Animal Senses #3: Sight

How does sight work? Every animal’s eye has light-detecting cells called photoreceptors, explains Ed Yong in Immense World. While photoreceptors vary, they all share a universal feature: they contain proteins called opsins. Opsins tightly embrace a partner molecule called a chromophore. The chromophore absorbs the energy from light and instantly changes shape. Its contortion forces the opsin partner to reshape itself, setting off a chemical chain reaction that ends with an electrical signal travelling down a neuron.

 

And yet, there are so many different ways of seeing:

“Animal eyes can be bifocal or asymmetric. They can have lenses made of protein or rock. They can appear on mouths, arms, and armor.”

 

This seems like a paradox:

“If all vision relies on the same proteins, and if those proteins all detect light, then why are eyes so diverse?”

The answer? Because light can provide a diverse set of information – it can hint at temperature, time of day, or depth of water. It reflects off objects to make them visible. It creates shadows which can be informative. Its enormous speed means information is instantaneous. To summarize the answer:

“Vision is diverse because light is informative in a multitude of ways, and animals sense it for myriad reasons.”

 

Eyes always involve a tradeoff between sensitivity and resolution.

“No eye can excel at both.”

An eagle can spot its prey at great distance (high resolution) but it can only do that during the day (low sensitivity). Hyenas can see well at night (high sensitivity) but they don’t see too well (low resolution). In case you wondered, we humans have very high-resolution eyes.

 

Eagles and vultures often crash into those tall wind turbines. How can birds with such great vision not see a turbine right in front of it? At the heights these bird fly, the glare of the sun is very high. So the birds have evolved to have huge blind spots in their front (to avoid the glare). This was not a problem since it was not like there was anything in the sky at those heights to bump into anyway. Until we humans started building wind turbines…

 

Animals even “see at different speeds”! This sounds nonsensical at first – doesn’t light travel at the same speed for all and practically instantaneous? True, but:

“Eyes don’t work at light speed.”

Photoreceptors take time to activate; and the electrical signal then takes time to reach the brain via the neurons. It is in these steps that animals show different speeds. In killer flies, it takes just 6-9 milliseconds for photoreceptors to activate + signal to reach the brain + brain signals to activate the muscles. In humans, it takes 30-60 milliseconds for the first step alone (photoreceptors to activate). Remember how TV works at around 60 frames per second (60 fps) to create the illusion of continuity? Well, flies see at 350 fps.

“To its eyes, a human movie would look like a slideshow.”

Conversely:

“The fastest of our actions would seem languid… Boxing would look like tai chi.”

The faster an animal is, the higher its visual fps (frames per second).

 

Night vision involves 2 challenges. The first one is obvious – there is so little light. The second one is very unintuitive:

“Photoreceptors can accidentally go off on their own, and at night, these false alarms can easily outnumber the real signals from actual photons.”

Yong summarizes the two problems for nocturnal creatures:

“They must overcome both the limits of physics and the messiness of biology.”

Two common solutions for the problem are to (1) Pool responses from multiple photoreceptors; and (2) Collect photons for longer before firing (“like a camera whose shutter is left open for a longer exposure”).

 

Creatures in the deep sea face the same problem of little to no light. So when we take our submersibles to study the deep sea, turning on its light can be blinding for those creatures who have evolved to see with the least possible light. A more considerate approach involves uses red light to see (because most sea animals can’t see red).

 

Until I read this chapter, I had no idea that vision is so varied.