The argument in support of the claim that animals have direct moral status and therefore have condition as sentient beings is rather simple. It goes as follows:
If a being is sentient then it has direct moral status. (Most) animals are sentient. Therefore (most) animals have direct moral status.
“Sentience” refers to the capacity to experience episodes of positively or negatively valenced awareness. Examples of positively valenced episodes of awareness are pleasure, joy, elation, and contentment. Examples of negatively valenced episodes of awareness are pain, suffering, depression, and anxiety. Internet Encyclopedia of Philosophy http://www.iep.utm.edu/anim-eth/