Adaptive Value

Adaptive value, which Tinbergen called survival value, refers to how the behavior might have served to increase the organisms’ ability to survive or reproduce, i.e. its evolutionary fitness [1].

Crypsis

The most obvious use of the cuttlefish’s ability to so dramatically alter its appearance is to remain concealed from predators.  This constitutes a form of crypsis, i.e. cryptic behavior.  A color or pattern is cryptic “if it resembles a random sample of the visual background as perceived by the predator at the time and place at which the prey is most vulnerable to predation” [2].  This carries with it two corollaries, namely that prey behavior and morphology can affect crypsis, and also that a given body pattern may be conspicuous nearby but cryptic at a distance, or cryptic to a predator while being conspicuous to a mate [2].

  • General Background Resemblance

Cuttlefish can alter their appearance to match their background in brightness, color, pattern, and texture.  There are three primary colorations adopted by cuttlefish in order to mimic their environement: uniform light, stipple, and mottle.  When selecting these patterns, the cuttlefish takes into accout both the graininess of the background and its own size [3].  Much of the cuttlefish’s ability to match backgrounds comes from its ability to accurately estimate how much light is reflecting off of a surface and then to match that amount of light [2].

    dynamiccamouflage

    An octopus blending in with an outcropping of coral using general background resemblance.   (From [4]).

  • Countershading and concealment of the shadow

Cuttlefish also exhibit countershading, which causes them to make their undersides lighter than their topsides.  This makes sense, since animals looking up would expect to see a light coming from the sky above the ocean.  This type of behavior is also known as self-shadow concealment because it acts to cancel out the differences in directional light caused by the fish’s shadow [5].  In fact, cuttlefish posses possess a countershading reflex (CSR) that causes them to make their upwards-facing sides darker even when they are disoriented [3].

  • Disruptive coloration

    Another manner in which cuttlefish’s color changing ability helps them stay concealed from predators is disruptive coloration, which breaks up the shape of the animal [2]. Cuttlefish, for example, have dark patterning around their eyes in a dark bar, which make their eyes less conspicuous.  Furthermore, the light and dark components of the cuttlefish’s coloration alter the prominence of different parts of its body, in a process that is called differential blending (because different parts blend in better with its background than other parts) [2].

    disruptivecoloration

    A cuttlefish on the rocky bottom of a tank displaying a disruptive pattern.  The large white spot in the middle of the cuttlefish resembles other rocks strewn about the bottom of the tank.  Image originally from [6].

  • Deceptive resemblance

Another strategy for camouflage is to mimic the appearance of inedible objects, which is called deceptive resemblance [3].  In some cases, deceptive resemblance can overlap with other camouflage strategies, for example appearing like a rock could constitute both general background resemblance and deceptive resemblance.

deceptiveresemblance

The cuttlefish in the center could be said to be mimicking a large stone, blending in with its background, or both!  Image originally from [7].

Communication

Another hypothesized manner in which the cuttlefish’s ability to change color is adaptive is that it provides a channel for communication.
In animal behavior, communication can be defined roughly as a three part process involving a sender animal sending a message via a signal (some way of conveying information or getting attention) to a second receiver animal who then processes and responds to the information [2].

Intra-Specific Communication

Although cuttlefish are generally regarded as non-social animals, it is still possible that they communicate to facilitate agonistic or sexual encounters [8].  The best documented of these displays is the Intense Zebra display, which is an agonistic display usually exhibited by sexually mature males.  Evidence suggests that this pattern is an honest signal of fighting intent, and contest winners have more highly contrasting patterns than contest losers [9].  Specifically, the darkness of the “face” (the area extending from between the eyes to the beginning of the first two arms) indicates the cuttlefish’s willingness to escalate the fight.  Although it may initially seem counterintuitive to enter into an agonistic interaction while signaling one’s intention to withdraw, the authors posited the following explanation for such behavior.  First of all, it would be dangerous to “bluff” and signal aggressive intentions if a cuttlefish lacked them, because doing so poses a very high risk of leading to a fight and ensuing injury and energy expenditure.  However, not presenting the Intense Zebra Display at all presents its own disadvantages.  Males blinded on one side do not see approaching males on that side and therefore fail to adopt an Intense Zebra Display.  When this occurs, the approaching male attempts to copulate, leading to fighting and inking (Ibid).  Thus adopting the Intense Zebra display is necessary to signal maleness in cuttlefish, who lack “visually overt sexual dimorphism.”    This conclusion is borne out by other data, as the Intense Zebra Display seems to be brought about by the sight of a rival male, and that male cuttlefish show more of the behaviors associated with this display to other males than they do to females [8].  In short, there is strong evidence that the Intense Zebra Display is a use of the cuttlefish’s ability to change its appearance in order to communicate with other cuttlefish, particularly in order to signal maleness and agonistic intentions.

Cuttlefish presenting an Intense Zebra Display.  Original image found at http://cephalove.southernfriedscience.com/wp-content/uploads/2010/09/zebra.jpg.

Although it may initially seem counterintuitive to enter into an agonistic interaction while signaling one’s intention to withdraw, the authors posited the following explanation for such behavior.  First of all, it would be dangerous to “bluff” and signal aggressive intentions if a cuttlefish lacked them, because doing so poses a very high risk of leading to a fight and ensuing injury and energy expenditure.  However, not presenting the Intense Zebra Display at all presents its own disadvantages.  Males blinded on one side do not see approaching males on that side and therefore fail to adopt an Intense Zebra Display.  When this occurs, the approaching male attempts to copulate, leading to fighting and inking (Ibid).  Thus adopting the Intense Zebra display is necessary to signal maleness in cuttlefish, who lack “visually overt sexual dimorphism.”    This conclusion is borne out by other data, as the Intense Zebra Display seems to be brought about by the sight of a rival male, and that male cuttlefish show more of the behaviors associated with this display to other males than they do to females [8].  In short, there is strong evidence that the Intense Zebra Display is a use of the cuttlefish’s ability to change its appearance in order to communicate with other cuttlefish, particularly in order to signal maleness and agonistic intentions.

Additionally, some scientists have postulated that cuttlefish use their ability to see polarization as well as their ability to produce polarized reflective patterns to communicate among themselves in a “private” communication channel [10]. This idea is very interesting, but first we should explain what polarization is!  Light can be visualized as waves, in which it behaves as if it were an electric field oscillating like a sine wave.  Each of these electric fields is accompanied by a magnetic field, which oscillates at the same frequency, but is tilted 90 degrees relative to the electric field.

lightwave

Original image found at http://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Light-wave.svg/1000px-Light-wave.svg.png

In normal light, there are many of these waves, and they point every-which direction, so the waves average out to make each direction have equal amplitude at any given time.  In polarized light, only waves pointing as certain direction are allowed.  Thus polarization of light looks something like this:

polarizer

 

In the first section of the image the light beam is unpolarized and oscillates in each direction equally.  As it passes through the first polarizer, light waves that don’t oscillate up and down are removed, and the resultant light beam is made up of waves oscillating only up and down.  (If it were to pass through the second polarizer, no light is left, because only light waves that oscillate side-to-side are let through, and none of those remain).

 

 

 

 

Original image from http://www.olympusmicro.com/primer/lightandcolor/polarization.html

 

Why does this matter to cuttlefish?  Cuttlefish possess iridophores, which reflect light in the same way that soap bubbles do [10].  What is more interesting is that these iridophores polarize some of the light that they reflect, and that cuttlefish can control how much of the reflected light is polarized [10].

armpolarization

True color image of cuttlefish arms (left) and false color view of the percent polarization of reflected light  (right).  Original image taken from [12].

It has also been shown that cuttlefish perceive linear polarization and even that they use it to assist them in predation [11].  It has also been shown that cuttlefish could send out reliable signals from the polarized light on their arms even when moving them [12].  So, since cuttlefish can polarize light, and they can see polarization, do they use polarized light to communicate?  What scientists know about vision in the cephalopod predators indicates that most probably cannot sense polarization, so if cuttlefish use polarized light to send information, they would have a “private channel” of communication [10].


Although it would be exciting if it were true, this hypothesis only has scant evidence to back it up.  One experiment has produced tentative evidence that female cuttlefish modify their activity levels, body pattern, and polarization when viewing other cuttlefish through a window that distorts polarization as opposed to one that doesn’t [11].  However, more definitive experimental confirmation of the hypothesis that cuttlefish communicate using polarized light has remained hard to come by.  This is in part because it is difficult to define a particular appearance as a visual signal meant to convey a specific message, and it is even harder to show that the signal was received [10].  This is especially true if the signal contains a message to remain concealed or that there is a possible predator nearby.  In sum, it seems that cuttlefish might have the ability to communicate with other cuttlefish via polarized light, but no one has produced solid evidence that they actually do.

Inter-Specific Communication

There is even evidence that cuttlefish use their ability to change colors for inter-specific communication.  In other words, cuttlefish may change their coloration in order to send signals to individuals of a different species.  Specifically, a recent study by Langridge et. al. investigated the use by cuttlefish of displays designed to scare off predators [13].  These kinds of displays are called deimatic displays.   Other examples of such displays are found in many other species.  The eye spots found on many species of butterfly and moth, which make them appear like large, predatory birds, are one example of a deimatic display.

eyespots

Example of eyespots in a butterfly.  Original image from http://www.123rf.com/photo_5713941_buckeye-butterfly-with-eye-spots.html

Another, more recent example is found in a species of squirrel that heats its tail when confronted by a pit viper, giving it a larger infra-red signature (which the viper can see), making it appear larger and alerting the viper to threat of being injured in a confrontation [13]. 

Squirrel without heated tail (top) and with heated tail in the presence of viper (middle). Original image found at http://acp.eugraph.com/news/news07/rundus.html

 

In cuttlefish, a deimatic display involves the generation of black eye spots and a dramatic flattening and spreading of the body (which makes the individual appear larger).  Experimenters found that juvenile cuttlefish would present a deimatic display almost every time they encountered a sea bass[13]. This supports the hypothesis that cuttlefish use their color-changing abilities to threaten predators.  Sea bass are teleosts, which hunt by sight, and hence could conceivably perceive a cuttlefish in a deimatic display as too large to attack.  Interestingly, cuttlefish did not present this display in encounters with dogfish or crabs, predators that do not hunt visually [13].  This suggests that cuttlefish can discern between different threats and tailor their response accordingly.

 

Juvenile cuttlefish exhibiting a deimatic display. Taken from[13].