A few mammals lay eggs, believe it or not (duck-billed platypus); others carry their young around in pouches (marsupials like the kangaroo)
But most of us, including cats, keep our unborn young inside, connected to Mom’s bloodstream through temporary tissue, called a placenta, until they’re ready to meet the world.
Placental mammals are dominant everywhere today except Australia–and it’s all because of the K/T (K/Pg) extinction.
How do cats reproduce?
Short answer: Sex: lots and lots of it. The feline reproductive organs are like ours in most ways but with some important differences.
Details: Puberty hits a cat anywhere between 3 and 18 months of age, depending on things like gender, breed, and whether it spends a lot of time outdoors.
An adult she-cat doesn’t get periods–her body doesn’t routinely release an egg every month. Instead, she goes into heat, generally every 2-3 weeks. Having sex triggers the ovulation.
For 10 days or so (unless she gets pregnant), the cat will be restless and very vocal. Cat breeders refer to this as “calling.” It’s a monotone yowl that can turn into ear-splitting howls if the cat is kept indoors.
She will also start rubbing her body against things, rolling on the floor, and making other body postures.
All this attracts tomcats, of course, and their noise gets her even more excited. Nevertheless, the caterwauling and fights around her don’t seem to influence which tom she finally allows to mount her.
Cats have the same basic sex organs that we do, and they have sex the same way, but it’s very quick. The tom gets in, ejaculates, and then the female yells, pulls away, and turns angrily on her partner. He will get beaten up unless he scampers away.
Biologists are fascinated by this sudden change of mood, which doesn’t happen in any other domestic animal. It’s not rejection or aversion–after a little while, she will pat the tomcat (who hasn’t gone very far) and they go at it again. This may go on up to 10 times in an hour!
Experts speculate that the she-cat’s strong emotions–as well as physical stimuli in the form of small barbs on the tomcat’s penis–somehow trigger those complex hormonal and neurological changes that result in ovulation. It takes about 24 hours for the egg to leave the cat’s ovary and pass into the fallopian tubes, where it meets sperm.
This delayed ovulation works well for such a solitary species. Instead of ovulating every month whether nor not there is a chance of pregnancy, a cat’s egg only becomes available when it might get fertilized.
Out in the wild, where cats rarely meet, pregnancy needs to happen more often than not in sexual encounters or the species will go extinct.
Once the she-cat is pregnant, the kittens develop in her fallopian tubes, not in her uterus, which is tiny. Unlike ours, a domestic cat’s fallopian tubes are long. The kittens line up in a row, each one wearing its own wrap-around placenta like a little cumberbund.
They typically are born 60-70 days after mating. At first kittens need a lot of care, but they are in much better postnatal shape than a young marsupial, which is born while it’s still an embryo!
Marsupials are animals like kangaroos, koalas, and opossums. They don’t have a way to feed their young inside the body for the full pregnancy.
Instead, they give birth very early and the little joey crawls to a nipple, which is usually in a pouch. Once there, it hangs on for several weeks or months, depending on the species, and continues to develop until it’s able to handle independent life.
What does the K/T (K/Pg) extinction have to do with it?
Short answer: There were a lot more marsupials than placentals, at least in North America, up until that catastrophe. Afterwards, placental mammals took over.
Details: Technically, the Age of Mammals didn’t begin when the dinosaurs went away 65 million years ago. Mammals and dinos got started at roughly the same time–some 200 million years ago, in the late Triassic.
Mammals just kept a much lower profile during the dinosaur heyday.
As far as paleontologists know, early mammals were small, none bigger than a modern beaver. However, what few fossils have been found thus far show that these mammals were surprisingly diverse.
A now-extinct group called multituberculates were the most successful ones at first–their nickname, “rodents of the dinosaur age,” tells you everything you need to know.
Egg-layers like the platypus’s ancestor didn’t leave much of a fossil record.
Since soft parts don’t fossilize, it’s difficult to classify long-extinct mammals, but paleontologists believe that both marsupials and placentals inhabited the swampy lowlands along North America’s vast Western Interior Sea during the late Cretaceous.
They may have lived elsewhere in the world, too–marsupials certainly got to South America somehow, and from there to Australia by way of Antarctica (which was not only ice-free but also green back in the day).
But the best fossil record of those times is in western North America. Late Cretaceous marsupial fossils are much more common than placentals here.
Then the world changed in a big way, and not only for the dinosaurs and great sea reptiles.
Ecosystems collapsed, and almost 90% of all known North American mammal lineages disappeared around 65 million years ago.
Just a couple marsupial groups there came through. Placentals were hit hard, too, but they came back, though it took hundreds of thousands of years. (This also happened with multituberculates, by the way, but after recovering from the big die-off, multituberculates went extinct some 25 million years later as the first true rodents–which are placental mammals–appeared.)
North American marsupials were now few and far between, though they did establish themselves successfully in South America and Australia. Instead, placental mammals inherited the northern continents, where cats and humans would eventually evolve.
There just isn’t enough paleodata available yet to show why this major mass extinction forced marsupial and placental mammals to switch dominant positions.
But the next time you hear that the extinction of the dinosaurs made room for mammals, remember that the actual event was more complex. Whatever happened during those times, it set the stage for today’s incredibly diverse world of mammals. And it’s the reason you’ll be putting Junior to bed tonight, not to pouch.
Featured image: Rick Kimpel, CC BY-SA 2.0.
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