Imagine you’re a puny peacock, rendered weak by bad genes or poor nutrition. You hope to attract a peahen, who mainly cares about the length of your tail. Growing a long tail would greatly enhance your sex appeal, but the encumbrance might prevent you from fleeing a predator that a fitter male could evade (and getting eaten dramatically reduces your chances of mating).
Will evolution program you to be honest, growing a lightweight tail that reflects your true fitness? Or will it have you risk life and limb by cultivating deceptively extravagant plumage?
The best strategy for peacocks — and for other animals when they communicate, from gazelles advertising their agility in front of lions to baby birds begging for worms —can be identified using game theory, the mathematical study of how different decisions affect the outcome of a game. In the evolutionary game, where the goal is producing offspring, rampant dishonesty is not an optimal strategy: If peacock tails are never honest, peahens will simply ignore that feature altogether. But perfect honesty, which was long thought to be the evolutionary strategy of choice, may not be ideal either, as some evidence now indicates.
In a new study, game theorists showed that partial honesty might be the best policy in animal communication. During computer simulations of evolving populations, researchers found that a fixed ratio of honesty to dishonesty sets in, where the “signalers” (peacocks) aren’t completely honest, and the “receivers” (peahens) aren’t completely trusting. “You can actually have a stable situation where you have partially honest communication,” said Kevin Zollman of Carnegie Mellon University, the lead author of the study.
Partial honesty works so well in theory that if biologists are able to observe it in practice among peafowl and other animals, it could replace a long-standing but broken picture of how animal communication works.
“It is an interesting idea supported by a theoretical model, which may be worthwhile to try and apply to the begging behavior of meerkats,” said Marta Manser, a behavioral biologist at the University of Zurich, by email. “However, it is not a straightforward test to do.”
It can be difficult to control for all factors in an animal’s behavior, Manser and other biologists explained, which makes it challenging to test the predictions of the new theory against those of the old one.
For decades, the canonical explanation for how and why animals communicate has been the handicap principle, first proposed by the evolutionary biologist Amotz Zahavi in 1975. According to this principle, all animal signaling comes at a cost, but animals are perfectly honest because the cost of deceit is too high. Puny peacocks, for example, do not grow long tails for the same reason that a poor man does not buy a Maserati: It costs too much. (The poor man risks bankruptcy; the peacock risks death.)
In 1990 the game theorist Alan Grafen proved mathematically that the handicap principle is evolutionarily stable. In a game in which a signal enhances the signalers’ attractiveness as a mate but reduces its viability (as is the case with peacocks’ burdensome tails), an equilibrium between cost and benefit is achieved when all signalers are perfectly honest. At that point, a liar that is introduced into the game — a weak peacock with a long tail, for instance — cannot gain an advantage. Backed by game theory, the handicap principle became “a panacea,” Zollman said. Whenever animals were observed communicating with each other, indicating that their signals were evolutionarily stable, scientists assumed the behavior must exact a cost.