Snug in the shell of a bird egg, unseen from the rest of the world, a curled embryo grows and develops into a creature that will one day soar the skies. To make it to that moment, unhatched yellow-legged gull babies may convey danger to their clutchmates by hearing the alarm calls of their parents and vibrating within their shells, communicating the peril to their siblings and delaying the onset of hatching.
Whether the embryos evolved this wriggling for that very purpose or it is a byproduct of development remains to be seen, but an "elegant experimental design" published in Nature Ecology and Evolution suggests that whatever the initial purpose, the behavior does indeed transmit information to their nestmates.
The team from the University of Vigo, Spain, collected a total of 90 eggs from a breeding colony of gulls (L. michahellis) on Sálvora Island and placed them in an incubator. They split them into artificial clutches of three eggs for a total of 30 nests and exposed two of the three eggs to the alarm calls of an adult gull or static noise during their last week of development. The third sibling remained in a sound-proof box unaware of the squawking adult.
The embryos exposed to the alarm calls vibrated more than the controls and hatched later. This was also true for the nestmates that were never exposed to the alarm calls but instead felt the vibrational intel of their siblings.
Even though they never heard the alarm calls, the "unexposed clutch mates showed altered prenatal and postnatal behaviors, higher levels of DNA methylation and stress hormones, and reduced growth and numbers of mitochondria (which may be indicative of the capacity for energy production of cells)," wrote the authors. "These results strongly suggest that gull embryos are able to acquire relevant environmental information from their siblings."
The higher stress hormones and changes in gene activity suggest the calls alerted the embryos to danger and delayed their hatching. The chicks, once free of their shells, were also quicker to crouch when they perceived a threat; however, whether such predator-danger behavior is actually helpful is unknown.
A commentary by Mylene Mariette and Katherine Buchanan from Deakin University note that bird and crocodile embryos can synchronize their hatching based on vocalizations and vibrations. It’s not a far reach to suggest that danger calls could stir something similar, although the differences seen could also be due to the control group’s lack of noise altogether. It’s also likely that the siblings are not intentionally communicating but rather gleaning information from wherever they can get it.
Still, Mariette and Buchanan say the research opens doors for further experimental tests on such behavior, including whether there are circumstances in which embryos may try to pass on misinformation to their clutchmates.
"Clearly, the world of prenatal communication is only now opening up to the investigation of when, where and how it pays to listen to the world around you."