Aposematism

Aposematism, 2006. Watercolor on illustration board

“Aposematism” is a type of protective signalling mechanism, usually but not always involving bright coloration, that draws the attention of predators. It might seem counterintuitive that potential prey would make themselves more conspicuous, but aposematic prey are defended by stings, venom, foul tastes, or other weapons, and they need to warn predators of that.

A yellowjacket wasp’s bright yellow grabs a bird’s attention, warning that this particular prey can fight back with a sting. A skunk’s stripe warns of a different, but equally unpleasant, defence. Bold stripes, bright colors, and other distinctive patterns help the predator associate the prey animal’s appearance with the unpleasant result. Like someone who gets the stomach flu after visiting a certain restaurant, they will avoid that food source in the future.

Familiar color patterns recur in aposematic animals. Honeybees and yellowjacket wasps both use yellow and black stripes. Although each species’ pattern differs, bees and wasps have successfully collaborated to make their predators strongly associate their shared defense, stings, with their shared yellow and black coloration. Resemblance across different aposematic species is called Mullerian mimicry. The mimicry is arguably more beneficial to the bee, because deploying its barbed stinger has fatal consequences, while the carnivorous yellowjacket can sting an attacker repeatedly and still escape in good health. But both hymenopteran families benefit from sharing one common, clearly understood warning signal.

Unpalatable butterflies and their caterpillars also use high-contrast black and yellow or black and orange patterns (some engage in Mullerian mimicry). Venomous snakes, like the coral snake, are yellow, black and red to warn their attackers off. Tropical poison dart frogs are yellow and black (some with stripes), but also hot pink, aqua, and neon orange. What do predators think of all these bright colors and patterns?

It turns out that chicks tend to avoid black and yellow food, even if they have never come in contact with it before. Schuler and Hesse (1984) painted mealworms with either a conspicuous yellow and black pattern or a drab olive color (created by mixing the yellow and black paint). They found that although newborn chicks would peck at the black and yellow striped mealworms as often as they pecked at the olive mealworms, the chicks did not eat as many of the black and yellow mealworms. Since the mealworms were identical (and harmless) except for the paint, the chicks had no incentive to ignore the black and yellow ones. Schuler and Hesse concluded that the chicks have a native aversion to black and yellow prey based on color alone.

To exploit such predatory prejudices, some animals mimic aposematic coloring despite having no defences of their own. The hornet moth (Aegeria apiformis) is a very convincing replica of a yellowjacket, though like all moths, it lacks a sting. If it’s lucky, the moth’s superficial resemblance to a more dangerous species will protect it from predatory birds. This type of mimicry is called Batesian mimicry. Bees and wasps, who are defended by stings, derive no benefit from the hornet moth’s Batesian mimicry of them. Mimicry can work against the model species: if a predator eats a Batesian mimic and experiences no ill effects, it may learn to doubt the reliability of yellow and black warnings, and begin to eat both defenceless mimics and defended model species. To avoid this problem, hornet moths must be significantly outnumbered by the authentic bees and yellowjackets encountered by the predator.

Coral snakes also have Batesian mimics. There are over a dozen species of American snakes with red, yellow and black stripes that resemble corals, more or less. Batesian mimicry apparently doesn’t have to be exact, just reminiscent enough of the genuine coral to give a predator pause.

The mnemonic “red on yellow kills a fellow, red on black venom lack” helps to distinguish venomous coral snakes, in which the red stripe is surrounded directly by yellow, from nonvenomous mimics such as the Mexican milk snake, with the red stripe insulated from the yellow by black. Unfortunately, this mnemonic is not always accurate. Its usefulness depends on geographic location, and much like the coral snake, it has many confusingly similar variations (“black on red, friend to Fred,” etc). In an episode of Seinfeld, Kramer reverses it so the “red on yellow” snake, the coral, is harmless. (He’s corrected by Elaine.) It also pops up occasionally as “black on yellow kill a fellow,” which is misleading, because black touches both red and yellow in the harmless milk snake. Check this helpful page for photos of coral snakes, some of their nonthreatening mimics, and a good explanation of the stripe pattern.

In evolutionary terms, it remains unclear how a species initially makes the transition from cryptic (camouflaged) coloration to conspicuous, aposematic coloration. Conspicuous prey seem more likely to be seen and eaten than cryptic ones, and it may take a predator several tries to learn that conspicuous prey should be avoided. If there are just a few conspicuous animals in a population, they should be picked off too quickly to reproduce. However, some recent studies modeling predator-prey interactions suggest that predators’ food preferences are complex, and conspicuous coloration may not be the fatal disadvantage it was once thought.

Humans have co-opted the high-contrast color combinations characteristic of aposematism for our own purposes. Warning signs (both traffic and industrial) tend to use yellow, red, or orange in combination with black. The familiar yellow caution tape depicted in this painting is so close to the natural coloration of the yellowjacket, the insect almost disappears against it. Note that this effect is a completely artificial inversion of the normal function of aposematic coloration, which should be the very opposite of camouflage. In the wild, against a background of green, brown, or grey, the yellowjacket would be clearly visible: “Danger! stinger here.”

Although yellow and black patterns do catch our attention, we aren’t chicks. These colors don’t make us lose our appetite (although mealworms might). Quite the opposite – yellow and red are often used with black as color schemes in the fast food industry (McDonald’s, Burger King, Carl’s Jr., KFC, Denny’s, Wendy’s, Taco Time, etc). In a recent article in Voice, David Barringer discusses the conflicting “danger” and “hunger” impulses evoked by yellow and red. One theory is that mammals tend to associate these colors with fruit, and picking them out against a dull background was an evolutionary key to success in foraging. Our eyes became optimized to identify fruit colors, and the colors might have innate positive food-related associations for us. Studies have shown red makes people hungry.

But it’s more complex than that, obviously. We learn simultaneous negative and positive associations for the red and yellow of fire (danger, burns, warmth, cooking); and the black-on-yellow “danger: flammable” sign has different connotations than a yellow-on-black sign for a flame-grilled hamburger. Barringer points out that many of the fast food joints using these colors could simply be emulating McDonald’s mid-century decision to go with gold, and benefiting from accidental association with an already well-known food brand — McDonaldsian mimicry. Yum.