Elaborate poses, tufts of feathers, flamboyant shuffles along an immaculate forest floor — male birds-of-paradise have many ways to woo a potential mate.
But now, by examining prepared specimens at the American Museum of Natural History in New York, scientists have discovered what could be yet another tool in the kit of the tropical birds — a visual effect known as photoluminescence.
Sometimes called biofluorescence in living things, this phenomenon occurs when an object absorbs high-energy wavelengths of light and re-emits them as lower energy wavelengths.
Biofluorescence has already been found in various species of fishes, amphibians and even mammals, from bats to wombats.
Interestingly, birds remain woefully understudied when it comes to the optical extras. Until now, no one had looked for the glowing property in birds-of-paradise, which are native to Australia, Indonesia and New Guinea and are famous for their elaborate mating displays.
In a study published on Tuesday in the journal Royal Society Open Science, researchers examined prepared specimens housed at the American Museum of Natural History and found evidence of biofluorescence in 37 of 45 birds-of-paradise species.
“What they’re doing is taking this UV color, which they can’t see, and re-emitting it at a wavelength that is actually visible to their eyes,” said Rene Martin, the lead author of the study and a biologist at the University of Nebraska-Lincoln. “In their case, it’s kind of a bright green and green-yellow color.”
In short, biofluorescence supercharges a bright color to make it even brighter.
Of all the birds-of-paradise species, three genera (Lycocorax, Manucodia and Phonygammus) showed no evidence of biofluorescence.
Dr. Martin said this could be because those birds tended to be much more monogamous, which would mean that there’s less need for elaborate displays and competition between males.
Also, because the researchers found biofluorescence in the blue-capped ifrit, which is considered to be the lone sister species to the birds-of-paradise, the most likely scenario is that all of these animals descended from a common ancestor that had evolved biofluorescent markings. The three genera that no longer have it, then, must have lost the trait somewhere along the way.
Earlier studies found that many species of birds-of-paradise evolved another color quirk — ultrablack feathers that absorbed nearly all the wavelengths of light that strike them.
The scientists found that many of these flamboyant birds seem to have sections of biofluorescence situated near or framed by the ultrablack. This juxtaposition likely enhances the entire display, which may signal to females that the performer is of good breeding quality.
While females of many of the birds-of-paradise species also displayed biofluorescent plumage, Dr. Martin said that the female patterns were much more subdued. Females in most of these species are the ones doing the choosing, reinforcing the idea that the males deploy their glowing for sexual selection.
Male birds also sometimes display their colors as a way to establish a pecking order. In other species, like snakes, scientists believe that biofluorescence serves as a way to blend in with bright forest backgrounds. So the next-level coloration is quite likely to serve more than one role.
Though it may seem obvious that biofluorescence would evolve many different times in the animal kingdom for a purpose, scientists caution against the assumption. After all, human teeth fluoresce under ultraviolet light, but that doesn’t seem to help us in any measurable way. Some might even argue that glowing teeth are a disadvantage.
To prove function, scientists look to a set of five criteria. This list includes whether an animal’s habitat has naturally available levels of the light wavelengths known to be absorbed, or whether an animal’s eyes are likely to be able to see the wavelengths that are re-emitted.
“It looks like birds-of-paradise meet four out of the five criteria for visual function,” Linda Reinhold, a zoologist at James Cook University in Australia, said in an email.
However, Ms. Reinhold said the fifth criteria — which assesses whether an animal’s behavior changes as a result of the biofluorescence — remains to be tested for birds-of-paradise. Of course, trying to observe finicky birds in remote habitats full of uncontrollable variables may mean that this particular test would not be resolved anytime soon.
Studying the birds in person would likely have other benefits.
“These birds in their natural habitat are going to be even more fluorescent than in museums,” Ms. Reinhold said.