What if parents could prepare their offspring to survive hot weather before they were even born?
Researchers at Clemson University and collaborating institutions in Australia, Spain and the United Kingdom have found evidence that zebra finch embryos can respond to special heat-related calls made by their parents, changing the development of blood vessels in the brain before they hatch.
The findings help explain a phenomenon that has intrigued scientists for nearly a decade: zebra finch chicks exposed to “heat calls” while still inside the egg grow up better adapted to hot conditions, despite never experiencing heat themselves.
“It’s really the idea that a sound might alter development while birds are still in the egg,” said Julia George, associate professor emerita of biological sciences at Clemson. “The idea that parents can give a signal to an incubating embryo and have that alter its development is really cool.”
Harsh environment
Zebra finches live in Australia’s harsh and highly variable climate, where temperatures can swing dramatically. During periods of extreme heat, adult birds produce distinctive vocalizations known as heat calls.
Previous studies showed that embryos exposed to those calls later develop traits that appear helpful in hot environments. As juveniles, they grow more slowly, which helps dissipate heat more efficiently. As adults, they pant less when temperatures rise, helping them conserve water, and they prefer warmer nesting sites than birds that were not exposed to the calls before hatching.
Researchers have also found evidence that heat-call birds produce less metabolic heat and show lower indicators of chronic stress.
“These birds were actually better at coping with heat,” said Prakrit Subba, a Clemson doctoral student in George’s lab who led the computational analysis for the study. “They were panting less and retaining more body water.”
Scientists suspected the calls were triggering changes in the developing brain, but they didn’t know how.
“Listening” before birth
To investigate, researchers collected zebra finch eggs and incubated them under identical temperatures. The only difference was what the embryos heard.
For the final four days before hatching, some eggs were exposed periodically to recordings of parental heat calls over about nine hours each day. Others heard ordinary zebra finch contact calls.
The embryos never experienced elevated temperatures.
Just before the birds hatched, researchers examined a region of the brain called the hypothalamus, which helps regulate body temperature, growth, metabolism and hormone production. Using RNA sequencing, they measured which genes were active and whether exposure to heat calls altered gene expression.
Big surprise
The team expected to find changes in neurons and hormone-regulating pathways.
Instead, they found something else.
“We were expecting to see neurons,” George said. “And we saw blood vessels instead.”
Many of the affected genes were associated with muscle contraction and cellular scaffolding. Further analysis revealed that the changes were concentrated in cells that form the brain’s vascular system, including vascular smooth muscle cells that help regulate blood flow.
These cells surround blood vessels and control whether vessels constrict or dilate. The researchers found evidence suggesting that heat-call exposure reduces the activity of genes involved in contraction, potentially making the vessels more likely to expand.
Preparing for future heat
Why would that matter?
The brain is one of the organs most vulnerable to heat stress, George said. Increased blood flow can help deliver energy and nutrients while also helping dissipate excess heat.
The researchers believe the epigenetic changes may represent a form of anticipatory preparation.
“If more blood can reach the brain, the brain has more resources available when it needs to handle future heat stress,” Subba said. Expanded blood vessels may also help the birds cool themselves more effectively when temperatures rise.
Importantly, the response was highly targeted. The vascular cells showing the strongest effects make up only a small fraction of the cells in the sampled brain tissue.
“It really appears to be targeting one specific system,” George said.
How can a sound do that?
One of the biggest unanswered questions is how embryos detect the signal in the first place.
Scientists have long assumed that hearing doesn’t develop in songbirds until after they hatch. So exactly how do embryos sense heat calls?
“We don’t know the answer to that,” George said.
One possibility is that embryos respond through mechanoreceptors, specialized sensory structures that detect vibration. Such receptors exist not only in the auditory system but also in blood vessels and other tissues.
The distinctive acoustic properties of heat calls may provide a cue that embryos can detect before hatching, though researchers say much more work is needed to understand the mechanism.
Lessons for a warming world
The study highlights a remarkable form of developmental plasticity — the ability of organisms to adjust their development in response to environmental information.
In essence, the embryos appear to be responding not to heat itself but to information about future heat.
The findings may help scientists better understand how animals cope with environmental variability and whether such adaptations can help species respond to rising global temperatures.
Still, George cautions that every adaptive system has limits.
The heat-call response likely evolved to help zebra finches cope with naturally variable Australian weather. But as climate change pushes temperatures higher and creates increasingly unpredictable conditions, it remains unclear whether such biological forecasting systems will be enough.
“They seem to have this intrinsic ability to adjust their physiology in anticipation of differences in temperature,” George said. “But I worry that system could break down if temperatures become too extreme or change too rapidly.”
