Science News - May 7, 2011

things can trigger yawns,” Guggisberg
says, “but then when you look at the
actual effect of yawning, then you have
more difficulties.”
Some researchers have suspected that
yawning makes the brain more alert.
Yet in human experiments, the brain’s
electrical activity didn’t increase after
a yawn, nor did heart rate or sweating,
hallmarks of alertness.

With hard evidence lacking for any clear bodily function for yawns, Guggisberg and others have turned their attention to one of yawning’s most peculiar features — its contagiousness.

A yawn is powerfully catching, says neurologist Fatta Nahab of the University of Miami in Florida, who has studied what happens in people’s brains as they “catch” a yawn. “Here is something where, essentially, it doesn’t matter who your volunteer is, how old they are, how young they are,” he says. “There’s no training involved. You show them a yawn and you’re going to get a response back.” This throat-jerk response may be governed by the prefrontal cortex, a part of the brain that becomes more active when people catch a yawn, Nahab and colleagues reported in Human Brain Mapping in 2009. Prefrontal lobes may repress the urge to yawn — until the brain detects one in someone else.

Nahab doesn’t think that neurons known to become active when a person watches someone else perform an action are required for contagious yawns. These “mirror neurons” aren’t any more active when people watch yawns than when watching nonyawn gapes. What’s more, infants are thought to have working mirror neurons, yet they can’t catch a yawn. “Babies can mimic facial movements, but they cannot mimic a yawn,” Nahab says.

Sending a message Because a yawn is so contagious, Guggisberg and colleagues contend in the April Neuroscience and Biobehavioral Reviews, it probably carries a message. And not always a nice one. “The funny thing is that yawns are socially inappropriate in all cultures,” he says. “I think this is because people actually do under-

stand the message: ‘Hey, I’m bored. Let’s
change something.’ ”
Gallup interprets the shame of a yawn
differently. To him, a stifled yawn “is an
example of a lack of the communicative
value,” he says. “It’s hard for me to
believe that yawning can reli-
ably transmit physiological or
emotional state,” especially
since it is often hidden.

Another argument
against the idea that
yawning sends a social
message comes from
spontaneous yawn-
ing, such as the
yawn that strikes in
the morning behind
a closed bathroom
door. Since no one
is around to receive
the message, it’s
unlikely that com-
munication is the
objective of these
solitary yawns,
Gallup says.
scientists made headlines with a study
that suggested dogs can catch yawns
from people, perhaps because dogs are
supposed to be in tune with their human
companions. Since then, two further
studies did not find the same effect.
One of those newer studies
tested dogs in their own homes and
found that the canines didn’t seem
to catch yawns from humans,
even from their familiar owners.
The paper, published in the
January Animal Behaviour,
casts doubt on the idea
of social yawn-
ing (and owner
empathy) for dogs.
“I’m afraid it’s not
looking good for
canines and con-
tagious yawning,”
says study coauthor
Sean O’Hara of the
University of Sal-
ford in England.
But evidence
favoring empa-
thy’s link to conta-
gious yawns comes
from a study in
chimps. Viewing
videos of famil-
iar chimps yawn-
ing induced more contagious yawns
than videos of yawning strangers,
Matthew Campbell and Frans de Waal of
the Yerkes National Primate Research
Center at Emory University in Atlanta
reported in April in PLoS One. They sug-
gest that contagious yawning might even
serve as a measure of empathy.
Whether or not yawning in other
species can explain why people do it, ani-
mals’ yawns have already helped solve
one big mystery— the evolution of life
itself. “Seeing a dog and horse and man
yawn,” Charles Darwin wrote in his note-
book in 1838, “makes me feel how much
all animals are built on one structure.” s
Anatomy of a yawn

Whether in humans, dogs or other animals, the mechanics of yawning are the same. It begins with a slow, deep inhalation.

The mouth and throat gape open; air streams in until the lungs are full. The yawn’s apex may bring a brief stillness, often accompanied by closed eyes and a stretch. As the yawn ends, air streams back out, the head, neck and diaphragm muscles return to their resting positions, and the mouth closes.

p ea s a

Yawn empathy

Part of figuring out whether a yawn carries a message involves knowing who can get it. For humans (and perhaps chimpanzees), contagion doesn’t seem to kick in until around age 5. Infants and preschoolers don’t catch yawns, not even when the yawns come from their own mothers, Ailsa Millen and James R. Anderson of the University of Stirling in England report in an upcoming Biology Letters. The youngsters still yawn spontaneously, just not on command. People with schizophrenia and autism aren’t as susceptible to catching yawns as others, studies show.

Some scientists think that contagious yawning reflects empathy and social skills, something human babies and people with autism might not have developed.