The panthers are not explicitly humanized; this is a central theme in the story, as the panthers are depicted as following their instincts, not acting "cruelly" or with "rage." However, the narrative does include some humanized language when discussing the panthers, as the reader is meant to sympathize with them rather than dismiss them as savage wild animals.
Theirs was no hideous or unnatural rage, as it is the custom to describe it... On their success in accomplishing that for which nature had so exquisitely designed them depended not only their own, but the lives of their blind and helpless young, now whimpering in the cave on the slope of the moon-lit ravine.
(Roberts, "Do Seek Their Meat From God," readbooksonline.net)
While the human child's life is more important than that of the panthers, the reader still feels empathy for the as-yet innocent panther cubs that are waiting for their parents. It is this connection, made as well with the settler and the crying child, the reader sees how parents care for their children, even when that care is entirely instinctual. The panthers are therefore not exactly humanized as much as they are made sympathetic; however, most readers will sympathize with the panthers by humanizing them, and so the empathetic language does, in a roundabout way, serve to humanize the panthers.