Horses: Maternal Ancestry Influence Foals' Gender

In a recent study, researchers investigated mares' maternal lineage and how it influences the gestation length and the gender of a foal, according to news release.

Knowing the parentage of their horses is a must for owners of sport and leisure horses. Horses with a good pedigree (ancestry) often have the ideal characteristics in terms of speed, physique and health.

Researchers Juliane Kuhl and Christine Aurich from the Graf Lehndorff Institute for Equine Science, a joint research institution of the Vetmeduni Vienna and the Brandenburg State Study in Germany, analyzed the data records for 640 pregnancies in 142 mares to examine the maternal lineage's impact on breeding.

The researchers found that the average length gestation (development/formation of the offspring), which in horses ranges between 320 and 360 days, varies from family to family. The gestation length of some maternal lineages was 10 days longer than in other families. The gestation length for male foals was longer than for female foals, and this depends on the variation from family to family, according to the researchers.

"We can still not predict the exact time of birth. The individual fluctuations among individual pregnancies are simply too large. But the information gained from the study can help us to narrow the possible range," Kuhl said.

"The length of gestation is also of interest for horse breeders. Ideally, a broodmare should give birth to a foal every year. Due to the average gestation which covers approximately 11 months, longer gestation lengths result in a delay in birth of the next foal. Breeders are interested in having foals born at the beginning of the year, as the horses will then compete better against animals born in the same year," Kuhl explained.

In the study, the researchers found that certain maternal lineages produce more female foals than male foals. The age of the mare also influenced this process. Young mares that had their first pregnancy when they were three years old produced mostly female foals - this also common for older mares. Middle-aged mares between the ages of four and twelve had a balance with number of female and male foals they gave birth to, according to the researchers.

"These results are important for horse breeders. They could possibly choose their mares depending on the desired sex of a foal," Kuhl said.

"We suspect that these effects are due to the differences in mitochondrial DNA. This specific DNA is inherited over the maternal line and influences cell metabolism and placenta function," said Christine Aurich, study director. "We also know that female embryos are more resilient. As 20 to 30 percent of early pregnancies are lost spontaneously, it is possible that male embryos survive less frequently. This could be a reason for the observed shift in the sex ratio." 

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