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Alright, so let's talk about scrotums!
A feature that is unique to mammals is the scrotal sac. Not all mammals have them, but everyone who has one is a mammal. That would be weird enough, but mammals thought this was so cool that they evolved it at least twice independently.
A feature that is unique to mammals is the scrotal sac. Not all mammals have them, but everyone who has one is a mammal. That would be weird enough, but mammals thought this was so cool that they evolved it at least twice independently.
Mammals, like other vertebrates started out with internal testes located near their kidneys, kinda like this:
The idea that mammals evolved external scrotums to keep sperm cooler is a bit backwards. Most animals, including those with higher body temperatures than bescroted mammals, have internal testicles and they reproduce just fine.
It’s more likely then that bescroted mammals sperm adapted to cooler temperatures due to external scrotums.
A hypothesis for why these sacks evolved in the first place has to do with the way mammals run around. Mammals tend to run in a way that causes the spine to arch and curl forwards, and it’s possible this motion may have caused trauma to the testes, reducing their fitness.
This was a problem for both the marsupials and the placentals, and both separately evolved adaptations to get around it. In marsupials the testicles moved to an external position. In placentals the testicles moved to an external position.
I know what you’re thinking. Someone clearly copied someone else’s homework here. But at least they had the decency to mix it up a little.
In marsupials the testicles moved in front of the penis:
And in placentals, the testicles went to the back with the vas deferens looping around the ureter:
Ultimately both configurations allowed mammals to scamper their way across the globe and into the modern day.
Diagrams are my own work. Some organs/structures have been omitted/simplified. The animal depicted is intended to be a generic representation of early mammals for demonstration purposes and not a specific taxa.
