Unusual creatures are more appealing: What guppies teach us about evolution

Females prefer to mate with unusual males. At least, that’s the case among guppies, small fish from South American rivers. Since their fertilization is internal, the males display a wide array of colors to attract females. It might seem that the most fashionable males are the most successful… but it turns out to be quite the opposite. So, why would a female guppy choose the most unusual male in the group?

The answer was found on the island of Trinidad, in the southern Caribbean, a veritable natural laboratory where guppy populations, separated by waterfalls and rivers, allow researchers to study evolution almost in real time. There, in 2007, biologist David Reznick, from the University of California, Riverside, launched the Guppy Project, in order to unravel some of the great mysteries of evolution.

After monitoring 10 generations of these fish, the researchers discovered that the females’ preference for unusual males didn’t improve the survival rate of their offspring. However, it did offer an indirect advantage: their sons — by inheriting these unusual traits — were more attractive and achieved more matings. This is known as the “sexy son” hypothesis. The indirect effect maintains the preference for the unusual and explains, in part, the dazzling diversity of colors and sexual characteristics that we see in nature.

These results, published in the journal Science in 2023, had a major impact. But what’s rarely mentioned is the work that took place behind the scenes, with teams of young people getting up at 5 a.m. to scour the jungle in search of fish. This is the case of Ignacio Paulin, who — having recently graduated in biology from the University of Oviedo, in Spain — was hired as a field assistant for the Guppy Project.

“I was used to the mountains of Asturias, where you can see for miles around. But I understood that the jungle was a place to focus on the small details, because your world shrinks to a tiny bubble,” Paulin notes. The biologist describes his experience: “Every [16 feet, there] was a new universe full of things to discover. I went [to Trinidad] for three months, but I immediately thought, ‘I’m not leaving here so soon.’”

At 6 a.m., he and his team were already in the car. After an hour’s drive, they began trekking into the jungle. “We were walking through the middle of nowhere. It’s undoubtedly the most remote place I’ve ever been to,” Paulin recalls. Each of them carried a backpack full of empty bottles. Upon reaching the river, they tried to capture as many adult guppies as possible. They would place them in the bottles and note the exact stretch of the river where they found them. Back at camp, the fish were transferred to small aquariums to be studied.

The next day, in the lab, the fish were anesthetized one by one and examined under a microscope, so that the researchers could read the color code tattooed on their skin, which served as their identification. If they had no code, it meant that it was their first time being captured: subsequently, a new code was tattooed and a DNA sample was taken. If they were already tagged, the database was consulted to record their weight and size, with a new photograph being taken before releasing them back into the river.

This routine has allowed researchers to track the evolution of guppy populations in Trinidad for years. They know how long each fish lives, how its appearance changes, where it moves, as well as its lineage. “Around 4,000 fish come through the lab every month. Since the project began, 115,000 individuals have been recorded. And, since many are caught multiple times, we now have over half-a-million records,” Paulin explains.

Although the fieldwork was fascinating, Paulin felt it wasn’t enough. He was drawn to studying animal behavior and wanted to formulate his own hypotheses. After getting to know Trinidad and its guppies firsthand, he decided to combine that experience with mathematics, so as to analyze the guppies’ behavior. He enrolled in a Master’s program in ecology, evolution and animal behavior at the University of Konstanz, in Germany. There, he met Alex Jordan, a researcher at the Laboratory of Evolutionary Behavior.

“One day, we were introduced. And he asked me, ‘Why are you here?’ I told him that I had just arrived from Trinidad and that I wanted to return [to the island, so that I could] apply computation to the study of animal behavior. [And I told him] that I needed a place to learn how to analyze all the data,” Paulin recounts. “It was amazing: we clicked from the start. He loved my proposal.”

During his graduate program, he returned to Trinidad, this time determined to study guppy courtship. To attract females, the males perform a ritual known as the sigmoid display: they position themselves in front of the females and perform a “dance,” by arching their bodies in an S-shape. During this display, some of their colors — regulated by hormones — intensify. It’s their way of pleading for a chance.

It’s worth noting, however, that male guppies aren’t always so chivalrous. These fish have another reproductive tactic: catching females off guard, darting past them like shooting stars… and fertilizing them unexpectedly. This tactic avoids relying on their consent (although the probability of successful fertilization is lower).

That’s why guppies end up resorting to courtship dances, even though they might be rejected. But the dance also carries a risk, because they become more visible to predators. And this is where Paulin’s research comes in: “I’ve recorded the fish’s courtship dances in different rivers on the island. Some have predators, while others don’t. Right now, we’re training a machine learning model to detect the fish’s posture and movements.” The biologist continues: “We want to study how the dance has evolved and see if its intensity changes between populations that live with predators and those that don’t.”

Currently, Paulin is at the Max Planck Institute for Animal Behavior in Germany, analyzing his data. He’s worried he won’t be able to return to Trinidad. For the first time since 2007, the Guppy Project has closed its doors. “The Trump administration decided to cut funding. It’s incomprehensible to let a project of this magnitude — in which so much has been invested — [just] die,” he laments. “Right now, there’s no one left at the station in Trinidad.”

“Beyond the science,” he details, “there’s the human aspect: we lived at the edge of the rainforest, near a small community that welcomed us for 16 years. We felt like part of them. I had a close relationship with the children; we played together and they showed me the rainforest. The thought that all of that could disappear forever makes me very sad,” he laments.

Scientific projects are much more than the articles published in a journal. Thanks to the Guppy Project, Paulin and hundreds of other young people found inspiration in Trinidad to launch their research careers. Today, at 26 years old, Paulin has extensive experience studying fish… and many questions left to answer.

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