How to wake up a chick sleeping in its egg

Their dads may be up with the sun, but it takes more than that to wake up a chick still in its egg. Unhatched chicks aren't roused from slumber by random noise, but they do wake up if they hear a chicken danger call.

Evan Balaban of McGill University in Montreal, Canada, says we still don't know when fetuses begin to experience sleep cycles the way adults do. It makes sense for the developing brain to stay in a state of suspended animation, similar to that of someone in a coma, to conserve oxygen. If the fetus brain is too active, it runs the risk of running out of oxygen and damaging itself.

To find out more, Balaban and colleagues looked at developing chicks still in their eggs. Unlike mice, the chicks in their eggs are separated from any influence by the mother's hormones, making them easier to study. The researchers labelled sugar molecules with radioactive tracers and injected the sugar into the chick embryos. When the brain is active, it uses the sugar and lights up with the radioactive tracer.

Using this method, the researchers found that the chicks' brains were fairly inactive until 80 per cent of the way through their development inside the egg. At that point, the brains began to take up the sugar in a regular cycle, suggesting they were passing through phases of sleep and wakefulness.

Alarm bell

The researchers tried to wake up sleeping chicks by playing loud sounds; the chicks just ignored these. But they woke up immediately when the researchers played the sound of a mother chicken screeching a danger call. It's the equivalent of someone shouting your name while you're asleep, Balaban says, as opposed to a truck passing by.

The researchers also found that the chicks were often moving even when their brain patterns indicated they were asleep. This suggests that lower brain functions might control the muscles before birth.

Marcos Frank of the University of Pennsylvania in Pittsburgh says that the study addresses an important issue. Babies of every species from flies to humans sleep a lot more than adults do, but we don't know why, he says. "If it's doing something important for brain development, even before birth, what happens when it's interrupted?"

Sleep disruptions could explain why babies who are born prematurely often have a higher risk of learning disabilities, the authors suggest. Balaban's next plan is to test whether waking up unhatched chicks causes changes in their behaviour after they are born.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2012.03.030

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