09/27/2026
🐴 In 1938 two Cambridge scientists put a Shire stallion to Shetland mares, and a Shetland stallion to Shire mares, and settled an argument that had run for centuries.
A. Walton and John Hammond used artificial insemination to make the crosses in both directions, and published what happened in Proceedings of the Royal Society B in 1938. Genetically, both sets of foals were exactly the same cross — the same two breeds, the same mix, in the same proportions. The only difference was which mare carried which foal.
The foals carried by the Shire mares were born about three times the size of the foals carried by the Shetland mares.
And it was not a temporary head start that evened out with time. The gap was still there when those foals were three years old.
It remains the classic demonstration that the dam's uterus — not the genes alone — decides how big a foal arrives in the world. A mare doesn't simply deliver what she was given. She sets the size of the room her foal grows in, and that room shapes the horse for years afterwards.
Every breeder who has watched a small mare safely carry a big stallion's foal has that 1938 experiment standing behind them, whether they know it or not.
Does that change how you'd think about choosing a mare?
