Imagine a two-dimensional enclosed space, where a murderous female ruler sits in the center. Contestants are thrown into the ring, and they seem puny compared to her, but one of them manages to overcome his competitors to maneuver in behind her without her noticing, and wins the right to potentially mate with her. This could be the plot of a bloody reality show – or a Squid Game challenge – but it's actually the way of life of the golden orb-weaver spiders (Trichonephila), common in Africa, South America and the Far East.
The fearsome females of the species spin large webs on which the males climb. They all strive to reach a particular point behind her – and wait for the right moment, when she’s distracted – to mate with her. While they’re approaching the female, the males are in grave danger. She is massive compared to them and can gobble them up with ease – and occasionally, that’s exactly what she does.
This behavior has been documented in a new study in which patterns shared by particle physics have been identified in the spiders’ behavior. It is the first time that a physics-based model has depicted the dynamics of competition over resources among animals. The findings were published on Thursday in the journal Proceedings of the National Academy of Sciences of the United States of America.
According to one of the article’s authors, Prof. Nir Gov from the Chemical and Biological Physics Department at Weizman Institute of Science in Rehovot, the idea for the study came during a meeting with Dr. Alex Jordan of the Max Planck Institute of Animal Behavior in Germany, in 2017. “The encounter was very interesting and a little scary, because he took me to see giant female spiders in his laboratory and the tiny males that he released onto their webs,” Gov says.
The researchers observed that the females remain in the center of the web, where they barely move, while the males are the ones to advance. They decided it would be interesting to try to depict the behavior of the males on the web in terms of physics. “We thought that the female is sort of like the nucleus of an atom, and the little males are like electrons. They are attracted to her but repel each other, because they are competing with each other,” Gov says.
Doctoral student Amir Haluts turned the idea into a full-blown physics model. He followed the movements of the males on the web, in relation to each other and to the female, and described the trajectories he identified in mathematical terms.
Because all of the males want to reach the same point, a conflict arises among them over a very precious resource – a location near the female that will ensure their evolutionary survival. Haluts discovered an interesting phenomenon: Despite the competition among them, in some cases, the males will behave as if they attract each other and move towards their rival. That is, the dynamics between them is more complex than relationships between electrons or other particles, among which there is usually only attraction or repulsion.
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When the male spiders are far from one another, they repel each other. But beyond a certain threshold, they realize that they will have to make contact in order to resolve this conflict – and decide who leaves and who wins. At this point, they are trapped in a “battle,” a momentary state in which all their attention is focused on one another, until one emerges victorious.
The researchers analyzed videos of the spiders’ behavior, and used them to encode the forces of attraction and repulsion in a model, according to their size and the distance between them. “It’s like formulating a law of nature out of their trajectories, the speed of their movements and the distance between them,” Gov explains. The results made it possible for them to predict the males’ real-life movements.
Conflicts between two spiders are resolved in two ways, with the outcome depending on size differences between the two. While in battles between mammals there are usually no significant differences in the size of two individuals of the same species, among golden orb-weaving spiders, some adult males are as much as 10 times larger than others. If two males of similar size face off, they will usually resolve it through physical combat (“They just beat each other up,” Gov laughs).
But according to the theoretical model developed by Haluts, if the size difference is large enough, there won’t be a physical battle. In this case, according to the model, the larger male will chase the smaller one away. For this study, this theory was tested in experiments conducted in the spiders’ natural habitat – the jungles of Panama. The researchers placed a large male and a small male on the female’s web, and found that the prediction of their behavior by the physical model was precise.
The males had to wage their wars and chases with great caution on the web, so the female wouldn’t notice the tiny invaders in her kingdom. “It’s like a game of red light, green light,” says Gov. “When she notices them, they freeze until she relaxes, and then they go back to competing.”
Despite the competition among them, in some cases, the males will behave as if they attract each other and move towards their rival
Existing models depicting struggles over resources among animals – whether over food, territories or a mate – are based on game theory, Gov says, and focus mostly on the possible outcome of each conflict in accordance with the identities and characteristics of the competitors. But these models do not take into account everything that happens before the winner is declared, nor do they depict the dynamics of the battle, which can significantly impact the outcome.
The researchers used the physics-based model they developed to calculate different scenarios. For example, what happens when a large male finds himself facing off against a number of smaller males? Previous studies found that in cases where a large male confronts several competitors, he has more offspring. But those same studies did not explain why more competitors increase the large male’s advantage.
The model that Gov and Haluts developed shows that when many small males compete for the female, they tend to entrap each other (“beat each other up”), while the larger male stays out of these frays on his way to the female. However, if one small male faces off against a large male, he will sometimes manage to evade his rival and be the one to reach the female.
The model, then, explains why sizes vary so much among male golden orb-weaver spiders. “It wasn’t clear from an evolutionary standpoint why the male golden orb-weaver spiders didn’t reach a certain optimal size best for competing for females,” Haluts says. “From the model, we realized that this species' webs are like isolated battlefields. The moment a male reaches the web, it becomes its whole world. Different conditions of evolutionary competition can develop in every such ‘world’; in some of them it’s better to be big, and in some it’s better to be small.”
The approach of using physics to explain animal behavior has grown more popular in recent years. Researchers treat every individual as a particle, and try to identify the forces that influence them and spur the behavior that manifests in experiments. According to Gov, so far such models have been developed only for describing the behavior of large groups of animals, such as flocks of birds or schools of fish. “What's special about the new study is that, for the first time, as far as we know, it is describing not the movement of a group of animals together in a situation where there is no conflict, but one in which there is competition over a resource."
The researchers added that the physics-based description of competition in this study not only relates to one specific system of one species of spider, but they believe that the model will also be able to describe competition over resources between other species in the future. They say they have already begun to develop a model that describes the competitive dynamics in a group of mice, and territorial combats among fish.