An article in Equus magazine on equine gestation length caught my eye. The headline that gender affected gestation length then made me curious. The research was performed on draft horse mares, and since Fell Ponies are sometimes grouped with drafts as cold-bloods, I wondered if the gestation length in my herd had similar gender bias.
The researchers at Obihiro University in Japan studied 209 gestations of 65 mares (a little over three pregnancies per mare on average). They found that average length of gestation was 334.9 days. They then looked at various factors to see if they correlated with gestation length. Gender of the foal was one factor that correlated, with colts being carried an average of 337.1 days and fillies averaging 332.7 days. The mare’s previous pregnancy length also correlated. “Factors that had no bearing on the gestation length included age of the mare, foaling month, number of previous pregnancies, barometric pressure, and hours of sunshine in the days leading up to foaling.” (1)
I of course have a much smaller sample size, just 18 foals where I know breeding date and foaling date. My data is also affected by the fact that I usually hand or pasture breed for several days, so in many cases there is the possibility of a 3-4 day imprecision in my data. In addition, half of the mares in my study only foaled twice for me; the others foaled 3 or more times. Generally, though, I found that my Fell Ponies did not track the findings of the Japanese research.
In my herd, the average gestation length has been 339 days, two days longer than the Japanese mares. In addition, there is no difference between the gestation length of fillies and colts. I have also not seen that particular mares follow a particular pattern in their gestation length. Instead, what is most striking is the month that the mares are bred, again unlike the Japanese findings. Mares bred in May or earlier average ten days longer gestation than those bred in June through September. Given our fickle spring weather, when it can seem more like winter than summer in April and May, the longer gestation may be due to the foals needing to be more mature when they hit the ground. Or it could be that the mares are spending more energy keeping warm during the last trimester and therefore less on growing their foals. My first foals also average a seven day shorter gestation than subsequent foals.
I’ve looked at other research on gestation length and compared it to my Fell Pony experience. If you’re interested, click here.
- Barakat, Christine and Mick McCluskey. “How gender may affect gestation,” Equus, volume 432, September 2013, p. 12
© Jenifer Morrissey, 2013