It occurs to me that in humans, we call the wrong person "the surrogate".
Countless examples in the animal kingdom of other species 'adopting' the newborn offspring born of another animal.
And in every case, we describe the adopting mother as the surrogate. She is the one 'standing in' for the missing actual mother. She becomes the surrogate, the stand in, when the real mother is dead, injured or missing.
Noone questions this. It's apparent to us as observers that the adoptive mother has taken on the role that naturally belonged to another, the mother that gestated and gave birth. They are the acting mother, they are the surrogate.
Isn't it telling, that in humans, we've reversed the naming rule? We call the actual mother 'the surrogate', 'the carrier', 'the birth mother' and confer the title of 'real mother' to someone else - or even erase any mother from the picture entirely.
In humans, we go to some length to vanish, diminish, and repackage the real mother as a minor walk-on, walk-off part, to comfort ourselves that there was some special role-reversal arrangement of nature in our species. It helps make us able to reassure ourselves that what would be unconscionable to do to a cat, take its kittens at birth, is acceptable in humans, because a human mother is deemed less important than an animal.