What does singer say about other philosophers' attempts to argue that only humans have moral worth?

The answer for this question is: That they come up with unjustified methods to include all humans while excluding all animals from moral consideration

Singer argued that if we never hear animals saying that they're experiencing emotional distress in a language that human cannot understand, doesn't mean that we have to ignore that emotion. He believed that in our everyday's life, human should reflect the emotion that felt by animals as if they're feeling what we're feeling.