麻豆传媒

PLAYING is a serious business. Children engrossed in a make-believe world,
fox cubs play fighting, or kittens teasing a ball of string aren鈥檛 just having
fun. Play may look like a carefree and exuberant way to pass the time before the
hard work of adulthood comes along, but there鈥檚 much more to it than that.

For a start, play can be dangerous, and even costs some animals their lives.
For example, 80 per cent of deaths among juvenile fur seals occur because
playing pups fail to spot predators approaching. It is also extremely expensive
in terms of energy. Playful young animals use around 2 or 3 per cent of their
energy cavorting, and in children that figure can be closer to 15 per cent. 鈥淔or
evolutionary biologists, even 2 or 3 per cent is huge,鈥� says John Byers from the
University of Idaho. 鈥淵ou just don鈥檛 find animals wasting energy like that,鈥� he
adds. There must be a reason for this dangerous and expensive activity.

But if play is not simply a developmental hiccup, as biologists once thought,
why did it evolve? There are scores of theories, but none is totally convincing.
The latest idea is perhaps the most audacious鈥攊t suggests that play has
evolved to build big brains. In other words, playing makes you intelligent.

Playfulness is quite a rare trait. It is common only among the mammals,
although a few of the larger-brained birds such as magpies and crows also
indulge. Animals at play often use unique signals鈥攖ail-wagging in dogs,
for example鈥攖o indicate that activity superficially resembling adult
behaviour is not really in earnest.

One of the most popular explanations of play is that it helps juveniles
develop the skills they will need to hunt, mate and socialise as adults. Another
is that it allows young animals to get in shape for adult life by improving
their respiratory endurance. Both these ideas have been questioned in recent
years.

Take the exercise theory. If play evolved to build muscle or as a kind of
endurance training, then you would expect to see permanent benefits. But Byers
points out that the benefits of increased exercise disappear rapidly after
training stops, so any improvement in endurance resulting from juvenile play
would be lost by adulthood. 鈥淚f the function of play was to get into shape,鈥�
says Byers, 鈥淚 would expect the age distribution of play to vary widely.鈥� The
optimum time for playing would depend on when it was most advantageous for the
young of a particular species to get in shape. But it doesn鈥檛 work like that.
Across species, play tends to peak about halfway through the suckling stage and
then decline to a low at weaning.

Then there鈥檚 the skills-training hypothesis. At first glance, playing animals
do appear to be practising the complex manoeuvres they will need in adulthood.
But a closer inspection reveals this interpretation as too simplistic. In one
study, behavioural ecologist Tim Caro from the University of California, Davis,
looked at the predatory play of kittens and their predatory behaviour when they
reached adulthood. He found that the way the cats played had no significant
effect on their hunting prowess in later life.

In another study, neuroscientist Sergio Pellis of the University of
Lethbridge in Alberta, Canada, scrutinised videos of rodents play
fighting鈥攖he most common form of social play in rodents. Despite
superficial similarities between this and the social, sexual and fighting
behaviour of adult animals, Pellis鈥檚 close examination of the play bouts
revealed no compelling link between play manoeuvres and adult tactics. 鈥淔or
rats, and probably other rodents,鈥� says Pellis, 鈥渢he primary function of play
fighting does not appear to be to provide practice for either sex or
补驳驳谤别蝉蝉颈辞苍.鈥�

So what is going on? Prompted by the observation that play seems confined to
the most intelligent animals, Byers looked at the behaviour and brain size of
various marsupials. He found that playful species such as the wombat have bigger
brains for their body size compared with their lazier kin, which include the
docile koala. More recently, Pellis has teamed up with Andrew Iwaniuk of Monash
University in Melbourne to show that in primates, the amount the brain grows
between birth and maturity reflects the amount of play in which each species
engages.

And earlier this year, Pellis, Iwaniuk and biologist John Nelson, also of
Monash University, reported that there is a strong positive link between brain
size and playfulness for mammals in general. It is the most extensive
quantitative comparative study of juvenile play ever published. Comparing
measurements for 15 orders of mammals鈥攆rom canids to dolphins, rodents to
marsupials鈥攖he team found larger brains (for a given body size) are linked
to greater levels of play. Likewise, animals with relatively small brains tend
to play less.

Byers believes that because large brains are less hard-wired and more
sensitive to developmental stimuli than smaller brains, they require more play
to help mould them for adulthood. Evolutionary neurobiologist Robert Barton of
the University of Durham agrees. 鈥淚 suspect it鈥檚 to do with learning, and
probably specifically with the importance of environmental input to the
neocortex and cerebellum during development,鈥� he says.

According to Byers, the timing of the playful stage in young animals provides
an important clue to what鈥檚 going on. If you plot the amount of time a juvenile
spends playing each day over the course of its development, you end up with an
inverted-U-shaped curve. This is the classic signature of a 鈥渟ensitive
period鈥濃�攁 brief developmental window during which the brain can be
modified in ways that are not easily replicated earlier or later in life. Think
of the relative ease with which young children鈥攂ut not infants or
adults鈥攁bsorb language.

Byers suspected that these play curves might coincide with a particular phase
of brain development known as terminal synaptogenesis. 鈥淚n many parts of the
brain, there is an overproduction of synapses [the connections between
neighbouring neurons] and then a specific culling,鈥� he says. 鈥淪ynapses that are
active are retained, while the ones that are less active end up being
诲别蝉迟谤辞测别诲.鈥�

To test this idea, Byers teamed up with biologist Curt Walker from Dixie
State College in St George, Utah, to see how the distribution of play with age
in cats, rats and mice fitted with the development of a part of the brain called
the cerebellum. Among other things, the cerebellum controls the fine motor
skills needed for eye tracking, stalking, pouncing and fleeing鈥攖he adult
activities that most closely resemble the play of kittens and rodent pups.

The researchers found that in all three species play was at its most intense
just as terminal synaptogenesis in the cerebellum reached its peak. Evolutionary
anthropologist Kerrie Lewis from University College London points out that since
new brain cells are seldom produced after birth, synaptogenesis is the most
likely way in which play could sculpt the developing brain.

But there are other possible mechanisms. 鈥淚t might also include things that
influence processing efficiency, like myelination,鈥� Lewis says. Myelin is a
fatty sheath that insulates the tentacle-like axons of nerve cells, improving
their ability to conduct electrical signals. Either way, play shapes the overall
architecture of the brain rather than individual circuits connected with
specific activities. 鈥淢ost likely, [animals at play] are directing their own
brain assembly,鈥� says Byers.

鈥淧eople have not paid enough attention to the amount of the brain activated
by play,鈥� says Marc Bekoff from the University of Colorado. Bekoff studied
coyote pups at play and found that their behaviour was markedly more variable
and unpredictable than that of adults. Behaving this way activates many
different parts of the brain, he reasons. Bekoff likens it to a behavioural
kaleidoscope, with animals at play jumping rapidly from one activity to another.
鈥淭hey use behaviour from a lot of different contexts鈥攑redation,
aggression, reproduction,鈥� he says. 鈥淭heir developing brain is getting all sorts
of stimulation.鈥�

Not only is more of the brain involved in play than was suspected, but it
also seems to activate higher cognitive processes. 鈥淭here鈥檚 enormous cognitive
involvement in play,鈥� says Bekoff. He points out that play often involves
complex assessments of playmates, ideas of reciprocity and the use of
specialised signals and rules. He believes that play creates a brain that has
greater behavioural flexibility and improved potential for learning later in
life. 鈥淚t鈥檚 about more connectedness throughout the brain,鈥� he says.

The idea is backed up by the work of neuropsychologist Stephen Siviy of
Gettysburg College in Pennsylvania. Siviy studied how bouts of play affect the
brain鈥檚 levels of a protein called c-FOS鈥攁 substance associated with the
stimulation and growth of nerve cells. He was surprised by the extent of the
activation. 鈥淧lay just lights everything up,鈥� he says. He speculates that by
allowing connections between brain areas that might not normally be connected,
play may be enhancing creativity.

All these findings paint a picture of how play might have originated. The
comparative study reported earlier this year by Pellis and his colleagues
suggests a 鈥渟tepwise鈥� relationship between increasing brain volume and the
evolution of play. The researchers suggest that minor changes in brain size
might not have required evolutionary changes in play behaviour, but at certain
threshold increases in volume, greater levels of playfulness evolved.

Lewis鈥檚 recent findings point to the intriguing possibility that different
types of play may have evolved at different stages in evolutionary history, to
allow the development of distinct regions of the brain. She looked at the
relative size of the neocortex鈥攚hich is responsible for social reasoning,
among other things鈥攊n primate species, and found that the larger the
neocortex in each species, the more social play they indulged in. But this
relationship did not extend to object or motion-based play. By implication,
Lewis believes, social play may help wire up the social brain, while other forms
of play do not. 鈥淚 think it鈥檚 reasonably safe to assume that different types of
play did emerge at different points in time, but possibly with some overlap,鈥�
she says.

The idea that play has evolved to build big brains certainly has its critics.
Like much of behavioural ecology, it rests on a scaffolding of correlations.
鈥淭he problem with correlations is that they don鈥檛 consider unknown third
variables,鈥� cautions Caro. 鈥淪o maybe brain size and play are both correlated
with metabolic rate or some other factor. Certainly, something about being
[warm-blooded] seems important for promoting play.鈥�

Even some of the researchers whose results seem to support the link between
brain building and play are cautious in their assessment of the theory. Siviy
believes there is not yet enough evidence to settle the question. But he thinks
the timing of play is convincing. 鈥淚t鈥檚 an ideal time to do some learning, to
make some modifications to brain circuitry,鈥� he says.

One of the strengths of the idea is its testability. Magnetic resonance
imaging techniques that identify myelin by-products, for example, should be able
to show whether play boosts myelination, as Lewis has suggested. What鈥檚 more,
measuring the volume and activity of certain parts of the brain is becoming
increasingly easy due to advances in non-invasive imaging.

If the theory is backed by experiment, what would it say about the way many
of us in affluent societies raise our children? We already know that rat pups
denied the opportunity to play grow smaller neocortices and lose the ability to
apply social rules when they do interact with their peers. Bekoff says play is a
sign of healthy development. 鈥淲hen play drops out, something is wrong,鈥� he says.
Children destined to suffer mental illnesses such as schizophrenia as adults,
for example, engage in precious little social play early in life. But can a lack
of play affect the creativity and learning abilities of normal children?

The answer is that nobody knows. When Byers searched the literature for
information on the relationship between childhood play and development in
different cultures, he found that no studies have been done. 鈥淭here鈥檚 not even
any great data on rate of play for any culture across ages,鈥� he says. Until such
information is available, assessing the importance of play will be slow going.
Meanwhile, our ideas about what constitutes a normal childhood are changing
fast.

鈥淜ids are discouraged from playing because they鈥檝e got to go to school,鈥� says
Bekoff. 鈥淭hey have all these things to do after school that adults think of as
play鈥攂ut Little League isn鈥檛 play, in many ways.鈥� Organised sports are too
structured to emulate spontaneous play, and there鈥檚 often so much pressure
involved that after-school activities aren鈥檛 even fun. With schooling beginning
earlier and becoming increasingly exam-oriented, play is likely to get even less
of a look-in. 鈥淲e have basically become a playless society,鈥� says Bekoff. Who
knows what the result of that will be?