Guppies adapt to predators after their release in rivers

Let’s say you had two rivers. One had predators, the other didn’t. Then let’s say you took several hundred guppies, the tiny colorful fish common to freshwater aquariums everywhere, and released half in each river.

What would you expect to find some years later?

Writing in The American Naturalist, scientists who conducted this experiment in Trinidad say they found that the guppies quickly adapted to the conditions in each river.

Eight years – between 13 and 26 guppy generations – after releasing 200 guppies into predator-filled and predator-free waters, the scientists found that the two fish populations had divergent reproductive strategies.

With predators present, females produced more embryos per reproductive cycle. They might have only one chance to procreate, the scientists reason, so better to do it all at once.

On the other hand, with predators absent, female fish produced fewer embryos. There, the mother fish conserved resources for the future.

To confirm that these adaptations did, in fact, make the guppies fitter, the scientists swapped a few to see how they’d fare in different environs.

Fish adapted to predators went into peaceful waters, and vice versa.

As expected, the scientists observed that the original inhabitants had a 54 to 59 percent better chance of surviving than the new arrivals.