Stress in the egg makes gull chicks fitter

Abstract

Biologists have known for years that conditions experienced during early development can dramatically influence health and longevity through to adulthood. When the environment becomes persistently challenging or too unpredictable, mothers pass on the stress hormones produced in response to difficult conditions to their offspring developing in the womb or egg. It is commonly believed that these increases in stress exposure have detrimental consequences for developing creatures long into their futures, such as the increased risk of illness and reduced survival. However, the stress communicated to a developing embryo by its mother could also be an advanced warning, which helps the youngster to develop in ways that might improve its chances of survival later in life. In new research led by Jose Noguera and his team at the University of Vigo, Spain, they provide evidence suggesting that this might be the case.Noguera and colleagues headed out to Sálvora Island, off the Atlantic coast of northern Spain to study a wild breeding colony of yellow-legged gulls. The team stealthily monitored the birds as they laid their eggs and selected the first egg from 46 nests – the first laid egg is often the only one that hatches successfully – before injecting some with a stress hormone, corticosterone. Soon after the chicks hatched the researchers took a small blood sample to investigate how the hormone boost had affected the developing chicks by looking at the DNA packaged in the birds’ red blood cells. Knowing that the ends of each DNA chromosome are tipped with specialised structures, telomeres, which protect them from wearing down and generally shorten with age, the team decided to find out how the stress hormone in the chick's eggs affected the length of the telomeres. In addition, the team checked the enzyme, telomerase, which is responsible for building and maintaining telomeres, to find out how well prepared the youngsters were for the rigours of life after hatching.The researchers found that the chicks that were exposed to stress hormones as eggs had more active telomerase and longer telomeres compared to the chicks that had not received a corticosterone boost. These results suggest that increased exposure to stress during embryonic life leads to changes that might improve the chicks’ survival prospects. Interestingly, using data collected previously from the same population of gulls, they also showed that starting life after hatching with longer telomeres increases the chances of chicks surviving for at least up to 8 days. This time window corresponds to the period of highest mortality in this species as gull chicks face constant danger, competing with their siblings for food while predators lurk in the colony.Noguera's study suggests that maternally derived stress hormones may be a key mechanism through which mothers transmit valuable signals to their offspring that could help them to program their biology in preparation for life on the outside. More broadly, the team challenge the common view that increased levels of stress hormones in the egg inevitably incur costs for the developing chick as a consequence of poor maternal conditions. It seems, instead, that the mother's stressful gift to her offspring may provide an opportunity to make them fitter, at least during the earliest and most challenging stages of life.