Your mare is due to give birth, and you anxiously count down the days, wondering if it will be a filly or a colt. You might, however, be able to guess what sex your foal will be—and we’re not talking about your typical fetal sexing techniques. Researchers in Brazil recently determined that a mare’s or stallion’s age can impact the sex of their offspring.

Guilherme Ribeiro Valle, a veterinary medical professor at PUC Minas Betim in Brazil, and colleagues conducted two independent trials examining two separate equine populations:

• Trial 1 included all male horses registered with the Brazilian Association of Mangalarga Marchador from 1990-2011 and an equal number of randomly selected mares from the same registry;
• Trial 2 included 253 Brazilian military police horses born from the matings of 119 mares with 16 different stallions from 1989 to 2010.

In both trials, researchers categorized data according to horses’ parental age and noted the age gap between mated mares and stallions.

“The first trial showed a theoretical tendency, because we used a 1:1 population and observed the distribution of males and females inside that ‘artificial’ population,” Valle said. “The second trial (employed) a real population, showing not a theoretical tendency, but a real tendency