One way to bypass the whole flawed concept of an intelligence scale is to recognise that we scale intelligence according to what traits are valuable to us. If we can recognise that, then we can go back and look at the original question and see that that the question is: "Why is human life worth more than animal life to us?"
We can then ask, "Why is a wolf cub's life worth more to the she wolf than a baby lamb?"
Indeed, the answer is obvious.