Thursday, November 17, 2011

Gender-Bending in the Animal Kingdom

An article in the New York Times this week inspired me to write about my favorite animal of all time, the cuttlefish.


Cuttlefish are cousins of squids and octopuses.  They're not native to the Americas (which is why many American readers may have never heard of them before), but they are common virtually everywhere else on the globe.  Like their cousins, cuttlefish are capable of extremely elaborate camouflage, a skill which can be and is used for hunting and communication, as well.  They're like chameleons, only about 10,000x cooler.  Why?

That's why.

Cuttlefish (and other cephalopods) have specialized pigment cells called chromatophores.  In the cuttlefish, these chromatophores are like little sacks of color that come in brown, yellow, and red.  Other animals, like chameleons, also have chromatophores, but what makes cephalopod chromatophores so unique is that they are surrounded by muscles, which are innervated by nerves.  Here's a fairly simple diagram:
When those muscles around the chromatophore contract, it expands the radius of the pigment cell, creating a visible dot of color, much like a pixel on a screen.  Cuttlefish skin is full of millions of these cells.  But the awesomeness doesn't end there.  Underneath the layers of chromatophores are colorless, light-reflecting cells called iridophores and leucophores, which reflect green, blue, pink, orange, and white.  The combination of all these various pigment and reflecting cells, along with the papillae that allow cuttlefish to change the texture of their skin by forming little bumps, creates a complete palette that allows the cuttlefish to blend into virtually any environment.  To demonstrate the point, here's a video of an octopus using the same type of skin to blend into its environment.

This complex camouflage is a rather useful strategy for cephalopods.  With the exception of the even less famous, but far more threatened Nautilus, cephalopods have very little protection from predators.  Their ancestors secreted large, thick shells that protected them, but squids have an extremely reduced internal shell (called a pen), and octopuses have no shell at all.  Cuttlefish have an internal porous shell called a cuttlebone, put even that doesn't offer much protection.  These animals are basically soft sacks of protein that everything (including humans) wants to eat.  While all cephalopods can secrete ink to make a quick escape from a predator, the camoflauge helps them to avoid being seen by predators in the first place.

Of course the byproduct of having millions of cells surrounded by muscles which are controlled by nerves is a big brain.  Perhaps what makes cephalopods so famous and charismatic is their intelligence.  You often hear stories about researchers and aquariums losing track of octopuses because they figured out how to escape from their tanks.  Cephalopods have also been shown to be able to learn visual cues to help solve mazes, and even use tools.

The trademark cephalopod cleverness brings me back to the original reason I wrote this post (aside from my fanatic love of cephalopods and talking about them).  One of the most amazing demonstrations of camouflage and intelligence in cuttlefish is seen in the case of "cross-dressing" in Australian Giant cuttlefishes, Sepia apama.  These cuttlefish are solitary, except for once a year where they have a mass mating frenzy.  During this time, the males who would usually like to go unnoticed put on these vibrant displays of colors and patterns to attract the females.  However, the females get the final choice in the mating game.  The way cuttlefish mating works is the male cuttlefish places a packet of sperm on the female's underside, which she later uses to fertilize her eggs.  Females will mate with a few males but they are also very picky about who even gets to mate, rejecting 70% of mating attempts.

Usually, it's the biggest, brawniest, most colorful cuttlefish that gets to mate with the female and later gets chosen to fertilize her eggs.  In part that's because he's big enough to be able to guard her and prevent other males from moving in.  Smaller males barely stand a chance against the largest, guarder males, so some of them employ a clever trick to get past their burly competitors by cross-dressing!  They assume the color pattern of the female and hide their uniquely masculine arms by curling them under, and then they just simply swim past the bigger male virtually unnoticed.  Unless, of course, it's so convincing that sometimes males will try and mate with these so-called sneaker males.  But the more important question is what do the ladies think?  Dr. Roger Hanlon looked into this question by subjecting the cuttlefish to paternity tests.  It turns out that when choosing who gets to fertilize her eggs, the female will select the sneaker males second to the big, brawny males, as if to give an "evolutionary nod" to the cleverness of the sneaker males.  That's not to say that she's consciously acknowledging the clever strategy (consciousness in animals is an entirely different discussion all together), but obviously there is an advantage to having this level of intelligence among cuttlefish, as it gets passed on to the next generation fairly reliably.

Who says the jocks get all the chicks?

Information from NOVA's Cuttlefish: Kings of Camouflage and Hanlon, RT; Naud, MJ; Shaw, PW; Havenhand, JN.  (2005).  Nature 433(7023) 212-212.

*Edit* It was pointed out to me that I referred to iridophores and leucophores as pigment cells, which is misleading as these cells do not actually contain pigment.  They are colorless but reflect certain wavelengths.  Leucophores reflect white light, whereas iridophores reflect pink, orange, green, and blue.