Yellow-legged gull eggs wriggle to warn nest mates about danger, induces survival traits

Yellow-legged gull chicks in their nest (image credit: Contando Estrelas)

Birds and animals emanating alarm calls to warn their offspring about possible predator presence is a common occurrence in the animal world. However, a new study published in nature Ecology and Evolution journal shows that Yellow-legged gull eggs which are exposed to such alarm calls can imbibe that information and pass it on to other nest mates through vibrations, inducing developmental changes at embryonic stage and also influencing anti-predator behavior in later life.

As part of the study, the researchers collected 90 eggs from a large colony of Yellow-legged gulls from Salvora Island in Spain and conducted an experiment to investigate whether alarm calls about predators ‘heard’ by the eggs induce anti-predator traits in the chicks when they hatch. They have created an experimental group – clutches of 3 eggs each, (totaling 45) which were exposed to alarm calls – and a control group –clutches of 3 eggs (totaling 45) which were not exposed to alarm calls. 

During the experiment, two eggs each from the experimental clutches were put in a sound-proof box and were exposed to alarm calls made by Yellow-legged gulls when they find a predation risk for the eggs from small carnivores like Minks. The third egg from each experimental clutch was not exposed to these alarm calls. The eggs which were exposed to the alarm calls were put back with the third egg which did not ‘hear’ the alarm calls. This was done four times a day between day 21 and 27 of incubation. The experiment found that the eggs which were exposed to the alarm calls vibrated extensively than the eggs in the control group. However, the third egg in the experimental clutch which was not exposed to the alarm calls also found to be wriggling like its nest mates who ‘heard’ the alarm calls.  

When the eggs hatched, the chicks from the experimental group and the control groups showed significant differences in their response to alarm calls and other aspects like hormone productions and limb size. Eggs which were part of the experimental clutches had prolonged period of hatching as compared to the control group who did not ‘hear’ any alarm calls. However, the delay in hatching time for the third egg in the experimental group which also did not ‘hear’ the alarm calls was on par with its mates who ‘heard’ the calls. The researchers claim that this indicates that the two eggs which were exposed to alarm calls of predator presence passed on that information to the third clutch mate through vibrations.

Apart from delayed hatching, the chicks from the experimental group also showed high stress hormone levels and quicker reflexes to alarm calls like faster crouching and hiding as compared to the chicks from the control group eggs. However, these characteristics were similar for the chick from the third egg in the experimental group which was not exposed to the alarm calls.

“Both embryos that were exposed to alarm calls and their unexposed clutch mates showed altered prenatal and postnatal behaviours, 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)”’ says the study.

The study reveals that the information and traits needed for survival in the animal world are passed on socially, even at embryonic stage. “These results strongly suggest that gull embryos are able to acquire relevant environmental information from their siblings”, says the study.

Jose C. Noguera and Alberto Velando from the Animal Ecology Group at the University of Vigo conducted the study.