It is nowadays considered a misconception that grizzly bears have poor eyesight and this misconception has been rebuked by science.
It is thought that grizzly bear vision is akin to that of humans.
However, I recently read the publication, ‘The Grizzlies of Mount McKinley’ by the famous early naturalist Adolph Murie (1981).
I have read other publications by the author and, overall, his perceptive and observational ability appears advanced which was why I was curious to notice references to the potentially poor vision by grizzlies in his publication.
It is possible that Murie ‘saw what he believed’ because he could have been told previously by individuals whose insight he trusted that grizzlies had poor eyesight (according to popular ‘mythology’).
However, some of Murie’s own observations made me wonder whether there might be an aspect to grizzly vision and perception that is not related to the eyesight being poor vs. good but rather to adaptive qualities of grizzly eyesight which sometimes confound the observations as grizzlies react to visual conditions in a manner that suggests inability to see clearly.
I will first quote the passages that I found worth evaluating in Murie’s publication (but that could be dismissed as erroneous assumptions on Murie’s part),
‘The pelage is usually grizzled, the tips of the hairs being light. This is not evident in many dark bears. The shade of color of a bear varies according to the direction from which the light strikes it, relative to the position of the viewer. The bear appears darker when facing away from the light because of reduced reflection. On one occasion a tourist told me that he had seen a bear on Sable Pass that was light on one side and black on the other. I had noticed this striking difference in this particular bear, too. Both sides were alike in color but as the bear shifted its position in relation to the light, the color tended to vary from blondish to blackish. The effect of light direction of this blondish bear was more extreme than noted in any other individual. Possibly this was due to some special character of the grizzling in this individual.’ [1; pp. 28].
‘Pelage color usually undergoes seasonal change. The grizzlies in Mount McKinley National Park emerge from hibernation with their autumn coats still in excellent condition. As spring progresses, the fur tends to fade and lighten in color. The long northern days and light reflection from snowfields probably accelerates this fading.’ [2; pp. 28]
‘The spring cubs are blackish or dark brown in their spring coats. By August they have acquired a new coat and show a lighter grizzling that is more pronounced in those cubs that were brownish in the spring.’ [3; pp. 28]
‘Bears are reported to have relatively poor vision, at least at long distances. My observations do not contradict this. Individual visual recognition, within families for instance, sometimes appears to be unreliable, especially if cubs become separated from mothers by several hundred yards. At such times there can be much hesitation on the part of the cub to rejoin the mother, even though they are in sight of each other. Olfactory reassurance that an adult is, indeed, its mother seems prerequisite for a cub to resume its usual activity within the family.’ [4; pp. 30]
‘In the field, I have frequently noted that the size of the cubs compared to their mother seems to vary a great deal from day to day. At times the cubs seem large, then again, small. I also have noted in examining several pictures of a family that the cubs seem large in some pictures and small in others.’ [5; pp. 39]
‘Once I saw two 2-year-old cubs near other older cubs and could easily recognize them as smaller. But when I saw these two by themselves, their smaller size was not obvious. It has always seemed to me that the closer one approaches a bear, especially a cub, the smaller it appears to be. I have been close to known 2-year-old cubs that seemed to be the size of yearlings.’ [6; pp. 39 – 40]
‘The mother of three spring cubs that had been visiting a garbage dump was found dead. When my friend, who had often seen the mother alive, saw the carcass, he exclaimed that this could not have been the mother of the three cubs because that mother was ‘huge’ and this dead one was small. Yet her dark color and the presence of the orphans made the identification clear. [..] But, as final word, size in big country is always deceptive.’ [7; pp. 40]
‘On 30 August this family again was discovered at the base of Mount Eielson, about 2 miles from where they had been seen in May. Three or 4 inches of snow lay on the ground. The mother was digging ground squirrels, her cubs huddled up about 100 yards from her, hidden by a growth of willows. Once she stopped digging, looked around, and dashed toward the cubs. She sniffed them as though to reassure herself of their identity and then returned to her digging.’ [8; pp. 74]
‘In general, one discovers a bear or a family off by itself, comfortably apart, feeding at ease. But in some favourite feeding areas, such as one on Sable Pass, one often finds that two or more bears are, by chance, feeding quite close to each another. They seem oblivious sometimes, but at other times are gauging the safety of the situation, keeping aware of the other bear’s position and moving accordingly, making a fine adjustment or departing from the neighborhood.’ [9; pp. 82]
‘Most grizzlies I have observed on McKinley are, so far as I know, on familiar ground and are not beset by the added worry of being in a strange area. On a few occasions I saw mother bears that had ventured beyond their familiar ranges. These mothers were excessively alert and apprehensive. On 22 August, 1962 I watched a dark female with her two spring cubs for several hours on the Polychrome Pass [..]. This was so far as I know, her first appearance in the area during summer. [..] She behaved as one would expect a bear in strange country. She seemed to be on edge, was ever watchful, and received several false scares. When, later in the day, she spied a young bear about 400 yards away, she hurried away immediately without trying to get a better look.’ [10; pp. 83]
***
I believe that these quotations attest to the following hypotheses that could be studied further:
- Brown bears (but particularly grizzly bears due to the unique, grizzled colouring of their fur) have developed a defense-disguise properties where it is difficult to assess the size of the individual (the size being a crucial factor in dominance and conflict avoidance). These properties confuse observers (including bears themselves) because bears are no longer easily individually identifiable from distance.
- Grizzly bears have adequate vision but their vision might have evolved diverse types of focusing (zooming in or out) due to the different foraging strategies as well as due to the bear morphology (head usually lower than shoulders but it can be raised and the bear can also sit up or stand on hind legs). It might be difficult for the bear to smoothly switch between one type of vision (near vision, lowered head) to another type of vision (distance vision, rising on hind legs), especially, under circumstance of stress or physical discomfort.
- There could be additional factors contributing to the two confounding conditions described above, such as, landscape (grizzly bears often are found on landscapes where the size becomes rather indefinite due to the vastness of the terrain or it is difficult to assess as relative to other objects because there is a lack of such prominent objects), fur colouring changes due to fading (grizzly habitats can be rather exposed to the Sun and they can have a prolonged snow cover period), changes due to maturation of cubs etc.
1
Disguise/defense
Body size is a property of dominance in bears.
As bears mostly attempt to avoid conflict by estimating the opposing force from distance, large body size or the ability to appear large (while, in truth, being smaller) can contribute to the bear’s dominance status and survival.
While is can be hardly useful in close proximity situations (e.g., it would not assist mating because the prospective mate has to approach the potential partner thereby discovering the true size), it can help bears (especially, smaller bears due to genetics, poor resource availability during crucial growth periods or due to current developmental stage) to avoid agonistic encounters by deceiving the opponent into assuming that their strengths are equal (while they are not) or even skewed toward the benefit of the ‘disguise-proficient’ bear.
If all bears appear larger than they are from distance, this is rather ineffective because the bears would simply have learned to adjust the flaws in their visions.
If only some individuals appear considerably larger, this could be advantageous to them in order to reach maturity, to protect cubs and to ensure other types of conflict avoidance that are usually accompanied with evaluating the opponent from distance.
The ability of cubs to appear larger than they are could also facilitate their survival by confusing adult male bears during the mating season and thus avoiding infanticide.
I have not yet been able to find hard data regarding the likelihood of infanticide in different age groups of cubs/juveniles but this ‘deception regarding one’s age’ might be beneficial if cubs are likelier to be killed than yearlings and yearlings are likelier to suffer infanticide than two year olds etc.
(Namely, it would be impossible for a cub to appear adult-sized but the cub could appear as an older cub/juvenile.)
Similarly, mothers might be better able to protect their cubs or to select optimal foraging grounds if they appeared larger to hostile males or other competitors.
This would provide a selective advantage – cubs that can pretend to be older and mothers that can pretend to be larger (and that pass on the ‘disguise’ genes) would contribute to higher survival (or survival of offspring, i.e., one’s genes) in ‘trickster-bears’.
Cubs that are not individually recognizable might additionally confuse adult males who attempt to determine whether the cubs have been sired by them.
This would not benefit actual offspring but it would benefit non-offspring because once the male bears have learned to recognize non-related cubs (and perhaps this occurs through visual mechanisms and not in close proximity), they cannot rely on this recognition because the next time they meet the cubs with the specific mother, the cubs appear different and the males cannot determine if those are even the same (unrelated) individuals.
The observations (4 – 7; 9) by Murie relevant to this hypothesis are made from the perspective of a human.
However, the contemporary research demonstrates bear vision to be comparable (or better) than that of humans.
Also, it might be important to consider perspective the height of the individual looking at the bear (or, more specifically, the positioning of visual organs relative to the object that is observed).
Human height can be greater than grizzly height if the grizzly has raised their head at ca. shoulder height (1 – 1.5 m) or it can be lesser if the grizzly bear is standing (2.5 m).
However, the positioning of the field of vision is relatively similar (compared to, for example, a wolf or a coyote who have to rather look up although this, of course, depends on the terrain).
Also, the positioning of the eyes in the head is frontal.
I do not suppose that grizzly bears would benefit from their prey perceiving them as larger because they would rather rely on not being noticed and they do not hunt by chasing ungulates extensively (during chasing, body size could induce panic and confound escape).
Thus, I think that if this phenomenon exists, it is rather directed at other grizzlies or perhaps competitors of a more broad array of species.
2
Specifics of bear vision
Bears spend a lot of time foraging on very small items that are found on the ground level.
Their vision is likely well adapted to discern details in near vision capacity.
However, distance vision is also significant because it allows to evade danger (e.g., approaching conspecifics) that is important in a non-territorial and non-cooperating species with dominance hierarchies and infanticide strategies.
Additionally, bears can alter their field of vision by keeping their head below shoulder level, raising it at shoulder lever (which is rather high) and also by sitting up or standing up on hindlegs.
This ensures a diversity of opportunities to focus on ground level (or small, nearby objects) or to let the sight travel far and to apprehend the more distant objects/subjects/conditions.
I believe that it is not as easy to switch between these modes and to adjust the brain processes so that the visual data were processed efficiently.
I base my assumption on the fact that, for example, my dog tends to adjust her ‘daily mode of vision’ to the landscapes we are frequenting on our walks.
If we have been walking in forests or other sites where the vision has to ‘zoom in’ or to be able to catch evasive movement amidst the trees, she becomes proficient at detecting other species etc. in this particular setting.
If after a while in the forest environment, we begin walking on roads in more open landscapes (agricultural fields), initially, she misses out on many of the animals that would be perceived through the use of distance vision and that are not detected by motion (because they normally react by standing still).
In a day or two, she, once again, becomes efficient at spotting these individuals.
Personally, it appears to me that switching from close to open mode of vision is more difficult than vice versa.
Thereby, it might not be easy for bears to zoom in and zoom out or to alter between the height of vision (ground level, shoulder level, standing up), either.
If there are stress factors involved (danger, protectiveness of cubs, novel environment), these switchings-between-modes might be even more difficult to process.
Accordingly, I think that the quotes 8 – 10 by Murie refer to the bear’s inability to smoothly transition from one mode of vision to another under stressful circumstances.
Bears might be aware of their relative lack of efficiency.
As indicated in quotes 9 – 10, the recognition of potential danger (proximity of strangers or foraging in a novel area) entices the bear to switch between different eyesight levels (low to the ground vs. raised head/standing up).
It is possible that bears do not have poor vision but that while they are zooming in (focusing on forage at ground level or on objects in their paws), they are less able to keep track of the more distant surroundings.
This could result in the situation mentioned in quote 9 by Murie where the bears can be oblivious to one another’s presence.
Of course, when bears are foraging, e.g., in willow groves or long grasses, the habitat itself impedes the scope of vision and the bear has to switch due to the determinants of the physical obstructions.
However, it might also happen that bears (unlike ungulates with their side vision) cannot feed and keep vigilance at the same time.
Still, it would seem that the bears should be able to apply vision and olfactory skills to the foraging activity while applying, for example, their hearing to the purpose of vigilance.
Personally, I suspect that bears, while focusing on near vision faculties, fall into some type of perceptive ‘trance’ where the world ‘shrinks’ and turns into a ‘microcosm’ (zoning in on the foraging objects and zoning out of the vaster surroundings – not unlike what humans attempt to achieve during meditating on objects).
That is to say, it might feel to the bear, subjectively, that they have been somewhat enveloped in the immediate surroundings (a perception that is formed while using near vision below shoulder level) and that the world outside of these surroundings has ‘ceased to exist’.
The bear is almost startled when the ‘vaster world’ reminds of itself.
It seems to me that the bears attempt to actively avoid this effect by alternating between low level and high level vision so that they do not become drawn in to the ‘microcosm effect’.
This bewilderment upon exiting the near sight mode perhaps partly revealed in the quote 8 by Murie where the mother seems extremely perturbed once she has extracted herself from the focus on foraging activity and she has discovered her cubs gone.
The anxiety, on this occasion, could be attributed to the concealment of the cubs in the willows.
However, her extreme reaction even disallows her to engage her other senses calmly (probably not that much time had elapsed for the cubs to have disappeared ‘for good’ without being able to alarm their mother if something bad were happening) and it reminds me of the state that we experience after awakening from a dream (with a start).
This drawn-in is likely enhanced if the foraging site creates a nesting effect (long grasses or a hollow, a place surrounded with fallen trees etc.).
Thus, I believe, when bears are emotional or altogether anxious, they have difficulties switching from near vision to distance vision although neither vision is impaired.
Also, I believe that bears have difficulty keeping vigilant of their surroundings while focusing on ground level activities (or small objects they are handing with their paws) which is why they can sometimes appear oblivious to other bears nearby or they can become highly startled.
It might also be the reason why they actively alter between foraging and vigilance because they are aware they might not be able to notice danger unless they change their focus.
3
I will not discuss at length how landscape can affect bear vision or our observations of bears and how different attributes can change with seasons or developmental stages etc.
It would be, nevertheless, interesting to examine if bear vision is different between populations that live in forested vs. mountainous and bare, open areas.
In fact, I believe one of the reasons why bears raise their head, sit up or stand up to assess the situation could be related to the frequent lack of adequate objects (trees, large stones etc.) that are unchanging and that can be used to gauge the relative size of the other bear.
This could become further aggravated if the perception of the size of objects considerably changes according to the head positioning and the bear has to process contrasting information acquired from several perspectives (angles).
Bears that live in such barren landscapes might have developed an ability to estimate the other bear’s size by comparing it to their own from their fixed point perspective.
It is not as reliable of a system as comparing to a third object (unchanging subject) but it could be more efficient than assessing the opponent’s (potential mate’s) size against the background of scattered small rocks and dwarf shrubs.
Also, it might be interesting to investigate effects of the brightness of the habitat on bears’ vision in different ecosystems.
For example, in bright conditions, bears might find it physically unpleasant to keep watch in the distance.
Perhaps, under such conditions, bears are likelier to use patches that do not create the ‘enclosure’ effect so that they could still look from below shoulder level in proximity of objects that are not as tall as trees are but that are sufficiently tall to provide some shade.
Notes by Ieva 