This Diminutive Reptile Plays Rock-Paper-Scissors

Side-blotched lizards probably don’t call the game that, but they play a version of it anyway. A new study explains the hidden biology that makes this possible.

If you live in the United States, chances are you’re familiar with the game rock-paper-scissors. You put out your hand in one of three gestures: clenching it in a fist (rock), holding it out flat (paper) or holding up two fingers in a “V” (scissors). Rock beats scissors, scissors beat paper and paper beats rock.

Americans by no means have a monopoly on the game. People play it around the world in many variations, and under many names. In Japan, where the game has existed for thousands of years, it’s known as janken. In Indonesia, it’s known as earwig-man-elephant: The elephant kills the man, the man kills the earwig and the earwig crawls up through the elephant’s trunk and eats its brain.

The game is so common that it exists beyond our own species. Over millions of years, animals have evolved their own version of rock-paper-scissors. For them, winning the game means passing down their genes to future generations. A study published on Thursday in the journal Science reveals the hidden biology that makes the game possible — and shows how it may be an important source of nature’s diversity.

The first clues that nature also played rock-paper-scissors emerged three decades ago in the dry hills outside Merced, Calif. Barry Sinervo, a biologist then at Indiana University, studied the common side-blotched lizard there. He would mark the lizards — named for the dark blue or black spot on their side, just behind the front leg — release them into the tall grass and catch the survivors to check up on them in later years.

Dr. Sinervo, who later joined the faculty at the University of California, Santa Cruz, and who died in 2021, grew fascinated by the strange mating habits of the lizards. At the start of every breeding season, the males developed one of three colors on their throats: blue, orange or yellow. And depending on their color, the males behaved differently.

Orange lizards aggressively fought other males to win territory and guarded as many as six females on their turf. They got into fights with blue males and regularly took away their females.

The blue males used a different strategy. They guarded a small territory, mating with only one or two females and cooperating with other blue males to fight orange males. Dr. Sinervo would sometimes observe a blue male die defending an ally in a fight against an orange one.

Yellow males were less aggressive still. They didn’t bother to guard any territory at all. Instead, they tried to secretly mate with females guarded by other males.

Blue males usually chased the yellow males off their small territories. But orange males, struggling to keep control over many females spread across a wide area, did a worse job of catching intruders. So female lizards guarded by orange males often ended up producing many offspring of the sneaky yellow males.

Dr. Sinervo tracked the male lizards over time and discovered that they went through cycles. The orange males had the biggest population for a year or two before giving way to the yellow, which then gave way to the blue, which in turn gave way to the orange again.

Back at Indiana University, Dr. Sinervo analyzed the results with his colleague Curtis Lively, an evolutionary biologist. They wondered if they could make sense of the results by thinking of the lizards as playing a game.

There’s a long tradition in biology of thinking about life as a game, a competition to leave as many descendants as possible. The players could be animals, plants or microbes. Biologists have borrowed equations from game theory to analyze how some strategies win out over others.

One day, as Dr. Sinervo and Dr. Lively drank coffee and puzzled over the data about side-blotched lizards, they realized that the males were playing a game of their own.

“I remember him jumping up in the coffee shop and shouting, ‘It’s rock-paper-scissors!’” Dr. Lively said of Dr. Sinervo in an interview.

In 1996, Dr. Sinervo and Dr. Lively published a mathematical model of the lizard version of the game. The way that the orange lizards could consistently beat the blue ones was akin to rock always beating scissors. The blue lizards beat the yellow ones like scissors beats paper. And, just as paper beats rock, the yellow lizards could win out against the orange males.

To create their model, the scientists assumed that each color was encoded by a variation in the genes of the lizards. When the males of one color had more success at mating, they passed down more copies of their genes and made the color more common — until the next color took over.

But Dr. Sinervo and Dr. Lively didn’t actually know how the different colors and behaviors arose in the lizards. In the 1990s, they lacked the tools that would let them answer that question. But in 2012, Ammon Corl, one of Dr. Sinervo’s former students, took on the challenge of discovering the inner workings of the game. It would take him 13 years to get some answers which, in the new study, he and his colleagues have now published.

Dr. Lively, who was not involved in the study, said it represented a major advance in understanding evolutionary games — one made possible by the huge advances in DNA sequencing over the last three decades.

“It was unimaginable to us in those days that something like this could be done,” Dr. Lively said.

Dr. Corl could not study side-blotched lizards in his lab the way researchers study more familiar species like fruit flies and mice. For reasons that scientists don’t yet understand, male side-blotched lizards do not develop colors in captivity.

“Controlled genetic experiments really couldn’t be done,” said Dr. Corl, who is now a biologist at the University of California, Berkeley.

Instead, he and his colleagues had to catch the lizards in the California hills, note the colors on their throats and take a sample of their blood to analyze back in the lab. But analyzing the lizards’ DNA posed immense challenges as well.

The standard way to analyze an animal’s genes started with isolating DNA from cells, and then breaking the molecules into little fragments. Researchers then made many copies of each fragment and read the genetic letters that made up each one. By comparing the fragments to a complete genome sequence, they could figure out where in the genome the fragment came from and could then pinpoint the variation.

But no one had ever sequenced a side-blotched lizard genome before, so Dr. Corl and his colleagues had no way to determine where each fragment of the animal’s DNA belonged. As a result, Dr. Corl and his colleagues had to spend years assembling a genome of a side-blotched lizard to serve as a reference for their research.

Once they had done so, the researchers could search the DNA of an individual lizard for a genetic variation that might determine its color. Try as they might, they could not find any clear-cut differences. Fortunately, more accurate methods for sequencing DNA became available in recent years, which Dr. Corl and his colleagues used to sequence a second lizard genome. At last, Dr. Corl and his colleagues could see even the tiniest genetic differences between the animals.

“The story crystallized, and it all fell into place,” Dr. Corl said.

He and his colleagues found that orange and blue males differed at just one spot in their genomes. The genetic variation between them turned out to be remarkably simple.

An orange neck, Dr. Corl and his colleagues discovered, is a recessive trait. In other words, lizards need to inherit two copies of the orange variant to develop that color. Otherwise, they become blue.

“It’s a near-perfect association between orange and blue,” Dr. Corl said. “This really couldn’t arise by chance.”

The researchers found that orange males produce lower levels of a protein called SPR. One of its jobs is to help make pigments. Another is to help create neurotransmitters in the brain.

“Potentially, this one gene could link changes in coloration and changes in behavior,” Dr. Corl said. “It’s kind of amazing that something like that could work.”

Then the scientists turned to the yellow lizards. Based on the original model that Dr. Sinervo and Dr. Lively built, they expected to find another genetic variation that produced the third color. But they found none — not in SPR, or in other genes. Genetically speaking, yellow lizards are indistinguishable from blue ones.

It’s not clear yet how two genetically indistinguishable lizards can end up developing such different colors in their mating seasons. Dr. Corl speculates that the animals might turn blue only if they manage to gain some territory. “They’re showing off their badge, saying, ‘Hey, I’m here, don’t mess with me,’” Dr. Corl suggested.

If that’s the case, then it’s possible that the species started off with blue and yellow males, and only later did orange ones evolve. Their new mutation made them so aggressive they could win against the blue lizards, but they were helpless against the yellows. The three colors settled into a stable cycle.

Erik Svensson, an evolutionary biologist at Lund University who was not involved in the study, said it offered a surprising resolution to the debate over the lizards. When Dr. Sinervo and Dr. Lively first argued that the lizards played a game, they proposed that the game was all about genes. But other researchers argued that the male lizards assumed different colors in response to cues in their environment.

Now it turns out both sides were right. It takes both flexibility and heredity to play the game. “It’s a wonderful twist,” Dr. Svensson said.

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