Superfetation in Goats

Superfetation in Goats

Superfetation in goats is a rare but possible circumstance when a doe gives birth to kids with different gestational ages. The simple explanation is that the doe somehow cycled into her next heat a few weeks after being successfully bred and was then bred again with both pregnancies continuing. This is common in some species of freshwater fish and a few small mammals such as the European brown hare. It is hypothesized in other animals but not proven. How could this happen? Why doesn’t it happen more often? We will need to first explore the goat reproductive system.

When a goat (or most other mammals) ovulates, the release of the egg from the ovary makes a spot that produces progesterone. If the egg is fertilized and implants, this spot, known as the corpus luteum, continues to produce progesterone throughout the pregnancy which prevents further ovulation, among other things. Progesterone also acts to prevent any future sperm or bacteria from entering the uterus by forming a mucus plug right inside the cervix (opening to the uterus). The body is rather good at preventing the possibility of superfetation, or another pregnancy occurring after the first begins. (Spencer, 2013) (Maria Lenira Leite-Browning, 2009)

While not impossible, there are several factors that must come into play for superfetation to happen in a goat.

The corpus luteum does not prevent the doe’s ovaries from releasing multiple eggs at the same time or within a day or two of each other. This can cause another interesting phenomenon of the same litter of kids having multiple sires. The buck’s sperm has a lifespan of only 12 hours, so being bred by multiple bucks is quite possible. This is called superfecundation.

While not impossible, there are several factors that must come into play for superfetation to happen in a goat. First, the progesterone levels must not be able to prevent ovulation. Whether this happens because the levels are lower than in a normal pregnancy or because the ovary was able to develop and release another egg regardless of hormone levels, we may never know. Because goats form a mucus plug on the uterine side of the cervix, sperm from another mating would need to somehow bypass this plug. A poorly defined cervical seal is possible and may allow this. Last of all, the sperm would need to somehow traverse the pregnant uterus which will be larger than normal with obstacles (developing kids) to overcome.

There are many biological processes that occur to prevent the possibility of superfetation, but we all know that nature isn’t perfect. Animals that have a bicornuate uterus (having two “horns” rather than one large body) have a higher chance of experiencing superfetation especially if the first pregnancy only has young developing in one horn. This would allow the fertilized egg to have a space in which to implant that wasn’t already supporting growth.

Superfetation can only occur in goats (or other animals) that have a heat cycle shorter than the length of pregnancy. Seasonal breeders cycle every 18-21 days during the “heat” season. Because there are three weeks between ovulations, a second pregnancy in superfetation would be underdeveloped when the first is ready for birth. It is unlikely that the underdeveloped kid would be able to survive. However, there have been a few documented instances of an animal giving birth to fully developed young several weeks apart.

Of the animals that experience superfetation as a normal part of their breeding, it is not expressed in the same way as accidental superfetation. The American mink and European badger experience superfetation in which breeding happens before the birth of the first litter, but the embryo experiences “diapause”. Diapause is when the embryo stops developing for a time before resuming development. Sometime after birth, the new embryos resume development. The European brown hare has a similar system in which they enter estrus shortly before giving birth. The fertilized egg implants shortly after the birth of the current litter. These forms of superfetation may be more properly termed “superconception” and “superfertilization” because neither have two fetuses developing at the same time but weeks apart in developmental age. (Roellig, Menzies, Hildebrandt, & Goeritz, 2011)

Superfetation is an exciting explanation for size discrepancies in the birth of kids. However, other factors can cause kids to be significantly different in size and yet have the same conceptual age. Genetic defects can cause one kid to be unhealthy, thereby smaller in size. Often kids are just different sizes even in the same conception. Does may abort one or more fetuses but retain others, carrying them to term. Some does may also steal the kids of another who birthed unobserved and birth their own at a later date, causing confusion.

While superfetation in goats may be rarer than many believe, it is hardly impossible. There are not many ways to prove a case of superfetation which is why it has not been extensively studied. A pregnancy would need to be followed with ultrasound imaging from the beginning to confirm superfetation. However, I don’t believe there are any “superfetation police” out there making sure that every claim is verified.

Have you experienced superfetation in your herd?

References

Maria Lenira Leite-Browning. (2009, April). Biology of Reproduction of Goats. Retrieved from Alabama Cooperative Extension System: https://ssl.acesag.auburn.edu/pubs/docs/U/UNP-0107/UNP-0107-archive.pdf

Roellig, K., Menzies, B. R., Hildebrandt, T. B., & Goeritz, F. (2011). The concept of superfetation: a critical review on a ‘myth’ in mammalian reproduction. Biological Reviews, 77-95.

Spencer, T. E. (2013). Early pregnancy: Concepts, challenges, and potential solutions. Animal Frontiers, 48-55.

Originally appeared in March/April 2022 Goat Journal and is regularly vetted for accuracy.

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