The stable-isotope analysis sounds fantastic. I'll look forward to reading more about a technique this detailed. But, the lines that really struck me from the article are these:
The cave lion diet, Bocherens says, appears to have been much more finicky than that of today’s lions, which eat just about anything they can catch.We often underestimate the behavioral plasticity of non-human animals. Yes, humans are particularly known for their ability to change their social organization or diet in order to fit their environment. But lots of other animals do this, too. Fallow deer can follow a herd- or harem-type social organization, depending on the population density in their region. A similar density-dependent effect on social organization has been identified in barn cats.
The results may provide new insights into why cave lions died out. When Europe’s climate began to warm about 19,000 years ago, the landscape gradually changed from chilly, open steppes to denser forests. That would have made an inhospitable habitat for reindeer and for the cave lions that depended on them for food.
Why should we assume, then, that cave lions had a biologically-determined more narrow diet than modern savanna lions (a narrow dietary adaptation that led to their extinction), when instead we could argue that cave lions had a narrower diet breadth for the same reason that Pleistocene humans appear to have had a narrower diet breadth than later human groups in the same region: the abundance of highly-ranked prey. There were lots of reindeer, and reindeer, due to their size, herd instincts, etc., were the top-ranked prey species for cave lions. Therefore, cave lions mostly ate reindeer.
We don't know if cave lions had the behavioral plasticity of modern savanna lions and could have diversified their diet, under favorable circumstances. When the reindeer went extinct, so did most of the other major prey species, and/or the species that remained, like elk or deer, were far less abundant and did not travel in large herds. Perhaps, if climate change had taken a different form (one in which some open habitat survived, and large groups of antelope had roamed the plains of Europe), cave lions could have also survived by diversifying their diet. The stable isotopes show that cave lions were reindeer "specialists", but that does not mean that they were biologically adapted to reindeer hunting, and therefore went extinct along with Dasher, Dancer, Prancer and Vixen.