By: Jim Maxwell, Ph.D
Those of us who are addicted to whale watching have almost certainly experienced the thrill of having a whale swimming so close to the boat that we could see its tubercles, barnacles, and scars; but what does the whale see looking back at us? We know that humpback whales are color blind, as described in a previous blog post, but how well can they see details? Can they tell one boat captain from another, or tell that you are wearing plaid golf shorts with a striped shirt? Maybe not. While whales have adapted superbly well to their ocean environment, their eyesight is quite poor compared to humans and many other terrestrial mammals. While we can’t know what whales see any more than we can know what they think, it is at least possible to estimate limitations on their visual ability. Broadly speaking, whales see details about ten times worse than humans and several times worse than your dog or cat. Obviously, it’s very difficult to perform eye exams on whales in the wild so if you wonder how scientists have been able to estimate their visual capabilities then keep reading for a more detailed explanation.
A LITTLE ANATOMY
Acuity is the ability to discern visual detail; for example, the ability to distinguish a B from an E on an eye chart. A whale’s eye has the same general anatomy as humans and other mammals. The cornea (front surface of the eye) and lens focus images of the world onto the retina in the back of the eye where photoreceptor cells, called rods and cones, convert light into signals that can be understood by the brain. Although the general anatomy is similar, the optics of our eyes and those of whales have evolved differently in response to very different environments. A primary difference is that light entering the eye of a terrestrial animal is refracted (bent) when entering the eye as it travels from the air into the denser aqueous medium of the eye, much as a stick placed vertically into a bucket of water appears to bend. In fact, refraction by the cornea in humans accounts for about 2/3 of the focusing power of the eye. The other 1/3 is supplied by the lens which lies just behind the cornea, iris, and pupil. In whales, on the other hand, because there is no significant difference between the densities of ocean water and the aqueous medium of the eye, the cornea does not significantly bend light entering the eye. To make up for the lack of corneal refraction, whale eyes and lenses are shaped much differently than in humans in order to ensure that images are focused onto the retina. In whales, the cornea is less round than in humans giving the eye a shape similar to a car’s headlamp, that is, fairly flat in the front and round in the back. The lens is also shaped differently and is nearly round, more like a fish than a human.
A BIT ABOUT THE RETINA
Another major difference between the eyes of whales and terrestrial mammals is the neuroanatomy of the retina. Humans and some other mammals, especially predators, have a high concentration of cones near the center of the retina, called the fovea, that is used in color vision and for seeing fine details. The rest of the retina is mainly populated with rod photoreceptors with cones scattered throughout. The rods are highly sensitive to light, so much so that they are fairly useless in bright light to humans, but they are essential for night vision. In short, during the day, human visual systems use cones, see color, and have good acuity. At night, our visual systems use rods, see in shades of gray, and have poor acuity. Whales are very different. If you read our post on color vision you know that most odontocetes (toothed whales) have only one functional cone type instead of two or three as in most other mammals (thereby precluding color vision) whereas most of the mysticetes (baleen whales) studied so far have no functional cones at all but rely completely on rods.
Do whales have something similar to a fovea? Very few studies have examined the photoreceptor layer in whales, but a number of research papers have shown that there are two areas in the retina with relatively high concentrations of another type of cell, the retinal ganglion cell. Ganglion cells are neurons that receive signals directly and indirectly from the photoreceptors and send visual information to other parts of the brain via the optic nerve. In the human fovea, very few cones (as few as one) converge onto each ganglion cell thereby preserving fine detail. On the other hand, in the periphery of the retina there is a great deal of convergence of rods onto ganglion cells, with up to 1000 or more rods converging onto each ganglion cell. This convergence is one reason why we do not see details well using our peripheral vision. Because the ganglion cells impose an upper limit on acuity, vision scientists have been able to develop formulas to estimate visual acuity based on the eye’s geometry, physical dimensions, and the concentration of ganglion cells. Researchers have found that whales have two areas in the retina with relatively high concentrations of ganglion cells that might represent areas of higher acuity.
So, now the million-dollar question: how well do whales see? We are almost ready to answer that question, but it is impossible to understand scientific measures of acuity without getting slightly more wonkish. If you don’t speak wonkish you may want to skip down to the conclusions.
HOW VISION SCIENTISTS MEASURE ACUITY
We are used to optometrists telling us that we have, for example, 20/40 vision, meaning that what a person with slightly poor vision sees at 20 feet, the average person can see at 40 feet. Vision scientists tend not to use this measure. Instead, they use either cycles per degree or minutes of arc. On average, the upper limit of human acuity is 1 minute of arc which is equal to 30 cycles per degree. To help you visualize what that means, imagine painting 60 very thin lines on your fingernail alternating white lines with black ones. If you hold your finger out at arm’s length your fingernail spans a visual angle of about 1.0 degree. Since there are 60 minutes of arc per degree, each line is 1.0 minute of arc wide. The average human, therefore, could just make out each of the 60 individual lines. In terms of cycles per degree, each pair of lines (one white and one black) constitutes one cycle. Therefore, 60 lines on your fingernail at arm’s length constitutes 30 cycles per degree. Thirty cycles per degree is, on average, the upper limit of human acuity. If there are more than 60 lines per degree, then the lines would no longer appear as individual lines but would blur together and appear as a uniformly gray field.
While it is hard to think of a good way of doing eye exams in any whale in the wild, acuity has been tested in a captive, trained dolphin at the University of Hawaii by Lou Herman and his collaborators. It’s beyond the scope of this blog to explain their methods in detail but has to do with the ability to discern a striped pattern from a uniform field. The conclusion was that the dolphin’s acuity was about 10 minutes of arc (3 cycles per degree) which matched the acuity calculated by counting ganglion cells. They also found, remarkably, that the dolphin could see nearly as well looking through the air as through the water when the targets were 2.5 meters away. The mechanism for how this is possible given the different densities of air and water is speculative and very complicated to explain and would require a separate blog that I promise never to write.
Because a dolphin’s vision is roughly ten times worse than a human’s, is it fair to say that whales have vision comparable to a human with 20/200 vision as some writers have done? Such a statement might be justified as a way of giving us a general idea of how poor a whale’s eyesight is but misleading because the mechanisms are so different. Most often a human’s poor vision is the result of poor optics, that is, when the focusing power of the lens and cornea is not well matched with the distance to the retina so that images are focused either in front of (near-sighted) or behind (far-sighted) the retina. While some whales might benefit from spectacle lenses, in general, a whale’s poor vision is the result of the spacing of the photoreceptors, the lack of functional cones and massive convergence of rods onto ganglion cells. To make an analogy with a digital camera (always misleading because cameras don’t work like eyes) you can have a blurry image either because the lens is out of focus or because the camera has too few pixels. Vision in whales might be somewhat comparable to a human with macular degeneration or a human with the extremely rare disorder of rod monochromacy, wherein they lack functional cones. About 1 in 30,000 humans are rod monochromats. Interestingly, their acuity is also about 20/200 (and they have an extreme sensitivity to light that, curiously, whales do not seem to have).
What does all this mean? It means that whales have quite poor vision by human standards and your plaid shorts and striped shirt may simply look gray, not mismatched. I should emphasize that we can never know what whales see. Scientists can examine that anatomy of whale eyes. They can use optical instruments to show that cetaceans such as dolphins are generally emmetropic, meaning that images are focused properly onto the retina. They can use photoreceptor and ganglion cell densities to calculate the upper limits of resolution and can even use trained dolphins to measure directly their ability to resolve a field of lines from a uniform gray field. They can compare this “front end”, that is, the input from the retina to human vision but none of this tells us how whales perceive the world. The visual system does not operate like a camera. Images of the world are not directly reproduced in the brain as they are on film or a CCD. Instead, at every step, from photoreceptors to ganglion cells, from brain stem to higher cortical areas, visual images are processed into characteristics such as lines, edges, texture, object motion, color, and contours to name a few. A large part of the human cerebral cortex is involved in processing visual information; the result of which is our visual perception of the world. Do you see blue the same way I see blue? Does a flower look the same to a dog or cat as it does to a bee? Does a mass of krill look the same to a whale as it does to a human? Given the limitations of the cetacean visual system, we can say that they cannot see as much detail as humans. On the other hand, they have successfully adapted to their ocean environment and see as well as they need to see in conjunction with their other senses to have survived for tens of millions of years.