Satisfying natural symmetries surround us – in snowflakes, starfish and butterfly wings. And our fondness for mirror images is also reflected in the design of exquisitely balanced buildings, such as the Taj Mahal and the White House. Asymmetries, however, also abound across many living things. Male fiddler crabs bear two claws that are starkly different in size, the larger of which they use to fight and flirt. Within flatfish, winter flounder host eyes on their right side while summer flounder have eyes on their left. Perhaps closer to home, differences between our left and right sides are the reason your left-handed friend struggles to operate that can-opener. To examine why asymmetries are so abundant in nature, Nicolas Adreani and colleagues from the Max Planck Institute for Ornithology, Germany, observed the nest architecture of red ovenbirds (Furnarius rufus).
Red ovenbirds are native year-round to eastern South America, inhabiting open habitats, including urban areas where they build remarkable domed nests resembling old adobe wood-fired ovens made from mud, straw and dung, featuring entrances that face either left or right. As these birds are cultural icons in each of the five South American countries where they are found, most inhabitants can recognize these large thick clay ‘oven’ nests, which appear on trees or man-made structures such as fenceposts and buildings. Taking advantage of this widespread recognition, Adreani and colleagues released a free multilingual smartphone app and recruited over 1200 citizen scientists from Argentina, Uruguay, Brazil, Bolivia and Paraguay through social media platforms. This team of citizen scientists – over 400 of whom are listed as co-authors – collected information about <12,500 nests spanning the bird's entire distribution. For each nest, the app provided the volunteers with an 8-step survey, collecting photos and information on nest location (automatic GPS coordinates), height above ground, whether the nest was covered or uncovered, and whether it was built on a natural or artificial structure. They also logged whether the nest was in a natural, rural or urban location, which direction the entrance was pointing, and whether the entrance was on the right or left side of the nest, which they referred to as ‘asymmetry type’.
With this information in hand, the authors found that nest asymmetry did not occur randomly, since there were 12% more right-entrance nests than left-handed nests. However, from the environmental data collected by volunteers, the scientists concluded that these factors alone could not explain the occurrence of either right- or left-leaning nests. Next, they set out to determine whether there was a genetic component that could explain the nest asymmetry.
Ovenbirds typically work together for 2–3 months to build their annual nest. Since they tend not to reuse the nests in the following years, it's common to see several nests clustered close together within a single territory, sometimes even one on top of the other! As these birds are monogamous, nests that were in close proximity within a territory and showed up on a single photo were likely to have been produced by one couple. The team measured how often right- and left-sided nests occurred in a territory and used that information to calculate the chance that couples built nests with the same asymmetries. They found that the pairs tended to build nests with the same sidedness, hinting at either a cultural or inherited basis for this behaviour. This discovery opens a whole new line of inquiry that could unravel an underlying genetic component for asymmetric architecture in nests and nature.