06/02/2026
In the summer of 2011, a five-year-old grizzly bear in Grand Teton National Park took one of her own mother's cubs and raised it as her own. Wildlife biologists who had been watching the family for years said they had never seen or heard of it happening before.
Grizzly 610 was born in 2006 in the first set of triplets produced by Grizzly 399, the bear we have covered on this page before. 399 is the most famous grizzly in the Greater Yellowstone Ecosystem, a prolific mother who raised eighteen offspring over two decades while living alongside the roads of Grand Teton. 610 grew up in that roadside territory, learned its rhythms, and when she reached maturity, she did what grizzly daughters do. She took over a portion of her mother's home range and started her own family.
In the spring of 2011, both bears emerged from hibernation with cubs. 399 had triplets. 610 had twins. Their territories overlapped in the Willow Flats and Pilgrim Creek areas of the park. For weeks, wildlife watchers and photographers tracked both families separately, choosing each morning between the mother and the daughter based on which one was visible from the road.
Then on July 18, visitors near Jackson Lake Lodge reported hearing what sounded like grizzly bears fighting in the timber. The following morning, a large male grizzly was seen in the area. Male grizzlies kill cubs. They do it to bring females back into estrus so they can breed. A boar in the vicinity of two mothers with five cubs between them is a crisis.
What happened next was pieced together from witness accounts and biologist observations over the following days. Steve Cain, Grand Teton's senior wildlife biologist, reported that people began observing a bear with yellow ear tags traveling with three cubs instead of two. On July 23, photographer Thomas Mangelsen confirmed the count. 610 was at Willow Flats with three cubs. 399 was at Pilgrim Creek with two. One of 399's triplets, an unidentified male, was now with 610.
610 had adopted her own sibling.
The cub swap was not forced. Nobody moved the cub. Nobody intervened. Cain and Mangelsen both observed 610 traveling with the adopted cub as though it had always been part of her family. The cub integrated into 610's group alongside her biological twins. She fed with them. She denned with them. She raised three cubs through the following winter instead of two.
Grizzly bear adoption is almost unheard of in the wild. Grizzly mothers are fiercely protective of their own cubs and typically aggressive toward unrelated bears, including juveniles, that approach too closely. A mother accepting a cub that is not her own, particularly while she is already raising a litter, contradicts the standard behavioral model for the species.
The fact that the adopting mother was the biological daughter of the cub's mother makes it stranger and possibly explains it. 610 and 399 knew each other. Their territories overlapped. They had encountered each other repeatedly since 610's independence. The adopted cub smelled like family because it was family.
The leading theory for why the swap happened centers on the male grizzly that was heard fighting near Jackson Lake Lodge on July 18. If a boar threatened both family groups simultaneously, 399 and 610 may have been in close proximity during the confrontation. In the chaos of a defensive response against a male, a cub could have ended up on the wrong side of the line. 610, rather than rejecting the extra cub, absorbed it.
Mangelsen, who had been photographing 399 since 2007 and 610 since her birth, said the two families were later seen meeting in the park on subsequent occasions. In June 2020, 399 with her four cubs and 610 with her two cubs were observed in the same meadow. The six cubs played together. The two mothers tolerated each other's presence at close range. Two generations of grizzly mothers, sharing territory, sharing space, and in 2011 sharing a cub, in a national park where the standard expectation is that adult grizzlies avoid each other or fight.
610 went on to produce multiple litters of her own. Twins in 2011, a single cub in 2014, twins in 2015, twins in 2019, and triplets in 2022. She inherited her mother's roadside territory and her mother's comfort with human proximity. She is still alive. 399 was killed by a vehicle near Cody, Wyoming on October 22, 2024, at the age of twenty-eight. The adopted cub's fate is listed as status unknown by the Grizzly 399 Project.
The adoption lasted one season and produced no dramatic outcome. No documentary was made about it. No book was written. It was a single data point observed by a park biologist and a photographer in July 2011, confirmed by ear tag identification and cub count, and noted in the record as an unusual occurrence. A five-year-old grizzly took her mother's cub and raised it through the winter because either instinct, recognition, or circumstance told her to, and nobody who watched it happen could explain exactly why.
Source: Grand Teton National Park / Jackson Hole News and Guide / Grizzly 399 Project / Thomas D. Mangelsen.
