Oddly, Predators Miss Flamboyant Guppies
The color pattern variation of the males guppies (left and right columns) is mostly genetic variation, not environmental. New data indicates that rare-colored males not only attract females (center column), they are also more likely to survive predation.
Credit: Kimberly Hughes

For male guppies, being unique is good for more than just attracting the ladies—it could mean the difference between life and death.

A new study finds that guppies with rare color patterns are less likely to be gobbled by prowling predators than their commonly colored counterparts. The finding is an example of how nature sometimes favors rare traits.

The four-year study, detailed in the June 1 issue of the journal Nature, involved manipulating the ratio of rare and common fish in 19 twin pools in three different streams in Trinidad.

At each site, one of the pools was manipulated so that guppies with pattern A outnumbered those of pattern B by a ratio of 3-to-1; in the other pool, the population was tweaked the other way, so that B outnumbered A. Each pool also harbored killifish or pike cichlids, both of which prey on guppies.

After about two weeks, the researchers collected all the adult-sized guppies. Rare males had higher survival at all three streams: 84 percent of the rare-type males survived the experiment, compared to only 69 percent of the common-type males.

Male guppies  don't normally engage in deadly duels, so the most likely explanation for the dip in population size is that some guppies were eaten, the researchers say.

"It's possible that guppy predators, which are known to hunt visually, may be more focused on common color patterns," said study team member Kimberly Hughes of the University of Illinois at Urbana-Champaign. "Predators can form 'search-images' of the most common prey types, and can be less efficient at locating and capturing prey that look different from the norm."

Another possibility is the guppies altered their behavior in response to the researchers' manipulations, and that the changed behaviors affected predation. More research will be needed to distinguish between these two possibilities.

The finding is an example of what biologist call "negative frequency-dependent selection," in which natural selection favors rare traits over common ones. It's been hypothesized that a similar process could explain why a small percentage of humans are left-handed . According to one idea, left-handed fighters have a surprise advantage over their right-handed adversaries.