Whales, unlike nocturnal rodents or ourselves, see the world in monochrome. Leo Peichl at the Max Planck Institute for Brain Research co-authored a paper with the nearly tragic title, "For whales and seals the ocean is not blue." Indeed, the first thing that we can know for sure about how whales see the world is that it exists only in shades of gray. The water we see as blue they would see as black. "They do want to see the background. They want to see animals on the background. And the animals on the background are reflecting light that's not blue," Johnsen explained. If we try to imagine what that might look like, Johnsen said perhaps we could picture a grayscale photograph of people wearing fluorescent clothes under a black light.
When it comes to the optics of whale eyes, the first difference we should note is that its cornea -- the outermost layer of the eyes -- doesn't help it nearly as much as ours helps us. We live in air, which has a different refractive index than the material of the cornea. When light enters our cornea, it bends inward. You know how pencils appear to bend when you put them in a glass of water? That's refraction, and our eyes exploit it to help focus photons on the central part of our retinas. Johnsen told me roughly 70 percent of the work of focusing light on our eyes is done by the cornea before the light even reaches the lens. But that's a clever terrestrial trick. In water, the refractive index of the cornea and water are roughly the same, which means that marine mammals don't get that pencil-going-into-water light bending help. "The lens has to do everything in the whale eye," Peichl said. While our lenses are flattish, theirs are circular in order to provide sufficient focus.
Now, when we talk about the resolution with which whales see the world, it helps to bring back the video camera metaphor for eyes. Whales, like other mammals, are trying to balance the sharpness of their eyes with their sensitivity. Sharp vision requires lots and lots of individual photoreceptors. But in low-lighting conditions, it's hard for the photoreceptors to gather enough photons. The image gets "noisy."
Photographers run into this problem all the time, too. What do you do in lower light settings? The first thing you might do is put a lens with the largest possible aperture on the camera to let in more light. The same goes for eyes: the whale's big cornea and large pupil opening means that it has a huge aperture. It's gathering up a lot of photons.
And it's got a biological mirror at the back of its eye, the tapetum lucidum, which is helping it capture even more light than our own eyes can.
But vision isn't all the optics. Other capabilities matter, too, like the size of the sensor that picks up all that light. We measure that kind of thing in the megapixels of the charge-coupled device, or CCD, in the camera. There's a similar principle at play in biological camera eyes. If an organism wants to see better, it has to have a lot of photoreceptors. More photoreceptors equals more pixels. A big difference here is that CCDs capture what hits them equally. Retinas have areas of greater or lesser rod and cone density that tends to coincide with where the light is being focused. This makes a lot of sense: Evolution has put the most sensors where the most light falls.
The light detecting system, however, is more complex than we find in any digital camera. Photoreceptors send their information to ganglion nerve cells, which integrate them, dynamically increasing the size of the photoreceptor. That increases sensitivity by cutting down on the noise problem, but it decreases the acuity because each "pixel" gets largely, i.e. it has to represent a larger portion of the physical world.
What's fascinating is that by looking at the ganglion cells, researchers can calculate the maximum resolution that a particular eye could have, inferring capabilities from the anatomy alone. That's helpful in species like whales where behavioral tests aren't generally possible.
The measurement that people tend to use here is cycles per radian, and it defines how well a given eye can discriminate between two lines next to each other. An eagle is up over 8,000 cycles per radian. A human eye registers an impressive 4175. A cat is down around 570. And researchers working with minke whales estimate that it is down with the rabbits and elephants at around 230.
Though it's probably not advisable to attempt a translation from this visual acuity to the more familiar units from your optician's office, I'm going to do it anyway. If normal human good vision is 20/20, a whale might rank somewhere like 20/240. That sounds pretty bad, but if you, like me, have a glasses prescription of -5.00, you almost certainly have worse visual acuity than a normal minke whale. (Of course, you can see colors, so count your blessings.)
But it's not easy to make the comparison between human vision and whale vision. It's definitely weirder than that. One fascinating aspect of cetacean eye anatomy is that it appears that whales don't have one central area for higher-resolution imaging like humans. Instead, they appear to have two areas of dense cell concentrations, according to a 2007 paper in the Anatomical Review. These match up with a strange feature of the cetacean pupil: It closes like a smiling mouth, and when it's very tightly constricted, it has two small circular areas that remain open.
Contrast that with the way our eyes work: when they constrict, the larger circle of our pupillary opening simply becomes a smaller circle, still focused on the on the fovea. For a whale using its eyes, two distinct spots would be in the best focus. I think that is impossible to imagine what it might be like to have two centers to one's vision.
Trying to imagine what a whale might see becomes even more difficult when we take into account the actual eye positioning for most whales. Whale eyes are located on the sides of their heads. This is roughly the opposite of our own visual system. We have two eyes facing forward with a ton of visual field overlap. Or as Herman Melville wrote in Moby Dick, "For what is it that makes the front of a man -- what, indeed, but his eyes?" His narrator is staring at a sperm whale head, a lifeless version of the same creature that Austin the photographer encountered.
Looking at the eyes, placed on opposite sides of the head, Ishmael wonders about the whale mind relative to our own:
How is it, then, with the whale? True, both his eyes, in themselves, must simultaneously act; but is his brain so much more comprehensive, combining, and subtle than man's, that he can at the same moment of time attentively examine two distinct prospects, one on one side of him, and the other in an exactly opposite direction? If he can, then is it as marvellous a thing in him, as if a man were able simultaneously to go through the demonstrations of two distinct problems in Euclid. Nor, strictly investigated, is there any incongruity in this comparison.
It is no surprise that we use the same word for refracting light into a particular location as we do for directing our consciousness to a particular idea or object: focus. We focus our attention. But what if there are multiple points of focus -- not just the two eyes, but the two focal points on the retina. To grasp after Melville's question, how could an organism make sense not just of its visual surroundings, but, its own sense of coherence or conscious unity? (I imagine the 90s sitcom, Herman's Head, in which four separate characters live within one guy's mind.)