Don't pretend to have the answer Grant, but I have done a bit research on heart score in Standardbreds over the years and there is an indication that standardbred mares are generally 2 to 4 points lower than their male counterparts, obviously there are exceptions to the rule and heartscore is not the be all and end all measure of whether a horse is up to it or not, but there is sufficient information out there to give this some credibility in my opinion.
I have no idea how thoroughbreds go in the HS department and if there is any gender factor at work.
Gait is a consideration also, for instance I believe we have seen more trotting mares competitive at the top level than pacing mares.
Another thing that has just entered my thoughts is the tying up problem that we see in many mares but fewer males, I've had to deal with many a mare tying up over the years, but have never really done much work on why it is the mares more so than colts and geldings?? If a larger percentage of mares never reach their full potential due to this problem then what impact does that have?
Temperament is a consideration but I would not think our standardbred mares are any more whimsical than their thoroughbred counterparts.
It's a great question and I look forward to hearing other people's views.
cheers,
Dan