They forget assumed species differences, that is.
In “Researchers Solve Mammoth Evolutionary Puzzle: The Woollies Weren’t Picky, Happy to Interbreed” ScienceDaily (May 31, 2011), we learn that DNA studies suggest that the woolly mammoth interbred with a “completely different and much larger” species 12,000 years ago (it went extinct 10,000 years ago). They suggest,
“We are talking about two very physically different ‘species’ here. When glacial times got nasty, it was likely that woollies moved to more pleasant conditions of the south, where they came into contact with the Columbians at some point in their evolutionary history,” he says. “You have roughly 1-million years of separation between the two, with the Columbian mammoth likely derived from an early migration into North American approximately 1.5-million years ago, and their woolly counterparts emigrating to North America some 400,000 years ago.”
Modern African elephants behave this way when species overlap, with the larger species getting most of the mates, they note.
The discovery raises the question how separate elephant species have ever been; notice that the researcher puts the word species in quotation marks.