In The Call of the Wild, how does Buck initially respond to the kidnappers when they try to tie him up with a rope? What can we infer about Buck’s nature from his initial response?

Initially, Buck's response to the dognappers is one of quiet acceptance. But that's because he doesn't yet know he's being stolen. He instinctively trusts Manuel, and so has no reason to think that he's up to no good. So when Manuel places the rope around his neck, he accepts it with quiet dignity. As we can see from his behavior, Buck may be a big, strong dog, but he's also quite trusting.
However, when Manuel hands the rope to a stranger, Buck knows there's something up. Now he starts growling menacingly. When the stranger begins tightening the rope Buck springs right at the man, who flings Buck over his back, pulling the rope ever tighter until the poor dog eventually passes out. The next thing he knows, Buck is in a baggage car, jolting from side to side as the locomotive on which he's traveling makes its way up to the wilds of Alaska.