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.

:D

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?

Images courtesy of aslo.org and the Florida Center for Instructional Technology.

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.

Lizards!

Lizards.

I grew up in the North--first New Jersey and then I went to college in Oberlin, Ohio.  So when I recently moved down to Florida to start a new job, one of the most exciting things I found about my new location was the new fauna.  Particularly all the little lizards running around!  The most common lizards to Florida belong to the Polychrotidae family, and are generally known as Anoles.  However, "lizards" are a broad and diverse group, with many different species with very different morphologies and life styles.  Here are a few fun facts about lizards:

• Lizards hear through the conduction of sound through small bones in their lower jaw vibrating a tympanic membrane (sometimes visible on the outer skin surface)
• Some lizards have prehensile (grasping) tails that aid them in climbing.
• Other lizards have more elongated tails that they can use for defense as whips.
• Some only have two limbs, while others are limbless (not to be confused with snakes).
• Some have webbed limbs and even specialized skin flaps that act as parachutes or gliders (I highly recommend that you go and watch that link, it's a really cool video from Animal Planet that I couldn't embed here.)
• Some have specialized fringes on their toes that allow them to run on water!  (See video)

However, being a nerd I find one of the most intriguing things about all lizards is that--unlike us--they are ectothermic, or basically cannot internally regulate their own body temperature.  This is what is generally referred to by the term "cold-blooded," although that term is considered a bit archaic by the scientific community due to the fact that cold-blooded animals do not have cold blood.  In fact, most of the time lizards probably have blood that is warmer than the blood of "warm-blooded," or endothermic animals (i.e. mammals and birds).

In order to understand the implications of being ectothermic, we should first consider the importance of body temperature in general, regardless of how it is regulated within an organism.  At it's most reduced, life is the result of many physical and chemical interactions occurring within the cells and tissues of the body.  The chemical interactions in particular (such as the transcription of DNA into RNA, and the translation of RNA into proteins; the breakdown of proteins and fats by enzymes; and the break down of ATP into ADP for energy... the list goes on), will function most efficiently within a certain temperature range.  If that temperature is too low, for example, then all those processes happen much slower.

What is it like to not be able to physiologically regulate your own body temperature?  Well for one, it vastly limits the type of environment you can live in.  While there are a few lizards that live in higher latitudes and altitudes, most lizards live in very warm climates.  This is because in order to keep their body temperature within an ideal range (somewhere around 40° C), they have to bask in sunlight and on warm substrates to keep warm, and then get to shade or be able to bury themselves in colder dirt or sand when the surrounding environment gets too hot.  In this way, you could consider lizards as behaviorally regulating their own body heat, but they still lack the physiological mechanisms by which endothermic animals regulate their own body temperature.

As you may guess, the environment ends up playing a very large role in determining the lizard's activities throughout the course of the day.  But as with any trait, it wouldn't have lasted so many generations if there wasn't a significant benefit.  Endothermic animals, in order to maintain their high internal body temperature, must have a very high metabolism to produce enough heat.  This high metabolism needs to be powered by energy gained from food, so a significant amount of the animal's time and energy must be allocated into foraging just to maintain their metabolic rate.  Lizards and other ectotherms don't have that problem; none of their energy intake needs to be put towards generating heat, so they can spend less time foraging and thus can allocate their energy towards rapid growth, social behavior, and reproduction.

The social behavior is what personally intrigued me by the Anoles of Florida.  If you watch these little guys for more than a few minutes, you will probably see them bob their heads up and down and extend a red projection from their neck, known as a dewlap.  This general behavior is seen in male Anoles, and is thought to serve to both establish territoriality and dominance over other males, as well as to attract females.  Moreover, the specific factors such as number of body push-ups, head bobs, and the degree of extension of the dewlap can convey specific signals within the same species of Anole.  It is also thought that the color of the dewlap varies between species, and that Anoles are able to visually detect the differences in color.  Some lizards are even capable of detecting ultraviolet wavelengths, thus adding another dimension to the visual spectrum that we cannot see ourselves.  Accordingly, the dewlaps of these lizards are able to reflect UV light.

In addition to being able to communicate with each other visually on a wider spectrum of visible wavelengths than we can, lizards (and many other animals) are also able to communicate using chemical signals called pheromones.  While the jury is still out on whether or not humans use pheromones to communicate, it is pretty well known that lizards can emit and detect such chemical signals.  In lizards, there seems to be some differences in the methods by which the two sexes employ the chemical signals.  Males seem to emit pheromones to communicate with other males, whereas females use their pheromones in a more male-directed fashion.  Lizards are able to detect these chemical signals using their olfactory system (sense of smell,) their gustatory system (sense of taste) and another sensory system a little foreign to us, the vomeronasal system, which specifically detects pheromone chemicals.  In lizards, the vomeronasal ducts open to the mouth, and they use their tongues to pick up the chemicals and then flick them over the olfactory and vomeronasal ducts.  This is the underlying reason for the tongue flicking behavior that you probably associate mostly with snakes.

There is quite a bit more interesting information about lizards, particularly in the shapes of their skulls and how sound is conducted through their lower jaws.  But in the interest of not writing a whole book on the matter, I'll cut this post off here.  But now you know a little more about the crazy lives of lizards!

Reference and disclaimer

Most of the information I find for the posts I write on this blog comes from a combination of knowledge that I gained through classes and through internet sources.  I generally link to these internet sources in the text of my posts, but sometimes I also get information from books (those archaic sources.)  Most of the information from today's post came from Lizards: Windows to the evolution of diversity by Eric R. Pianka and Laurie J. Vitt (University of California press, 2003).  The rest came from my own notes or internet sources where linked.

Thanks for reading!