Senses

 

There is a relatively well-established body of common knowledge about the sensory capacities of bears: that they have an acute, if not unmatched, sense of smell and that their eyesight and hearing is so-so if not underdeveloped. Some of these perceptions are supported by research and some not. In this section I provide an overview of evidence pertaining to the vision, hearing, and olfaction (i.e., smell) of bears--which includes the giant panda (see  Evolution). Where appropriate, I also provide illustrative anecdote from my own field observations. And, as in most of the background sections, much of the information that I provide is comparative. The only way to understand the sensory abilities of bears is in context of the sensory abilities of other species, especially other carnivores, scaled to account for the effects of body or brain size. Most organs, including sensory organs, are large simply because the animal is large, without any direct correlation with acuity of the associated senses. As a result, most of the graphs I present below emphasize deviations in the size of structures relative to what one might expect from body size alone--which is a better indicator of sensory development.

Eyesight

 

There is not much known, at least in a reliable scientific sense, about bears' eyesight. The graph at left shows the size of eye relative to the size of brain for a number of mammal species. Each dot represents an average for a single species. The large brown dots denote bears; the reddish dots canids (e.g., dogs); and the orange dots felids (e.g., cats). It is noteworthy that both of the represented bears species (polar and brown) fall well below the main trend line, which suggests that bears have relatively under-developed eyes--and perhaps eyesight--compared to most other mammals. Hence, the notion of myopic or near-sighted bears may be true.

 

More certainly, laboratory research has established that bears have color vision. Black bears have been shown to reliably differentiate colors along all of the major gradients, with the possible exception of discriminating green from red. Similarly, pandas have been shown to reliably differentiate green, red, and blue. Added to this, of course, is an ability to differentiate shades of gray, from black to white.

On a more anecdotal note, it is clear from observing bears that they rely more on their nose than they do on their eyes in many situations. When encountering a human they often seem to peer uncertainly at the person, and even approach to get a better sense of what they are encountering. On the other hand, bears clearly rely on their vision to coordinate actions of the paw and mouth during the close-range acquisition of food--as when they are eating seeds, berries, or spawning fish. Along these lines, I've observed an interesting interplay among the senses, as when a bear is differentiating among plants with and without edible roots. Such a situation occurs when grizzlies in Yellowstone are seeking out yampa roots in meadows populated by yarrow, which is a plant of similar size and flower color to yampa. The involved bears seem to first approach a likely plant on the basis of vision (tall, white-flowered, and umbel-like), and then switch to their nose to make a final determination of whether the plant is yampa or not.

Hearing

 

As with eyesight, inquiries into the nature and acuity of hearing among bears have been relatively limited. Perhaps the most comprehensive treatment of carnivores in general has been done by Rebecca Ruprecht. She was especially interested in how sensitivity to emitted sounds scaled with body size. The results of her work are shown at right. Again, each dot represents a different carnivore species, with bears denoted by a brown dot. The up-down bars straddling each dot denote the range of frequencies to which different animals during different tests were most sensitive. The overall point here is that dominant sensitivity (in Hertz) declines with body size, which is not too surprising given the biomechanics of vocalizing and auditory organs.

 

In addition to showing the results of Ruprecht's work, the graph at right also summarizes (the vertical beige bar, far right) the range of documented frequencies either emitted by bears as different sounds, or to which individual bears were evidently sensitive. The match to Ruprecht's results is fairly good, especially the median frequency (the horizontal line towards the bottom of the bar). That said, the range of emissions and detections documented by other researchers obviously encompass higher frequencies as well. 

Bioacoustics aside, it is clear that sound plays a significant role in the lives of bears. Bears are surprisingly vocal and have been documented to emit a wide range of different calls and other sounds. They are known to whine, moan, yelp, grunt, snort, chuff, and hum. Cubs seem to more often hum. You will often hear snorting and chuffing during a close encounter with a distressed bear. There is also roaring, which is common when bears fight each other.

 

On a complementary note, bears also seem to use audio cues to home in on foods. For example, in Yellowstone, grizzlies probably use their hearing to find seeds of the whitebark pine which they extract from cones cached by squirrels in larders (see Whitebark pine). I am convinced, based on a lot of field work, that bears often find the squirrel's caches by listening for squirrel chatter--which is typically in response to a threat--and then homing in on it. You will usually find a cone cache at the base of the tree containing an enraged squirrel. Similarly, there is compelling evidence that grizzlies home in on gunshots during hunting season as a means of finding the remains of elk left behind by hunters. A number of hunters have also been jumped by bears while trying to lure bull elk in with the sound of a simulated elk bugle, suggesting that the bears were homing in on the bugling in hopes of ambushing a bull elk. In either case, hunters seem to put themselves at risk of a close encounter with a bear simply because of the sounds they make.

Smell

 

There are several physical features that indicate the acuity of smell (or olfaction) in a given species. No one indicator is definitive, but in combination they provide a good picture of an animal's sense of smell. The structures most commonly assessed are the olfactory bulb (the organ governing smell, located toward the front of the brain) and the olfactory turbinals (key parts of the nasal cavity associated with uptake of odors). In additional, some researchers have looked at the size of the cribiform plate (which separates the nasal cavity in the snout from the olfactory bulb in the brain) and the number of holes through it (foramina) accommodating the nerves associated with olfaction.

 

The bar graphs immediately left (thanks to the work of John Gittleman and others) summarize differences among carnivore species in the size of the olfactory bulb relative to the size of the brain. The thick black horizontal line is the median for each carnivore family. As you can see, the olfactory bulb of bear species (the ursids) is actually relatively small in comparison to the canids (the dogs) and hyenas. Which would suggest that bears do not have an exceptional sense of smell.

The graph to the immediate right shows the size of the olfactory (panel B) and respiratory (panel A) turbinals relative to skull and body size, respectively. The respiratory turbinals are associated with the capacity to inhale and condition air for respiration. In both graphs the gray dashed line shows a relationship fit to data including bears (the brown dots), and the black line a relationship fit only to the non-bear species. The reason for excluding bears derives from the fact that there are no other terrestrial carnivore species of comparable size that can provide a meaningful contrast to bears, which means that bears will almost exclusively drive the fit at larger body sizes--which precludes a meaningful comparative assessment. (The data come from research published by Patrick Green in association with Blaire van Valkenburgh and others.)

 

So, focusing on deviations from the main trend defined by the solid black line: When it comes to intake of air for respiration and thermoregulation, it looks as if brown bears (U. arctos, with females and males differentiated) are exceptionally well-endowed. Other bear species less so. When it comes to the olfactory turbinals, it looks like polar bears (U. maritimus) are the best-endowed species, with all of the other bear species falling near the size you would expect given the size of their skull.

 

The conclusion from this might be that most bear species, with the notable exception of polar bears, are not gifted with comparatively ample olfactory turbinals, lending weight to the idea that most bears do not have an exceptional sense of smell. However...

The graphs immediately left focus on structures identified with the amount of olfaction-related enervation--indicated by the size of the cribiform plate, and the number of nerve-associated holes (formina) in it. The bottom-most plate repeats an analysis relating olfactory turbinals to skull size (as immediately above), but using a different data set. The beige dots identify bear species, the gray dots a number of other carnivores. All of these data and the topmost two illustrations are thanks to Deborah Bird and colleagues.

 

The analysis relating size of olfactory turbinals to size of skull basically reiterates the results above, suggesting that bears are more-or-less average in this regard (that is, the dots denoting bear species fall close to the main trend line). By contrast, the features more closely associated with enervation (cribiform plate and foramina within) place bears well above the main tend line, suggesting the bears have comparatively ample enervation related to olfaction (i.e., smell).

 

So what does this all mean? It depends upon how much weight you give to the role of enervation versus the olfactory bulb versus physical structures that amplify uptake of scent in a overall reckoning of olfaction. Weight of evidence would suggest that most bears, including brown bears, don't have exceptional adaptations for smelling, with the possible exception of augmented conductive nerves related to olfaction. Put another way, the available evidence does not support (certainly not unequivocally) the idea that bears have a truly exceptional sense of smell compared to other carnivores.

 

That said, there is no doubt that smell plays a huge role in the lives of most bears, perhaps more so in the case of the highly predatory polar bear. There is ample evidence that bears "follow their noses" to find food, especially prey, carrion, and subterranean delectables. There is also ample evidence that bears--which are largely solitary--communicate with each other primarily through scent--through the sensing of chemical stimuli (chemosensation). Of relevance to this last point, there is now a large body of research documenting use by bears of marked trees (either by scratching or by rubbing their backs) to deposit distinctive chemicals that other bears detect and use to map their social world.