The origin and evolution of sex organs in mammals

Yesterday we looked at the origin and evolution of the human face.

Today we hit below the belt
We’re going for the gonads. If you’ve ever wondered the ins and outs of where, how and why your privates arose, this is your primer.

This topic also comes under the subject of reptile evolution because mammals are reptiles, according to the large reptile tree. This is heresy, of course, bucking decades of tradition. However, now the amniote/reptile tree has a new topology simply by adding taxa, 361 at last count. This experiment can be repeated by anyone wishing to go to the trouble of doing it.

The rest of this post is standard, straight from the textbook. No heresy involved.

Evolution does its own thing
It’s fairly common knowledge that basal reptile genders are pretty similar internally. In other words, it’s generally hard to tell males from females. Often, the only difference is the substitution of uteri and ovaries for testicles. Externally a single opening, the cloaca (or sewer), expels urine, feces and eggs. The same holds true for basal mammals, like the egg-laying monotremes (Fig. 1, image from Peters 1991). The uterus is the organ that nourishes eggs and produces a secretion that hardens to form a shell around each one. In males an extensible penis was present for internal sperm deposit. It was not a conduit for urine at this stage.

Figure 1. Monotreme sex organs. There is very little difference here, other than the uteri and ovaries in place of internal testicles.

Figure 1. Monotreme sex organs from Peters 1991. There is very little difference here, other than the uteri (uteruses) and ovaries in place of internal testicles. That all changes in marsupials and placentals.

Marsupials (metatherians)
In marsupial females things are a little different. The rectum splits off from the urine and birth canal creating its own opening in the body wall. The birth canal is shorter than the cloaca and splits to form twin vaginas.

In most mammals (whales and manatees are notable exceptions) the testicles need to be kept cooler than body temperature, so in marsupial males the testicles are outside the body wall in a sac-like scrotum. The scrotum of the male and the pouch of the marsupial female arise from the same structure. Hormones turn them into distinct body parts during maturation.

In marsupials the penis becomes a urine channel as well as a sperm channel. The penis is generally kept within the body, extending only for mating. Here (Fig. 2) it is extended, and yes, it has two tips.

Figure 2. Placental mammal sex organs.

Figure 2. Marsupial and placental mammal sex organs.

Placentals (eutherians)
In placental females the twin vaginas fuse to become one. The uteruses (uteri) remain paired and their walls thicken to support larger embryos (fetuses). Humans have but one uterus, formed by the fusion of the pair.

Most male placentals “drop” the testicles slightly behind the penis from their origin near the navel in marsupials. The penis is typically outside the body wall, but often sheathed against the belly/abdomen. In bats and primates it become pendulous, losing its body wall attachment.

Most male placentals (eutherians) develop a bone (baculum, os penis) in the penis, which lends support during mating. This bone is still present in primates, but greatly reduced in the gorilla and chimpanzee, and, of course, lost in humans.

In other reptiles
The penis developed independently several otherwise unrelated lineages. Some turtles have them. Some birds have them. Others don’t. Crocs have a penis. Lizards and snakes have paired hemipenes. Sphenodontids do not have a penis. So, the situation in pterosaurs is a toss-up. Click here for a broadcast video (with lots of commercials) on dinosaur sex featuring some of your favorite paleontologists.

References
Peters D. 1991. From the Beginning – The Story of Human Evolution. Little Brown.

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