Research suggests that older mares are more likely to have fillies

Researchers at the Pontifical Catholic University of Minas Geras in Brazil have found a potential link between a mares age and the gender of her offspring. Specifically that older mares are more likely to have fillies.

The study included looking at breeding records of nearly 60,000 horses born in Brazil from 1990 to 2011. These horses were all Mangalarga Marchador, a Brazilian riding horse. During this process they looked at; the offsprings gender, mares age, stallions age and the age gap between mare and stallion. They then did the same analysis on a smaller group of horses, 253 horses, of different breeds foaled between 1989 and 2010.

The results of this study should be taken with a pinch of salt, but it did discover some interesting trends. The most interesting being that the mares age has a greater influence on the offsprings gender than the stallions.

Mares aged 5-10 years old when they have a foal are more likely to have a colt. 1.17 Colts born for every Filly. This is possibly linked to the idea that Male sperm is 51% likely to fertilise an egg vs 49% for female sperm. For mares aged 10-15 years old, mares have 0.88 Colts for every Filly and mares aged 15-20 have 0.74 Colts for every Filly. So the older the mare is, the more likely she is to have a filly.

Stallions aged 5-10 years old produce pretty much the same number of Colts and Fillies. However, Stallions age 10-20 are slightly more likely to have a filly, before leveling out again over 20 years to be fairly equal.

When they looked at the age gaps between the mare and stallion, when they are a similar age (within 5 years of each other) they produce roughly the same amounts of colts and fillies. But when we start to look at gaps greater than 10-15 years, we see trends similar to the mare trends. A younger mare with an older stallion produces 1.53 colts for every filly. But an older mare with a younger stallion produces 1.29 fillies for every colt.

Why does this happen?

We have to take these results with a pinch of salt because we don’t understand why this happens. Mares only have female eggs, so it isn’t like they might be more likely to ovulate different gendered eggs at different stages of their life. Whereas stallions produce both male and female sperm, so it would make more sense if they had a bigger impact on the offsprings gender.

I would be really interested to see if there is more research into this area in future, especially if we can dive more into the why this is happening. But it is interesting reading for anyone who is waiting for their mare to foal.

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Last Updated on 17/05/2022