Everyone has
heard the idea that certain males may overcompensate for a lack of reproductive
ability by being boisterous, overconfident, and overly aggressive. Perhaps their lack of reproductive
ability changes their neuro-chemical balance or physiology in ways to help them
have a greater chance of attractive a mate to improve odds of passing on their
genetics. Until recently there was
a lack of scientific evidence backing this claim, however Cambridge University
recently completed a study on Howler monkeys that shows there is a direct
relationship between how loud they are and the size/reproductive ability of their
testes.
Howler monkeys
are one of the loudest animals in the world and they use this roar to attract
potential mates. The size of the
male hyoid bone in the neck varies considerably and allows them to utter a deep
guttural roar that can be heard up to 3 miles away. When comparing the monkeys at varying ends of the noise
spectrum, loudest to quietest, Cambridge researchers found an intriguing fact;
males with the loudest roars also had the smallest testes and the least ability
to produce sperm. The opposite was
the case for the quietest monkeys.
Additionally, researches found that the loudest males tend to live in
distinctly different social groups in which they are a dominant figure that has
exclusive mating privileges with several females. Males with better testes and lesser vocal abilities lived in
larger groups with as many as 6 additional males where all females are
available to be mated with. These
two models show the different of “power of voice” over “quantity/quality of
sperm” and how each model directly relates to the unique abilities of each male
to potentially reproduce.
This research is
a great example of sexual selection that was first proposed by Darwin in
1871. It reinforces the fact that when
it comes to when it comes to reproduction it is rare to find the highest level
of reproductive fitness in one individual. Other evidence has been seen linking investment in
large bodies, bright colors, or physical weaponry to a decrease in reproductive
ability, but howler monkeys have show the first relation of a trade off between
vocal capabilities and sperm production.
Although human
beings are quite different from howler monkeys, we still share similar genetic
traits passed down from primate lineage.
Could it be that louder, more aggressive males are perhaps unknowingly
making up for what they are lacking in?
Perhaps our stereotypes weren’t so off after all.
Citation:
1.
Dunn et al. 2015. Evolutionary Trade-Off between Vocal Tract
and Testes Dimensions in Howler Monkeys. Current Biology, 1-6, DOI: 10.1016/j.cub.2015.09.029
-First
read on www.sciencedirect.com
Although "lack of size" is often associated with one's reproductive fitness, history would also associate this phrase with one's stature. Known as the "Napoleon Complex", males are also known to express an overly-aggressive behavior to compensate for their lack in height. You would never guess where this phrase originally stems from...okay, it's Napoleon Bonaparte. This power crazy man was the Emperor of France and compensated for his 5'2" frame by launching wars and other conquests. Similarly to Emperor Bonaparte on the battlefield, shorter NBA referees have been shown to officiate basketball games far different than their taller peers. For example, in a 2014 study, shorter referees were far more likely to call personal fouls. It is possible this could have an impact on the outcomes of NBA games, especially those that are high profile. Let's just hope, when the Bucks make it to the NBA Finals, the "Napoleon Complex" is left to the wayside.
ReplyDeleteGift, R., Rodenberg, R.M. (2014). Napoleon Complex: Height Bias Among National Basketball Association Referees. Journals of Sports Economics. DOI: 10.1177/1527002514535168
My question is how does testosterone play into all of this - did the researchers look into hormone levels of the animals? If a male has smaller testes, does he have less testosterone than a male with larger testes, and therefore theoretically have less aggressive characteristics than the latter? In that case, it wouldn't make sense that these animals are more boisterous and aggressive. Or perhaps it doesn't matter, and there is, as you suggested, a change in their neuro-chemical balance and physiology to be overly assertive and confident in order to attract mates. Since I'm not a man, I'm no expert on testosterone and how that necessarily relates to genital size, but I think that it would be interesting to investigate that relationship. It would also be interesting to look into how different females react to loud/small testes vs. quiet/large testes males, and whether they have innate instincts to mate with males who have better reproductive abilities or they respond more to the overconfident males. I know that as humans there is so much personal preference that factors into mate choice, but maybe we can answer some questions about that preference by looking to our monkey relatives and how they decide.
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