Apparently wolly rhinos are on the brink of extinction and there are only two left in America, a brother and sister, that what, zoologist are trying to get to mate. Isn't there some instinctual taboo the siblings will detect preventing them from mating? Surely this can be argued on Darwinian grounds right? I just cannot see how selective pressures layered down in their genotypes would allow them to mate easily, or "naturally" I mean. Or is it common for (mammalian) siblings to mate in Nature?