Why is it more wrong to kill intelligent animals?

This Guardian articleby John Sweeney argues that because the intelligence of elephants is similar to that of humans, it’s wrong to kill them.

I don’t understand this argument; if a creature’s right not to be killed is correlated to its intelligence, then humans with developmental disabilities have less right to live than those who don’t.

I suspect that Sweeney wouldn’t agree, because his argument seems to be less about logic or morality and more about what could be...