Biology concepts – fluctuating asymmetry, mate selection,
honest signal, directional asymmetry
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Facial symmetry supposedly plays
a role
in physical beauty, but may
represent
developmental stability, an
ability to resist
disease, and of genetic
strength.
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Bilateral facial symmetry is thought to play a role in what
people think is pretty, and pretty plays a role in mate selection. Why? Because
symmetry implies a stable embryologic development and good genes – or maybe
not. As Shakespeare wrote in A Midsummer
Night’s Dream, “The course of true love never did run smooth;” it applies
here.
Bilateral asymmetry takes three forms; fluctuating asymmetry, directional
asymmetry, or antisymmetry. In
terms of humans and pretty faces, it’s fluctuating asymmetry that we’re talking
about.
Take a sharpie and draw a line from the top of your head
down to the split in your legs – no, don’t really do it, this is just a thought
demonstration. If you were to measure the distance from that midline to the
same point on each of your eyes or ears or any body part, the length on the
right will be close to the length on the left, but probably not exactly the
same.
The same would be true if you measure the heights from the
floor to the bottom of some body part like your eyes, or if you measure the
length of fingers on each hand. The differences represent fluctuating asymmetry (FA), and your
neighbor’s will be different from your own – we say that FA is what gives faces
character.
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President Lincoln had a
defect called facial
micronesia. The left side of
his face was much
smaller than the right. The
middle image
above is his face. The left
image is his right
side flipped and placed
against his right side.
The right image is made from
two left halves of
his face. Everyone’s right
face and left face
look different; his is just a
greater than average
difference.
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In other words, the more instability there is in the
developmental environment, the higher the FA could be. What might stop fluctuating
asymmetry from becoming large? Some scientists believe it to be good genes. The
instability in the womb could come from disease, toxins, parasites, drugs,
stress… who know how many things might be involved.
But if your immune system is strong – partly a genetic
trait, then perhaps you could control in utero diseases and FA could be kept to a minimum. For just about any stressor you can imagine,
there could be a genetic response that, if healthy and strong, could minimize
the damage from the stressor.
In terms of picking a mate, unconsciously sensing low levels of
FA suggests to your primitive brain that the genes of this
individual might be more adapted to the environment - good potential parent. And that’s all that really matters to an organism at the basal level – having
strong offspring.
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A 2005 study in Jamaican
teenagers showed that
those teens that were rated
as better dancers had
less FA. As always, cause and
effect is elusive.
Did symmetry make them better dancers, or
were they perceived as better dancers because
they were more
symmetric? Regardless, good
dancers do
have a reproductive advantage.
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A 2012 study in rhesus monkeys built upon the result of a
previous experiment in that macaques stare at symmetric faces longer than
faces with higher FA. This was supposed to mean that they preferred the
symmetric faces. And there is evidence to suggest that our brain does find
symmetry in objects, scenes, and art more pleasurable.
So the 2012 study measured both the FA and the health of
female macaques. Using veterinary health standards, number of wounds and weight
gain over first four years of life, the females with less FA had the best health.
This supported the hypothesis that better genes result in better health.
To my mind, there is also the possibility that less FA leads
to being treated better within the group, which results in better overall
health. Since primates prefer symmetric images, a symmetric face (the seat of
social communication) might result in more food offered, fewer fights, and/or
less stress – and therefore a better overall health rating. This idea suggests that
better health is a result of symmetry, not the other way around. In this case, wouldn’t
facial symmetry be a dishonest signal?
Is anyone studying this?
There is evidence that supports this notion, or at least
lessens the strength of the symmetry and fitness hypothesis. A 2015 study in
Senegal found no link between malaria rates and FA in teenagers. This suggests
that FA doesn’t predispose to malaria (lack of fitness meaning higher
susceptibility), and that malaria
does not increase FA.
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FA isn’t just a measure of
“beauty.” A 2013 study
showed that urban lizards
display more FA than
rural lizards. The hypothesis
is that urban living
increases exposure to stress
and pollutants, and
this manifests as increased
FA. If true, tracking FA
could be used a measure of
pollution and its effect
on wildlife.
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One could make a strong argument that humans have invented
their way out of needing to rely on finding mates with low FA. Our technology, medicine, and brains have helped us to
overcome our environment (sometimes to our detriment) so that a different set of
traits might be more telling as to the fitness of partners. Intelligence,
puzzle solving, emotional quotient and even tendency to maintain monogamy might
be more desirable today. But the instinct to look for low FA has been stuck in our brain by
evolution and it doesn’t care about logic.
Why do I bring up emotional quotient and monogamy?
Some studies reviewed in 2010 show that lower FA in human males and females correlates with more
extramarital or extra-significant other affairs (called extra pair coupling,
EPC). Males with more symmetrical bodies sought out more EPC and females who
engaged in EPC were more likely to seek out men with low FA. Not a great
commercial for symmetry.
It isn’t just humans that sense symmetry and use it for mate
selection. The primate studies above show that it extends to macaques, but
birds do it was well (as do probably animals in other phyla). Peahens look for symmetric tails in peacocks and barn swallow females look for
makes with symmetric and long tail feathers.
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The left butterfly is paler; it's a territorial male. The
right speckled wood butterfly
male is non-territorial.
He flits from area to area looking for the
females not
in a sun spot. If he competes
with a territorial male –
he always loses. He has more
FA than a territorial
male, but no directional
asymmetry.
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This European and North African butterfly has two male
morphs. One is territorial, it's a paler color and has fewer spots on it’s wings.
The other morph is darker, has dark spots and is non-territorial. The territory
they fight over is a sunny spot where the light comes through the forest
canopy.
Females are more likely to be in the sun, so territorial
males will fight over a sunny spot territory. The contest between males is aerial; combinations of acrobatics and duration determine the winner – except when
the territory is being defended. Sound a bit confusing?
In 1978, a paper addressed this. The author found that the
defender of a territory ALWAYS won the flying competition. It’s a weird way of
determining who will probably mate more often if they already know the outcome.
A non-territorial male will compete, but will always lose.
The only time the competition isn’t rigged for the
defender is when the spot has no owner, or both males believe it to be their
territory. Then the flight contest is much longer and more intricate. This is where
the asymmetry comes in.
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The territorial speckled wood
butterfly males seek
out sun shafts in the forest.
This is where the girls
will be, but it also makes
them better at the flying
competition. Their time in
the sun warms them up
according to a 1998 paper.
Being ectotherms, warmth
means they will fly with more
energy and win
more contests.
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If asymmetry helps win the territory when it is up for grabs,
then it will provide more contact with females. This means that this asymmetric (uglier?) male will
have more reproductive success and his version of asymmetry will be passed on,
if it's genetic.
FA is generally not considered genetic,
but directional asymmetry is. The fluctuating asymmetry in the males is low,
but the asymmetry that helps win competitions seems to be directional, so it
could be genetic. The asymmetry that helps flying is slight, so it is a
middling asymmetry that should be passed on.
The fact that females and non-territorial males have more asymmetry
suggests that keeping asymmetry low is energetically costly (think about it).
Directional asymmetry in the wings has a functional
advantage so it is worth the cost, whereas FA is allowed to get
larger. It just so happens that directional asymmetry and antisymmetry are
our subjects for next week.
For more information or
classroom activities, see:
Fluctuating asymmetry -
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