Why does a lizard do push-ups on top of a rock? This was one of the behaviors that first caught the attention of Barry Sinervo, a professor of ecology and evolutionary biology at the University of California, Santa Cruz.
Every spring, he and his team hiked the hills near Los Baños. They were armed with fishing rods, but they were not looking for fish. Among the rocky outcrops and grasses, there lived a dense population of side-spotted lizards (Uta stansburiana), and Sinervo had mastered the art of capturing it. He could catch more than 100 individuals a day and was no match for any of the students he challenged every time they went out into the field. Of course, all the lizards were released after study in the laboratory.
After several years of working in Los Baños, Sinervo described the social and evolutionary dynamics of side-spotted lizards. It turns out that they are a single species, but the males sport three different colors on their throats. Some have orange, others, yellow and others, blue. The most curious thing is that, depending on the color, they have a different trick to reproduce.
The orange ones are the bullies. Larger and stronger than the rest, they establish territories with many females, which they defend aggressively. As in the case of other vertebrates, this aggressiveness is linked to higher levels of testosterone.
The yellow ones are the elusive ones. Instead of establishing territories, they sneak into each other’s territories and try to breed with some unattended female. Part of their success lies in looking like them. Since receptive females also have a yellow throat, they can easily go unnoticed.
The blue ones are the good guys. They establish small territories and are monogamous, so they can focus all their attention on guarding a single female. These individuals cooperate to defend themselves against males of other colors. They even warn their blue neighbors that there are intruders nearby by doing push-ups on a rock.
Based on the idea that in nature the best adapted wins, we could think that, over time, one of the strategies would work better and one color would predominate over the rest. But with the lizards of Los Baños this does not happen, since, according to Sinervo, their populations have been playing the game of rock-paper-scissors for millions of years.
The game works like this: the oranges beat the blues. With their aggressiveness, they manage to defeat each one and usurp their territory along with their female. However, as they have such large territories, they cannot keep a good eye on all their females and are defeated by the yellow ones, who sneak in without problems. In turn, the blues beat the yellows, who have nothing to do against a one-female type.
Therefore, the success of each strategy depends on the frequency of the others. When the blue ones are the most numerous, the orange ones begin to be more successful and usurp their throne. But then it is the yellows’ turn to grow, since their strategy is favored in a world of oranges. When the yellow ones are in the majority, the blue ones, with their territory well defended and their female well secured, begin to proliferate. Thus, the frequency of the three colors oscillates in an eternal dance.
Females also have their role. They usually give preference to the least abundant color, because it is the one that will be most successful in future generations and, therefore, their offspring will be more likely to reproduce. It’s like betting on the winning horse, but with a foresight for the future. In this way it also contributes to one color being surpassed by another.
In 1996, Sinervo published this discovery in the magazine Nature and it went on to become his most famous article, as it was the first time that an example of the “rock, paper, scissors” dynamic was found in nature. Since then, similar relationships have been discovered in more species.
For example, a team from the National Museum of Natural Sciences (MNCN-CSIC) and the Pyrenean Institute of Ecology of Jaca, demonstrated in a 2014 study that in the Pyrenees there is also a lizard (Viviparous Zootoca) playing rock, paper, scissors. In this case, the males are differentiated by the color of the belly: yellow, orange or white. As with side-spotted lizards, each color has a different life strategy in which they lose against one type and win against another.
But let’s return to the Sinervo lizards, because they still brought more important news for evolutionary biology. As already mentioned, blue-throated lizards cooperate to defend their territory. This species only lives one generation, so if it does not have offspring during the reproductive season, it never has any. Despite this, there are blue lizards that put themselves in danger to help others by doing push-ups in broad daylight. Additionally, when there are many oranges, some blues are too busy defending their mates and fail to reproduce. We are talking about altruism, giving up one’s own reproduction to help others. But how is it possible that evolution allows such behavior?
Richard Dawkins, in his classic book The selfish gene from 1976, predicted the altruistic behavior of these lizards, except that instead of being a throat it was a beard and instead of being blue it was green. According to this author, what is important in evolution is not the organisms, but the survival of the fittest gene. Therefore, the altruism gene survives by helping another organism with that same gene, even if some of the organisms in which it is found do not reproduce.
However, for this to occur, it is necessary that individuals carrying the altruistic gene have traits, such as a green beard or a blue spot, that function as recognition labels. This is what Dawkins called the Altruistic Greenbeard Effect. Male blue-throated lizards were one of the early discoveries that supported Dawkins’ ideas.
Barry Sinervo continued to catch lizards in Los Baños and bring them to his laboratory for the rest of his professional career. Concerned about the drastic decline he observed in lizard populations, he decided to dedicate his last years to researching the consequences of climate change on these animals. Lizards are ectotherms, they depend on ambient temperature and that makes them especially vulnerable to global warming.
As he says in a TED talk, it was like a punch in the stomach for him to learn that the Western scaly lizard, with which he had done his doctoral thesis in 1984, had already become extinct. “This is just basic math, temperatures are rising, lizards are going extinct.” In a study published in Science In 2010, Sinervo estimated that, if we do not act, by 2080, 20% of the world’s lizards will have become extinct.
In March 2021, when Barry Sinervo was 60 years old, cancer decided to end his life among lizards, but he passed the baton to an entire team eager to continue with his line of research, so that we can understand the qualities that emerge from evolution and so that the games that have been developing for millions of years among the recesses of the rocks do not cease.
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