View Full Version : Animal litters: Identical or Fraternal?

06-20-2000, 09:27 AM
Generally, in animals than normally give birth to litters, are the offspring "identical" (genetically the same, from the splitting of a single fertilized embryo), or "fraternal" (the result of several fertilized ova and sperm)?

Or is it often a mixture of the two?

Do different species have different types of litters, in the indentical vs fraternal sense?

What about dogs? I can't recall ever seeing a litter of dogs, but my feeling is that some are colored differently than others, and therefore are likely fraternal. This recollection could be totally from works of fiction, though.

light strand
06-20-2000, 09:43 AM
Litters of animals are fraternal. Your dog produces many eggs which are in turn fertilized by the neighbor's mangy mutt, producing puppies that can either look like your dog of the neighbor's. The same goes for cats, rats, rabbits and other "litter" producing critters.

There are however, identical twins in the animal world. Cows often give birth to identical twins. Identical twins are usually only found in animals that generally give birth to a single offspring. This is because only one egg is produced in a cycle.

06-20-2000, 09:56 AM
As a kid growing up in the country I've raised chickens, rabbits, beagle puppies and lots of kittens and I have no recollection of any incidence of identicals. However, I'm sure that it's possible.

There was an article with a picture in a local paper recently that featured a kitten born with 2 faces (would you call that conjoined or siamese twins?); so it does appear the the zygote can split in lower mammals. I don't think that this paper is on-line yet, but if I can find the article I'll post it.

06-20-2000, 10:10 AM
I remeber reading that Amadillos and humans are the only 2 mammals that give birth to identical twins.

06-20-2000, 10:24 AM
Armadillos always give birth to four identical young the only mammal known to do so.

All four young develop from the same egg and they even share the same placenta!
Source: http://msu.edu/user/nixonjos/armadillo/

The above page also mentions that Armadillos are able to delay the birth of their offspring up to nearly two years. They do this in times of stress and surprised researchers when their 'virgin' Armadillo gave birth long after it had been captured.

06-20-2000, 10:25 AM
There are some exceptions. Armadillos have the interesting feature of nearly always giving birth to identical quadruplets.

Obviously, as stated, a litter of dogs is mostly fraternal since mixed breed matings are usually greatly varied. I wouldn't be greatly surprised to learn that occasional fertilized ova splittings can occur, leading to pair of identical pups in the litter. Anyone know for sure?

The armadillo is the only one I know off the top of my head. Any other species in which the fertilized ova splits as a normal circumstance?

06-20-2000, 10:29 AM
While I think most mammals are capable of producing twins dogs, cats and the like are fraternal.

In fact, a litter of kittens might have several fathers. If several males succeed in mating with the female in the alotted time period each male may fertilize an egg.

Since fraternal twins are possible in humans is it conceivable that a human female can give birth to two children each with a different father (given that she had sex with two different men in short order)?

06-20-2000, 10:47 AM
In regard to the last question, Cecil answered it. You can have fraternal twins which have two different fathers. The word to describe this is "superfecundation."

There was a thread about this earlier under the name "Zoological nomenclature."

I know because I started it.

06-20-2000, 10:59 AM
The answer is in the number of eggs released. Typically, when a cat, dog, mouse, or other litter-producing animal ovulates, several eggs--not just one--is released. I suppose you could consider them fraternal in the sense that they were all born at the same time, but as mentioned, they may not even have the same father. (There have been rare cases of this happening in humans as well.)

Identical twins do occur in other species, however, as rare as it is. This is where you get a lot of the Ripley's freaks--the two-headed snake, the 5-legged, 2-tailed calf, the calf (this one is the freakiest of them all to me) with its partially absorbed twin sticking all four legs straight up into the air from the back of its surviving twin, etc. Like conjoined twins in humans, the pair's eggs did not completely separate, and the melted-together look results.

Have you seen a pic of that calf? I need to find a link to it and post it here. It is beyond freaky (and its birth killed its mother, not surprisingly).

06-20-2000, 11:11 AM
Have you seen a pic of that calf? I need to find a link to it and post it here.

Please don't. The two-faced cat was freaky enough for me :eek:. In case you hadn't heard, that cat died suddenly a couple of days ago. BTW, several years ago I saw what I'm sure was a two-headed snake in the wild. I think it was a common gardner snake, although my memeory wants me to believe it was a giant python :).

06-20-2000, 11:53 AM
Here's a pic of the kitten: http://dailyradar.com/peepshow/2000-06-12.html

Wood Thrush
06-21-2000, 04:45 PM
so it does appear the the zygote can split in lower mammals.

It has to split. Only defective and dead zygotes don't split.

Ike Witt
06-21-2000, 04:59 PM
Does anybody have a theory on how it is advantageous, in a evolutionary sense, for an animal to give birth to identical quadruplets?

06-22-2000, 12:23 AM
Twins in cows are not produced "often" and are rarely, if ever, identical.

A producing milk cow in a family-farm setting has a 10 month gestation and is bred every 12 to 20 months, depending on her production and other factors. My father milks about 50 head. I can remember maybe 8 sets of twins produced at the home place. I'll let someone else do the math to figure out how "often" that is.

As a side note, if the twins are of opposite sexes the bull calf will almost always be sterile.

06-22-2000, 11:18 AM
In regard to the last question, Cecil answered it. You can have fraternal twins which have two different fathers. The word to describe this is "superfecundation."

The word 'slut' comes to my mind. :D

light strand
06-22-2000, 04:14 PM
Perhaps "often" was a bit strong. But in some breeds of cattle it isn't that unusual, and they are sometimes identitical.

Here is an article about Texas Longhorn Cattle and this farm experienced 9 sets of twins in two weeks!

It's a really good article on this subject.

06-22-2000, 07:45 PM
Interesting read, bio-brat.

I've just emailed the Ag department of the University of Minnesota, and if I have time tomorrow I'm going to try to call our local extension office. Will also give dad a ring and see if he has any more info, since the guy is a) always right, and b) if he's wrong refer to "a". :)

9 sets of twins in 2 weeks is, of course, an anomaly, but now I'm curious what the frequency really is and if there's a difference in breeds. The article was about Longhorns and our farm is Holsteins, so maybe that's where the difference lies.

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