Would it even be responsible to save the Northern White Rhino right now? Maybe wait a few years, let the current ones die off, then breed several of them using Southern White Rhino surrogates could be one plan.
If they could use southern Rhinos as surrogates, doesn't that imply the southern and northern white rhinos are the same species? So we are not talking about the extinction of a species here, only one less habitat for a species? (Not that this somehow makes it acceptable of course.)
Surrogacy is simply using the womb. It is not interbreeding, so it does not necessarily imply they are the same species.
There are many definitions of species. If they are able to interbreed and create viable (i.e. non-sterile) offspring, they might be considered to be the same species. However, there are cases where offspring are viable, but less fit for a variety of reasons. In this case, many biologists would still consider them separate.
The definition of "species" varies based on the biological specialty you're talking about (it's the least arbitrary of the taxa, but it still is kind of arbitrary). Cladists/geneticists tend to have a view close to the one you mention ("interbreeding with viable non-sterile offspring defines a species"), but even they will say that definition gets too simplistic. Behavioral biologists generally have a more stringent definition of "species" that involves the population's ecological niche.
But, either way, surrogacy is not breeding; it would be a northern egg and sperm implanted in a southern rhino.
Not going to dig up a cite right now, but I'm pretty sure that females of one species have successfully gestated embryos of another species, if they are reasonably closely related.
The distinction being that in surrogacy, there is no requirement to mix DNA to make viable offspring. There are likely immunological compatibility concerns at the placental boundary.
I was looking up the differences in them a few minutes ago and there really isn't any, so much so that I'm not concerned with losing the Northern White Rhino sub-species. It's not a good thing, but we do have the required pieces to bring them back and if we don't then there is an identical animal which still has a decent sized population.
I think the tolerance between species is highly variable, but "Fetuses of the Giant Panda have been grown in the womb of a cat by intercurrently inserting panda and cat embryos into the cat womb." http://en.wikipedia.org/wiki/Interspecific_pregnancy
However, wikipedia says: "Following the phylogenetic species concept, recent research has suggested the northern white rhinoceros may be an altogether different species, rather than a subspecies of white rhinoceros, in which case the correct scientific name for the former is Ceratotherium cottoni. Distinct morphological and genetic differences suggest the two proposed species have been separated for at least a million years."
The Southern White Rhino has a population of about 20,000 (Endangered).
TIL about the Woolly Rhino: http://www.rhinos.org/rhinos/extinct-woolly-rhino
Would it even be responsible to save the Northern White Rhino right now? Maybe wait a few years, let the current ones die off, then breed several of them using Southern White Rhino surrogates could be one plan.