It might seem counter-intuitive to keep an important male reproductive organs exposed to the elements outside of the body, however scientists have multiple theories about why testicles are not kept safely tucked away inside the body.
Temperature control is the most obvious answer.
Sperm production is at its most effective at 35° Celsius, which is two degrees below the temperature maintained inside the human body.
Organs that perform best at 37°C are shielded by bones inside the cavity of the body, including the brain and the kidneys.
However, there is some disagreement within the scientific community about the temperature thesis.
It’s unclear whether the testes descended because they needed to be cooler than the rest of the body, or whether the organs evolved to perform at that temperature because they were external.
Scientists have multiple theories about why testicles are not kept safely tucked away inside the body
The cooling thesis originated at Cambridge University in the 1890s.
Scientist Joseph Griffiths experimented on dogs, pushing their testicles back into their abdomens and stitching them in place.
Less than a week later, Griffiths discovered the organs had degenerated, with the tubules where sperm production occurs constricted, and sperm virtually absent.
He attributed this to the higher temperatures inside the body, spawning the theory.
The research was picked up in the 1920s by Carl Moore at the University of Chicago.
Building on the work of Charles Darwin, Moore argued that when mammals had transitioned from cold to warm-blooded animals, the internal temperatures severely hampered sperm production.
Males who were born with testicles outside of their body became the most successful breeders and passed on their genetic material, perpetuating the trait.
However, those who oppose the theory point to mammals that still keep their testicles inside the body and continue to successfully reproduce.
Many mammals with internal testicles, like elephants and birds, have a higher core temperature than human beings and primates – which some say discredits the temperature theory.
Some academics believe the cooler temperatures might be to stop DNA from mutating, while others believe keeping sperm below body temperature allows the warmth of a vagina to function as an activating signal for the swimmers.
It is possible external testicles can be explained by the so-called handicap theory, which posits that if a female has to choose between two male suitors who have bested all other competition, she would choose the one who had to surmount the greatest odds – as this hints at an even greater strength.
For example, climbing Mount Everest is impressive, but climbing to the summit with one hand tied behind your back is even more impressive, right?
The controversial theory goes some way to explaining a number of problematic evolutionary phenomena, like male birds’ colourful plumage and songs that seem designed to attract predators.
If the handicap theory is correct, the genes for scrotums were passed along because being able to function with these crucial organs suspended on the outside of the body impressed potential mates.