Anomalies of Reproduction & Acrimonious Behavior of a Mother Bear

Photo: Grizzly bear #533, pictured with her three, 3-year-old cubs after emerging from her den during 2008. The Interagency Grizzly Bear Study Team has only been privileged to documented two radio-collared females, whose cubs accompanied them for a third year. Typically, cubs will separate from their mothers in the spring when they become 2-year-olds. This image was capture by Steve Ard during an aerial flight over Cougar Flats, in Yellowstone National Park, May 1, 2008.

​The Interagency Grizzly Bear Study Team (IGBST) has been radio-marking and monitoring grizzly bears in the Greater Yellowstone Ecosystem (GYE) since 1975. Over the several decades of monitoring, there have been various aspects of grizzly bear ecology and biology which prove to be increasingly rare. Since 1975, the team has collared and monitored 900+ unique grizzly bears. One of the rarities that has been documented only twice since 1975 in radio-marked bears, is cubs accompanying a female for a span of longer than two-years.
 
Typically, grizzly bear females will separate from their cubs when they are two-years of age, generally in the spring. During this time, males bears actively seek out a female companion sometimes pressuring the separation of cubs from their mothers, inducing the female into estrus.
 
Another somewhat related phenomenon which actively plays a role in reproductive ecology, is sexually selected infanticide or referred to as SSI. In Yellowstone and the GYE, this has been documented on various occasions. This process is a reproductive strategy utilized among brown and black bears throughout North America, not only endemic to the GYE. It involves a dominant male being removed, or inaccessible during mating. This allows for a new subordinate male to enter the picture. The subordinate male will kill newly born cubs, or sometimes yearlings sired by the now absent dominant male. The female will then be induced into estrus, allowing her to breed earlier.
 
Grizzly bears have the lowest reproductive rates among terrestrial mammals. Generally speaking, a sexually mature female can have litters of cubs about every 2 ½ years. However, those females that retain their offspring for additional seasons throw a curveball into the picture. There are benefits for the offspring, but not so much for the female.
 
During 2008, the IGBST identified a radio-marked female, who had kept her cubs until they were 3-years-old. Grizzly #533 was one of the few radio-marked females that the study team had recorded in nearly three decades of research, to have her offspring accompany her an additional year. More recently, an unmarked sow grizzly frequenting the East Entrance of Yellowstone National Park, was also documented with a cub that accompanied her for the same duration of three-years. Based on the frequency of this phenomenon over the decades, it appears to be a rare occurrence. However, this behavior (anecdotal) seems to have become more common in the past several years (more sightings of unmarked grizzly females accompanied by subadult offspring).

Photo: Images taken from the Yellowstone Grizzly Family Chart 2017. This sow (pictured on the left) was accompanied by her cub (right) for a third year during 2018. Photos courtesy of Jort Vanderveen

Additional Information: Grizzly #533 was first captured on July 29, 2006 at Cold Springs, in Caribou Targhee National Forest (CTNF) at 14 years-old. She was fitted with a radio-collar and at the time, was unaccompanied by any offspring that year. However, during 2007, the IGBST observed 533 with three 2-year-old cubs; again accompanied by three 3-year-old cubs in 2008 at Cougar Flats in Yellowstone National Park (YNP). The following year, 2009, she was documented and observed  with three cubs-of-the-year (COY). 

On June 28, 2009, grizzly #533 had a run in with hunters.  Shots were fired, but she managed to escape unscathed. DNA evidence later confirmed that she and her COY were involved in the incident.

(http://www.thewildlifenews.com/2009/07/14/island-park-grizzly-that-attacked-hound-hunter-found-alive-and-well-with-her-3-cubs/)

In 2010, #533 was observed with two of three cubs remaining (yearlings). In 2011, she was not observed with any cubs, and she later cast her collar. In 2012, she was captured at Moonshine Mountain, CTNF and collared again. That year, she was unaccompanied by any cubs. During 2013, it would prove to be a similar repeat of 2009. Grizzly #533 was surprised from her day bed by a researcher during July 2009. She proceeded to attack the researcher, later running away from the scene.

https://idahonews.com/news/local/researcher-attacked-by-grizzly-bear-in-eastern-idaho

Later that summer, grizzly 533 was killed. She was documented as a mortality in the 2013 IGBST Annual Report as:

Unique ID: 201320
Bear: -
Sex: F
Age: Adult
Date: 2013
Location: ID
Certainty: Known
Cause: Human-caused. Under investigation.

Grizzly #533 was 21 years-old at the time of her death.