Masters of Disguise

September 22, 2020

Written by: Nitsan Goldstein

One of the most remarkable adaptations of land and sea creatures alike is the ability to rapidly change their appearance in order to blend into their surroundings. The process is not yet entirely understood, and while it may seem like magic, scientists are beginning to uncover the biological processes involved in camouflage. Not surprisingly, an animal’s ability to alter its appearance is dependent on a coordinated and sophisticated nervous system response that is not all that different from the way our own nervous systems respond to our surroundings. 

Let’s consider swinging a bat. In order to hit the ball, the batter’s eyes must track the ball’s movements as it comes towards them. Is the pitch a bit high or low? How fast is the ball getting larger, that is, how fast is it coming towards you? When this visual information reaches the brain, the brain makes extremely fast calculations to determine exactly which muscles in the shoulders, arms, hands, hips, and even legs and feet have to be activated to contact the ball at the right time. The brain then sends electrical signals to those muscles to execute the movements. The average fastball in Major League Baseball travels at about 92 mph. The distance from the pitcher to the batter is about 60 feet. This means that the whole process— taking in the visual information, calculating proper movements, sending signals to the muscles, and executing the movements— occurs in less than half a second. Though swinging a bat and camouflage seem like completely different skills, they actually require similar nervous system processes carried out with high speed and precision. 

To understand the neuroscience behind camouflage, we will focus on one species called the cuttlefish. Cuttlefish are mollusks from the class Cephalopoda along with squids and octopuses. They inhabit warm ocean waters like the Mediterranean and the coasts of East and South Asia. Cuttlefish are known as the “chameleons of the sea” because of their remarkable ability to change their appearance to communicate or blend into their surroundings. To watch cuttlefish camouflage in action, check out this video.

Step 1: Visual Information

            In order to camouflage, cuttlefish must be able to see well enough to match the colors, patterns, and even textures of their surroundings. There’s only one problem- cuttlefish do not have color vision! How can an animal match its surroundings without seeing color? The answer is not entirely clear, but scientists think that they use other properties of light to determine how to best blend in. For example, cuttlefish can see the polarization of light, or the angle at which light reflects off a surface. In fact, they have the most acute polarization vision ever measured, with the ability to detect changes in light reflection as small as one degree¹. Another strength of cuttlefish vision is the ability to distinguish contrast between light and dark². This aspect of their vision is critical for blending into surfaces such as pebbles on the ocean floor. All this visual information is sent to the brain, where the next step of the response occurs. 

Step 2: Calculating the response

            Once the visual information reaches the brain, how does the cuttlefish determine the appropriate behavioral response? Again, much of this process is still being investigated, but theories have been proposed based on recent research. Scientists have determined that cuttlefish have about 40 body patterns that can be used alone or in combination^2,3. Examples of these patterns include “white posterior [towards the back] square,” “white head bar,” and “mottle.”² Cuttlefish do not blend into their surroundings “pixel by pixel”. That is, they do not identically mirror the shapes and colors of the area underneath them. Rather, they take in visual information of their surroundings, or the “background”, and then make a statistical calculation to determine the best pattern for them to adapt out of their repertoire of possibilities. The brain receives information about the spatial frequency of an image (how much the light varies across a certain distance) and the polarization of light to infer certain characteristics of the image such as contrast and depth. The brain then determines what type of background is present- is it a continuous surface, or made of discrete objects like pebbles? The brain calculates which combinations of the 40 possible colors and patterns best matches the background and, finally, determines what signal to send to each of thousands of muscles on the animal’s body to execute the chosen pattern². 

Step 3: Motor commands

            The cuttlefish brain has made an incredibly complex calculation in a matter of milliseconds. Now it is time to execute the camouflage by sending neural signals to the rest of the body. The surface of the cuttlefish is lined with thousands of pigment cells called chromatophores. Chromatophores vary in color and are usually a light yellow when they first appear and become darker brown over time⁴. These cells are surrounded by tiny muscles that can expand or contract them to vary the brightness of that individual chromatophore. The tiny muscles surrounding the chromatophores are each controlled by a small number of motor neurons. To execute a certain pattern, the brain sends electrical signals to these motor neurons and individually sets the size of each chromatophore that is needed to achieve the desired color and pattern⁴. 

Recently, one group of researchers used moving cameras to track naturally behaving cuttlefish and measure the size of each chromatophore as the cuttlefish adapted to a pebbled background⁴. They found that there was actually a stereotypical sequence of patterns generated when the cuttlefish shifted their appearance. The cuttlefish started out in a uniform dark brown pattern and ended up with a light stripe along their center. However, when the scientists looked across several of these transformations, they noticed that, consistently, the cuttlefish turned uniformly light first, then darkened the areas that were not in the light stripe⁴. This consistent sequence suggests that there are stereotypical neural signals that are sent when the animals camouflage, potentially as they learn how to best adapt to their surroundings. 

While camouflage is a remarkable feat that still seems somewhat magical, the neural processes involved are very similar to those we use every day. Our brains use different properties of light, such as color, to adapt to our surroundings. We make incredibly complicated calculations to determine our best course of action, and constantly send signals to thousands of motor neurons that control tiny muscles that together accomplish our goal. And our brains execute these functions in a fraction of a second without our conscious effort. So next time you swing a bat, or even type on a computer, think about how complicated what you are doing is and you’ll realize that the brain is a remarkable machine that directs every aspect of survival whether it’s blending into the ocean floor or going about your normal day.

References

1. Temple, S.E., Pignatelli, V., Cook, T., et al. High-resolution polarisation vision in a cuttlefish. Curr Biol. 22, R121-R122 (2012).
2. Kelman, E.J., Osorio, D., & Baddeley, R.J. A review of cuttlefish camouflage and object recognition and evidence for depth perception. J Exp Biol. 211,1757-1763 (2008).
3. Kelman, E.J., Baddeley, R.J., Shohet, A.J., & Osorio, D. Perception of visual texture and the expression of disruptive camouflage by the cuttlefish, Sepia officinalis. Proc Biol Sci. 274,1369-1375 (2007).
4. Reiter, S., Hülsdunk, P., Woo, T. et al. Elucidating the control and development of skin patterning in cuttlefish. Nature 562, 361–366 (2018). 

Cover Photo from Wikipedia Commons and Borazont