Flashiness isn't an attribute we're used to associating with chameleons, but according to a new study of the evolution of chameleon color change by Devi Stuart-Fox and Adnan Moussalli, it's time to rethink the cliché of chameleons as unobtrusive critters who change color to blend with their surroundings.
While environmental cues can trigger temporary color changes in creatures as diverse as crabs, octopuses, and fish, chameleons are the most familiar color changers. Among the more than 150 chameleon species, the ability to change color ranges from subtle shifts in shades of brown to remarkable ensembles of orange, blue, green, and even ultraviolet colors that are invisible to humans. Investigations of the function of chameleon color change have yielded two main hypotheses. According to the camouflage (or crypsis) hypothesis, natural selection led to an increased ability of the chameleon to match a variety of backgrounds to escape predators. The alternative hypothesis proposes that color change evolved to facilitate social communication among chameleons of the same species. Under this scenario, color change enables chameleons to flash conspicuous color patterns that are highly detectable to other chameleons, while appearing camouflaged at other times.
To determine which selective pressures may have driven the evolution of color change, and toward which colors selection favored, Stuart-Fox and Moussalli compared the coloration and color-change capacities of 21 lineages of southern African dwarf chameleons (Bradypodion spp.) in a series of field-based behavioral trials. The authors chose to work with dwarf chameleons because the different species vary tremendously in the range and types of colors they can display, and because the evolutionary relationships within this group are known. Male dwarf chameleons use color to signal dominance to other males in aggressive contests and to woo females. If a male loses a contest or is vigorously rejected by a female, he displays an alternate set of submissive color patterns. Although chameleons also change color when confronted by a predator, because the color range is greatest when male dominant and submissive coloration is contrasted, the authors used this measure to estimate each species' capacity for color change.
To measure the degree to which increased capacity for color change correlates with crypsis or signal conspicuousness, Stuart-Fox and Moussalli first staged contests between male chameleons on a perch in their natural habitat and then took color measurements of the dueling chameleons and the surrounding vegetation using a technique called reflectance spectrometry, which measures the proportions of light wavelengths (and thus color) that reflect from surfaces. As soon as the chameleons showed either clear aggressive behavior—including head shaking, displays in which chameleons puff out their throats and flatten themselves to appear larger, or chasing and even biting —or submissive behaviors, such as fleeing or flipping to the underside of the branch, the researchers measured the color and brightness of the top, middle, and bottom portions of their flanks, along with other high-contrast regions (see the Figure). By pitting each male against several others, they obtained measurements for about five individuals in each of the 21 lineages, enabling them to measure species-specific color change. To measure background coloration, the authors took reflectance readings of the leaves, branches, grass, or vines where the chameleons were caught. They also took median reflectance measurements of the vegetation most common to each species. Using these data, along with data collected in previous studies regarding the spectral sensitivities of photoreceptors in the eyes of both chameleons and birds, the authors were able to estimate just how conspicuous each individual would be in a variety of environments, when viewed by both other chameleons and their bird predators.
Chameleon species that showed the greatest capacity for color change also had dominance signals that were most conspicuous to other chameleons—because they showed a highly contrasting color pattern and each color contrasts markedly with surrounding vegetation. There was no correlation (based on evolutionary relationships) between increased color change capacity and the variety of backgrounds that chameleons must match in order to be camouflaged. Furthermore, species that showed only limited color change live in open habitats with dense ground vegetation such as grasslands and heaths. But this might not be simply due to predation, because the display colors of these species are just as conspicuous to bird predators as the display colors of species in other habitats. Together, these results support the hypothesis that dramatic color change evolved in dwarf chameleons largely as a strategy to facilitate social signaling rather than camouflage.
The authors suggest that limited color change might have evolved originally to function in thermoregulation or camouflage but that the dramatic changes in hue shown by some chameleon species evolved subsequently to enable chameleons to communicate using bright, flashy colors. These findings have broader implications for the evolution of animal social signals, since they support the importance of selection for signals that are highly detectable within the animal's environment and demonstrate how studies that take animal visual systems into account can be used to understand the evolution of signal diversity in animals.
- 1. Stuart-Fox D, Moussalli A (2008) Selection for social signalling drives the evolution of chameleon colour change. D. Stuart-FoxA. Moussalli2008Selection for social signalling drives the evolution of chameleon colour change.
For sheer breadth of freakish anatomical features, the chameleon has few rivals. A tongue far longer than its body, shooting out to snatch insects in a fraction of a second. Telescopic-vision eyes that swivel independently in domed turrets. Feet with toes fused into mitten-like pincers. Horns sprouting from brow and snout. Knobbly nasal ornaments. A skin flap circling the neck like a lace ruff on an Elizabethan noble.
Of all its corporeal quirks, the chameleon is most defined by one, noted as far back as Aristotle: color-changing skin. It’s a popular myth that chameleons take on the color of what they touch. Though some color changes do help them blend into their surroundings, the skin’s changing hue is in fact a physiological reaction that’s mostly for communication. It’s the lizard using colorful language, expressing itself about things that affect it: courtship, competition, environmental stress.
At least that’s the belief today. “Even though chameleons have attracted attention for centuries, there’s still a lot of mystery surrounding them,” says Christopher Anderson, a biology postdoctoral associate at Brown University and a chameleon expert. “We’re still piecing together how their mechanisms actually work,” from the explosive projection of the tongue to the physics of the varying skin colors. (Learn more about chameleons at a website Anderson runs.)
“Even though chameleons have attracted attention for centuries, there’s still a lot of mystery surrounding them.”
Christopher Anderson | biology postdoctoral associate at Brown University
Scientists recently have made important discoveries about chameleon physiology by watching the lizards in captivity. Their future in the wild, meanwhile, is far from certain.
When the International Union for Conservation of Nature (IUCN) released a new Red List assessment of chameleons last November, it ranked at least half the species as threatened or near threatened. Anderson is a member of the IUCN Chameleon Specialist Group, as is biologist Krystal Tolley, a National Geographic grantee whose expeditions in southern Africa have documented new chameleon species and vanishing habitats. (Read Tolley’s blog posts from her expeditions.)
In Afrikaans, says Tolley, chameleons have two common names. One is verkleurmannetjies, which means “colorful little men.” The other, trapsuutjies, translates as “treading carefully.” That refers to the lizards’ odd, slow gait—but also could be read as a plea to conserve the curious species and their home terrain.
Rostral protuberances—such as this one on a long-nosed chameleon—help individuals of like species identify each other and can be used as jousting weapons.
How Chameleons Change Color
About 40 percent of the 200-plus known chameleon species are found on the island of Madagascar. Most of the rest live on the African continent. Thanks to DNA testing, some chameleons that look nearly identical have been found to be genetically distinct. More than 20 percent of the known species have been identified in just the past 15 years.
Given their many odd traits, chameleons “have always intrigued naturalists,” Anderson says. Because the lizards often died on the journey from Madagascar and the African continent to Western laboratories, early herpetologists could only guess at how live chameleons worked. That yielded theories that seem laughable now, he says: “It was once thought that the chameleon tongue projected because it inflated with air or filled with blood, like erectile tissue.”
“It was once thought that the chameleon tongue projected because it inflated with air or filled with blood, like erectile tissue.”
Anderson studies chameleon feeding in intricate detail. Using a camera that captures 3,000 frames a second, he turned 0.56 seconds of a chameleon eating a cricket into a 28-second instructional video on projection mechanics. (See videos of chameleon tongue projections.)
Stored in the lizard’s throat pouch is a tongue bone surrounded by sheaths of elastic, collagenous tissue inside a tubular accelerator muscle. When the chameleon spies an insect, it protrudes its tongue from its mouth, and the muscle contracts, squeezing the sheaths, which shoot out as if spring-loaded. The tongue tip is shaped so that it acts like a wet suction cup, grabbing the prey. The tongue recoils; dinner is served.
Scientists have more to learn about tongue projection, Anderson says. His research suggests that in some chameleons, it may go even farther and faster than previously thought.
The understanding of chameleon coloration also has changed over time—and dramatically earlier this year, when Michel Milinkovitch’s research was published. Scientists had long thought that chameleons changed color when skin cell pigments spread out along veinlike cell extensions. Milinkovitch, an evolutionary geneticist and biophysicist, says that theory didn’t wash, because there are many green chameleons but no green pigments in their skin cells.
So Milinkovitch and his University of Geneva colleagues began “doing physics and biology together,” he says. Beneath a layer of pigmentary skin cells, they found another layer of skin cells containing nanoscale crystals arranged in a triangular lattice.
By exposing samples of chameleon skin to pressure and chemicals, the researchers discovered that these crystals can be “tuned” to alter the spacing between them. That in turn affects the color of light that the lattice of crystals reflects. As the distance between the crystals increases, the reflected colors shift from blue to green to yellow to orange to red—a kaleidoscopic display that’s common among some panther chameleons as they progress from relaxed to agitated or amorous. (See a video Milinkovitch’s team made of a panther chameleon color change.)
An insect succumbs to a foraging Calumma chameleon, whose extremely sharp vision allows it to project its long tongue with pinpoint accuracy.
New Ways to Hide
At age seven, Nick Henn got his first chameleon. Twenty years later the hobbyist and breeder keeps as many as 200 of them in the basement of his business in Reading, Pennsylvania.
Rows of wire-mesh cages contain plants for climbing and sandy floors where females can lay eggs. Lights and misters simulate the lizards’ native climes. Arranging the cages is as tricky as seating warring factions at a United Nations summit. To keep the animals from riling each other, Henn places females where they can’t see males, and males where they can’t see females—or rival males.
Ember, a young male panther chameleon, is a so-called red bar, a variety that’s native to the Ambilobe district in northern Madagascar. His torso has red and green zebra stripes plus an aqua blue racing stripe along each side. When Henn opens Ember’s cage and prods him to climb onto a long stick, he “gets grumpy,” which Henn knows because the chameleon’s red bars get a little brighter.
Henn carries Ember on the stick around a corner to the cage inhabited by Bolt, an adult male blue-bar panther chameleon and the largest lizard in Henn’s collection. When Henn opens the door, and Bolt sees Ember, the response is immediate. By the time Bolt has advanced a few inches, his green bands have turned vivid yellow, and his eye sockets, throat, and spiked spine have changed from green to red orange. Ember becomes redder—but as shows go, Bolt’s is far more flamboyant. For good measure, as Bolt crawls nearer, his mouth gapes wide, displaying bright yellow gums.
“I keep envisioning the little chameleons clinging to their branches as that forest is getting chopped.”
Krystal Tolley | biologist and National Geographic grantee
Henn retreats and puts Ember back in his cage. Had he not, he says, Bolt might have tried to ram or bite Ember, whose skin almost certainly would have changed to brown—the color of crying uncle. (A 2014 study concluded that chameleons developed this fade-to-drab submissive ability because their “slow-moving lifestyle severely restricts their ability to rapidly and safely flee from dominant individuals.”)
Though all chameleons change color, some species don’t change dramatically enough to cow observers. However, almost all chameleons do have another technique for physical intimidation: They can make themselves look larger. They narrow the width and increase the height of their bodies by unfolding their jointed, V-shaped ribs to elevate their spine. They also can look more massive by coiling their tails tightly and using their tongue apparatus to expand their throats. Turning this profile to its nemesis, the lizard looks significantly bulkier.
In the cages where Henn keeps female chameleons, one named Katy Perry—salmon pink because she’s ready to mate—is next door to one named Peanut, pink with dark bars because she has already mated and is gravid, carrying eggs. If Katy were approached by a male that impressed her with his courtship colors and bobbing, swaying dance, she might submit to being mounted. If the same male approached Peanut, she would become intensely darker with bright spots and open her maw menacingly at him. If he persisted, she’d hiss or try to bite him.
Both male and female chameleons are polygamous. Most species are egg layers, but some deliver live young in clear, cocoon-like sacs. Chameleons do no parenting, so the young are on their own as soon as they’re born or hatched.
To avoid the birds and snakes that hunt them, chameleons have evolved novel ways to hide. Most species are arboreal; when they narrow their bodies, they’re slender enough to hide on the opposite side of a branch. If ground dwelling chameleons see a predator, Tolley says, some “play leaf,” contorting their bodies to look like crumpled leaves on the forest floor.
Chameleons can hide from some threats but not from the slash-and-burn agriculture destroying their habitats. The IUCN lists nine species as critically endangered, 37 as endangered, 20 as vulnerable, and 35 as near threatened.
The entire life cycle of the Labord’s chameleon lasts roughly a year. Some chameleon species may live a dozen years in captivity, but less than half that in the wild.
Identifying New Species
Tolley and her team have identified 11 new chameleon species since 2006, in South Africa, Mozambique, Tanzania, and the Democratic Republic of the Congo. The Massachusetts-born professor has studied the lizards in Africa since 2001 and works for the South African National Biodiversity Institute in Cape Town.
When a genetics study confirms that a chameleon is a new species, “it feels like you’re not just writing some random scientific paper that nobody will read,” Tolley says. “You’re accomplishing something—this is going to be forever.”
In the next breath she notes that “at the same time as thinking, ‘Wow, this is so cool,’ it was awful. I keep envisioning the little chameleons clinging to their branches as that forest is getting chopped.”
Describing it, her voice breaks. “I could not help thinking, I wish we’d never found them,” she says. “Because if this doesn’t stop, they’ll soon be extinct.”
Panama-based Christian Ziegler trained in tropical ecology before becoming a photojournalist. His natural history, conservation, and science images have won World Press Photo awards and other honors.
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