What with polar bear populations higher than they were 50 years ago and with many bears onshore during the ice-free season, a few polar bear attacks are to be expected – but how does the behaviour that drives those attacks compare with their closest evolutionary cousin, the grizzly?
I’ve done some summer reading on this topic, which I’ve summarized below. The results may surprise you.
Figure 1. From a recent article in the DailyMail (July 8 2014) about the inquest into the polar bear attack on a group of Eton schoolboys in Svalbard, Norway: “The camp site where Horatio Chapple was mauled to death. Witness Matthew Burke said: ‘The bear reared up and then slammed itself down on to Horatio.’” Read it here.
First, a quick evolution refresher course: according to both fossil and genetic evidence, Asiatic and American black bears are the oldest living species of bears (Herrero 1972), with grizzlies (aka brown bears) and polar bears arising sometime later (the exact timing depends on the kind of information you use and how the analysis has been done: there are a variety of answers).
Fossil data indicate grizzlies arose less than 1 million years ago and polar bears even more recently (oldest polar bear fossil ~130,000-115,000 years old) while genetic data suggest both are much older (Figure 2), see my previous posts listed at the end of this essay.
Figure 2. A summary of bear evolution according to the fossil record, relative to climate change over the last 5 million years or so (based on proxy data from the Vostok ice core in Antarctica). Living species (Asiatic and American black bear, brown bear/grizzly, polar bear) have only existed for 2 million years at most, according to the fossil record; but dates of polar bear and grizzly speciation based on genetic analysis vary widely.
Bear attack behaviour compared
I’ve composed a simple diagram (FIg. 3 below) to sum up the behaviour of bears as it relates to their interactions with humans (especially attacks on people), compared to their evolutionary relationship:
Figure 3. A summary of bear behaviour by species as it relates to attacks on people, contrasted with the evolutionary relationship of the bears to each other.
Black bears preferred “defense” of their cubs is to lead them away or up a tree – or just leave them to their own devises (Herrero 1985) – and seem to be rather easily driven off a carcass or other food source they’re feeding on, sometimes preceded by a bluff charge or two. Black bears are, however, well known for occasionally initiating purely predatory attacks on people (Herrero 1985, 2003; Herrero and Fleck 1990; Jonkel 1978; Sheldon 1997, 1998, 2001).
“We judged that the [black] bear involved acted as a predator in 88% (49 of 56) of fatal incidents. Adult (n=23) or subadult (n=10) male bears were involved in 92% (33 of 36) of fatal predatory incidents, reflecting biological and behavioral differences between male and female bears.” (Herrero et al. 2011)
Polar bears are known to defend their cubs rarely — 5 out of 6 known attacks in one study occurred at or near the site of the females’ maternity den in spring. Herrero and Fleck (1990:30) made this statement about polar bear attacks on people at den sites:
“This low incident rate and the fact that all incidents occurred in or near denning areas suggest that polar bear females are less aggressive in this regards than are grizzly bear females, but more aggressive in defense of young than are black bears (see Herrero 1985).”
Few (if any) polar bears attacks are associated with defending a kill but predatory polar bear attacks on people are legion (Clark 2003; Dyck 2006; Gjertz and Persen 1987; Stenhouse et al. 1988).
Grizzly behaviour towards people is quite different — females defend cubs of all ages and both sexes defend precious food sources ferociously (not only their own kills but those “taken over” from hunters or other bears). While a grizzly attack that started out as a defense of cubs or food may occasionally turn predatory (with intent to kill and consume), this is a rare outcome with grizzlies.
Oddly, polar bears appear to be more like black bears in their responses to humans (as opposed to their reaction to other polar bears1), especially when it comes to predatory attacks.
Subadult male polar bears more commonly initiate predatory attacks than other age classes, although as the fatal August 2011 polar bear attack on a group of adventuring UK students on Svalbard demonstrated, old bears may also treat people as easy prey. From a report on the inquest into that attack, by a male polar bear (July 10, 2014)
“Subsequent examination of the polar bear established its age to be around 24 years old. Bears in the Svalbard archipelago rarely reach the age of 25.
High Court judge Sir David Steel’s report produced on behalf of the BSES revealed polar bears of that age generally weigh some 400kgs. The polar bear that attacked weighed roughly 250kgs, and was thus seriously underweight.” [my bold]
In other words, polar bears (and black bears to a lesser degree) are known to treat humans as prey. Polar bears will stalk you, kill you and eat you as if you were a rare sort of seal and black bears occasionally do the same, as if you were a deer.
The fact that grizzlies rarely do this is rather curious, given that grizzlies have a closer evolutionary relationship to polar bears than to black bears.
Even in areas where they are hunted, grizzlies will beat up anyone or anything that violates their space, threatens their cubs or their food – sometimes with deadly force.
But grizzlies rarely initiate predatory attacks (i.e., killing and eating humans without a provocation that sets off the attack), although there are some exceptions (see Shelton 1998, 2001). For example, see this historical note, from Steven French’s “Bear Attacks” chapter (in the 2001, 4th edition of “Wilderness Medicine”) about a situation that spawned killer grizzlies (click to enlarge):
A few examples of bear attacks on people, by species:
Polar bear: Winter 1880-81, Bering Sea; December 1990, Alaska;
November 2013, Churchill; July 2014, Hudson Bay;
Grizzly: September 2014, Calgary
Black bear: May 2014, N. Alberta
Do evolutionary differences explain this conundrum?
Grizzlies evolved in the company of other large and powerful Pleistocene predators (including now-extinct short-faced bears, and cave bears) that would have been serious competitors for food, and very real threats to cubs.
So the hyper-defensiveness of grizzlies, who have always lived in rather open habitat, makes evolutionary sense.
The smaller size of black bears and their greater evolutionary age means that they have a long history of being at a serious disadvantage against larger predators of all kinds, including grizzlies and large Pleistocene bears – so their propensity to flee and climb trees rather than fight makes sense.
And it even makes sense that some black bears undertake some quite spectacularly vicious acts of predation on people – including starting consumption long before the victim is dead (Shelton 1998). Presumably, some black bears find out that people are not only ‘not a threat’ but almost as defenseless as the young deer and moose that form an important part of their diet in some areas.
In a similar way, the lack of defensive response by polar bears fits their ecological niche, since they evolved in a habitat where they were virtually the only big predator around — and their enhanced predatory nature fits with their almost totally carnivorous diet.
It just seems curious that grizzlies are so much less predatory than polar bears — and less so even than black bears.
Another oddity of nature (for the moment, anyway).
Footnote 1. Bear-to-bear behaviour, of course, is another matter – all species will vigorously defend themselves, their cubs, or their food, from other bears (or other predators, like wolves).
But in the interactions between bear species – say between a black bear and a grizzly or a grizzly and a polar bear, the aggressiveness of the grizzly stands out.
And as I’ve noted before with regard to hybridization, one of the reasons that grizzly x polar bear hybrids are almost always the result of a female polar bear mating with a male grizzly is because between the two species, grizzlies are the more aggressive – grizzlies dominate polar bears in encounters between them.
Female grizzlies will take on male polar bears more than twice their size – and win.
In the wild, a female grizzly would be far too aggressive to stand still for the sexual advances of the demure (relatively speaking) polar bear male. Zoo matings are another story, especially if the individual bears have lived together for a while.
While it may be possible for black bears and grizzlies to mate successfully and produce fertile offspring, I’ve never heard of such a cross – suggesting they are black bear x grizzly hybrids are rare to non-existent.
Related posts:
ABC bears and polar bear evolution – and an adventure August 8, 2013
Is it plausible that Polar bears are 4-5 million years old? Part 1 August 19, 2012
Is it plausible that polar bears are 4-5 million years old? Part 2, Hybridization August 22, 2012
Is it plausible that polar bears are 4-5 million years old? Part 3, Sea ice August 23, 2012
New genetic study confirms polar bears survived several warm Interglacials March 17 2014
Eemian excuses: the warm was different then, polar bears were fine November 12, 2013
Polar bear spin reaches new heights as geneticists promote their work March 24 2013
Polar bears cavorting with ABC brown bears not supported by geological and fossil evidence March 27 2013
References
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