Researchers in New Zealand are suggesting that a foal’s birth season may affect his development more than we think. Their study* finds that at puberty, spring-born Thoroughbred fillies and colts were older and heavier than their autumn-born counterparts. The age at onset of puberty in spring- and autumn-born foals was 291 to 408 days and 212 to 270 days respectively. The weight at puberty for spring-born foals was 666 to 902 pounds, compared to 611 to 758 pounds for the autumn-born babies. The researchers concluded that the spring-born foals reached their minimum threshold weight to support puberty during the winter months, but “remained reproductively inactive until after the stimulus of increasing day-length occurred.”

*Onset of puberty in pasture-raised Thoroughbreds born in Southern Hemisphere spring and autumn. Brown-Douglas CG, Firth EC, Parkinson TJ, Fennessy PF.