If you’re looking for love, it pays to stand out from the crowd. Or at least that’s how it works in some parts of the animal kingdom. Scientists have found that in several species – green swordtail fish, Trinidadian guppies, fruit flies, Poecilia parae fish – ladies overwhelmingly go for the guy that looks different from the rest.
But the reason for this attraction to novelty has remained a mystery. So my colleagues and I used the Trinidadian guppy to investigate the psychology behind why many females have an affinity for the unusual.
Male features that attract females
The guppy has long been a workhorse for biologists like me who are interested in understanding the mating decisions that animals make and the evolutionary forces behind those decisions. Male guppies attempt to woo females using courtship dances that show off the elaborate color patterns adorning their bodies. The females of the species are color pattern connoisseurs, carefully choosing among their suitors based, in large part, on their visual appeal. This tendency has made the guppy an excellent model for studying mate choice.
Many types of animals exhibit what evolutionary biologists call directional preferences, an attraction to more of a certain thing – think bigger antlers, a longer tail or brighter color spots. And there are evolutionary theories that help make sense of these preferences. If a male can grow more extreme features, that can be a sign that he is in good physical condition, has good genes, or would make a good parent.
What’s less clear, though, is why females should value unusualness in a mate.
When puzzling over why this mating preference arose, it occurred to me that attraction to novelty fits with a simple kind of learning called habituation. Psychologists have long known that when an animal is repeatedly exposed to some stimulus – be it a sound, a touch or in this case a visual pattern – it responds to the stimulus less and less. This occurs because the nervous system starts to “tune out” repetitive information. Since repetitive information is usually unimportant, habituation helps to free up the animals attention for other more important things.
What’s interesting about habituation is that it’s pervasive: Virtually every animal species, including human beings, can habituate to a wide range of things. It’s the reason why the noise from the air conditioning unit seems loud and distracting when it first turns on, but before long you barely notice it.
I wondered if this might be happening in guppies. If females are tuning out the color patterns they commonly see, then a male with a different-looking pattern is going to have a huge advantage attracting mates.
Mitchel Daniel, CC BY-ND
Testing whether habituation is what’s happening
To test this idea, I had to determine whether preference for novel-looking males meets four key criteria of habituation.
First I needed to see whether repeated exposure to a stimulus – like the noisy air conditioner, or a particular color pattern – made the animals less interested in that thing. Scientists call this “responsiveness decline.” To test it, I took a group of female guppies and exposed them to a series of males that looked alike. Then, I observed how the females behaved toward a male with the now-familiar pattern.
Male guppies are persistent suitors, and females mostly ignore their courtship dances. But when a male catches their fancy, females will turn and approach the male, which can lead to copulation. The effect of exposure to males was striking: Females already familiar with the pattern responded about half as often to male courtship compared to females that had never seen any male color patterns before. I also found it took just 12 minutes of exposure to reduce female responsiveness, which shows that female interest is fleeting.