Polar Bears In Depth
Reproduction
Page 2 of 4
Testes of male polar bears reside in the abdomen for most of the year. They descend into the scrotum in late winter, and remain there through May. Descent of the testes permits spermatogenesis, which is thought to occur from February to May (Erickson 1962; Lentfer and Miller 1969; LØnØ 1970). LØnØ (1970) reported that male/female pairs were observed as early as 8 March and as late as 20 June. According to histological examination of testes and ovaries, LØnØ (1970) further concluded breeding could last into July. Deteriorating ice conditions preclude scientific observations in most polar bear habitats by June, so the frequency of summer breeding cannot be easily documented.
Lentfer and Miller (1969) concluded, from presence of mature spermatozoa in epididymides, that male polar bears in Alaska may be able to breed as early as 3 years of age. Presence of sperm also guaranteed reproductive capability until at least age 19 years (Lentfer and Miller 1969). A recent study in Greenland found that 2 of 7 two-year old males, 5 of 10 three-year-olds, and 4 of 9 four-year-olds had some spermatazoa in epididymides (Rosing-Asvid et al. 2002). Although spermatazoa occurred at low density in the younger bears, all bears ? 5 years old, except for one very thin individual, had produced abundant spermatozoa and appeared capable of breeding. Lentfer et al. (1980) observed males 311 years old in consort with estrous females, confirming at least the age of earliest breeding ability for male polar bears. It should be noted, however, that excessive hunting in Alaska just before and during the time those observations were made had all but eliminated prime males (aged >10 years) from the population (Amstrup et al. 1986). Subsequently, few male bears that young have been observed with females. Since 1980, the proportion of prime males in Alaskan waters has been high (Amstrup 1995). Presently, large males weighing 400500+ kg are abundant in this region. Three and 4-year-old bears typically weigh 250 kg, and would not be able to compete successfully for mates with the now-abundant large males. Currently, young males must have very low reproductive output despite their apparent reproductive potential.
Productivity of polar bear populations appears to be largely dependent on numbers and productivity of ringed seals. For example, in the Beaufort Sea, ringed seal densities are lower than in some areas of the Canadian High Arctic or Hudson Bay. As a possible consequence, female polar bears in the Beaufort Sea usually do not breed for the first time until they are 5 years of age (Stirling et al. 1976; Lentfer and Hensel 1980). This means they give birth for the first time at age 6. In contrast, across many areas of Canada, females reach maturity at age 4 and produce their first young at age 5 (Stirling et al. 1977b, 1980, 1984; Ramsay and Stirling 1982, 1988; Furnell and Schweinsburg 1984).
Craighead and Mitchell (1982 :527) reported that in grizzly bears "reproductive longevity approximates physical longevity." Female polar bears, on the other hand, may show a reproductive senescence long before the end of their lives. Derocher et al. (1992) calculated an average age of first breeding in the Hudson Bay area of 4.1 years. Productivity, assessed by estimated pregnancy rates, remained high between 5 and 20 years of age and declined thereafter (Derocher et al. 1992). Unfortunately, long-term monitoring of individual polar bears is uncommon and data addressing senescence are few. One 32-year-old female in the Beaufort Sea was monitored for the last 25 years of her life and seen annually during her last 10 years. This bear was in extraordinary condition nearly every autumn. Although she was not recaptured during the autumn of her 30th year, she was observed standing next to a 400-kg female that was captured that season. The 30-year-old female appeared larger, but still did not enter a den that autumn. Despite her apparent excellent physical condition, she last produced cubs at age 22, suggesting a prolonged reproductive senescence. Some contrary evidence also is available. One 29-year-old female in the Beaufort Sea was clearly in estrus (based on turgidity of the vulva) and traveling with an adult male in the spring of 2001. Derocher et al. (1992) also indicated that some females retained reproductive competency throughout life. The reproductive longevity of brown bears and polar bears appears to be fertile ground for further research.
Lentfer and Miller (1969) concluded, from presence of mature spermatozoa in epididymides, that male polar bears in Alaska may be able to breed as early as 3 years of age. Presence of sperm also guaranteed reproductive capability until at least age 19 years (Lentfer and Miller 1969). A recent study in Greenland found that 2 of 7 two-year old males, 5 of 10 three-year-olds, and 4 of 9 four-year-olds had some spermatazoa in epididymides (Rosing-Asvid et al. 2002). Although spermatazoa occurred at low density in the younger bears, all bears ? 5 years old, except for one very thin individual, had produced abundant spermatozoa and appeared capable of breeding. Lentfer et al. (1980) observed males 311 years old in consort with estrous females, confirming at least the age of earliest breeding ability for male polar bears. It should be noted, however, that excessive hunting in Alaska just before and during the time those observations were made had all but eliminated prime males (aged >10 years) from the population (Amstrup et al. 1986). Subsequently, few male bears that young have been observed with females. Since 1980, the proportion of prime males in Alaskan waters has been high (Amstrup 1995). Presently, large males weighing 400500+ kg are abundant in this region. Three and 4-year-old bears typically weigh 250 kg, and would not be able to compete successfully for mates with the now-abundant large males. Currently, young males must have very low reproductive output despite their apparent reproductive potential.
Productivity of polar bear populations appears to be largely dependent on numbers and productivity of ringed seals. For example, in the Beaufort Sea, ringed seal densities are lower than in some areas of the Canadian High Arctic or Hudson Bay. As a possible consequence, female polar bears in the Beaufort Sea usually do not breed for the first time until they are 5 years of age (Stirling et al. 1976; Lentfer and Hensel 1980). This means they give birth for the first time at age 6. In contrast, across many areas of Canada, females reach maturity at age 4 and produce their first young at age 5 (Stirling et al. 1977b, 1980, 1984; Ramsay and Stirling 1982, 1988; Furnell and Schweinsburg 1984).
Craighead and Mitchell (1982 :527) reported that in grizzly bears "reproductive longevity approximates physical longevity." Female polar bears, on the other hand, may show a reproductive senescence long before the end of their lives. Derocher et al. (1992) calculated an average age of first breeding in the Hudson Bay area of 4.1 years. Productivity, assessed by estimated pregnancy rates, remained high between 5 and 20 years of age and declined thereafter (Derocher et al. 1992). Unfortunately, long-term monitoring of individual polar bears is uncommon and data addressing senescence are few. One 32-year-old female in the Beaufort Sea was monitored for the last 25 years of her life and seen annually during her last 10 years. This bear was in extraordinary condition nearly every autumn. Although she was not recaptured during the autumn of her 30th year, she was observed standing next to a 400-kg female that was captured that season. The 30-year-old female appeared larger, but still did not enter a den that autumn. Despite her apparent excellent physical condition, she last produced cubs at age 22, suggesting a prolonged reproductive senescence. Some contrary evidence also is available. One 29-year-old female in the Beaufort Sea was clearly in estrus (based on turgidity of the vulva) and traveling with an adult male in the spring of 2001. Derocher et al. (1992) also indicated that some females retained reproductive competency throughout life. The reproductive longevity of brown bears and polar bears appears to be fertile ground for further research.