Rotund, plump, hefty — go ahead and pick the fat synonym and it’ll likely aptly describe Grizzly 566, the second-heaviest grizzly bear ever documented in the Greater Yellowstone Ecosystem.
The 19-year-old male was well known to biologists in Yellowstone National Park, where the big bear resides. As a 3-year-old in 2007, he weighed in at 232 pounds. During a 2010 handling, the boar had plumped up to 393 pounds. His weight stayed in that range, registering at 381 pounds when caught and immobilized at age 9 in 2013.
Then a decade went by without Grizzly 566 coming onto biologists’ radar.
Yellowstone biologists nabbed him once more on Oct. 15 while trapping bruins for routine grizzly bear monitoring, according to Interagency Grizzly Bear Study Team leader Frank van Manen. It was the tail end of a verdant summer following a long-lasting winter in the Northern Rockies. Grizzlies in the Yellowstone region eat upwards of 266 species in four animal kingdoms, and options for foraging were evidently plentiful in 2023 — and especially so for this bruin.
Grizzly 566 weighed a whopping 712 pounds.
“You don’t come across animals of this size very often,” van Manen said.
In fact, he said, the only heavier Yellowstone-region grizzly bear ever documented was encountered all the way back in 1977. That beast of a bruin was a 715-pound male.
In some respects, Grizzly 566’s near record-breaking heft isn’t completely surprising. Male grizzly bears don’t reach their peak body size until age 14, van Manen pointed out.
Plus, a couple other grizzlies handled in 2023 had relatively high body fat percentages, he said. Ordinarily, by October, females reach 28% to 30% body fat and males are just a little bit fatter — it’s 32% or so of their fall body mass.
Boars are the fatter sex because they need extra reserves for when they emerge from the den. That’s breeding season: Males are more interested in getting to know female grizzlies than extensive eating, and they’re actually losing “quite a bit” of body mass in April and May, van Manen said.
Grizzly 566 figures to be set up well for ursine philandering come next spring.
“He had 41% body fat,” van Manen said. “It looks like the highest [body fat figure] we’ve had before was 43%.”
Were Yellowstone-region grizzlies as a population fatter than average in 2023?
Tough to say, van Manen said, though the rate of fat gain appeared “pretty much normal” this year, he said. The leanest time of year is the end of June, when grizzlies typically range from 15% to 20% body fat. Because dozens of grizzlies are captured across the ecosystem throughout each spring, summer and fall, comparing grizzly bear fat levels from one year to the next isn’t straightforward statistically.
Taking a longer view, however, the science on grizzly bear body fat is more clear. Grizzly fat accumulation rates have not changed over the decades, nor is there any correlation to population density, recent research has found.
That says a lot, van Manen said, about the remarkable plasticity of grizzly diets: Even as major food sources like whitebark pine, cutthroat trout and some ungulate populations have declined, bears are still packing it on.
“The way these animals are gaining fat from June through October hasn’t changed,” he said. “These are incredibly resourceful animals and they’re finding calories on the landscape.”
Interested in everything that happens in Wyoming!
What is a full grown male Grizzley Bear’s normal blood pressure measured on the forearm?
Fans of grizzlies might want to read “Biography of a Grizzly” by Ernest Thompson Seton, published about 1900. Try used book stores. Seton illustrated his books (gifted artist), was Ontario’s Provincial naturalist. Fabulous book.
saw a large grizzly in october of 2022between yellowstone forever amd cook city
it was huge
living close to yellowstone for 4 years was wonderful
lived in park in 1975 never saw a beat
How did they weigh him?
Y’all see a photo of a big Grizzly bear that weighs over 700 lbs. and the science shamans and biologists who study these bruins claim it is the second heaviest bear ever documented in the Greater Yellowstone . What I see is a sterling example of government Grizzly researchers who do not know what they do not know. Which puts ALL of their computer modelling and science-based policy decisions under skepticism if not outright suspicion.
There are a plentitude of griz heavier than 712 lbs. that have been known over the years in and around Yellowstone . They walk among us. The bear on exhibit at the Meeteetse Museum affectionately called ” Little Wahb ” after the fictional bear in Ernest Thompson Seton’s eponymal novel was reported to 900 lbs. by the Billings gazette. That is an exaggeration . It was 800 give or take. Several bears in the Greybull River Valley above Meeteetse observed since the 1870’s have been north of ” huge “.
For Grizz known to weigh in a half a ton or more, you gotta go back to Yellowstone Park proper during the era when the Park Service actually fed bears as an evening tourist attraction out behind Old Faithful Inn. They built an amphitheatre for that , and there would be as many as 60 bears show up for the banquets. Well known Cody man Cliff Spencer who was a sharp photographer in the early-mid 20th century has a 2-frame panorama of one of those feedings that shows five dozen black and griz bears in a scrum of restaurant garbage and roadkill elk carcasses. I’ve seen that photo: there is a Griz standing upright on its hind legs towering above all the other bears, and she was in the center dining alone. That whole social hierarchy thing at work . The giant Old Sow always ate first, get in line Bruno. She looked half again bigger than the next larger bears.
Here is where I say the largest known bear weighed at Old Faithful during the dump feeding years came in at 1164 pounds. But even that is not the largest Ursus Horribilis specimen ever known. The so-called Fitzgerald Bear taken in the wilderness of Alaska tipped the freight scales at 1600 lbs and stretched to 10 feet tall… same species as Yellowstone bears, just the heftier northern variant. That is a big bear. Sumo bear.
Again my poin t here: Greater Yellowstone Grizzly Bear researchers do not know what they don’t know. Keep that in mind when the likes of Frank van Manen and his ilk start waving their arms at the PowerPoint screen and start spouting some numbers.
You are correct. I won’t hazard a guess as to how many 700 Ibs. plus grizzlies are out there, because I have no clue. Consider this: Fish & Game said “Little Wahb” bottomed out an 800 pound scale, so he could have weighed more than that. Also, just a few years ago Fish & Game euthanized a grizzly in the same area for livestock predation and they said in a newspaper article that he made “Little Wahb” look small!
What a trophy!
Wow – he is BIG and beautiful. Thank you for the great article.
May the Gods Bless and keep the Silver Bear healthy.
Larry Majerus
Impressive weight gain , be interesting to know his actual diet and not assumed
AWSOME!