Great bowerbirds are known for their dramatic mating displays and elaborate constructions. Now researchers have determined males of this crafty species build staged scenes that make themselves look larger or smaller than they actually are.
As a result, the scientists believe great bowerbirds are the first known non-human animals that create scenes with altered visual perspectives for viewing by other individuals. In this case, those other individuals are female great bowerbirds seeking mates.
Architects, set designers and artists frequently employ the technique when creating certain paintings, gardens, amusement parks and other constructions that feature optical illusions. But we're relatively new at this.
"Bowerbirds have been doing it longer than we have," lead author John Endler told Discovery News. "Good human perspective didn't get started until the 15th century."
Endler, a professor of sensory ecology and evolution at Deakin University, and colleagues Lorna Endler and Natalie Doerr studied great bowerbird bowers in Queensland, Australia.
Each male-made bower consists of an avenue -- two rows of tightly packed sticks with a stick floor -- that opens onto a court. The court functions as a stage where the male displays for females.
The avenue ensures females can only see the court from one viewing angle. Males carefully line their courts with pebbles, bones and shells, such that the absolute size of these objects increases with distance from the avenue entrance and the female viewers.
This design could lead females to "perceive the court as smaller than it is and therefore perhaps perceive the male as larger than he is," according to Endler.
The male then tries to wow the female with his show.
"The visual display includes his presenting his expanded bluish-purple nuchal crest waved at the female, and then turning around and waving a colored object towards the female," he said. "This is often accompanied by a strutting walk around the avenue."
If impressed, the female will mate with the male right on "stage."
For the study, published in Current Biology, Endler and his team rearranged the pebbles and other objects in male courts.
"When we reversed the gradient, putting smaller objects further away and larger objects closer to the avenue, the birds put the gradient back in three days," he said.
In the future, he and other researchers hope to see if all of this architectural effort is related to mating success, and if other animals might use forced visual perspective too.
It's possible that many organisms are optical illusion masters. But are their creations art?
"This depends upon the definition of art," Endler said. "Flowers produce things to be looked at and attracted to by pollinators. Is that art? Fiddler crabs produce towers to attract females. Is that art? Spiders spin orb webs to attract prey. Is that art? These could (all) be called art."
Great bowerbirds and humans, however, may be the only ones to produce art with perspective.
"(The study) demonstrates that these birds are using not just structure and color arrangements, but are also manipulating the geometry of these arrangements," David Westcott, a rainforest ecologist at the Commonwealth Scientific and Industrial Research Organization of Australia, told Discovery News.
"Part of the appeal of this paper is that it pulls one more brick from the wall of human difference," Westcott added. "Perhaps these birds are living simple lives, but they are engaged in complex responses to their world."