What's the best way for a lonely guy to get a date? If you're a Splendid Fairy-Wren (Malurus splendens, native to Australia), your best bet might be to frighten the object of your affection.
You've learned all about the birds and the bees; now it's time to learn from them.
Lots of research has shown that animals reduce their sexual behaviors when predators are around. After all, it isn't just potential mates who would see or hear an elaborate mating display, but also potential predators. The male splendid fairy-wren does something a little bit unusual, though. Gray Butcherbirds (Cracticus torquatus) are known to prey upon splendid fairy-wrens and their nests. Almost without fail, whenever the call of the gray butcherbird is heard, male fairy-wrens begin singing! Specifically, they sing a short trill referred to as a "Type 2 song." In fact, researchers Greig and Pruett-Jones have gone so far as to refer to the double-call as a "duet."
What would cause the male splendid fairy-wrens to respond to the calls of their predators in such a strange way? One possibility is that the response of the fairy-wrens is an alarm call, meant to warn others nearby of the presence of a predator. One experiment ruled out that possibility, though: male splendid fairy-wrens give their Type 2 songs in response to hearing the vocalization of the butcherbird, but not in the presence of silent butcherbirds. Another possibility is that the Type 2 songs are meant to scare the butcherbirds away, but another experiment indicated that this wasn't the case either, as males were no more likely to sing their Type 2 songs upon seeing a silent butcherbird. It seems, then, that there is something special about the vocalization of the butcherbird that causes the splendid fairy-wren to sing its Type 2 song.
It could be that the Type 2 song functions as a territorial display, to keep other males away. If the Type 2 song is used as a territorial display, Greig and Pruett-Jones predicted, then males would respond to Type 2 songs coming from encroaching males in an aggressive way. This, however, is not the case either.
Alternatively, it might be the case that the Type 2 song functions as a mating display: a way to attract the attention of the females. In this case, the Type 2 song may occur following the butcherbird call simply because that predatory call is particularly salient; in other words, if females become more aware of their surroundings after a butcherbird call, then they will be better able to detect the males' Type 2 song that immediately follows. This predicts that female splendid fairy-wrens should become more alert and attentive following the butcherbird call. If this is the case - if butcherbird calls essentially facilitate the detection of the Type 2 song - then females should be more responsive to the Type 2 song when it follows a predatory butcherbird call, rather than in isolation.
To address this question, the researchers conducted a series of experiments utilizing pre-recorded butcherbird calls and splendid fairy-wren Type 2 songs. They found that females did preferentially attend to the calls of the butcherbird compared with other non-predator calls. In addition, females tended to sing more in response to the males' Type 2 song when it was preceded by the butcherbird call than to solo Type 2 songs. It is not clear what the meaning of the females' response call is - it could be positive ("here's my phone number, call me!") or negative ("not a chance, dude!"), but whatever its purpose, it is certain that it occurs in response to the males' Type 2 songs.
So when the female splendid fairy-wrens are scared by the predator calls, they seem to become more responsive to male Type 2 songs. This phenomenon has been recorded in humans, as well, but in reverse.
In 1974, University of British Columbia psychologists Donald G. Dutton and Arthur P. Aron were studying human attraction, and in one particularly brilliant experiment, they took advantage of a local river.
There are two bridges along the Capilano River in North Vancouver. The Capilano Canyon Suspension Bridge is a five foot wide, 450 foot long, bridge made of wooden boards attached to wire cables that run from one end of the canyon to the other. The authors noted that it has several anxiety-inducing features, including, "(a) a tendency to tilt, sway, and wobble, creating the impression that one is about to fall over the side; (b) very low handrails of wire cable which contribute to this impression; and (c) a 230-foot drop to rocks and shallow rapids below the bridge." This served as the "experimental" bridge. By contrast, the "control" bridge was "constructed of heavy cedar, this bridge was wider and firmer than the experimental bridge, was only 10 feet above a small, shallow rivulet which ran into the main river, had high handrails, and did not tilt or sway."
Male travelers, naive to the experimental set-up, would pass by along either bridge, and after crossing, would be approached by an attractive female experimenter, who would ask them to fill out several questionnaires including the Thematic Apperception Test, which requires individuals to write a story based on a series of pictures. Not only were the men who walked across the fear-inducing shaky bridge more likely to include sexual content in their stories than those who crossed the stable bridge, but they were also more likely to flirt with the experimenter and ask for her number. It's as if they misunderstood their anxiety-induced physiological arousal - elevated heartrate, sweaty palms, and so on - as reflective of sexual attraction and desire. This is not to say, of course, that the mechanism underlying this phenomenon in fairy-wrens and humans is necessarily the same, even if the outcome is analogous. Whatever the mechanism, it turns out that human men, as well as female splendid fairy-wrens, are more likely to be receptive to or interested in mating-related behaviors following a fearful situation.
Perhaps that's why the scary movie is such a common dating ritual.
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
Greig, E., & Pruett-Jones, S. (2009). A predator-elicited song in the splendid fairy-wren: warning signal or intraspecific display? Animal Behaviour, 78 (1), 45-52 DOI: 10.1016/j.anbehav.2009.02.030
Dutton DG, & Aron AP (1974). Some evidence for heightened sexual attraction under conditions of high anxiety. Journal of personality and social psychology, 30 (4), 510-7 PMID: 4455773