Elephant Testicles!
Hey Li!
Today, I did a deep dive into testicles!:) I read that elephants don't have scrotums, they have internal testes. I obviously I became a bit obsessed with this factoid.
Why did I become obsessed? Well, first of all, I had only heard of internal testes in mammals in whales and dolphins. Most other vertebrates βΒ fish, amphibians, reptiles, birds β also have internal testes, but most mammals, I thought, have scrotums. Anyway, this obviously became a very long rabbit hole...
A little bit about my obsession with dolphin testes: Dolphins have internal testicles and this is one of the many physiological adaptations I find amazing about them. Remember that summer I spent at the Duke Marine Lab studying marine mammals? During my time in Beaufort, I did a necropsy (animal autopsy) on a dolphin that had died locally. One of the things I found to be most incredible during the necropsy was seeing how, on the inside of the male dolphin's dorsal fin were little holes. These holes were for blood vessels that took blood to their dorsal fins and then brought it back down to their testes in order to keep them cool.
As you may have heard in popular culture, sperm production in many mammals is optimal at temperatures are lower than body temperature (this is in part why boys are told not to do work with their warm laptops on their laps β don't want to π₯ those β½β½!)
Anyway, dolphins evolved from land-dwelling creatures with external testes and scrotums. However, like many marine mammals, they have internal testicles probably because, when they went back to the water, being streamlined was selected for. And so, as they became more hydrodynamic over generations, dolphins' testicles were reabsorbed into their abdominal cavity. Supposedly, to deal with the fact that sperm production is optimal at a lower body temperature, they have this blood that cools down in their dorsal fin (which has more surface area) and then travels down to their testicles to cool them down.
But elephants have a different thing going on. They evolved from animals like manatees and dugongs. These animals β and, it turns out, many mammals β do not have scrota and externalized testes and neither did their ancestors. In fact, the original condition in mammals is to have internal testes!
Some mammals internalized their testes over evolutionary time (e.g. rhinos and whales) but most mammals with scrotums who testicles are inside them, retain the original mammalian condition! Platypuses, sloths, armadillos, elephants, elephant shrews, moles, elephant shrews, and bunch of other related mammals don't have external testes. So the question becomes, why would any mammals evolve scrotums? It seems dangerous to have your prescious future offspring housed in a fragile ball sac outside your body! Why would this ever happen and why didn't it happen to elephants?
The most common hypothesis is about cooling, that sperm production is optimal at cooler temperatures and therefore testicles moved outside of the body. This idea came from an experiment in the 1920s where a scientist sewed dog testicles back into their abdomen π¬ and all of their sperm died and their sperm-production centers in their testicles were all messed up.
But it's really a chicken-or-the-egg kind of question? Did sperm who prefer lower temperatures evolve before or after scrotums evolved? Also, how does this cooling hypothesis account for all of the mammals with internal testes? Like, if being warm is such a problem than why do so many mammals have it?
It turns out in fact, that scrotums (aka external pouches for testicles) evolved twice βΒ in marsupials and in placental mammals like us. We know this because in marsupials, their balls are in front (that's right!) of their π!!!
One current compelling hypothesis came out of observations that rowers exerting themselves in races were having some of their prostate fluid come out in their urine. They think this was because the cyclic abdominal work was pushing stuff from inside their testicles out into their penis tubes. This his was happening because unlike the heart or different parts of the digestive tract, the testicles have no sphincters or valves. This led some scientists to think that maybe, other animals that are increasing the pressure in their abdominal cavities, would benefit from not having testicals inside their abdomens. This would make particular sense in animals who gallop especially on all fours, crunching their stomachs up and down as they run like a cheetah.
Maybe these running animals could reproductively benefit from having testicles outside, decreasing the stress and compression on these precious parts.
Some further evidence for this galloping hypothesis: none of the mammals who have internal testicles (e.g. sloths, moles, elephants) are real gallopers.
So...maybe it was galloping, not temperature that selected for scrotums. Either way, if you look at an elephant from behind, you'll never see what, based on our bodies, you may expect to see!
Some great resources:
P.S. Here's the next letter!