Don’t Mess With Me! How Collared Lizards Fight

WHEN MALE COLLARED lizards square off with one another in rocky areas of the Great Plains and deserts of the Southwest, the message they send is clear: What you see is what you get.

• Michael Lipske
• Dec 01, 2008

Patrolling the edge of his territory, an adult collared lizard that spies a rival male usually flashes a warning by gaping his jaws. But he isn’t flaunting his pearly whites. Instead, his open-mouth threat display lets his rival see the highly developed jaw muscles that put the "ouch" in a collared lizard’s bite. "The machinery is displayed," says A. Kristopher Lappin, a California State Polytechnic University biologist who studies threat displays among collared lizards in Oklahoma’s Wichita Mountains National Wildlife Refuge. "When they gape, you can see the muscles that are primarily responsible for how hard they can bite."

Over the years, scientists have catalogued a vast array of apparently threatening body postures, facial expressions and sounds produced by different species during competition over territory, mates, or food--picture a hissing cat or a dog baring its teeth. Researchers assumed that these displays signal a willingness to fight. But in the case of the collared lizard, they sometimes also convey information about the effectiveness of the flaunted weapon.

Collared lizards aren’t big; minus the tail, a male’s body may measure four inches in length. But the reptiles can deliver a nasty nip. Lappin has caught specimens that have scars or fresh wounds showing the outline of another collared lizard’s jaws. Some collared lizard scrappers even have broken bones from fights.

Fortunately for the lizards, fights are actually rare, probably because their mouth-gaping threat display persuades one lizard to back down. In fact, Lappin’s research on collared lizard threat displays is the first demonstration of how an animal, by showing off its weapon, can provide a rival with honest information about how effective that weapon will be in a fight. As Lappin wrote in a recent paper, by displaying their jaw muscles, rival males provide each other with a "reliable, body-size independent, biomechanically based index of weapon performance."

Lappin’s point about the lizard’s message being "body-size independent" is important because male collared lizards (and it’s the highly territorial males, rather than females, that engage in most threat displays) of similar size can vary greatly in how hard they bite.

"And with these lizards, how hard you can bite really matters," he says. In a previous study, Lappin showed that collared lizards that bite harder also have bigger territories and greater access to female lizards.

To measure their biting ability, Lappin persuaded collared lizards to clamp down on a force transducer, or bite bar, a metal gizmo that resembles a tuning fork and converts the strength of a lizard’s bite into a readable signal. By comparing bite-force measurements from each lizard in the study with measurements of the breadth of its jaw muscle complex, Lappin determined that the size of the jaw muscles was a better predictor of biting force than the size of a collared lizard’s head or body.

He also found that when the lizards gape, they expose bright patches at the corners of their mouth. Only visible when they open their mouth, the skin patches help delineate the outer edge of the jaw-muscle complex. The patches also reflect ultraviolet light, which collared lizards, unlike people, can see. Lappin argues that the skin patches function as "amplifiers" of the message the lizards send with their gaping threat display. "They’re accentuating the jaw muscles during the display."

So when a collared lizard opens his mouth and shows off his awesome adductor mandibulae muscle complex, is he telling a rival male something like, "Hey, pal, check out the specs on my painmaker"? That’s basically Lappin’s theory--that the gaping displays "provide honest information to an opponent about your abilities," information that may help both lizards avoid a risky fight.

A watery world away from collared lizard country, another small animal may also draw attention to its threat display with special lighting effects. Mantis shrimp, an order of ocean crustaceans with special claws for smashing or spearing fish and other prey, have color patterns or spots that are unique to each species. While those spots are easily seen in sunlit shallow water, they are less visible in the dim light on the ocean floor. But at least one species of mantis shrimp, Lysiosquillina glabriuscula, has solved that problem with spots that fluoresce yellow-green in the blue murk of deeper water. Although many ocean creatures (such as corals) exhibit fluorescence, this is the first known case of fluorescence being used to enhance undersea signaling.

"When it goes into a full threat, those spots are prominently displayed," says Roy Caldwell, a University of California–Berkeley authority on mantis shrimp. That full-threat posture consists of raising and spreading its mouthparts as well as the mantislike front legs that include its weapon system.

Mantis shrimp have to be able to read each other’s signals to stay alive, says Caldwell. "These are animals that can kill an opponent in a single blow," says the scientist, and those opponents can be other mantis shrimp. In one study, Caldwell found that about 30 percent of the animals he was looking at "bore significant injuries" inflicted by their fellow mantis shrimp. Whether competing with each other or scaring away a predatory fish, the point of a mantis shrimp’s threat display is the same, says Caldwell. "It’s a presentation of weapons and trying to look as big as possible."

What about threat displays among other animals? "When a deer waves his antlers or when a baboon male gapes his mouth and shows off his teeth to another male or when a dog growls and puts its ears back, what information is really being conveyed there?" asks Lappin.

Pour too much beer into a couple of higher primates in a bar, and if they’re not joking and slapping each other on the back, they may quickly default to body language instantly recognizable to the student of threat displays. According to Lappin, whose experience watching lizards gape at one another in the wilds of Oklahoma has given him an eye for all manner of rivalry, "If two human males are interacting and they’ve had too much to drink, they’ll tend to stand tall," puffing their chests and raising their voices, he says. "It may lead to a fight. Or maybe one will back down, kind of cower down a little bit, not give as much eye contact."

Just like a couple of lizards.

Writer Michael Lipske is based in Washington, D.C.

Web Exclusive

For Crayfish, Size Does Matter

Unlike the honestly intimidating weaponry of the male collared lizard, the claws of one species of crayfish ward off intruders in a novel way: by bluffing. A team of researchers has discovered that male slender crayfish--palm-sized crustaceans that live in Australian streams--possess one of the few known examples of "dishonest" weaponry in the animal kingdom. When two males meet and wave their claws in a showdown over territory, the one with bigger claws nearly always prevails--even though the smaller-clawed crayfish are often stronger.

"Male crayfish lie and cheat their way to the top," says study author Robbie Wilson of the University of Queensland in St. Lucia, Australia.

A confrontation between slender crayfish looks less like a fight than a curious dance: One male sticks his claws into the sand while the other male taps the claws, sizing them up. Then the two switch roles. In almost every case, one crustacean eventually gives up and retreats without a struggle. The researchers measured the pinching strength of 32 males and then paired them off in a series of face-offs. Almost without fail, the larger-clawed crayfish was dominant, even when the smaller-clawed but stronger opponent could have won in an out-and-out battle of brawn.--Hannah Schardt