Excellent post Lore.
I do find many of the WTS "teachings" to be erroneous and make no sense from a biological standpoint. I mean they have some real gall to make such assertions about things when there is no evidence supporting these claims, except evidence to the contrary.
"The cow and the bear themselves will feed; together their young ones will lie down. And even the lion will eat straw just like the bull. And the sucking child will certainly play upon the hole of the cobra . . . They will not do any harm or cause any ruin in all my holy mountain."-Isa. 11:7-9.
The quote from Isaiah seems to be a spiritual fulfillment. This is entirely my own opinion and I have no sources to back it up. I just want to be clear on this.
Since the text mentions YHWH's "holy mountain" and Mount Zion is often illustrated as being in heaven (not the literal Mount Zion in Jerusalem, which was little more than a hill) it would seem to me that this is a spiritual type parable. People who were once "bullish" in nature, ones who have calm peronalities (like a cow), ravenous "Lion"-like people etc would be have their innate personal traits transformed by God's Holy Spirit. Or to put it another way, they would make themselves over in the new personality and likeness of Christ.
So to that effect these once dangerous and "beastly" persons are now harmless. A child could play upon the home of a once dangerous viper (who has been made over) and have no fear of harm.
This text seems to portray the personalities of those resurrected during the millenial reign and the peaceful way in which they would conduct themselves. So it's literally as if they (the animal like people) had changed their diets.
People are often compared to animals in the Bible, The Lion of Judah for example. So it seems to me that this is what Isaiah was alluding to and not a very complex biological change to all animal species.
Peace and Love,