A European study of warmblood horses showed that maiden and aged mares were more likely to give birth to female offspring.
“Breeding maiden and aged mares should not be done with the aim to produce male offspring because the sex ratio might be female-biased in these mares,” the Austrian and German researchers reported in the open-access journal, PLOS ONE.
They also found that the maternal lineage significantly influenced the length of pregnancy and the sex ratio among foals.
“To the best of our knowledge, this has neither been proven for the horse nor for other mammalian species so far,” Juliane Kuhn and her colleagues reported.
“The mare explains 13 to 18% of this variation whereas the sire contributes only 2 to 3%,” the research team reported.
They noted that the length of pregnancy was highly variable in mares. “Selection against maternal lineages with prolonged gestation length could be an interesting approach to avoid the gradual postponement of the birth date of offspring in mares with every year.”
The researchers said they set out in their study to investigate if pregnancy length and fetal sex ratio were influenced by the maternal lineage. Both, they noted, showed high variability among individual mares.
They used data from 142 warmblood mares from the Brandenburg State Stud at Neustadt, Germany.
The stud has been continuously recording lineages of mares since 1946, with some able to be traced back to the founding of the stud in 1788.
In all, 119 mares were classified in six different maternal lineages with records going back 10 or more generations. Another 23 mares belonged to smaller maternal lineages.
Records showed the average number of live foals produced per mare was 4.6. The live foal rate was 83.5% and the average length of pregnancy was 338.5 days, plus or minus 8.9 days. The pregnancies ran from 313 to 370 days, which represented a range of nearly two months.
They found that gestation length was affected not only by maternal lineage, but also by the individual mare, the age of the mare, the year of breeding, the month of breeding, and the sex of the foal.
“Of the 640 foals born alive at term, 48% were male and 52% female. Mare age group and maternal lineage significantly influenced the sex ratio of the foals, too,” they said.
Their findings confirmed previously reported research showing that the month of breeding and the sex of the foal affected pregnancy length.
“Although the relatively short breeding period used on the stud, with the majority of mares bred within an interval of three months, reduced the occurrence of seasonal effects, gestation length was shorter when mares were bred very early or late in the year.”
The average duration of pregnancies that produced colts was found to be longer than those for fillies.
Kuhl J, Stock KF, Wulf M, Aurich C (2015) Maternal Lineage of Warmblood Mares Contributes to Variation of Gestation Length and Bias of Foal Sex Ratio. PLoS ONE 10(10): e0139358. doi:10.1371/journal.pone.0139358
The full study can be read here.
The original research was published under a creative commons license.