January 2006

Mare May Influence Sex Of The Foal

Of course, it’s still X and Y chromosomes, but it may also come down to mom’s body condition.

Maybe both sire and dam play a part in determining a foal’s sex.

There’s an ecological theory, the Trivers-Willard hypothesis, that predicts (among other things) that mothers in poor body condition will produce more female than male offspring, and vice versa. The logic behind the theory is that since males require a higher plane of nutrition to develop into large, strong, competitive individuals, it would be advantageous for the species to favor production of male offspring when conditions are ideal.

How they accomplish this isn’t clear, but the levels of many hormones, particularly those produced by fat cells, are known to fluctuate depending on body condition and feeding. This could be the signal that determines whether uterine conditions are most favorable for males…