Mylute wrote: ↑Sun Sep 29, 2019 9:48 pm
A lot of you are probably already aware of this but for those who aren't I learned today that thoroughbreds seem to conceive twins more often than other breeds. Around 20% of ovulations in TB mares result in twins. There's no explanation as of yet.
Also, mares who conceive twins are more likely to be repeat offenders. So Ms. Lyon, keep an eye on Songbird!
I'd love to know if horses end up with the same "about 4x" increase in chance that humans do of having another set of twins.
And OMG, if the Jockey Club rules allowed putting an embryo back into the original dam,, I could so see a secondary market for the extra embryo in other breeds where donor eggs are allowed. Heck, we'd probably have that secondary market for extra embryos today if they could reliably flush out just one embryo from a thoroughbred mare and not risk losing the other.
Urban Sea and Secretariat carried the "twin gene"!
JUST KIDDING ... but given how DNA research is finding all sorts of gene-based regulatory mechanisms for proteins and hormones, and multiple egg production is often a byproduct of specific hormonal surges, there's almost certainly a genetic factor or interplay of multiple genes that leads to increased chances of multiple eggs being produced. If nothing else, the fact that certain breeds of sheep and goats are known to produce primarily twins (or in some other cases, primarily singles), points to a strong genetic influence.
My explanation for the increase in thoroughbreds is that the genetic factor associated with increased chances of multiple ovulations has drastically increased its frequency because it's no longer "naturally" restricted. It historically restricted its own spread by killing many of the twin pregnancies that get produced, and injuring or killing the mares who had a twin pregnancy.
Thanks to ultrasounds and research into timing and new techniques, twin conceptions can be detected early, pinched, and you end up with a healthy single foal and healthy dam, both passing on the genes that lead to the multiple ovulation in the first place. Those foals aren't dying any more, and their dams aren't being pensioned early because of the higher risk of another twin pregnancy. Even though it's just one foal coming out of the twin pregnancy, that foal carries the genetic legacy for higher rates of twinning.
There are other factors with the multiple ovulations as well, and they're seen in other mammals, too. Age is one, human women over 35 have much higher chance of multiple ovulations and with horses IIRC it starts in their mid to late teens. That's partially mitigated by how an overall lower conception and pregnancy success, which keeps the actual rate of twin conceptions & twin preganncies lower. We are managing the reproduction of our thoroughbreds well into their late teens and twenties, keeping them healthy and doing what can be done to increase successful pregnancies in spite of being the horse equivalent of middle-aged, so those mares are being bred during periods of their lives when they're more likely to produce the extra eggs.
Also, one area that's been researched in humans with some conflicting results is increased twin conception after having stopped hormonal birth control methods. The thinking there is that the body responds to the removal of hormonal suppression with a hormonal spike, triggering extra eggs. Given how often fillies and mares are on treatments to suppress or manage/regulate their heat cycles, whether it be for racing or to time things for breeding, it's not an unreasonable theory - just needs some research to see if it it's so.
Take all the above and then factor in how frequently thoroughbreds are linebred, the shrinking of the gene pool, the impact one prominent sire (and by extension, his dam) can have down the road -
Trying to remember figures from when I researched this a whiles back, IIRC the more managed a breed's reproduction and subsequent removal of risk/downside from a twin conception, the higher the incidence of multiple egg ovulations. The native UK pony breeds have something like less than 1% naturally occurring multiple-egg ovulations. Warmbloods have percentages that tightly track the frequency of thoroughbreds being used for outcrossing.
I'd love to get some long-term historical data and do a rigorous analysis of it. We'd need at the minimum decades of veterinary records for establishing frequencies before the advent of technologies that let us detect and manage twin conceptions.
)
