WHY do we yawn? For no reason, apparently.
It's an essential part of normal lung
development in unborn babies, but serves no
purpose in adults, say two researchers in the
US.
Other theories on the significance of yawning
have ranged from social cues to a craving for
oxygen ("The big yawn", New Scientist, 19
Decembe 1998, p 72). But Richard Roberts of the
Genetic and Prenatal Diagnostic Center in Signal
Mountain, Tennessee, thinks differently after
studying hour-long ultrasound scans of fetuses
with possible medical problems. The scans showed
that fetuses yawn and hiccup as early as 11
weeks.
Roberts and his colleague Will Blackburn of
the Greenwood Genetic Center in South Carolina
think these actions reduce pressure in the lungs
and clear out webs of tissue that might block
the air ways. Fetal lungs secrete liquid that
makes up the amniotic fluid, a long, with-
urine. If it can't escape the lung, the airways
overdilate and are damaged. Babies with
congenital obstructions that prevented fluid
escaping are sometimes born with deformed
lungs.
"There is no reason why babies and adults and
children need to yawn and hiccup," says Roberts.
He believes these movements persist in adults as
a "leftover" behaviour that serves no
purpose.
"I find it interesting," says Robert
Provine of the University of Maryland in
College Park, who believes adult yawning is a
social cue to switch activities. He and others
have speculated in the past that yawning might
ensure proper development of a fetus's skull.
But he notes that fish and turtles also yawn, so
it might merely be an activity of an ancient
ancestor that's now redundant, although possibly
still serving a secondary social role in
adults.
Fetal Yawning In Utero at 23 Weeks
Gestation
