For mother brown bear in a Swedish forest, protecting cubs means risking their lives against human hunters. A brown bear has to raise the defenseless cubs near villages to protect themselves against adult male brown bears, according to a study published on June 22 in the journal Proceedings of the Royal Society B.

Apparently, during the annual mating season, from early May to mid-July, adult male bears that seek to mate with female bears kill cubs that are not their own. This is called selected infanticide, a behavior aimed to trigger estrus or sexual heat among female bears.

Without the cubs around, female bears will be more sexually receptive to males. Normally, it takes 18 to 30 months after birth for brown bear mothers to come into heat, when their cubs are already independent.

brown bear

To protect these brown bear cubs, the mother has to live near humans. Credit: Natures Images

Sexually selected infanticide is not unique to brown bears. The behavior is also common among big cats, primates, bats and birds.

Researchers say that bears generally avoid areas close to humans. To prevent infanticide, however, young brown bear mothers decide to overcome their own fear and live near humans during this time.

The research team sought to measure the exact distance that determined the success of young female brown bears. They used GPS to track 26 bears from 2005 to 2012.

Overall, 16 mother bears successfully raised cubs but 10 did not. On the average, successful brown bear mothers resided 780 meters or 2,500 feet from villages.

On the other hand, residing half a kilometre away or 1,210 meters away from villages led to failure. The mothers who avoided humans did not experience the same success of raising their cubs.

After mating season, the mother bears change behavior, says study co-author Sam Steyaert from the Norwegian University of Life Sciences. These bears go as farther away possible from humans during the hunting season.

In case an adult male brown bear spots a female brown bear and her cubs, the mother tries to fight him. Unfortunately, this usually results to  the death of the mother and her cubs.