A black-eyed guppy (Poecilia reticulata) defends a morsel of food from its calmer, silver-eyed counterpart. ROBERT HEATHCOTE UNIVERSITY OF EXETERIn a study published on June 4 in Current Biology, researchers report that Trinidadian guppies (Poecilia reticulata) swiftly alter their iris color from silver to black when behaving aggressively towards smaller guppies. “You’ll see these little angry fish swimming over food patches, attacking anything coming over them,” Robert Heathcote, a coauthor on the paper, tells The Atlantic

Black- and silver-eyed guppies (Poecilia reticulata) feed on a food patch. ROBERT HEATHCOTE UNIVERSITY OF EXETER

Heathcote’s team conducted behavioral trials using smaller, guppy-like robots with black or silver irises to see how larger guppies would respond to the eye color. The researchers found that when guarding food, the black-eyed, smaller imposters attracted larger, blacker-eyed competitors. The researchers believe the fish show their “honest”...

R.J.P. Heathcote et al., “Dynamic eye colour as an honest signal of aggression,” Curr Biol, doi:10.1016/j.cub.2018.04.078, 2018.

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