Signaling Theory: How Prey Animals Communicate with their Predators

(A longer discussion of animal communication can be found in my book.)

It’s no surprise that animals communicate with one another, but we normally think of animal communication as between members of their own species. It turns out that predators and their prey have evolved elaborate systems of communication as well.

I am dangerous!

A few prey animals have evolved ways to let their potential predators know that they are dangerous and it might be better if they moved on and picked another target.

The rattlesnake is probably the most striking example of this. Rattlesnakes are themselves predators, but their distinctive rattle call is certainly not part of their hunt. Why would they want to advertise their presence to their prey?!

Rattlesnake-Picture1

Particularly as juveniles, rattlesnakes are the frequent targets of hawks, eagles, crows, raccoons, coyotes, skunks, and even other snakes. With the exception of whipsnakes (who have evolved immunity to rattlesnake venom), all of these predators run a very high risk when hunting the very poisonous rattlers.

The only hope of nabbing a rattlesnake without dying in the process is to sneak up on them. When a threat is spotted, rattlers shake their tail and make a very distinct and conspicuous sound. Clearly, the predators have learned this signal because they almost always retreat, rather than risk being dealt a fatal injury.

I am poisonous!

While I would like to focus on intentional communication in this post, I would be remiss if I didn’t at least mention something called aposematic coloring. This is a phenomenon in which a prey animal evolves an anti-predator defense mechanism AND a way to communicate that defense to their predators.

The most famous example are the poisonous tree frogs of the Amazon rain forest. These little guys secrete extremely potent toxins into their skin. So toxic are these secretions that Amazonian tribes wipe their arrows on the backs of these frogs to make a truly lethal weapon.

Dendrobates pumilio Strawberry poison dart frog

The value of the poison is obvious: if a predator eats the frog, she will then die. However, even when the poison works as it should, the frog is still dead. An even better way to use a poisonous deterrent is to train predators not to eat you in the first place.

poison-dart-frog1

This is where their bright coloring comes in. The poisonous frogs have evolved obnoxious coloring so that they are as recognizable as possible, the exact opposite of camouflage. This coloring then “trains” the predators to recognize and avoid the frogs.

Blue-poison.dart.frog.and.Yellow-banded.dart.frog.arp

This training is not about true learning: the predators wouldn’t survive the “learning” process. Instead, it is evolutionary learning. Through mutation and selection, the predators evolve the avoidance behaviors. Predators with no aversion to the frogs eat them and die, while predators who are genetically disposed to avoid them survive and leave offspring.

Once this training is in place, the poisonous tree frogs are largely left alone by their predators. The “communication” is complete.

One last thing about coloring. Nature has a common tendency to reward cheaters and poison advertising is no exception. After one species has gone to the trouble of evolving poison and then training predators to avoid eating them, others can piggyback. There are also species of tree frogs that are brightly colored but with no poison. These are nature’s freeloaders and this term is called Batesian mimicry.

cb41ab5d510e2e2808c8286caada9852

(Can you tell which of these frogs are poisonous? Neither can their predators. Answer: the top row are all poisonous species, the bottom row are all nonpoisonous mimics.)

I am fast and healthy!

For some prey species, it would be silly to feign aggression and pretend that you could fight off your predator. Imagine a mouse trying to convince a cat that he’s big and tough and will fight back.

CatMouse1R_468x399

However, some prey animals have evolved means to tell their would-be killers, “I see you. I’m faster than you. Don’t bother trying to chase me because I’ve got a head start and you’ll just be wasting your energy.” This phenomenon is called signaling theory and these displays usually involve a prey animal engaging in feats of strength to show predators that they are strong, fast, and/or alerted to their presence.

One of the most famous examples of signaling theory is a behavior in gazelles called stotting or pronking.

The main predator of gazelles is the cheetah. The cheetah can sprint faster than the gazelle, but has less endurance and cannot maintain speed as well while turning. This means that the cheetah must sneak up on the gazelle stealthily and make a sudden surprise attack if she is going to catch him. If the gazelle has a head start, the cheetah has no chance.

cheetah-gazelle1

When a gazelle spots a stalking cheetah, he will start jumping very high, straight up in the air. It’s a rather remarkable sight, actually. At first blush, this seems kind of stupid. Here is this gazelle being stalked by a cheetah, and when he notices, rather than running away, he makes himself incredibly obvious to the cheetah.

Springbok_pronk

However, what happens next demonstrates the purpose of the stotting: the cheetah gives up the hunt and walks away. The stotting is the gazelle’s way of telling the cheetah that he sees her, has a head start, and that a chase would be futile.

I am honest!

Stotting is often referred to as an “honest” signal because, since the gazelle has to be in good physical shape to perform the signal, it is a true display of fitness. This is a fascinating example of co-evolution because the gazelle has evolved to perform the signal and the cheetah has evolved the ability to interpret it. Both species benefit from this communication because they’ve been spared the bother of a fruitless chase.

The gazelle is happy to evolve a way to avoid having to outrun the cheetah every time, and the cheetah is happy to evolve a way to reduce its record of unsuccessful hunts. Chases are energy-expensive after all and they are also are very loud and obvious. After a chase, every potential prey animal in the area is suddenly aware of the cheetah. They get one shot. If it doesn’t work out, there probably won’t be any successful hunts the rest of that day.

Communicating About Predators

Stotting also communicates other things to nearby animals. Of course, the slotting warns fellow gazelles of a cheetah in the area. That may have been the reason that the behavior first evolved and then Cheetahs learned it later, but we’ll probably never know for sure. Further, stotting may be part of the courtship behavior of gazelles. Given its utility in avoiding both predation and in saving energy, stotting seems like as good a display of fitness as any other.

natgeo-springbok2

The many species of gazelle are not the only animals that stot. Their cloven relatives impalas, antelopes, and wild sheep are all thought to stot. Although domestication seems to have diminished stotting in adult sheep livestock, young lambs are prone to periodically engage in bizarre spastic jumping behavior that seems playful and may be the remnant of the stotting instinct. Other forms of pursuit-deterrent signal have been discovered in motmot birds, Eurasian jays, rabbits, mice, curly-tailed lizards, and even guppies.

As I’ve written about before, Diana monkeys have been shown to give specific calls to warn their fellow monkeys of specific kinds of predators. Their vocabulary includes distinct calls for each of their main predators.

diana-monkey-family

Interestingly, two of the Diana Monkey predators – leopards and eagles – hunt by surprise attack. Accordingly, they have “learned” what the calls mean and when they hear the monkeys making those calls, they give up the hunt. Thus, these calls also function as pursuit-deterrents for those predators, not unlike slotting in gazelles.

However, chimpanzees also hunt Diana monkeys, but they do so through sustained stalking and chase, not surprise attacks. Consequently, they are not at all dissuaded by the warning calls. For the chimps, they don’t rely on the element of surprise anyway so they could care less if the Dianas are aware that they are being hunted.

While this Diana monkey alarm system almost certainly evolved as a warning to conspecifics, the predators have “learned” what they mean. For eagles and leopards, the game is up when they have been spotted and so they give up and move on.

Signaling theory depends on the signals being truthful. What if Diana monkeys went around making eagle calls randomly, just to protect themselves in the off-chance that an eagle was nearby? After a while, the eagles would lose their training and no longer be dissuaded from attacking based on the calls alone. The dishonest Diana monkeys would find themselves to be victims of eagle attacks, possibly even more often than chance alone because the calls might actually attract the eagles. If the cheating behavior were genetic, dishonesty would quickly be bred out of the population and balance would be restored. Thus, honesty is self-perpetuating in signaling theory.

Eavesdropping

 Interestingly, some animals appear to have evolved to “eavesdrop” on predator-warning calls made by other species. There is a species of iguana on the island of Madagascar that has evolved very respectable hearing, despite the fact that they don’t hunt using sound, nor do they communicate among themselves using any auditory communication.

Oplurus-cuvieri

They do, however, respond to the predator-warning calls of a species of flycatcher bird that also lives on the island. Both the iguanas and the flycatchers are sometimes preyed upon by large raptors—birds of prey. The iguanas never evolved a warning call for raptors because they didn’t have to. They could just listen out for the calls made by flycatchers.

Just like the nonpoisonous frogs that mimic poisonous ones, cheaters can prosper.

-NHL    (@nathanlents)

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