Friday June 17, 2022


Birds of a feather flock together, but when guppies gather, they get really bad worms.

New University of Pittsburgh research shows that fish that group together to avoid being eaten run the risk of breeding nastier parasites—a pattern that’s likely common across the animal kingdom and may even be the case for some human diseases.

“There are so many animal hosts that shoal or flock or herd for defense against predation,” said lead author Jason Walsman, a postdoctoral biology researcher in the Kenneth P. Dietrich School of Arts and Sciences. “Predators driving hosts into the arms of increasingly deadly parasites should happen a lot—it’s probably happening in hundreds or thousands of species every day.”

The species Walsman studies is guppies on the island of Trinidad, an unlikely star of decades of evolution research. In some areas, these guppies are eaten by all manner of bigger fish, so they bunch together for safety; others live a relatively stress-free life in upstream utopias, protected by impassable waterfalls from the hungry predators downstream. These differences make them the perfect study species for understanding how predators steer the path of their prey’s evolution.

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