Thursday, May 15, 2008

GRIZZLY AGGRESSIVENESS: HONED BY EVOLUTION

The potential aggressiveness of the female grizzly helps ensure the survival of her young. A female with older offspring is shown here. Photo by Janine Cairns-Michael.

So why is the grizzly a more bellicose bear? It is instructive to look back in time and examine the “neighborhood” that the grizzly bear species “grew-up” in. The fossil record indicates that the first grizzlies were found in North America towards the end of the Pleistocene epoch (geologists tell us this epoch ended about 11,550 years ago, spanned about 1.8 million years and covered the earth’s most recent period of repeated glaciations). The receding glaciers left luxuriant grasslands in their wake that were home to numerous herbivores, like bison, mammoth and yak. Where there are herbivores, there will also be great predators to cull the herds. Such was the case in the Pleistocene and the grizzly was one of these predators. But this was a rougher neighborhood than the modern day grizzly inhabits. There were massive saber-toothed cats, dire wolves and a behemoth of a bear known as the giant short-faced bear (Arctodus simus) – this was a massive creature that is proposed to reach a weight of 2,000 pounds.

While the grizzly foraged in the open grasslands, usually adjacent to forested areas, the black bear was (and still is) a resident of the woodlands - a habitat that is critical for the survival of their young. A black bear cub can climb trees days after leaving the maternal den. By scurrying up trees, neonates are able to elude most predators. In many cases, the mother black bear will climb up the tree behind her young or may remain at the base of the tree to dissuade a potential threat from pursuing her offspring any further. Because of their arboreal abilities, momma black bears rarely have to fight off the enemies of their cubs.

Now consider the grizzly. This species is not an adept climber. Even if they were, spending time in the open grassland habitats preclude the likelihood they are going to be able to out climb an adversary. Therefore, in order for the grizzly to survive and protect its vulnerable young in this more exposed Pleistocene ecosystem, it was the female’s aggressiveness that determined whether her offspring survived or not. Those mother grizzlies that turned into a raging ball of fury when their young were threatened passed more of their genes (including their aggressive inclinations) into the next generation. If she had a disposition more like that of a black bear sow, her young would not last long on this hostile, unforgiving community.

Natural selection has apparently honed the female grizzly into a more volatile creature – unfortunately, when it comes to getting along with H. sapiens their defensiveness is a big strike against them. While a black bear sow with young of the year is more likely to run for cover when encountering a human hiker, a mother grizzly is more predisposed to initiate a preemptive strike (that is, take out the potential threat before that threat can harm her cubs).

Male grizzlies can also be aggressive toward people, but usually only when they are taken by surprise. If a grizzly (boar or sow) knows your coming, more often than not, they will vacate the area in order to avoid contact with their most formidable rival - man. But if the boar is suddenly face-to-face with a human, it may do as the female with cubs does; that is, take out the menace to prevent harm to itself. That said, in most cases, a surprised bear will simply bluff charge, hoping that intimidation will be enough to prevent the threat from attacking or run away. On rare occasions, usually when the person behaves inappropriately, the bear may make contact.

Many grizzly attacks can also be attributed to defense of a food source. The script for these episodes usually goes as follows: unknowing human stumbles onto a food cache (usually the carcass of a large animal), grizzly that is resting nearby guarding its prize, or feeding on it, attempts to drive off potential competitor (said human). Bear bites and slaps unsuspecting interloper around a few times and returns to its post. A subsequent assault may occur if the human does not behave appropriately (e.g., gets up attempts to flee, fights with the bear). In most cases, the curl up and “kiss your ass goodbye maneuver” (that is, play dead) is the most appropriate defensive strategy in these situations.

It was one of a family of grizzlies that were defending their food that mauled a jogger, 54-year old Dennis VanDenbos, around Jackson Hole last summer (2007). Janine and I encountered this ursid family unit (a sow with three larger offspring) in the willows along the road early one morning last spring. The family had been hanging around in this area, to the delight of many tourists, for a long time and had not been implicated in any aggression towards human observers. But this morning, the 350-pound sow, along with her cubs, were feeding on an elk carcass, when the unsuspecting Mr. VanDenbos stumbled across the bruin bunch and their freshly-killed ungulate prize. When confronted by the bears, VanDenbos yelled at the bears and when that didn’t work, he hit the ground and played dead. One of the bears ran forward and bit and clawed the man (for more on this story click here).

It may be surprising to some readers that grizzly attacks rarely have a predatory component. Of all the serious human-grizzly conflicts that have occurred in Alaska, only 1 % of these attacks ended in the victim being fed upon. In fact, the majority of attacks (about 88 %) were of short duration, lasting no longer than 60 seconds. The typical encounter is typically fast and furious.

While black bear attacks may be less frequent than those inflicted by grizzlies, bear biologist Dr. Stephen Herero has documented U. americanus assaults are often motivated by predation. If the black bear presses home an attack, it is more likely an attempt to eat you than defend progeny or a resource. In fact, Dr. Herero recommends fighting, tooth and nail, against a marauding black bear, not playing possum. In the last couple years, there were two cases of black bears preying on humans: one case in Utah (click here for more) when a black bear killed a 11-year old boy (2007), and one case in Tennessee, where this species killed a 6-year old (2006) (click here for more).

So how dangerous are bears, especially grizzlies? We will investigate this in the next ursid post.
Copyright (2008) Scott W. Michael

1 comment:

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