So a mule has more hardiness constitution and will to live even though they are losing teeth at 4 years old and need masses of feed to keep them alive. Bred off "hill" ewes that never seen a hill and marketed purely for the colour of their face because obviously the darker ones milk better. Are you having a laugh?I don't think there is much evidence to show that Mules are more prolific than either Swaledale or Blackface ewes, when both are kept under good nutritional regimes.
Just to put a few rough figures on expected scanning percentages of mature ewes all fed a similar adequate diet.
Swaledale or Blackface - 175%
BFL - 240%
Mule - 190%.
I would guess the increased Percentage in the Mule is due to hybrid vigour rather than heritabilty.
And another surprising result if you take imaginary figures for ''hardiness, constitution and will to live''.
Swaledale or Blackface - 100
BFL - 30
Mule - 120
Which shows that hardiness has an inverse heritabilty.