Biology concepts – internal asymmetry, neuroendocrine,
reproduction, hormone, absence symmetry
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This is the Decatur County
Courthouse in Greensburg
Indiana. A mulberry tree has
been growing out of the
steeple for over a hundred
years. Nature has things out
of place, sometimes for a
good reason, sometimes
through sheer tenacity.
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A couple of examples - in December of 2013, a University of
Arizona team announced that they had found a planet orbiting a star about 300
light years from Earth. No problem, we’ve found many exoplanets. But to our
current understanding, it shouldn’t be there.
Planet HD 106906b is 11x the size of Jupiter and travels in
a path 650x larger than Earth’s orbit around the sun. This makes it the largest planet we have
noted orbiting such a long distance from its star. Our current hypotheses of
planet formation can’t account for this planet.
Usually a planet will accrete from the left over material in
the debris disk that forms a star. But a planet this large would take too long
to form in that manner. And the outer edge of a debris disk, where this planet
is located, doesn’t have enough material to build a large planet. So why is it
there?
Perhaps it was a failed star, formed at the same time as its
star. Many binary star systems form by quick accretion in two areas of the same
disk. But binary star systems usually have a mass ratio of 10:1 or less. HD106906
(the star) and the presumed “failed star” (HD 106906b) have a ratio of
>140:1. There’s no good reason for it – yet there it is.
Now for a less universe-shaking example. My family and I
live in Indianapolis, the largest city in the United States without a navigable
waterway. Yet, there's a US Naval Station located on the near north side of
the city. Built in 1936 as a WPA project, the Heslar Naval Armory housed
sailors that trained on Lake Michigan (200 miles away) each summer.
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This is the Naval Armory in
Indianapolis. There is a river
behind it, but the last time
anyone tried to navigate the
White River from the Ohio to
Indianapolis they ran
aground and the entire boat
had to be disassembled.
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In terms of reproduction and neuroendocrine function,
there’s a true disconnect with the testes and their function. Animal
testes do a couple of important jobs. They produce the male gamete cells for
reproduction, but they also produce sex hormones that control reproduction and
secondary sex characteristics.
If the testes are so important, why are they housed in harm's way, outside
the body cavities? You think you know the answer, but really you
just know a portion of the story. The testes are housed in the scrotum instead
of inside the abdominal cavity. Think how silly it would be to suggest that a woman’s
ovaries should be housed in a sac outside her body – yet there are the male
testes, in a sac outside his body.
One weird theory is that large testicles and scrotums are a
sexual dimorphism ornament; it costs energy to make large testicles, so the
larger they are, they better health the animal must have to be able to afford
to invest so much in ornament size. To take advantage of this as a way to draw
mates, they have to be seen. Therefore, they're outside the body. It's an example of the handicapping hypothesis
– an organism handicaps itself by investing so much energy in one thing, but it
pays off in a reproductive advantage.
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Testicle size varies
according to species and breeding
strategy. The right whale has
the largest testicles,
about 500 kg (1100 lb) each, 10x
larger than those of the
blue whale. In general, monogamous males tend to
have smaller testicles. A paper from 2013 showed that
testicle size in humans is
negatively correlated to
parenting ability. Those with
smaller testicles were more
likely to be nurturing
fathers.
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This idea of scrotum as ornament is understandable, but if
this isn’t the reason, then what is? Believe it or not, the answer explains
several testicular asymmetries.
It’s all about temperature – male gamete cells survive only
a short time at normal body temperature, so you save their active period for
when they need to swim to the egg. By having them outside the abdominal cavity,
they are cooler. The skin of the scrotum is thin, and the arteries of the
scrotum and testicles lie right next to the veins; more opportunity to pass
heat to blood that is moving away from the testicles. Everything is geared to
lowering the temperature a few degrees.
Believe it or not, temperature is also the reason why they
are positioned asymmetrically. In the majority of men, the right testicle
doesn’t hang as low as the left. A 1997 paper actually studied this and showed
that about 62% of right-handed men and 58% of left-handed men had a left
testicle that hung lower. But the number of men with a lower right testicle was
exactly the same in both right handers and left handers (around 21%). What
changed was the percentage of men that had testicles that were positioned
horizontally in the scrotum.
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The vervet monkey (Cercopithecus aethiops) is an old
world
monkey that lives Acacia woodlands of the
mountains
of southern Africa. There are five subspecies,
all of
which are threatened. They are used in research,
because
they have hypertension, anxiety and alcohol
dependence
as humans do. You can see why someone
might
get the idea that testes and scrotum might be a
sexual
ornament for mate selection.
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Again we go back to temperature. There's a muscle called
the cremaster. It responds to
several signals, including temperature and adrenaline and testosterone. This
muscle can raise or lower the testicles independently of one another. In a
dangerous situation, the testicles are drawn up closer to the body to reduce
chance of injury. In cold temperatures they are drawn up in an effort to keep
them warm.
As we said above, the scrotum holds the testicles outside
the body to minimize gamete activation until needed. But they don’t want to be
too cold either. There is a small range of temperature in which function is
optimal, and each testicle needs to be at that temperature. So, the body senses
the temperature and the cremaster moves the testicle closer to and then farther
away from the body to modulate temperature. Believe it or not, they both are
constantly moving in independent orbits!
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As with this statue of
Hercules, the Greeks got it wrong.
Very accurate in their
sculptures, they most always had
the left testicle hanging
lower than the right in their
male nudes. But, they also
had the left testicle as larger.
They fell prey to the idea
that bigger would hang lower.
My thoughts on this – someone
went around examining
and measuring the testicles
on Greek sculptures?
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This leads to another asymmetry in testicles – one is
usually bigger than the other. It varies amongst the species of the world, but
in most mammals, the left is usually slightly larger than the right. But we humans
are exceptions, in men, the right is usually larger than the left. It’s larger,
but hangs higher – an example of antigravity?
The working hypothesis is that one testicle is dominant,
usually the right in humans. This testicle contributes more to reproduction and
plasma testosterone levels, and is affected more by changing levels of
luteinizing hormone. But the other one is just as important. It is evolution’s
back up plan. If the dominant testicle is injured, the other one takes over. By
reducing it’s role in normal function, it reduces the risk of use problems
developing; when it is needed, it is fresh and ready to go.
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The leatherback turtle has a
skylight in its skull (2014).
The bone is so thin that
sunlight can reach the pineal of
the epithalamus. It has light
sensing receptors, just like
the eye. In fact, most vertebrates
have this ability. We
don’t since our cerebral expansion
has buried the
epithalamus and hypothalamus
deep on our brain.
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In animals that have a distinct breeding
system, the sizes of the testicles can change. Just before the breeding season,
the testicles may swell to double or triple their normal size. This occurs in
the many bird species, and amazingly, their brain senses the day length
directly. A 2000 paper looked at mallards, but other papers have shown in other
species as well that the hypothalamus of many non-mammal vertebrates has light
sensitive cells and can detect lengthening days right through the skull!
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This ground beetle is one of
thousands of carabid
beetles. They are predators,
fast and have nasty
secretions as a defense. And
most them only have one
testicle. Interestingly, it’s
always the right one.
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So now you can ask not only, "why are they located there," but also, "does
there really need to be two of them?" Next week, we shouldn’t leave out the
girls. Ovaries have just as many exceptions as testes.
For more information or
classroom activities, see
Not many classroom activities
involving testicles. Sorry.
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