I saw Jeff's post of several days ago with the video clip of twin boas, and was reminded of the twins we had in one of our litters a couple years ago:
Then an interesting thought came to mind as I realized I had taken for granted the fact that these twins are not identical (I didn't notice in Jeff's video if those twins were identical or not). What I realized was that I was thinking of twins in human terms, where fraternal (non-identical) twins result when two ova are released by the ovaries, fertilized, and implant in the uterus (these are technically called dizygotic [two zygotes] twins). In boas and other animals that routinely produce multiple offspring per birth event, we don't think of the babies as fraternal twins, but they are.
So then, how does one get apparently fraternal twin boas from a single egg sac? The theories that come to mind are:
1. Identical, or monozygotic (one zygote), twins result when a single fertilized ovum splits into two genetically identical parts. This would seem to be a case where in boas twins could develop within a single sac (interestingly, in humans some identical twins develop in a single sac, sharing a placenta and amniotic membrane, while other identical twins develop with completely separate placentas and sacs). Since physical appearance is influenced by environmental factors and not just genetics, “identical” (monozygotic) twins can actually look very different – though it is difficult to conceive of environmental conditions that could be any less different than inside the same egg sac! Monozygotic twins are always the same sex – our twins pictured above are both male.
2. A third, hybrid type of twinning has been theorized called polar body twinning, and occurs when an unfertilized ovum splits into two parts, and each part is fertilized by a different sperm. The twins have identical genetic material from their mother, but different genetic material from the father (via the two fertilizing sperm). This would seem to be another case with potential for boa twins to develop within a single egg sac. Because in boas the sex of the offspring is determined by the egg rather than by the sperm as in humans, polar body twins in boas would always be the same sex (in humans they can be the same or opposite sex).
3. The membranes of two fertilized ova merge and the fraternal twins develop within the unified membrane with umbilicals joined to the unified egg mass. Seems unlikely, as ova are not designed to do this (in fact, they ARE specifically designed NOT to do this), but Mother Nature seems to provide odd exceptions to just about everything.
4. A single ovum with two nuclei is fertilized by two sperm. No idea as to the likelihood of such an ova occurring, but the fertilization by two sperm seems highly unlikely because normally after one sperm penetrates the ova, changes occur almost instantly that make the cell membrane impenetrable to other sperm.
Another commonality between Jeff’s twins and ours is size – the twins were much smaller than their littermates (ours caught up to their littermates in their first year). However, I don’t think this helps resolve any of the possibilities as it is conceivable for each of the theoretical cases above that the twins would have access to only about half the material resources that their other littermates had while in the egg sac.
Other ideas? Anybody really know anything about this? All of the above with regard to boas is really just thinking out loud….. With issues of such importance bouncing around in my head during the wee hours, it’s no wonder I haven’t had time to post more often – LOL!
Steve Reiners
