The Stoics cannot deny that animals perceive things in relation to each other things (e.g. the scent coming from that direction). The dog must minimally connect the scent a direct if we are to explain the animal following an animal's trail. And it is also true that if the animal is to act on a stimulus it must connect the stimulus with a desire or experience. The dog must connect the scent with its desire or hunger. However, I do not agree that the animal must believe that the scent is coming from a given direction or even believe that it is hungry (though that is how we would interpret and articulate the grumbling and sensation from our stomach if we dogs). But by Sorabji’s account the connection implies predication which in turn implies, syntactically, a proposition, hence the "that-clause" and his view that animals must have propositional attitudes. Sorabji fails to recognize that connectivity need not imply predication.
Rubarth, Scott, "Animal Perception in Early Stoicism: A Response to Richard Sorabji" (1999). The Society for Ancient Greek Philosophy Newsletter. 456.