Wednesday, February 28, 2007

Why Are Twins Rare in Arabian Horses?

Whether a mare is an Arabian horse or another breed, the odds of carrying twin horses to term are rare. I wrote about the statistics related to the occurrence of twin foals in horses in my post Some Basics About Arabian Twin Foals. This post also covered the usual outcome of a mare carrying Arabian twin foals but didn't address the physiology responsible for it, the WHY of Mother Nature and horse twins.

Many species of mammals are equipped for multiple pregnancies, twins or more. If you think about it, those species have offspring which are very immature at birth. They usually are blind and unable to defend themselves for days, even weeks. They are usually smaller animals with denning behavior that hide their young away from predators pretty easily. Even if predators do find their den, the gestational length and the time it takes for these species to mature is short enough that the population is constantly replenishing.These offspring continue to mature in their nest or den after they are born until they are able to protect themselves.

On the other hand the Arabian horse, like other horses, is a large animal, even at birth weighing in at approximately 100 pounds. Size makes it difficult to hide a newborn foal for any length of time from predators. The length of gestation in the horse is about 345 days. Horses don't usually mature to breeding age until about two. Given all of these thing, the horse foal needs to be born more mature than the offspring of multiple pregnancy species. The horse foal must have the capabilities to defend itself and keep up with the other horses for the survival of the species. Horse foals are born with some semblance of sight(shadows mostly so they can dtect movement), great hearing, and swift flight reflexes. Horse foals have the ability to stand and defend themselves by fleeing, biting, kicking, rearing or striking in just a matter of an hour or two, sometimes a little longer. Without these capabilities newborn horses would most surely be lost to predators. Without surviving newborn horses, the species of horses as a whole would be lost.

To accomplish this miraculous feat, Mother Nature designed the reproductive system of the (Arabian) horse and other horses. This system carries the equine fetus a much longer gestational period. It also gestates the Arabian foal (or other horse foals) to a much greater degree of maturity than the offspring of multiple birth species. The equine uterus provides the large blood supply needed to accommodate this amazing growth through the placenta. Fetal development is dependent on this long term rich nourishment for proper gestation. What that translates to in layman's terms is simple. In the horse if the blood supply to the developing foal is compromised, then the development of the foal compromised.

In the normal horse pregnancy, the placenta of the foal is attached to the entire uterus of the horse. The blood supply is established through a network of tiny blood vessels between the uterus and placenta as well as major veins and arteries. In the event that a (Arabian) mare settles twin foals, normally each foal will have it's own placenta sharing the one uterus. Some horse twin pregnancies will have a 50% - 50% split of the uterus, some a 60% - 40% and still others a 70% - 30% split of the uterus. What this means is that the resulting fetus of the horse only has a proportionate percentage of placenta size which diminishes the blood supply proportionally as well. Without 100% of the blood supply, the development of each of the horse twins is severely compromised. Obviously the (Arabian) twin horse with the 70% space in the uterus has 70% of a normal placenta with access to only 70% of the normal blood flow. This twin horse has the greatest odds for survival. Its counterpart twin horse with 30% of a normal plancent with access to only 30% of the normal blood flow has the lowest odds for survival.

This explains why full term twin Arabian horses and other horses are born with the same issues as premature or dysmature foals. A twin horse pregnancy may go full term according to the calendar. From a gestational point of view the resulting twin boals will be not be mature like normal foals. Because of improper nourishment issues related to the lack of blood flow, twin foals at full term are still under developed. These twin foals will manifest the same symptoms as a single pregnancy premature foal. Twin foals that are born as the result of a premature delivery will have even greater odds against the horses' survival.

Taking all of these things into account, the odds are definitely stacked against twin horses surviving at all. Between being affected by the varying degrees of blood flow issues and the length of gestation, there is no best case scenario for the survival of both twin horses. Even in the case of the twin foal fortunate enough to have received 70% of the room with 70% normal blood flow, that horse is at a great disadvantage. It's truly a miracle that even a single twin foal sometimes survives a twin pregnancy in horses. Let alone the unique miracle that happens when both twin horses survive this fluke of Mother Nature.

Don't forget today is Blog Carnival day. There are lots of great posts. Experiements in Training Equines

1 comment:

  1. Hi MiKael

    Just popped in for my daily bit of education. Always so interesting, thanks so much.

    Hope you are well and the horses arent bouncing off the walls. Longing for spring!