A new international study involving researchers from the Doñana Biological Station (EBD-CSIC) reveals that young birds begin preparing to cope with high temperatures even before they hatch. The research, carried out on the zebra finch (Taeniopygia guttata), shows that while still developing inside the egg, embryos respond to a specific call produced by their parents only during hot weather. Hearing this call activates a distinct set of genes from those triggered by other parental vocalisations, including genes involved in regulating blood flow and fluid balance, two processes that play a key role in cooling the brain.

“It is remarkable that sound alone, perceived from inside the egg, can alter how the brain functions,” says Mylene Mariette, researcher at the Doñana Biological Station.

To investigate this phenomenon, freshly laid eggs were removed from nests and incubated under optimal conditions in artificial incubators. At the same time, speakers built into the incubators played the “heat call” to one group of eggs and other parental calls to another. On the day before hatching, the embryos' brains were examined to determine which of the thousands of genes expressed in the brain had been activated or switched off.

This study builds on a decade of research led by Mariette demonstrating that the “heat call” prepares developing chicks to cope with high temperatures by modifying different aspects of their physiology and behaviour. However, this is the first study to examine the changes that this acoustic signal induces directly in the brain. 

Initially, the research team expected to identify the common mechanism underlying all the previously observed changes. To do so, they focused specifically on the hypothalamus, a brain region that plays a central role in hormonal regulation and the detection of body temperature. Instead of orchestrating the body's preparation for heat, however, they discovered that the brain itself appears to be preparing to protect its own function.

The findings came as a surprise. “We expected to see major changes in the genes regulating hormonal functions, particularly those involved in stress responses, growth, metabolism and temperature regulation,” explains Julia Georges, lead author of the study. “At first, it was disappointing to find that those changes were only very modest,” she says.

The brain appears to protect itself

The real surprise came when the researchers discovered that the strongest response was not related to hormonal regulation but rather to genes involved in muscle contraction and cytoskeletal dynamics. These genes were expressed primarily in the muscle cells surrounding blood vessels in the brain and in cells responsible for regulating the movement of fluid into the brain.

The results showed that chicks exposed to the “heat call” developed greater flexibility in the blood-brain barrier, the structure that controls the exchange of substances between the bloodstream and the brain. This feature is also observed under conditions of extreme heat. However, in this study the changes occurred solely in response to the acoustic signal, without the embryos ever being exposed to high temperatures.

“It is remarkable that both heat itself and the ‘heat call’ affect the same trait in the same direction,” says Mariette. “The cells that make up the blood-brain barrier account for only about 2% of all brain cells, the vast majority being neurons and glial cells. Even so, the effect was very clear.”

The research team will continue investigating what these changes in gene expression mean for an individual's resilience to heat stress.

The study was published in the Journal of Experimental Biology and is the first paper arising from the PhD thesis of first author Prakrit Subba.