The word, exotropia, is not found in my computer dictionary. But it is found clearly on the web (e.g., wikipedia). It describes the situation were one’s two eyes, rather than looking at the same thing, are deviated outwards. In humans, exotropia is uncommon and is considered to be a problem to be corrected, for the two eyes then send two different images to the brain.
Bears have a marked exotropia at times. While the role that it plays in bears is speculative, the situation seems quite common and is apparently used to combine images to get a wider field of view.
I tried to learn about exotropia in bears, but found only one reference on the web. More on that below.
I start with two supposedly normal views of a bear’s eyes, one from black bears, one from grizzly bears. In these pictures, they are both presumably seeing the world in stereo for the pupils of each eye are looking at the same things. One assumes that, as with people, this is a bear’s default way of viewing the world.
A black bear looks at the world in stereo.
A grizzly bear looks at the world in stereo.
The next two pictures were used in recent previous postings, but now they are used to discuss exotropia. To see the effect, cover one eye of a bear to see where the other eye is looking, and then switch and cover the other eye. With each animal, the eyes are clearly divergent.
This black bear was fishing in a spawning channel.
This grizzly bear was fishing in a different spawning channel.
The only other reference I was able to find was one by a Vancouver-Island photographer and blogger, Wayne Barnes, who commented on the condition in 2012 and noted that V.I. black bears all seem to have divergent eyes (tofinophotography). On Vancouver Island, there were no grizzly bears and he had no data from the mainland.
My observations extend the condition to the mainland, and to grizzly bears. This suggests that it has remained a characteristic of bears since before the two species separated, about five million years ago. As the situation seems common in both black and grizzly bears, it is likely favourable for bears and can give them a wider field of view.
An intriguing observation indeed! To me these two bears seem to be looking at the viewer (you) while at the same time scanning for other points of interest/concern. Thanks for sharing.
Once again Alistair perceives what most of us miss! I am wondering if the exotropia in bears is an unwanted, uncontrollable affliction as it appears to be in humans, or if in bears it is a voluntary and useable advantage – as Trevor’s comment might allude.