Sealed within eggshell how can chicks prepare for the world into which they are about to hatch with no obvious direct communication channel across the shell. Adult zebra finches produce distinctive high-pitched warning songs – known as 'heat calls' – when panting during a heat wave and experiencing the calls can leave developing chicks better prepared to cope with heat after hatching. But no one knew whether the rapidly repeating warning songs can directly change the chicks' brains. Julia George (Clemson University, USA) and colleagues report in Journal of Experimental Biology that experiencing heat warning calls when still in the egg alters the activity of genes involved in the blood-brain barrier in a region of the chick's brain – the hypothalamus – that controls how the body responds to overheating, preparing the tiny developing birds for hot conditions when they hatch.
To find out how the zebra finch warning songs change developing chick brains, Mylene Mariette – then at Deakin University, Australia – played recordings of heat calls to developing zebra finch embryos, as though the parents were chattering to them, during the last few days before the chick was due to hatch.
'How zebra finch embryos detect heat-calls has not been tested yet, but could possibly involve vibration sensing or other modes of detection, in addition to hearing', says Mariette.
Just before the chicks hatched, she collected samples of the chicks' brains, jetting them off to Queen Mary University of London (QMUL), UK, where Katy Palios (QMUL) and the QMUL Genome Centre prepared the samples for analysis.
Then, Prakrit Subba (Clemson University) began the painstaking task of identifying which genes were switched off or on, to produce the changes in the youngsters' brains triggered by the parents' heat calls.
However, the team was surprised when almost none of the hormone genes in the chick's hypothalamus, which they had suspected would be switched on after experiencing heat calls to offer protection, were activated more.
'This was initially disappointing', says George.
Instead, genes involved in cell structure and muscle contraction – including the gene for Tropomyosin 1, an essential protein involved in muscle contraction – were activated much less after the developing chicks were played zebra finch heat warning calls.
The team then looked closer at the cells that make up the hypothalamus and realised that the warning songs had altered the activation of genes in the muscles lining the blood vessels of the hypothalamus, which form an essential component of the blood-brain barrier.
These changes would prevent the blood vessels from maturing fully before hatching, keeping them flexible, which could allow the chicks to be more adaptable to heat waves after hatching.
Most importantly, the changes were not caused directly by rising temperatures; Mariette had made sure that the chick embryos remained at a comfortable 37.5°C while developing.
'Remarkably, the heat calls produced by the parents altered the development of their offspring', says George, adding that the changes in the hypothalamus blood vessels resulting from the warning songs could protect the chicks in later life by altering the blood flow in the brain during dangerous heatwaves.
'In the hypothalamus, heat calls seem to act mainly on blood vessels rather than altering the hormone-producing neurons', says George. And this could be extremely helpful given how vulnerable the circulation in the birds' brains is to heat stroke.
But George adds that these changes can only offer protection if the parents' warnings accurately predict the conditions that the chicks will encounter after hatching, 'a match that may break down under rapidly changing climates', she warns.
