Polar bears and grizzly bears occasionally interbreed in areas where their ranges overlap due to climate change, producing offspring that are fertile and can themselves reproduce. The hybrids have intermediate characteristics and have been documented in the wild. Both bear populations remain genetically distinct despite the possibility of interbreeding.
Lions and tigers, in the rare situations where they have been kept together in captivity, can produce offspring—ligers and tigons—that are almost invariably infertile. This case represents a different end of the interbreeding spectrum: species closely enough related to produce viable first-generation offspring but too diverged for those offspring to reproduce.
The Neanderthal-Denisovan case appears to fall closer to the wolf-dog or bear end of this spectrum than the lion-tiger end. The evidence for Neanderthal ancestry in Denisovan genomes, and for Denisovan ancestry in Neanderthal genomes, implies that the hybrid offspring of these encounters were at least sometimes fertile and contributed to later generations in at least one of the parent populations. This is not a guarantee of fertility for any specific individual, including Denisova 11 herself, but it is evidence that the groups were not so diverged that all hybrid offspring were sterile.
The modern-animal comparison should not be pushed too far. The social and ecological contexts of human populations are vastly more complex than those of wolf or bear populations, and the specific mechanisms by which archaic human groups came into contact, reproduced, and integrated or excluded hybrid individuals are not known from any preserved evidence. But the comparison is useful for situating the phenomenon itself—occasional interbreeding between closely related but genetically distinct groups—as something that is observable in the modern world rather than as an impossibility or an exceptional anomaly.