Murres (guillemots in Britain) like to nest on seaside cliffs. This defense against predators, however, comes with the risk of their eggs rolling off. Murres are also known among ornithologists for the unusually pointy shape of their eggs, more teardrop than chicken egg. The connection seemed obvious, but it took the intervention of a 3D printer to prove that the birds' unique egg shape helps prevent it from rolling off the cliff.
Common and thick-billed murres are distributed along the coastlines of most of the Northern Hemisphere. Dr Mark Hauber of the University of Illinois notes in the Journal of Experimental Biology that their nesting ledges can be “as shallow as the egg is long.” Between the occasional clumsy parent and the force of coastal winds, eggs that moved too easily presumably paid a high evolutionary price.
Birds with eggs shaped in ways that made them slightly less prone to falling had a breeding advantage, on which natural selection could work. Still, proving murres' eggs really are best suited to preventing their unhatched offspring from plummeting to the rocks below has been something of a challenge.
Indeed, a study published in Science last year shows fear of falling is not the only factor in egg shape. After examining 50,000 eggs from 1,400 species, the authors concluded that the parental birds' style of flight influences egg shape more than where it nests.
“Very little is known about how the murre egg shape affects its stability and viability in this setting,” Hauber said in a statement. “But earlier studies failed to isolate specific features of the eggs – such as elongation, asymmetry and conicality – to robustly test this hypothesis.”
Hauber decided to test out a wide variety of egg shapes and see how they rolled under different sloping conditions. What might once have been a difficult task was made easy with the use of 3D printers, with silicone innards to mimic the weighting effect of the yoke.
"We found that conicality – the degree to which the pointed end of the egg mimicked a cone – suppressed egg displacement on surfaces inclined more than 2 degrees," Hauber said. The difference was minor for flatter surfaces.
Having a pointy end may be murre eggs' most obvious feature, but others also matter. The greater the ratio between an egg's length and its girth, the wider the circle in which it rolls, increasing the chance a bump will take it off the cliff edge. Similarly, the distance between the pointy end and the egg's widest part influences rolling radius.
All this makes it difficult to balance up the various factors and produce the least rollable egg, at least without creating something with flat sides. However, the slow march of evolution appears to have done the trick for murres.