Biology concepts – internal asymmetry, neuroendocrine, reproduction, hormone, absence symmetry
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.
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.
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.
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.
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.
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.
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!
According to a 2008 paper, if the testicles hung side by side, they would warm each other; the maximum surface area of the scrotum for temperature dissipation would not be utilized. Therefore, one hangs below the other for maximum independent temperature regulation. Apparently it isn’t a deal breaker, because 20% of men have testicles at equal heights; perhaps they have a slightly lower reproduction rate.
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.
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.
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!
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.
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.