Some animals use visual or auditory cues to signal “I’m a good catch” or “I’m strong—don’t mess with me.”
So why do so many species”—like crows, dolphins, and people—lack these seemingly important quality signals? They probably evolved in smaller social groups where everyone already knows what everyone else is like, a new study suggests.
“Judging rivals by their ornaments is sort of like judging a book by its cover,” says Michael Sheehan, assistant professor of neurobiology and behavior at Cornell University and coauthor of the study published in the journal Behavioral Ecology. “You’ll get some information about the book, but you’ll learn much more by reading it. Animals that repeatedly interact with the same individuals essentially get a chance to ‘read the book’ and learn about their rivals.”
As species evolved, quality signals and social recognition have been antagonistic, with selection depending on the social structure in which the animals lived. Evolving in small social groups favors social recognition over external signals.
In a troop of baboons, everyone knows who the alpha male is. But Gelada monkeys that live in huge herds in the highlands of Ethiopia carry a bright red chest patch that indicates status. The Gelada chest patch is thought to act like a lion’s mane, advertising the strength of a male to unfamiliar rivals that might challenge him for his pride.
Each form of signaling requires an investment: It takes energy to grow a bigger mane or develop a more flexible vocal system, but social recognition requires more learning, which is energetically costly too. Experiments with fruit flies have shown that flies that had to learn a task died quicker when later placed under stress. The benefits of the signaling system have to outweigh the costs both for senders and receivers as traits co-evolve.
“What this broadly tells us is why some animals are so brilliantly decorated and have invested a lot in display,” Sheehan says, “and why social species lack these designs. We might notice [physical] things about a species that would give some sense of their social system, and if you know their social system you could predict their signaling.”
But, it’s not an all-or-nothing game. For example, when a songbird establishes its territory, quality signaling plays a major role. But birds typically learn about their territorial neighbors and then depend on individual recognition instead of quality signals. In the case of the Gelada’s chest patch and lion’s mane, the message is likely meant just for unfamiliar rival males.
“Humans are an interesting case,” Sheehan says. “Cultural adaptation may have outpaced biological adaptation.” We evolved to use social interaction, but we have incorporated quality signals into the culture, from military uniforms to the number of windows in an office.”
This way of thinking opens new ideas to explore, researchers say. The development of quality signals might predispose a species to move away from living in small social groups, and vice versa.
“It’s a way of thinking about this but still largely untested,” Sheehan says.
Thore J. Bergman, associate professor of ecology and evolutionary biology at the University of Michigan, is coauthor of the study. The National Institutes of Health, the National Science Foundation, and the University of Michigan supported the work.
Source: Cornell University