Bowerbirds woo mates with a lot of effort and a little illusion

Inherently, there’s a problem with the way we view the world: our world is three dimensional, while the image projected onto our retinas is just two dimensional. Therefore, without even being aware of it, we use clues to figure out distance and perspective in our environment. When an object gets smaller, it is becoming farther away; when it gets larger, it is approaching us.

New research in Science this week shows that we aren’t the only species that uses visual clues as a means to an end: male bowerbirds’ mating success depends on their ability to create a false sense of perspective.

Male great bowerbirds (Ptilonorhynchus nuchalis) have a tough job. To woo females, they must construct an elaborate bower, centered around two parallel walls of sticks planted vertically in the ground. In front and behind the bower are two “courts,” connected by a path called an “avenue” between the two walls. The male builds the courts by collecting stones, bones, and shells, and arranging them on the ground. Called “gesso,” these objects cover the ground of the male’s court, where he displays colorful objects and shows off for potential mates.

But in building these courts, the males are very careful in how they arrange the gesso. The gesso is organized so that the smallest objects are toward the front of the court, and the largest objects at the end of the court, farthest from the female. This creates a forced perspective, since the natural relationship between size and distance is violated. If you looked at the court from where the female stands in the middle of the avenue, all the objects in the gesso would appear to be the same size, despite the increasing distance.

The males certainly aren’t creating this size gradient by accident; if the gesso is rearranged to a random or even a reversed gradient, the male will quickly restore it to its initial order. And they are careful about the gradient for good reason: the arrangement of a male’s gesso actually predicts his mating success. Males with precise size gradients are much more likely to win females than males with smaller gradient slopes are. An even stronger statistical relationship exists between a male’s mating success and the regularity of the pattern created by the gradient, as measured from the center of the avenue.

To be fair, we don’t know enough about bowerbirds’ vision to say that they experience this illusion in the same way we do. However, the fact that the relationship between mating success and the regularity of the gesso pattern from the female’s vantage point is so strong (an adjusted r squared value of 0.96) indicates that the female’s perception of the court is integral to her decision whether or not to mate with the male. The regularity of the gesso might be the end goal, or the regularity might enhance the male’s display in another way.

Humans have long used illusions to entice mates, but there’s much less evidence of mate choice illusions among other species. While we know that some animals, such as pigeons and parrots, are sensitive to illusions, we don’t yet know enough about many species’ sensory systems to understand how they respond to illusions. For bowerbirds, we know that the creation of forced perspective drives the ladies wild—we just don’t yet know how or why.

Science, 2012. DOI: 10.1126/science.1212443  (About DOIs).

Listing image by Photograph by Tatiana Gerus