In an environment rife with dangerous predators, parents want their offspring to be born ready for life's challenges. Biologists have known for years that parents can affect their offspring's development in the womb or egg by sending signals via their hormones and behaviour. But can developing siblings also share information about the external environment with each other before being born? Exciting new research from Jose Noguera and Alberto Velando from the University of Vigo, Spain, suggests ‘yes’. The researchers found that yellow-legged gull eggs may move and vibrate in a way that siblings can detect through their shells to communicate about predator threats.

Developing eggs don't sit idly by, waiting to hatch. They jiggle when the embryos move in response to external sound or contact from their parents and siblings, and these good vibrations have been shown to help nests hatch more synchronously. But whether eggs can warn their siblings about lurking predators and what effects these communications might have on chick development was unknown. Noguera and Velando collected wild yellow-legged gull eggs and brought them to a field lab to answer this question. Raising the eggs in nests of three inside cosy incubation chambers – no parents needed – the researchers assigned the nests to one of two groups. The first group was exposed to recordings of the shrill alarm calls of their parents when threatened by predators, while the other group listened to white noise only. Then, the researchers added another level of complexity to their experiment to be sure that they were recording genuine cross-talk between the nestmates. Only two eggs from each nest heard the alarm calls or white noise inside a sound-proof box for 12 min each day, while the third egg was excluded.

The researchers discovered that the eggs were certainly affected by the parents’ alarm calls, as they developed in ways that might enhance their survival. Compared with the white noise nests, the chicks that had heard alarm calls as eggs were quieter and quicker to hide when they experienced alarm calls after hatching. They also had increased levels of stress hormones and more of their DNA was methylated, which suggests that they have altered gene expression patterns to help them cope with danger. However, these survival benefits also came at a cost. The chicks exposed to scary sounds inside the egg grew more slowly, possibly because of excessive stress hormones or because they had fewer mitochondria in their blood cells (an indicator of energy production ability). Most excitingly, the chick inside the third egg developed exactly like its nestmates, despite not hearing the frantic parental alarm calls while developing inside its egg.

The eggs were clearly sharing information before hatching, so the third egg was equally well prepared to deal with predator threats as its nestmates. Noguera and Velando also filmed the eggs before they hatched and scored their vibrations from the video footage. The eggs that had been exposed to their parents’ alarm calls vibrated more than the eggs that had only heard white noise and the third egg, which had not heard the alarm calls, mimicked the other eggs’ vibrations; the vibrations are a likely means of egg-to-egg communication.

More research is now needed to understand exactly how external cues like shell vibrations cause such major changes in an animal's development, but it is clear is that nearby siblings can eavesdrop on each other and gather important information that helps programme their biology in preparation for life on the outside.

Bird embryos perceive vibratory cues of predation risk from clutch mates
Nature Ecol. Evol.