The Broodmare Influence

of a viable foal. In case of pregnancy loss abortion or stillbirth the data were excluded.
The study showed that gestation length was not only affected by maternal lineage but also was significantly influenced by the individual mare her age the year of breeding month of breeding and the sex of the foal. The two influencers of the sex ratio of the foals were the mare age group and the mares maternal lineage.
The researchers found that the mean gestation length for mares was longer in their first pregnancy compared to subsequent pregnancies. First time broodmares were bred at three years of age. The same tendency longer gestation times was also seen in aging broodmares those over 12 years of age.
Why longer gestation shows up in younger and older broodmares is thought to be influenced by differences in endometrial and placental function. The endometrium is the innermost glandular membrane in the uterus an environment that is perfect for a fertilized egg to embed itself and begin development. Kuhl says that the endometrium and the placenta are essential for nourishing the fetus in the uterus. Research though has shown that in maiden mares the contact area of the placenta with the endometrium is smaller and less developed. Curiously female embryos appear to have the ability to cope and survive in this environment better than male embryos.
In the horse the uterine environment at conception and during early pregnancy has a pronounced effect on sex ratio of the offspring says Kuhl. Findings of the present study are thus in agreement with the hypothesis that the early female embryo has mechanisms that counteract its death under detrimental uterine conditions. In further support of this suggestion female pregnancies seem to have a better placental adaptation to adverse conditions than male ones. Age related alterations in endometrial quality and function and consequently fertility are wellknown in mares. Together with the findings of studies on malefemale differences using in vitrocultured embryos we can assume that female embryos are better at coping with suboptimal conditions as provided by the uterusendometrium of maiden or aged mares leading to the female biased sex ratio in maiden and aged mares.
Kuhl says that the bias toward having fillies in younger and aged mares as shown in their study dovetails with similar findings from different wild ass subspecies. However for mares between four and twelve years of age the foal sex ratio is balanced.
These results
are important for horse breeders Kuhl believes. They could possibly choose their mares depending on the desired sex of a foal.
What actually drives the coping mechanism of female embryos in the earliest days of development is still unclear.
We suspect that these effects are due to the differences in mitochondrial DNA says study director Christine Aurich. This specific DNA is inherited over the maternal line and influences cell metabolism and placenta function. 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. But it is also possible that embryo survival is influenced by differences in placental function.
Kuhl adds that horses are a polygynous species which means one male the harem stallion lives with a group of females. Past studies have suggested that where one sex has more variable reproductive success such as males in a polygynous species dams in good condition compared to those in poor condition would be advantaged by producing more of the stable sex. Kuhl says that the nutritional state of the mare seems to be communicated to the embryo via the uterine environment.
Studies on semiferal horses have shown a femalebiased sex ratio in years following breeding seasons with poor food availability or in mares with poorer body condition or losing weight around conception.
The reason for the differences in gestation lengths of male foals being longer than females is also still unclear.