Mama brown bears have a big problem. During mating season, males can be very aggressive to cubs that are not theirs, killing about three of every ten cubs. Mama bears do what they can to protect their cubs. Sometimes they try to defend them, but males are larger and stronger than females. Males can grow about 8 feet tall (2.4 meters) and weigh about 700 pounds (320 Kg). Fortunately, after the mating season ends, cubs are no longer victims of male bear attacks.
Mama bear with her cub. Courtesy Lisa Hupp/US Fish & Wildlife Service
This situation did not seat well with mama bears (no kidding!), and I was intrigued (and happy!) when Sam Steyaert, a brown bear scientist in the Scandinavian Brown Bear Research Project in Tackåsen, Sweden, told me that mama bears had come up with a strategy to keep their cubs out of danger from male bear attacks. I had to hear about it!
Steyaert and his colleagues keep track of brown bears’ whereabouts in Tackåsen with GPS tags on collars attached to the bears. Every morning, Steyaert grabs a cup of coffee and sits at his desk in front of a computer. He clicks a button, leans back on his chair and sips his coffee while the GPS data from the night before downloads into his computer.
“The software plots on a map series of dots that represent the locations of a bear throughout the night, and connects the dots,” Steyaert said. “It does this for each bear we are tracking, which includes males and females.”
Steyaert can now see which bears crossed paths and which stayed apart.
“During summer and fall, male and female bears cross paths often. They remain in the same area to feed, getting ready for winter,” Said Stayeart. “But when I looked at the data from spring and early summer, the mating season, I found something quite different.”
Steyaert saw that from early May to mid-July, some mama bears with cubs do not cross paths with males. These females and cubs do not eat, sleep or travel in the same areas male bears roam, even if those areas have abundant food. This pattern repeats every season. Steyaert thought that mama bears had found a way to protect their cubs by staying away from the males during the time they were aggressive toward cubs. Now, to prove it, Steyaert joined forces with Boss.
Steyaert and Boss, crime scene investigators
The GPS data showed that some mama bears and their cubs moved to an area male bears did not travel to, but also that other mama bears with cubs did remain where males lived. Steyaert’s plan was to compare both groups of mama bears with cups regarding cub survival. Would cubs that stayed away from males survived more often than those that crossed paths with males?
Every morning, Steyaert studied the GPS data collected the night before, identifying mama bears that had crossed paths with males. Then, he went out into the forest to see with his own eyes what had happened when males, females, and cubs came close to each other.
Steyaert went to the forest with Boss, his bear-tracking dog. Boss is a Jämthund, a type of dog typically trained for moose and bear hunting. Boss looks a lot like a wolf with a curly tail. He helps Steyaert find bear tracks, scat, and sometimes bears that are dead or alive.
When Boss and Steyaert found an area where males and females had crossed paths, they tried to figure out what had happened. Had there been a fight? What had happened to the cubs?
After analyzing many crime scenes, Steyaert confirmed his initial suspicion. Mama bears that stayed away from males kept more cubs alive than mama bears that had encountered males. But, bears can travel long distances. They could have easily followed females that wanted to stay away from them. Why didn’t the males follow the females?
The answer was in the GPS data plotted on maps. Mama bears that stayed away from males had chosen areas of the forest that were close to people’s homes. These areas, however, were not so close that people would become concerned about having brown bears almost in their backyard. Males, on the other hand, try to stay as far away from people as possible. For male bears, man is the top predator.
I was fascinated by these findings. Mama bears had modified their behavior in a way that gave their cubs a better chance of surviving male bear attacks by using a ‘human shield.’
Thank you for reading. I am looking forward to preparing another science true-story for you next time!
Ana María
Ana María S. Rodríguez
Interested in Sam Steyaert’s original science paper about this work? Here is the reference:
S.M.J.G. Steyaert, et. al., Human shields mediate sexual conflict in a top predator,” 2016, Proceedings of the Royal Society B, 283 (online publication).
Here you can find more information about brown bears.
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