As a research studying maternal behavior, I come across a lot of sex & reproduction research. As a (very) general rule of thumb, most small mammals are either sexually receptive or parentally responsive - your sex circuits remain on until you have offspring to tend to, at which point your parental circuit overrides your sex circuit so that you can tend full-time to your kids. These opposing mechanisms are evolutionarily strategic - it's bad form to be out reproducing with wanton members of the opposite sex while you should be taking care of your kids back in the nest - your kids are carrying your DNA, so its most advantageous to ensure their survival. If you don't have kids back in the nest...well, get out there and have some fun!
So, as sex and parental research can be considered two sides of the same coin, I end up learning a good deal about one while studying the other. And recently, I stumbled across something startling.
Apparently, a male elephant lacks a scrotum.
Not only is this rare amongst mammals, but it turns out that this "missing part" is one of the most convincing pieces of evidence suggesting that modern-day elephants arose from an aquatic ancestry.
There's a good deal of paleontological evidence suggesting that elephants and sea cows (e.g. manatees, dugongs) shared a common, semi-aquatic ancestor in the group Tethytheria. As the groups diverged, one becoming terrestrial and one becoming fully aquatic, the male reproductive tract retained striking similarities between the two emergent species.
First, the testes of male elephants remained intra-abdominal. In humans and many other terrestrial mammals, testes descend into a scrotal sack, where they remain at a slightly cooler temperature than that of the human body. As warm temperatures can reduce both testicular weight and sperm count (and, actually, has been suggested as a possible method of contraception), testicular descent is important for most terrestrial mammals to retain optimal reproductive function.
But excessive cooling can also threaten sperm production, and the cool temperatures of aquatic habitats make an inhospitable environment for spermatogenesis. So, the testes of most aquatic mammals and fish never descend, instead remaining at a nice cosy intra-abdominal location where they can be kept warm.
As it turns out, elephants have always had intra-abdominal testes - researchers can't find any evidence (e.g. this or this) that their testes ever descended and were subsequently retracted. So, elephants are considered a primary testicond species - their testes never descend and they have never been considered scrotal animals.
Whales, however, do show residual structures suggestive of retracted testes and scrota - a supposed reminant of their former terrestrial ancestry. In elephants, this isn't the case.
So...why is this so interesting?
Not only is primary testicondia unusual in mammals, it reflects a major adaptation with evolutionary significance. Elephants' phylogenetic history has presumably altered the mechanisms currently in place that support spermatogenesis at slightly elevated (core body) temperatures, due to the intra-abdominal location of the testes. And, it paints an elegant picture (if male reproductive anatomy can be considered, uh, "elegant") of the supposed origins of these grand mammals.
As Gaeth et al. summarize:
[elephant testes] were adapted to its aquatic environment and...some of these unusual anatomical adaptations have persisted in present-day terrestrial elephants.
Cocktail-party conversation, if I've ever heard it :)
Bizarre semi-related link: elephants are afraid of bees
note these points too...
elephants and members of sirenia like dugong have no pareital pleura.
both have a bifid heart apex...
no sweat glands...
similarity in melecular genomic studies also...
Oh noes! Not the Aquatic Proboscidean Hypothesis!
Just kidding... I actually haven't heard much about this before, although I know that male elephant testes were inside their body rather than in a scrotum (which makes them a prime candidate for the Behemoth in Job - "His tail [penis] sways like a cedar; the sinews of his thighs are close-knit.")
As far as I'm aware, the early Eocene mammal Moeritherium is considered to be one of the earliest elephant relatives known, and may have been semi-aquatic like a hippo. Moeritherium was not a direct ancestor to elephants, instead representing a sister group that left no descendants and died out, but it might provide clues as to what close relatives that did give rise to elephants might have looked like.
Are we sure this isn't just a cube-square issue? I mean, they can get up to a pretty fast gallop, and ...
On the other hand, perhaps mammals with dangling testes can't evolve to be larger than a certain size. Now *that* would be an interesting evolutionary hypothesis. Would explain why the dinosaurs got so big, and land mammals never did.
If elephants were semi aquatic and that is the reason for internal genitals, are the hippos genitals internal and if not, does that negate the theory?