When predators approach, yellow-legged gulls sound the alarm by voicing a singular distress call: “ha-ha-ha.” This warning reaches embryos still encased in their shells, triggering vibrations that enable the unhatched chicks to convey the presence of danger to their less-developed nestmates. Upon breaking free of their shells, both groups of baby seabirds exhibit behavioral and physiological adaptations designed to help avoid predators. Meanwhile, those not exposed to adults’ alarm calls—either directly or through siblings’ shell vibrations—emerge from their eggs with a marked lack of survival instincts.
These findings, newly detailed in the journal Nature Ecology & Evolution, suggest that unhatched embryos are not only highly attuned to danger, but also capable of sharing this information with nestmates.
As Rebecca Calisi Rodríguez, a bird behavioral biologist at the University of California, Davis who was not involved in the study, tells NOVA Next’s Katherine J. Wu, “Any information an animal gleans from its environment … will help it prepare for the world it’s about to be born into. But the mind-blowing thing here is that there’s communication between siblings … in a way that changes how all of them develop.”
According to the New York Times’ Knvul Sheikh, study co-authors Jose C. Noguera and Alberto Velando, both of Spain’s University of Vigo, assessed embryo-to-embryo communication by collecting 90 gull eggs laid on the small island of Sálvora over the course of three days. The pair transferred these eggs to incubators, organized them into clutches, or groups, of three, and placed them in a series of different situations.
In some instances, researchers separated some eggs from the nest and exposed those embryos to warning calls. Per Scientific American’s Jennifer Leman, these eggs began vibrating and continued to do so upon reuniting with their siblings, ensuring that unexposed clutchmates whose sense of hearing had yet to develop also received the alarm. Eggs in a control group underwent similar separation, but rather than hearing distress calls, they were exposed to static white noise. Ultimately, the New York Times' Sheikh explains, the researchers created two classes of embryos: one in which “informed” eggs had the opportunity to pass along information to a “naïve” sibling and another in which all three siblings were naïve.
In a Nature News & Views article accompanying the study, scientists Mylene Mariette and Katherine Buchanan write that chicks either directly or indirectly exposed to adults’ warnings developed behavioral, physiological, morphological and molecular changes—among others, delayed hatching, quieter and quicker crouching behavior, elevated levels of the stress hormone corticosterone, and a slower-growing tarsus leg bone. Members of the control group, on the other hand, vibrated less than their informed counterparts and showed none of the changes seen among the experimental clutches.
“These results suggest a degree of developmental plasticity based on prenatal social cues, which had hitherto been thought impossible,” Mariette and Buchanan report.
Although chicks tuned in to siblings’ vibrations found themselves better-equipped to respond to danger, Science Alert’s Michelle Starr notes that this advantage came at the cost of reduced energy production and growth. Still, Sheikh writes for the New York Times, the informational advantage represented by nestmates’ warnings “could mean the difference between being eaten or not.”
For now, the Guardian’s Nur Pirbhai writes, the scientists remain unsure exactly how the embryonic information-sharing process works. Moving forward, Noguera says, he and Velando hope to determine whether unhatched chicks are able to learn other information regarding their external environment, such as the number of eggs in their nest and threats posed by environmental or social factors. According to Scientific American’s Leman, the researchers also hope to assess embryo-to-embryo interactions in avian species where chicks compete for resources.
Speaking with NOVA Next’s Wu, Noguera adds that the exchanges seen in the study may not represent intentional conversation. Instead, he posits, there is a significant chance that the gull eggs are simply absorbing all available information.
As Calisi Rodríguez concludes to Wu, “We’re subconsciously gathering information from each other all the time. There’s that saying: ‘By failing to prepare, you are preparing to fail.’ By having these particular capabilities, these organisms are preparing to succeed.”