Biology question of the day

Here it is:

Can kittens be identical twins?

Now cat litters are usually bigger than two, but that doesn’t mean that identical twins or even triplets (which result from the splitting of a single fertilized egg) can’t occur. The only way to test this, of course, is not similarity in appearance–as with Jerry Coyne the Sixth and Not Jerry Coyne, who were littermates–but using genetics: their genomes should be virtually identical.

I’ve done a brief but inconclusive Google search (only about 3 minutes) and don’t have an answer. I throw this one out to the readers–and I’d like genetic evidence.

This of course goes for other animals; some of us already know, though, that nine-banded armadillos always produce identical quadruplets, but I haven’t investigated how often identical twins occur in various animal species. Readers can weigh in here, too.


  1. BobTerrace
    Posted June 12, 2017 at 5:35 pm | Permalink


  2. GBJames
    Posted June 12, 2017 at 5:36 pm | Permalink

    I have absolutely no expertise here. Therefore I pronounce the one true answer.

    Of course they can!

    • Posted June 12, 2017 at 6:03 pm | Permalink

      I pronounce you correct with the same lack of qualifications.

  3. Mark R.
    Posted June 12, 2017 at 5:43 pm | Permalink

    I would think it was possible…I did a search for dogs and found this.

    DNA tests are the only conclusive way to prove identical twin dogs, and no reports exist of dogs proven to be genetically identical. Conjoined animals are often identical twins. Reports of conjoined dogs would be consistent with the idea that they are identical twins because conjoined twins usually result from an incomplete split of a fertilized egg that would have produced identical twins. But although conjoined kittens are not that unusual, conjoined puppies are rarely if ever reported.

    • Posted June 12, 2017 at 5:55 pm | Permalink

      I am not talking about conjoined kittens, though, but it does constitute SOME evidence.

    • Posted June 12, 2017 at 6:38 pm | Permalink

      Are conjoined kittens also called Siamese twin kittens?

      • Posted June 12, 2017 at 6:42 pm | Permalink

        Only if they’re Siamese kittens, in which case the politically correct term is “Twin Siamese kittens.”

        • ladyatheist
          Posted June 12, 2017 at 7:21 pm | Permalink

          Found via a pubmed search, yes, and they can be conjoined as well:

        • Posted June 12, 2017 at 9:46 pm | Permalink

          But if I wanted to be a non-pc boor I’d say “Siamese Siamese twins”.

          • ThyroidPlanet
            Posted June 13, 2017 at 1:08 am | Permalink

            I N C E P T I O N

            … I always wanted to do that.

          • Simon Hayward
            Posted June 13, 2017 at 7:42 am | Permalink

            and if they were born in Thailand then 3x Siamese…

          • Gnu Atheist
            Posted June 13, 2017 at 11:01 am | Permalink

            Et cetera, et cetera…

            • Stephen Barnard
              Posted June 13, 2017 at 11:14 am | Permalink

              I get it. Obscure movie reference. Nice.

  4. Bobby Lieberman
    Posted June 12, 2017 at 5:44 pm | Permalink

    Interesting question.

    Several animals such as the Marmorkreb (German Crayfish) can reproduce via parthenogenesis producing offspring that (I think) are genetically identical… not a monozygotic twin, but genetically identical all the same 🙂

    • Bobby Lieberman
      Posted June 12, 2017 at 5:48 pm | Permalink

      *Identical to the parent

  5. Posted June 12, 2017 at 5:52 pm | Permalink

    We had a litter of kittens that I caught. And one pair of kittens shared the same sac. So we always called them identical twins. And they looked it. That’s not sufficient evidence?

    • Posted June 12, 2017 at 5:55 pm | Permalink

      I don’t think so. I want GENETICS!

      • Gregory Kusnick
        Posted June 13, 2017 at 12:22 am | Permalink

        If monozygotic twins are rare enough that the false positive rate of the test exceeds the actual frequency of true positives, then genetic testing may not be sufficient either.

        • johnw
          Posted June 13, 2017 at 10:18 am | Permalink

          With deep sequencing the false positive rate would essentially be zero. That cat genome is ~ 2.7 Gb, and theoretically every base pair would constitute an individual comparison. It’s possible with highly parallel deep sequencing in fact to tell monozygotic human twins apart genetically, due to the hundred or so de novo mutations that we all have. I expect cats are similar.

    • Simon Hayward
      Posted June 13, 2017 at 7:57 am | Permalink

      I was thinking about this in the middle of the night when the damned cat decided to wake me up. I have no data on cats. However I’ve looked at thousands of fetal rats and dozens to hundreds of mice. If you open up a gravid uterus in these rodents the pups are lined up along the horns like lima beans in a pod. Each pup is in its own amniotic sac with its own placenta. Monozygotic twins should share a placenta and be in the same sac. I think the anatomy should be sufficient – you don’t really need genes if you can open the uterus (to quote my postdoc advisor, to a student from a mol bio lab “I don’t need gene expression to tell you the thing on the end of your arm is a hand, anatomy will suffice”)

      I do not recall seeing two mice or two rats in the same sac. That being said I’ve not really been looking for that, but it would be very obvious. (Will put out the question at lab meeting this morning and report back)

      This certainly reflects individual species, as you note. Without the genetics it would be difficult to impossible to tell once animals are born, since genetically identical individuals could have very different markings when colors are due to melanocyte migration (random) or X-linked inactivation (ditto)

      • Simon Hayward
        Posted June 13, 2017 at 9:56 am | Permalink

        Following up (as threatened/promised), none of us has seen monozygotic twins in rats (could of course be a strain specific thing, and experience limited to SDs and a few F344s. One technician who used to work on a lot more mice says she has very occasionally seen monozygotic twins in mice (two pups in one sac). Not sure what strain this was.

        With inbred strains all individuals would be essentially identical anyhow, so the genetics would not help – although the anatomy does!

    • Posted June 13, 2017 at 11:49 am | Permalink

      A brief internet search reveals this: “While fraternal twins (2 eggs and 2 sperm) are always surrounded in their own sacs and have their own individual placentas, 70% of identical twins may end up sharing a single placenta.”

      So you did find identical twin kittens.

      Further searching reveals whether identical twins share the same amnion and chorion depends on the timing of the split between them.

    • Posted June 13, 2017 at 11:53 am | Permalink

      A brief internet search found this, about humans: “While fraternal twins (2 eggs and 2 sperm) are always surrounded in their own sacs and have their own individual placentas, 70% of identical twins may end up sharing a single placenta.” (Other sites had similar information.)

      Basic mammalian biology is pretty much the same, so I would say the kittens you found were identical twins.

      • Posted June 13, 2017 at 11:54 am | Permalink

        Sorry about the double post! PCC(E), feel free to remove one.

  6. ThyroidPlanet
    Posted June 12, 2017 at 6:00 pm | Permalink


  7. Posted June 12, 2017 at 6:10 pm | Permalink

    Well I dunno about domestic cats but I do know that the genetic bottleneck in Cheetahs was discovered when it was found that Cheetahs don’t reject skin grafts even from what were thought to be distantly related individuals. Not twins, of course, but really closely related.

    Dobrynin, Pavel , et al. Genomic Legacy of the African Cheetah, Acinonyx jubatus Genome Biology 16.1 (2015): 277

    • Posted June 13, 2017 at 1:15 pm | Permalink

      Knowing what happened to the genetically homogeneous Tasmanian devils (they are infecting each other with tumor cells by biting), I hope cheetahs abstain from biting!

  8. Bobby Lieberman
    Posted June 12, 2017 at 6:29 pm | Permalink

    Researchers in Canada have conducted an extensive genetic study on polar bears living in the Western Hudson Bay, producing a number of findings that contribute to our previously limited understanding of these elusive ice bears’ mating patterns. Among the most surprising discoveries was the detection of a pair of genetically identical twins – the first such case ever reported in any species of bear.

  9. Posted June 12, 2017 at 6:33 pm | Permalink

    My question – what’s the best way of finding out? Would one have to take genetic tests of hundreds or thousands of kitten litters in the hopes of finding for duplication, or is there some more practical short cut?

    (I know even less than most people here.)

    • Posted June 13, 2017 at 1:18 pm | Permalink

      The testing should be limited to kittens with identical coloration (except the spotting, which appears randomly). If a kitten is black and his litter-mate is grey, they are apparently no identical twins.

  10. Craw
    Posted June 12, 2017 at 6:33 pm | Permalink

    Seems so.

  11. Stephen Barnard
    Posted June 12, 2017 at 7:25 pm | Permalink

    Am I going to have to go back to the shelter to swab Not Jerry Coyne before someone adopts him?

    By the way, after they took Not Jerry Coyne away at the shelter to examine him, the technician (a lovely young woman) announced that he’s a boy; and furthermore (and this is interesting), that most orange cats are males. If that’s true, it shifts the Bayesian odds in favor of the proposition.

    • Stephen Barnard
      Posted June 12, 2017 at 7:37 pm | Permalink

      I have to correct myself. A little research on why orange cats are mostly male (which is true) isn’t relevant.

      Found this interesting, well written blurb on cat genetics:

      • Stephen Barnard
        Posted June 12, 2017 at 9:48 pm | Permalink

        Money quote: “I personally believe that orange tabbies are the most outgoing, lovable of all the cat colors.”

        I don’t have enough experience with cats to evaluate that claim. Opinions? He has a less charitable, but carefully hedged, opinion about calicos.

        The essential claim is that coat color correlates with behavior and personality. Is there any evidence for that in cats or other animals?

        • Posted June 13, 2017 at 7:23 am | Permalink

          True in our case; but with a miniscule sample size!

        • Posted June 13, 2017 at 1:26 pm | Permalink

          Maybe inbred cats tend to be more passive, and non-inbred ones to be more lively, which in an unfamiliar environment means “aggressive”. Calicos are heterozygous for at least one gene, so they cannot be 100% inbred.

      • Posted June 13, 2017 at 1:23 pm | Permalink

        Most of the genetics there is wrong, however.

  12. eric
    Posted June 12, 2017 at 8:32 pm | Permalink

    Here is an article that discusses monozygotic twinning in general. WHile it doesn’t mention cats, it does have some interesting tidbits:

    1. Twinning occurs in sheep, and at different rates in different breeds of sheep.
    2. In mammals, its associated with a higher likelihood of failure than other pregnancies. Probably one of the reasons it’s rare.
    3. Various groups have evidently tried to create viable twinned embryos in mice, sheep, cows, and rabbits, with “very limited success”.
    4. Fraternal twins are slightly more likely (51%) to be male. Identical twins are slightly more likely (51%) to be female…unless they are monoamnionic. In which case, they are 70% likely to be female! The authors aren’t sure why, though they cite some possible explanations.

    • eric
      Posted June 12, 2017 at 8:40 pm | Permalink

      As a side note, I also searched for monozygotic twinning in canines…and got a lot of hits on comparing teeth between twins. Evidently, identical twins can have different teeth, and there’s some studies as to why/how.

      • Simon Hayward
        Posted June 13, 2017 at 10:40 am | Permalink

        If they had the same teeth they would have to keep taking them out and passing them to the twin so they could both eat. A selfish twin might run off in possession of the teeth and allow its sibling to starve 🙂

        • Posted June 13, 2017 at 1:33 pm | Permalink

          Still better than the mythological Graeae sisters, who are 3 and share a single eye.

  13. Posted June 12, 2017 at 8:44 pm | Permalink

    Well, this is more fun than grading papers.
    Of course there is the unnatural case of CopyCat, the cloned kitteh. But you want the more natural kind I am sure.
    Looking around, I easily found a link for identical twin d*gs that is verified by genetic testing: .
    No twin cats are verified, except for CopyCat have I found. However, here is anecdotal evidence for a case: Here, the claim is that two kittens were born sharing an umbilical cord. This is evidence of identical twin cats from a developmental standpoint, since identical twins forming from splitting an early embryo often still share extraembryonic tissues like the cord or the placenta. Shared extraembryonic tissues is highly unlikely except for genetically identical twins. This claim of twin cats would need to be verified by a genetic test.
    Back to work. *Sigh*.

    • Richard
      Posted June 13, 2017 at 4:39 am | Permalink

      You should retire. It’s wonderful. 🙂

    • johnw
      Posted June 13, 2017 at 10:12 am | Permalink

      I expect the dog situation applies to cats too…. And in both species the twins would usually go unrecognized due to the in utero environment influence on coat coloration. The puppies looked very similar but had some obvious differences in their markings.

  14. Stephen Barnard
    Posted June 12, 2017 at 9:06 pm | Permalink

    I’d guess that Jerry Coyne the Sixth and Not Jerry Coyne aren’t identical twins, despite their morphological similarity, based on their markedly distinct behaviors and (subjectively) personalities.

    • Posted June 12, 2017 at 9:14 pm | Permalink

      That would not necessarily be true in humans.

      • Stephen Barnard
        Posted June 12, 2017 at 9:39 pm | Permalink

        And it’s probably not *necessarily* true in cats, either, but it’s indicative, unless you’re one of those people who think genes have no influence on behavior.

        • Posted June 12, 2017 at 9:52 pm | Permalink

          Its just that there are plenty of identical twins that are still different in various characters, including personality and physical characters. The genes are identical, but they can be slightly different in how they are expressed.

          • Stephen Barnard
            Posted June 12, 2017 at 10:00 pm | Permalink

            But (and this is a big but) identical twins (at least human one) are far more likely to be similar in psychological metrics than unrelated individuals, siblings, or even fraternal twins. That alone is enough to weight my guess as to the identical twinness (what’s the technical term?) of Jerry Coyne the Sixth and Not Jerry Coyne.

  15. W.Benson
    Posted June 12, 2017 at 10:09 pm | Permalink

    I tried (for 10 mins) but found nothing.

  16. Posted June 13, 2017 at 12:09 am | Permalink


  17. Posted June 13, 2017 at 12:55 am | Permalink

    Related to this subject is another I’ve sometimes wondered about and finally, thanks to this post, looked up the answer to. (Sorry about that preposition.) Turns out that superfecundation is quite common in cats and dogs.

  18. nicky
    Posted June 13, 2017 at 1:05 am | Permalink

    I did not find genetic studies in cats either. However, I would be very surprised if there would be a complete absence of monozygotic twins in cats. The ‘joined cats’ are a good indication.
    So Jerry, we’ll genotype Jerry the VIth and Not Jerry 😊.
    Might be a first! (and hence publishable ! 😆)

  19. Posted June 13, 2017 at 3:12 am | Permalink

    I found this but…

  20. TJR
    Posted June 13, 2017 at 5:45 am | Permalink

    We need an experiment. Find some possibly-twin cats, raise them all separately and then introduce them to their possible-twins in a few years time.

    If they are wearing the same clothes as each other and have similar-looking partners then they are identical twins.

  21. jrhs
    Posted June 13, 2017 at 8:20 am | Permalink

    I was led to the following sites after a Google search using Chinese.

  22. Posted June 13, 2017 at 12:15 pm | Permalink

    I realize I had assumed that all “looks alike” animals of the same age were in fact monozygotic multiples. Guess not.

    I have no information either way, but I am reminded of the gerbils we had when I was in elementary school. There were 3 in the pet store – two that looked alike (all brown and skinny) and one which was fatter and white-splotched. My sister decided we should have two that looked different so we got one of the two all-browns and the partial-white. We always assumed that the two all-browns were (identical) twins, but perhaps not. We also wondered for years what had happened to the third brother, but that’s another story.

  23. jimroberts
    Posted June 13, 2017 at 5:19 pm | Permalink

    In humans, the ratio of monozygotic to fraternal twins was known well before the possibility of genetic testing. Monozygotic twins are always same-sex, but we expect fraternal to be about 50% same-sex, like siblings of separate birth. For some proportion of monozygotic, somewhere between 100% and 50% which can be rather easily calculated. If we had sufficient data, a more complicated calculation would tell us the proportion of monozygotic cats born.

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