All animals, living or extinct, are bilateral, meaning they have an even number of limbs—two arms, two legs—to help them move through the world. But some animals have evolved clever workarounds to this biological code: Kangaroos, for example, use their tails as a fifth limb to help launch them forward.
Now, scientists are intrigued by another species that has evolved to think outside the bilateral box: Lovebirds. These small, brightly colored parrots use their beaks as a third limb to help them climb trees, according to research published this week in the journal Proceedings of the Royal Society B.
Scientists had observed parrots using their beaks while climbing in the past, but they weren’t sure whether the beak was actually moving them forward, or if it served more as a stabilizing hook to help them stay balanced.
To more fully understand the birds’ unique gait, researchers brought six rosy-faced lovebirds into the lab, then used high-speed cameras and sensors to track their movements at various incline angles. After analyzing the data, they found that lovebirds began using their beaks as a third limb periodically once the pitch grew steeper than 45 degrees. On completely vertical surfaces at a 90-degree angle, they used their beaks in this way 100 percent of the time while climbing.
They also found that the birds’ beaks provided nearly as much power as their legs in their cyclical tripedal gait. When adjusted for weight, the beaks’ power was equal to or greater than the power of a rock climber’s arms or other climbing primates’ forelimbs.
Lovebirds use their beaks as propulsive limbs, which makes them more like kangaroos and spider monkeys, which use their tails to help propel themselves forward, and less like crocodiles and giant anteaters, which simply drag their long tails behind them while walking, the researchers write.
“For them to take their faces and integrate it into their stride cycle is pretty incredible,” Melody Young, an anatomist at the New York Institute of Technology and the study’s co-author, tells the New York Times’ Veronique Greenwood.
It’s not clear exactly why parrots evolved this way, but researchers suspect it’s because they alternate their legs when they walk and, thus, cannot hop up and down tree trunks. Their neck muscles and nervous systems likely had to adapt to this distinct style of climbing, the researchers say.
“Parrots rarely hop and appear to have lost this ability somewhere in their evolutionary history,” the researchers write in the paper. “The use of the beak as a propulsive third limb may be either a consequence of, or the causal factor driving, the loss of this ability.”
Though having an odd number of limbs is a so-called forbidden phenotype, lovebirds—like kangaroos and a handful of other animals—seem to have found a way around nature’s propensity for symmetry. So in addition to being lovable household pets, lovebirds are also a cool scientific anomaly.