The Friday the 13th and Nightmare on Elm Street movies aren’t typically thought of as mating strategies. But putting on a scary movie is a trick as old as drive-in theaters for encouraging one’s date to jump in fright and snuggle in just a little bit closer. Birds, so far as we know, aren’t into horror movies, but field research published late last year by two University of Chicago researchers suggests that the “scary movie effect” used by humans may also be a mating strategy for some bird species.
Stephen Pruett-Jones, associate professor of ecology and evolution, has been traveling to Australia for more than 20 years to study the splendid fairy-wren, a bird with a very progressive social life. While splendid fairy-wrens are socially monogamous, with males and females forming lifelong pairs, they are sexually promiscuous, mating almost entirely with partners outside of their home relationship. Fairy-wrens also form unusually complex family groups, with young birds sticking around the home territory to help their parents raise offspring rather than flying off to start their own family as soon as possible.
Over the course of studying these unique behaviors, Pruett-Jones and others observed another odd quirk of the splendid fairy-wren. When disturbed by the call or sight of a predator or threat – such as the butcherbird, or a clumsy human researcher – the small, blue males of the species don’t fly away and hide as you might expect, but sing their own distinct call, called Type II song. While a graduate student working with Pruett-Jones, Emma Greig studied the timing of those calls and found that the male fairy-wren responds almost instantaneously to the song of his predator.
“The male begins his Type II call immediately after the butcherbird begins to call, so they’re basically right on top of each other,” Pruett-Jones said. “It sounds like a duet.”
But the purpose of that communication remained mysterious. Was it an alarm call to other fairy-wrens in the area? A display of their bravery and physical fitness to attract mates? Or an effective means of capturing the attention of any females in the area?
The latter theory was tested by another round of field experiments, conducted at Brookfield Conservation Park in South Australia. Using an iPod filled with bird-call audio files, Greig played different songs for female fairy-wrens, using Type I song (a territory-marking call), butcherbird songs, and Type II songs in various combinations. The females responded most strongly when the butcherbird-Type II song combination was played (listen below), as measured by looking in the direction of the call and responding with their own song.
“We have shown that females do, in fact, become especially attentive after hearing butcherbird calls,” said Greig, currently a postdoctoral researcher at Cornell University. “So, it seems that male fairy-wrens may be singing when they know they will have an attentive audience, and, based on the response of females, this strategy may actually work!”
The experiments suggest the males may use predator calls as an alerting signal, similar to how a person would might get someone’s attention by prefacing their comments with a loud “Hey!” In fairy-wrens, the “vocal hitchhiking” of a male’s song onto the butcherbird call appears to benefit both species. While males increase their chances of attracting female attention, females find out where potential mates are located in neighboring territories and are potentially primed to mate by predator-induced fear.
“The most exciting possibility is that Type II songs have a sexual function, and that females are more easily stimulated by, or receptive to, displays after being alerted by a predator, such that the male’s song is especially attractive,” Greig said.
Another possibility is that the male fairy-wrens vocal hitchhiking is an example of the handicap principle, a theory of sexual selection where animals signal their physical (and genetic) fitness by intentionally putting themselves in danger. But in looking at several measurements of physical condition – such as age, social status, body condition, and blueness – Greig found no differences between males who sang Type II songs after a predator call and those who did not.
“All males, regardless of age, color, or other measures of individual ‘quality,’ gave Type II songs with equal frequency, which suggests that singing after a predator vocalizes may not be as costly a behavior as you might imagine,” Greig said. “Contrary to what you might expect, singing after a predator call may actually be quite safe: the male fairy-wrens know where the predator is located, and he also knows that the predator isn’t actively hunting at that moment, but is instead singing its heart out.”
The possibility remains that male fairy-wrens who frequently sing Type II songs are more successful at mating than those who sing less often, a question that could be answered by tracing the paternity of offspring, Pruett-Jones said. Whether vocal hitchhiking is a rare behavior limited to a handful of bird species or a more universal mating strategy is also being tested by Greig, who is currently in Australia braving the floods and conducting further research. Though the typical animal reaction to the presence of a predator is simple – they flee – more nuanced experiments may reveal a scary movie effect elsewhere in nature.
“We suspect it may be more common than previously appreciated. It’s definitely rare, but people just haven’t followed up in these other species to do these kinds of experiments,” Pruett-Jones said.
Greig, E., & Pruett-Jones, S. (2010). Danger may enhance communication: predator calls alert females to male displays Behavioral Ecology, 21 (6), 1360-1366 DOI: 10.1093/beheco/arq155