The she-bear who nurses little baby Atalanta in the wilderness isn't just any old bear. We're thinking she's got a whole bunch of symbolism hiding under that furry hide. For one, bears are a symbol of the warrior in many cultures. So, it makes sense that Atalanta, who would grow up to be a girl warrior, would be nursed by a female of the species. (Hm, does bear milk grow tough babies? Maybe we should market it.)
The she-bear is also a sacred animal of Atalanta's fave deity, Artemis, goddess of the wilderness and the hunt. In some cults of Artemis, young girls used to actually dress up as bears as a part of certain rituals. (Ca-yoot!) Though the story doesn't usually say specifically that Artemis tells the bear to go save infant Atalanta, ancient Greeks might possibly have inferred this because of the animal's association with the goddess of the hunt.
The story of a future hero getting reared in the woods by a benevolent beast would be all too familiar to the Romans, who believed that the legendary founders of Rome the martian twin (that is, the children of Mars) Romulus and Remus, were supposedly raised by a she-wolf.