Complex sentence: For the rearing of young animals alone, the dams are responsible for all whales, some species of whales there are, however, so-called "aunts" who also occasionally suckle the pups.
Split into simpler sentences: For the rearing of young animals alone, the dams are responsible for all cetaceans. In some species of cetaceans, there are so-called "aunts" who also occasionally suckle the pups.