Australian zebra finches (shown) make rapid peeps when the temperature rises. These “heat calls” reprogram their yet-to-hatch babies’ brains in preparation for the heat.
David Cook/flickr (CC BY-NC 2.0)
Zebra finches sing their young into biological preparedness for hot weather, all before they even leave the egg.
As the heat punishes sun-crisped Australian woodlands, the adult birds make a rapid, peeping “heat call”. That signal kicks off genetic changes in unhatched baby zebra finches’ brains, researchers report June 11 in the Journal of Experimental Biology. The tune appears to give developing finches a physiology-bending forecast, giving them a leg up once they emerge into the broiling conditions on the other side of the eggshell.
A decade ago, behavioral ecologist Mylene Mariette and her colleagues discovered that exposure to these heat calls in the egg shortly before hatching changed how the chicks dealt with high temperatures. They grew more slowly, preferred warmer places to nest and seemed better equipped to handle hot conditions.
But it was unknown how hearing a simple song could trigger these kinds of physical and behavioral changes in the young. Mariette, of Deakin University in Waurn Ponds, Australia and Julia George, a neuroscientist at Clemson University in South Carolina wanted to know if the songs might initiate changes in the hypothalamus, a small region of the brain heavily involved in regulating metabolism and responses to heat.
Hear the finch’s heat-induced call
This high-pitched, rapid peeping is the “heat call” of the Australian zebra finch. The effect the call has on the developing brain’s vasculature may make the chicks more resilient against heat stroke. But the impact lasts the birds’ entire life.
“Some of the consequences of [the call] are permanent. The birds will choose warmer nest sites as adults if they are exposed to heat calls as embryos, and they will produce more offspring in those warmer environments,” says study author Julia George.
The researchers raised developing zebra finches (Taeniopygia guttata) at a consistent temperature, but exposed about half to playback of an adults’ heat call and the other half to a different control call for a few days. The team removed the embryos from the eggs and euthanized them. They cut out a small sample of the hypothalamus and extracted the RNA from the tissue. RNA is the messenger that DNA sends to carry genetic information to the protein-making components of the cell. So, by comparing levels of different RNAs, the researchers can see how certain genes are ramped up or down in their production of proteins.
The team thought there would be changes in hormonal genes within the hypothalamus in response to the heat calls, says George. But instead of hormonal genes, the heat calls dampened genes that regulate the contraction and dilation of blood vessels in the brain. The researchers think this helps the chicks dissipate heat in their brains.
“It’s important that the temperature of the brain is kept cool, even in hot temperatures,” George says. Modifying the blood flow system in the brain could protect against dangerous ailments like heat stroke.
Rather than the calls changing the function of the hypothalamus across the board, only a select set of systems vulnerable to high temperatures seem to be targeted. Only about 2 percent of the RNA in the hypothalamus shows this impact of the heat calls.
“[The heat calls] could be like a little weather forecast that allows [the finches] to fine-tune their physiology to be better suited to the environment right after they hatch,” George says.
Other types of sound-triggered preparatory changes are known in yet-to-hatch birds. For instance, yellow-legged gulls that hear adults’ predator warning calls while they’re in the egg slow their growth and are more prone to crouching when they hear those calls after they hatch.
Alexandra Cones, an evolutionary behavioral ecologist at Ludwig Maximilian University of Munich who was not involved with this research, wonders if different genetic and cellular pathways can be used to adapt to the heat on the fly. Her own research demonstrated that adult house sparrows can change how their chicks’ metabolisms respond to temperature. Cones and her colleagues never uncovered which exact mechanism was responsible for this effect, but says that the “acoustic-to-thermal pathway” revealed in zebra finches “forces us to consider a broader range” of possibilities for how this kind of flexibility evolved.
Zebra finches may not be alone in their brain-bending musical abilities, George says. Perhaps there’s a whole playlist of heat-busting birdsongs yet to be discovered.
