Supernormal stimuli make birds ignore their eggs (and then their chicks), cause fish to fight dummies (while live competitors go uncontested), and can drive humans back to sugary snacks. Find out how these special stimuli use just the right trigger to make an animal destroy itself.


We assume that creatures evolved by making the best choices for survival and subsequently passing their genes along. We survived by being sharp: having the ability to spot a mate, a competitor, or a child in need. But in some cases, animals become attuned to one particular cue instead of making a choice based on overall value. This one cue, properly applied, can drive an animal to self-destruct.

The stickleback is a freshwater fish in Holland. On its belly is a wide red stripe. Each male stickleback has a territory and defends it from other males in an attempt to impress females. Toss in a different fish, and the males will ignore it completely. Toss in a piece of junk, ditto. But if you paint the underside of that junk red, the males will always fight it. They will fight it even if it looks nothing like a fish or when other fight-happy male sticklebacks are around. The brightness of the stripe triggers the sticklebacks' urge to fight, and since humans can make a brighter red than sticklebacks, they can control the males' behavior with paint colors.


Though fish are known to engage in this kind of stimuli and attack dummies and mirrors, birds are the champions of this. From mating to roosting to bringing up chicks, birds will tune in to all kinds of supernormal stimuli. When you put caps on the heads of male chickadees that are (sadly devoid) of the crest that it requires to impress a mate, females will flock to them and ignore chickadees with actual crests. Birds will try to roost on "eggs" as large as themselves, as long they're roundish and colored correctly. Bigger eggs mean bigger chicks. When those chicks hatch, however, mother birds will feed a fake baby bird on a stick, as long as the "baby's" beak is bigger and more vividly colored than their actual babies.

Even insects will get in on the act. Ants will welcome in and serve a massive enemy ant many times their size as long as it's coated in the juices of their fellow ants. They'll even drag a living and kicking one of their fellows over to an ant graveyard if it is dabbed with a scent that ants give off when they die. Meanwhile in the more picturesque butterfly kingdom, male silver-washed fritillary butterflies will scorn females of their own kind to mate with a rotating cylinder if it's colored correctly. Many different species seem to have cheat codes. Punch them in, and they'll over-ride anything else.

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Experts believe that these cheat codes aren't limited to animals with brains the size of loose change. Humans are also wired to respond overwhelmingly positively to things like sugar, salt, and fat. We have brains that tell us that we don't need something, but our bodies overrule us. We absolutely need another piece of cake, followed by potato chips, followed by more cake. Unlike the animals, though, we create our own supernormal stimuli, inventing them and resupplying them, at the cost of our own health. Then again, cake is better than a rotating cylinder. Maybe a little self-destruction is good.

Image: PsyWW. Via The Scientist.