron 0.2%. For the sample from the female with older cubs, with 0.97% ash, the ash was composed of calcium 14.5%, phosphorus 10.8%, potassium 5.0%, sodium 12.8%, magnesium 1.1% and iron 0.2%. (J30.48.w1)
  • Milk samples from seven wild polar bears, sampled July-August, and October, contained mean 47.6% total solids (range 35.8 - 59.6%), including fat 33.15 (23.8 - 48.4%), casein 7.1% (5.2-9.6%), whey protein 3.8 % (2.2 - 4.6%), lactose 0.3% (0.2 - 0.5%), ash 1.4% (0.8 - 1.9%), calcium 0.29% (0.21-0.34%) and phosphorus 0.23% (0.16-0.28%). (J332.53.w1)
  • At Denver Zoological Gardens, milk samples from one female contained 23.4% fat at 11 weeks and 30.1% fat at 27 weeks. (J23.39.w1)
  • Milk from ten females out on the ice pack with cubs-of-the-year contained vitamin D at 0.0042 +/- 0.0073 nmol/g; milk from a captive female in the den contained vitamin D at 0.14 nmol/g. (J54.17.w1)
  • Milk from polar bears contained, for 52 samples from bears with cubs aged three to 29 months, 33.1% solids, 10.7% protein, 7.4% casein, 3.4 % whey, 30.0% total fats (of which 39.5% was saturated, 55.2% monounsaturated and 4.7% polyunsaturated), 1.1 % ash, 1.3% carbohydrates, and had a gross energy of 16.1 kJ/g. A single sample taken from a bear with 13-day-old cubs contained 14.9% fat. (J54.25.w2)
  • Milk from 10 polar bears, sampled at 210-870 days lactation, contained 36-60% dry matter and 12.6-20.5 kJ/g gross energy. (P17.57.w2)

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Sexual Maturity

Source Information

SUMMARY: Females may be seen pairing at 3.5 years in some areas, with first cubs born when the female is four, but usually the first mating is at four to seven years, with the first litter born when the female is five to eight. Males may be sexually mature in terms of sperm production by three years of age, but are unlikely to compete successfully for a female, and therefore to actually breed, before six years old.
  • In the fourth year. (B144)
  • Average about 5-6 years. (B147)
  • Females may be seen pairing with males from about 3.5 years. (D244)
  • Females first reproduce at four to eight years, but on average first cubs are produced at six to seven years.
  • In the Beaufort Sea, first breeding age averages five to six years, with first cubs therefore at six to seven years, but individuals may first breed as young as four years or as late as eight years old. (D244)
  • In the eastern Canadian Arctic, the age of first breeding is four years. (D244)
  • A study in the Hudson Bay area, Canada, 1980-1985, found that no three-year olds were accompanied by cubs and only 5% of four-year-olds were accompanied by cubs in summer or autumn, compared with nearly 50% of bears five years old or older being accompanied by cubs. (J46.214.w2)
  • Females first mate at four or five years old. (B285.w4)
  • In females, first mating occurs at four years with litters born when the females are just five years old; in some areas (Alaska, western Canadian Arctic), usually a year later. Maximum fecundity is from 10-19 years. (B406.37.w37)
  • In many areas of Canada, polar bears first mate at 4.5 and produce their first cubs when they are five years old, but in some areas first mating is a year later. The sparse data available indicates that females may become reproductively senescent after about 20 years of age, although one female was apparently in oestrus (travelling with a male, vulva turgid) at 29 years of age. (B490.27.w27)
  • While litter size and weight declines in females older than 14-16 years of age, some females still have cubs to at least 27 years of age, although with smaller litter sizes, and in captivity reproduction to at least 37 years has been reported. (J46.234.w1)
  • First mating occurs when females are four to seven years old in the wild. (V.w100)
  • Males may have sperm present in the epididymis from three years to 19 years of age; it is unlikely that males mate at earlier than six years of age. (D244)
  • Males may be sexually mature at three years old but are unlikely to compete successfully for a female, and therefore to actually breed, before six years old; they may be able to breed to about 19 years old. (B406.37.w37)
  • Males may be capable of spermatogenesis as early as three years of age and to at least 19 years old, with abundant sperm in bears five years old and older. Males as young as three years old have been seen consorting with females, at least in a period when excessive hunting had reduced the numbers of older males. More typically, males three or four years of age, which would weigh less than 250 kg, would not be able to compete for females against older males weighing 400-500 kg or more. (B490.27.w27)
  • A male polar bear in a zoo successfully mated with an Ursus arctos - Brown bear at 64 months old (cubs born when both bears were 72 months old.) (J23.9.w1)

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Male Seasonal Variation

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SUMMARY: Male polar bears show yearly regression and recrudescence of the testes. Spermatogenesis occurs from February to May or possibly June; testes are largest during the breeding season (e.g. in May), but smaller by October. There are also seasonal variations in hormone levels such as testosterone, LH and prolactin, all of which are high in April and low in October.
  • Spermatogenesis occurs February to May or possibly June. (D244)
  • Testes descend from an intra-abdominal position in late winter, remaining there through May. Spermatogenesis is thought to take place February to May. (B490.27.w27)
  • Testis size varies seasonally. Wild polar bears captured in the Canadian arctic showed seasonal variation in testis size. In May, testes were measured as 39.4 +/- 3.5 cm, whereas in October testis size was significantly smaller (P = 0.002) at 27.3 +/- 2.0 cm. In April and July, testis size was intermediate (larger in April than in July). (J367.123.w1)
  • Serum testosterone varies seasonally, being elevated low in March, peaking in spring (April to May) then decreasing to a low level by August and remaining low to November (samples were collected March to November). There was a positive correlation between age and serum testosterone concentration during the breeding season, but not in October. (J371.38.w2)
  • Serum testosterone, prolactin and LH vary seasonally. In polar bears in the Canadian arctic:
    • Serum testosterone was 5.8 +/- 0.8 ng/mL in April, 1.7 +/- 0.5 ng/mL in May and 1.1 +/- 0.2 ng/mL in October. (J367.123.w1)
    • Serum LH was high in April (0.14 +/- 0.04 ng/mL), lower in May (0.09 +/- 0.01 ng/mL), July (0.10 +/- 0.02 ng/mL and October (0.08 +/- 0.00 mg/mL). (J367.123.w1)
    • Serum prolactin was high in April (1.9 +/- 0.3 ng/mL), highest in May (2.5 +/- 0.2 ng/mL), lower in July (1.3 +/- 0.1 ng/mL) and lowest in October (0.8 +/- 0.07 ng/mL) (J367.123.w1)

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Longevity / Mortality

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SUMMARY: Maximum longevity has been suggested at 20 - 25 years, or 25 - 30 years, and 45 years has been reported for a zoo bear. Annual pre-weaning mortality of cubs is about 10 - 30%; annual mortality rates for adults are 8 to 16 % and for younger animals may be 20%. A critical time is when the cubs first become independent. It is common for at least one member of large litters (three cubs) not to survive to weaning. Mortality is due to starvation, intraspecific predation and accidents. Starvation is the most important cause of death, particularly for inexperienced and physically disabled bears when seal numbers drop suddenly.
  • The sex ratio at birth is even. (D244)
  • Maximum longevity is suggested to be 20 - 25 years, or 25 - 30 years. (D244)
  • Potential longevity in the wild is about 25 - 30 years. (B147)
  • 34 years seven months in captivity. (B39.w8)
  • A female in a zoo was still alive at 45 years of age. (B147)
  • Based on studbook data, the average longevity in zoos is 18 years, and the oldest recorded individual was 41 years old. (D315.3.w3)
  • In the wild, one individual of 31 years of age and another of 33 years have been recorded. (D315.3.w3)
  • Longevity of females is mid-20s to early-30s, males early-20s, occasionally late-20s. (B285.w4)
  • A hybrid with Ursus arctos middendorffi (Ursus arctos - Brown bear) lived to 38 years one month. (B39.w8)
  • Preweaning mortality of cubs is about 10 - 30%. (D244)
  • A study in western Hudson Bay found that cub-of-the-year survival between spring and autumn varied annually, averaging 53.2% and varying from 39.0 to 100.0%, with whole-litter loss between spring and autumn of 30.8%; only 38.0% of females did not lose any cubs. Minimum cub survival from one autumn to the next was 34.7%; this was related to cub mass, maternal mass, and maternal condition; it was estimated that cub survival during autumn was 83.0%. Survival for the whole of the first year of life was no more than 44%. (J30.74.w1)
  • In Svalbard, for the period 1972-1980, calculated survivorship for cubs during the period three to 20 months of age was 0.48. Suggested reasons for low survival included cannibalism by abundant large males, and unfavourable ice conditions. (J40.49.w3)
  • In Svalbard, unpublished data indicated an annual adult survival rate of 0.91 for the period after 1977.(J40.49.w3)
  • Mortality is due to starvation, intraspecific predation and accidents. (B406.37.w37)
    • Starvation is probably the commonest cause of death and occurs when seal numbers change suddenly. Those most likely to starve are inexperienced bears and those with a physical disability. (B406.37.w37)
    • Causes of death of cubs include predation by adult males and accidents. (B406.37.w37)
    • Mortality of members of large litters is common; usually in triplets one is smaller and trails the others. The number of litters of triplets in a given area is higher in spring than it is by autumn. (B406.37.w37)
  • Polar bears sometimes kill one another in fights over mates, and sometimes predate one another. Male polar bears will kill cubs and sometimes, but not always, eat them. The reasons for this behaviour are not fully understood. (B490.27.w27)
  • Bears with serious jaw injuries due to fights or failed predation attempts are unlikely to survive long-term as their ability to catch prey will be seriously reduced. (B490.27.w27)
  • Unusual causes of death reported in wild polar bears include gastric dilatation and volvulus and gall stones occluding the bile duct. (B490.27.w27)
  • A study along the western coast of Hudson Bay, Canada, and around the Churchill dump, 1981-1983, found on bear dead with two pieces of lead batteries in its stomach; the bear was thought to have poisoned itself. (J30.63.w2)
  • Every year some bears are destroyed as "problem bears" around Churchill. (J30.63.w2)
  • A female and her cubs died when their den collapsed. (J343.44.w1)
  • Subadults 2-5 years old have lower survival rates than do adults. (B490.27.w27)
  • Subadults which have left their mother but not learned to hunt effectively may starve to death. Old bears which become too feeble to catch food also would starve and die from old age. (B490.27.w27)
  • Annual mortality rates for adults are 8 -16 %. For younger animals (cubs to subadults) mortality rates may be 20%. (B406.37.w37)
    • The time when cubs first become independent from their mother is a critical time for survival. (B406.37.w37)
  • Annual mortality rate of subadults has been estimated at 3 - 16%. (D244)
  • One adult annual mortality rate estimate was 8 - 16%; typically it is assumed to be 8 - 12%. (D244)
  • The natality rate of females is reduced after about 20 years of age. (B147)
  • Females may remain reproductively active to e.g. 21 years of age, but are in peak reproductive condition at 10 - 19 years. (D244)
  • A female in a zoo gave birth at 36 years 11 months old. (B147)
  • Females are reproductively active into old age. (B285.w4)
  • A study in the Hudson Bay area, Canada, 1980-1985, found that while females 20 years old or older still produced cubs, compared with younger females they appeared to have higher levels of cub loss before summer - 12% of females with cubs-of-the-year in spring were at least 20 years old, but only 4% of bears with cubs in summer were this old. (J46.214.w2)
  • A study of temporal variation in reproduction in western Hudson Bay for the period 1966-1992 found that cub mortality increased from 25.0% in 1980-1984 to 50.9% in 1987-1992. (J30.73.w2)

Age Estimation Techniques: 
  • Annuli in the cementum round teeth are the most reliable guide to age. (D244)
  • Body size, skull characteristics, reproductive status, dental development, tooth wear, and counting cementum rings are all used, often in combination for the most accurate determination of age. (B406.37.w37)
    • In spring, at the time families first emerge from maternity dens, cubs weigh 10-15 kg (22 - 33 lb), yearlings about 45 - 80 kg (100 - 176 lb), while two-year-olds weigh 70 - 140 kg (154 - 309 lb). By August, cubs-of-the-year weigh 50 - 110 kg (110 - 40 lb), yearlings 90 - 160 kg (198 - 353 lb), two-year-olds 170 - 200 kg (375 - 441 lb). Note: weights vary with area and year. (B406.37.w37)
    • For polar bears over two years of age, the single most reliable indicator of age is the annuli of cementum around the teeth. However, the layers are not as consistent or easy to read as in other bear species, probably because these bears show more continuous activity through the year. Processing involves decalcification of an extracted tooth, sectioning with a freezing microtome, staining with toluidene blue and immediate reading microscopically. If a photomicrograph is then taken, a permanent record is provided and other workers can confirm the reading. (B406.37.w37)
  • Tooth wear and the height of the enamel line on the canines can be used for estimation. Additionally, a first premolar tooth can be extracted, sectioned, and the age determined from the cementum layers. (J40.35.w1)
  • Annuli in the cementum of an extracted vestigial premolar are used for ageing. (J46.234.w1)

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Authors & Referees

Authors

Dr Debra Bourne MA VetMB PhD MRCVS (V.w5)

Referee

Andrew Derocher (V.w100), Ellen Dierenfeld (V.w16)

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