A mule results from a male donkey and mare, each of which has a different number of chromosomes. A mule is something that shouldn’t happen, but it does. However, a mule is sterile, even though all mules have the same number of chromosomes. Why?
A corollary problem I have is this. Horses were native to America (how any life could be native to America is another topic). During the ice age, they migrated over the Bering Straits. All of them. (They were reintroduced by the Spaniards.) My question is how did all the horses happen to move north to migrate out of America? Especially during an ice age when it was bitterly cold up there.
The mule is sterile because it doesn’t have the correct number of chromosome pairs…it gets N chromosomes from one parent, and N+1 from the other. So the sperm and eggs the mule forms won’t be carrying the right genetic information when they split.
Now, horses and North America. Yes, North America is the homeland of the horse family, and yes, horses were re-introduced by the Spaniards. However, they didn’t all move out during the ice age. Lots of them stayed here. They just went extinct here, while they stayed alive in Eurasia and Africa.
And the Beringia was mostly ice-free during the ice ages. There were several expansions and contractions of the ice sheets. Central Alaska was ice-free continuously, and there were many times when the glaciers retreated enough for a corridor from Alaska to the rest of North America to open. The Bering straights open and close when sea level rises or falls.
Alaska at that time had bison, mammoths, mastodon, lions, cheetahs, short-faced bears, giant badgers, horses, saiga, ground sloths, two species of musk-ox, camels (probably more like giant llamas), dire wolves, dirk-tooth cats…
Alaska at that time was mostly grasses, instead of tundra or taiga, allowing it to support a much more diverse group of herbivores. So travel while travel directly from North America to Asia wasn’t always possible, one could travel from North America to Alaska when the corridors were open when the glaciers were small, and travel from Alaska to Siberia when sea levels were low enough when the glaciers are large.
Can’t entirely answer your question, but it is important to bear in mind that the ancestors of the modern horse were more akin to rodents than the impressive beasts we know today. Eopis Hippus, (IIRC the latin form), evolved and grew as food sources and predators changed. Today, horse trainers play upon the two driving instincts of the horse that have given rise to its survival: safety in the herd (stable and paddock are substitutes) and flight from threat (hence the explosive speed). All this is to say that perhaps increased body mass was a result of survival of the fittest in the northern climes, of which you speak. Just wanted to make sure that you weren’t imaginging Clydesdales galloping across the tundra. From my understanding, it just didn’t happen that way.
To expand on Lemur’s reply, another problem leading to sterility is that the homologuous chromosomes from the different parents are dissimilar enough that they have trouble synapsing during meiosis I. But the “extra” c’some is a bigger problem.
I have a related question: I just recently learned about hennies; ie, the result of a male horse (stallion) and female donkey (jenny). My mom said she thought that mules are most often male while hennies are most often female, and that while mules are sterile, hennies aren’t. However, horse and donkey husbandry wasn’t anything she or her folks cared about (they were dirt farmers), so she didn’t know for sure. Do the Teeming Millions?
All the information I’ve seen on the hinny (the usual spelling I believe) said that they were sterile. (I’m not an expert on the subject however.)
This article, * Ponderings on Hinnies*, from a magazine on exotic livestock, says that in veterinary studies at Cambridge (England) and Cornell (USA) showed a 14% pregnancy rate resulting from cross-breeding attempts between a horse and a jennet (and of that 14%, only a low percentage, again around 14, resulted in live births.) The author draws the conclusion that a hinny is much more difficult to produce than a mule.
Another problem with hinneys. Horses and donkeys have different social systems. A mare is generally fairly tractable…when it’s time to mate, she’ll mate, and if a donkey is around, he’ll do. But female donkeys are much more ornery and apt to put up a fight. A stallion is likely to have his ego bruised when she tries to kick the crap out of him, whereas male donkeys expect it.
Re: Eohippus/Hyracotherium. Yes, the ancestors of horses were small, the size of cats. But that was waaaaay back at the dawn of the age of mammals. The ice ages are very recent, in fact they are still going on…we still have a whole continent and one very large island still covered permanantly with ice, and it’s not clear that the glaciers are going to continue to retreat. The geologic record shows that the glaciers raced back and forth over Canada and Northern Europe many times…there were times that were mostly glaciated but with a few warm spells, and times that were mostly warm with some glacial spells, all changing back and forth apparently chaotically.
Anyway, only 15,000 years ago my home would have been under an ice sheet a mile thick. Are we headed for a permanent warm spell, or will we have another glacial spell soon? Let me know when you’ve figured it out and I’ll arrange your Nobel prize…
Wait, where was I? Oh yes, Eohippus. Yes, Eohippus was small, but the horses that crossed the Bering land bridge were pretty much like modern horses in the broad sense…horses/zebras/donkeys. No, they weren’t Clydesdales, although there was one species of giant horse (I forget the genus…) that was bigger than our modern Equus caballo. Anyway, if you saw these ice age horses you’d think they were ponies, unless you thought they were donkeys or zebras…we do know that the ones in Europe weren’t heavily striped because of the cave paintings, but the others could have been as gaudy as modern zebras.
I thought all mules were female. Any specialists in the house?
“Mule” is also a general term for any sterile crossbreed, I hear.
BTW…Anybody ever heard of a “cabbit”? Supposedly half cat and half rabbit? Friends of mine swear to have seen them, but a biologist friend says it’s impossible.
(Oh, and someone tell Cecil, I’ve got a pet jackaloupe–I caught him in my cantalope patch.)
Chromosomes - donkeys have 62, while horses have 64. Mules have 63. Though generally infertile, there are apparently very rare instances of foals produced by female mules or hinnies.
Further search reveals that many sterile crossbreeds are possible between various animals in the horse family in spite of mismatches of various numbers of chromosomes. It is possible to produce a zebra hybrid with either a horse or a donkey (called a “zebrule”). The mismatch of chromosome numbers is quite large in this case:
Apparently then, from the answers received, mules can be either sex and do engage in sex with other mules and/or hinnies, or whatever. A corollary question is: do they enjoy it? (well, who knows, I guess.)
If not all horses mirgrated through the Bering land bridge and if the horses during the ice age were near the size they are now, how did all of those who stayed here become extinct? There were no humans around to exterminate them.
On the contrary, it was the appearance of humanity on the Americas that is generally credited (if credit is the verb we want) with the elimination of all large land mammals in the Americas. Notice the short list of large mammals posted by lemur686. Nearly all of them disappeared contemporaneously with the invasion of humans. Bison survived, as did moose, wapiti, several varieties of bear, and some others. However, the mammoths and mastadons were clearly hunted to extinction and it is probably the horses were, as well.
(E d’mann, every “cabbit” I have ever seen was a manx kitten. Manx cats have slightly longer back legs (or, at least, hips that are angled slightly higher) than other cats and, as kittens, they have a lolloping walk that is reminiscent of a rabbit. Cats and rabbits cannot breed.)
They went extinct for the same reasons that all the other pleistocene megafauna went extinct. Not only did we lose horses, we lost ground sloths, mammoths, etc, etc, etc. And humans moved into North America just as the ice age was ending. Not all species go extinct because of humans, though! Humans didn’t exterminate the dinosaurs, did they?
But it is an unsolved question. The trouble is that we had a major climate change…the ending of the ice age…at the same time humans moved in. Of course, there were lots of glacial retreats before, and the pleistocene animals didn’t go extinct then. Perhaps the combination of changing climate and the presence of humans was the last straw.
My understanding was that the large American land mammals died out at the end of the ice age, contemporaneous with the arrival of humans. In other words, they survived the ice age only to be wiped out by a new, dangerous predator. If it’s an issue, I can go try to find a citation.