Sex of the foetus determines the time of weaning of the previous offspring of captive plains zebra (Equus burchelli)

We examined if captive plains zebra (Equus burchelli) mares change the timing of weaning according to the sex of the suckling foal and of the new impending foetus. We observed 19 captive plains zebra foals, in three different formations, at the Dvůr Králové Zoo. The earliest time for weaning of any foal was recorded at the age of only 243 days. The latest suckling that was ever recorded occurred 83 days before the dam's next delivery, when the foal was at the age of 355 days. We found that pregnant mares weaned their foals sooner than the non-pregnant mares. Survival analysis revealed that the sex of the foetus was the most influential factor in determining the weaning age of the current foal. Hence, if the foetus was a male, then weaning occurred sooner than if the dam was carrying a female foetus. This finding is the first of this kind among ungulates. The reproductive state of the mare and the sex of her foetus should be considered when interpreting weaning times of equids.