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Mating and Reproduction


Brown bears have a polygamous mating system where males mate with multiple females and females with multiple males. Females are in estrous, or in heat, from mid-May through early July. During this time, both males and females travel long distances to find mates. In general, males that are larger, older, and have the highest genetic variation tend to be the most successful breeders. Young males have greater success in northern Sweden, possibly because illegal hunting removes competition from the larger males. Even still, the larger of the young males in that area tend to be more successful breeders.


It is possible that females mate with multiple males to protect their cubs from of infanticide, or being killed by male bears. Indeed, multiple mates can result in shared paternity, which

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occurs in 14% of litters with >2 cubs and 28% of litters with >3 cubs. Furthermore, males do not kill their own offspring, and the mother can ‘hide’ paternity better by mating with multiple males. Mating with multiple males may also promote sperm competition, i.e., competition for paternity takes place between sperm cells from several males. It is also plausible that females can choose paternity during or after the act of mating, which is physiologically possible as the act of mating induces ovulation in bears.


Interestingly, female bears have delayed implantation, meaning fertilized eggs attach to the uterine wall later in the fall, after the pregnant female enters the den. Cubs are born between late December and early February. At birth, the newborn cubs weigh only 250-600 g, which is extremely small compared similarly sized mammals. 


In Scandinavia, females commonly give birth to between 1 and 4 cubs, who stay by her side the first year and den with her the following fall. Most cubs leave their mother during the following estrus season, when they are 1.5 years old, i.e., a month or two after the family has emerged from the den. In northern Sweden, about half of all litters stay with their mother an extra year, or until they are 2.5 years old, which is similar to patterns observed in brown bears in North America. The trend in southern Sweden is that more litters are staying longer, until they 2.5 years old. We are unsure why this pattern is changing, but it may have to do with the increased density of bears in the area, or because hunting regulations protect family groups. 


In Scandinavia, females first give reproduce between 4 and 5,

although there are slight differences in age between the more northern 5-7 years) and southern (4-6 years) populations. Female productivity begins to decrease after they reach 20, and they stop reproducing altogether after 28. Bears can live for 30 years in the wild, although it is rare for individuals in hunted populations, such as Scandinavia, to live that long.

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