The only problem was, the costs didn’t always exist. “If the handicap principle is true, people thought they should be able to observe the cost of signaling in the various signaling systems observed in nature,” said Michael Lachmann, a theoretical evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology in Germany. “And that hasn't always panned out.”
Consider a baby bird that wants to grow as big and strong as possible. The chick should beg its parents for food whether it is hungry or not, unless the begging is costly. But careful experiments over the years have revealed that for most bird species, begging is cheap. “It’s easy to do and doesn’t attract predators,” Zollman said.
In Grafen’s signaling game, the low signal cost should lead to runaway begging among chicks, causing this mode of communication to lose meaning and disappear. For cheap signals to be stable, evolution must be playing a different game.
In a 1997 paper, the theoretical biologist Carl Bergstrom and his colleagues identified another conundrum: In order for honesty to be stable, the cost of the signal — even an honest one — has to be so steep that both signalers and receivers would be better off if the signal had never evolved. It is unclear to scientists how this situation might arise. “Simply identifying an evolutionarily stable strategy isn’t enough to tell you whether evolution can actually end up there,” said Bergstrom, of the University of Washington, who also co-authored the new study.
In the January issue of “Proceedings of the Royal Society B,” Zollman, Bergstrom, and Simon Huttegger of the University of California, Irvine, show that a partial-honesty model solves both problems. It is evolutionarily stable for both costly and cheap signals, with an equilibrium point that is reachable in a computer-simulated game.
For peacocks, partial honesty works like this: High-quality males will always grow long tails, but low-quality males will be polymorphic, with some growing long tails and some short. “So if you see a peacock with a long tail, it’s more likely he’s of high quality than low quality, but you’re not sure,” Zollman said. Meanwhile, peahens won’t fall for every long tail that sashays by. “If she sees the long tail, she sometimes chooses to mate with him and sometimes declines.”
Partial honesty works similarly when the signal comes cheap. Hungry chicks will always beg, and not-so-hungry chicks will sometimes beg too. “The parents should sometimes but not always feed a chick that’s begging, and should never feed a chick that isn’t begging,” Zollman said.
It’s a winning strategy in theory, but further research is needed to determine if real-world birds actually use it. Hugh Drummond, a biologist at National Autonomous University of Mexico who studies parent-chick interactions among blue-footed boobies, said parent boobies sometimes ignore intensely begging chicks for up to 20 minutes before suddenly coughing up a morsel, behavior that is seemingly consistent with the partial-honesty model. “But is the parents’ responsiveness capricious?” Drummond asked. “Or are they waiting for food to be sufficiently predigested to be ready for regurgitation?”
Biologists must unravel these subtle interpretations of animal behavior in order to answer larger questions about the evolution of honesty and deceit. "Although predictions of Zollman's model are superficially clearcut," Drummond said, "tests of them could get mired in ambiguities."
Reprinted with permission from Simons Science News, an editorially-independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the computational, physical and life sciences.