[For readers w/ no time to enjoy the finer things in life and click the cite, two identical puppies were born, and they’re as cute as all the other puppies you might run into.]
And only one kind of armadillo, at that, according to the cite. Which leaves a ton of mammals on God’s green earth.
The article mentions “why” evolution played out the way it has, because of the load on a single placenta would not be optimal (or optimal-enough, evolutionarily) for the individual.
So what gives with us? And the armadillo?
Or should that be “what gave” at some time in the primate development did the genotype change?
And, as usual, what’s that got to do with whatever the hell happened in the evolutionary tree of armadillos.
Humans don’t regularly have identical twins, either. I’d guess that the rate is probably about the same for dogs and humans, except nobody bothers to keep track how common it is for dogs (especially since several puppies in a litter will look a lot alike, anyway)
The article makes the point that it may not be as rare as all that. This delivery was only spotted by chance as the bitch was having a caesarean . Lots of animals have twins, and for the most part, there is no one around counting placentas. Even if a farmer thinks that twin lambs are identical, he is unlikely to go to the expense of testing them.
Perhaps the reason that human twins survive has to do with our size relative to our offsprings’ size, and us not having litters. In order to have big brains, humans have tiny, helpless babies who are very proportionately small compared to the adult. Large adult mammals have pretty large offspring, and tend to have just one, while smaller ones have litters. It might give us more margin for extra investment in another baby if one happens to show up.
On top of that, we also can deliver lots of extra nutrients to a mother in many parts of the world. I’ll bet anything that twins happen more often in first world countries, and that applies especially to identical twins.