This paper broadly summarizes our scientific knowledge about the sentience of animals, particularly from a "welfare-aligned" perspective.

The notion that some animals are sentient is becoming more accepted every day, and continues to spread internationally. But how much evidence is there to support this claim? And what does it actually mean to be “sentient”? This paper summarizes what we currently know about what is called “welfare-aligned sentience,” the capacity of certain animals to consciously experience positive and negative sensations and emotions that matter to them.

To begin with, the author defines welfare-aligned sentience through various criteria. For example, he notes that sentient animals can respond to a variety of sensory inputs with behaviors. For example, they might shy away from the heat after touching a hot surface or approach food that smells good to them. Differentiating among nervous systems, he notes that when an animal has a more sophisticated nervous system (like mammals, birds, and fish), they can have subjective experiences (also called ‘affects’), which include emotions. Being sentient means experiencing both positive and negative affects. Those affective experiences matter to the animal, either because they are important for survival or because they reflect how the animal feels.

From there, the author notes that if we want to consider an animal as sentient, they have to be conscious, i.e. aware of their feelings and sensations. We also have to take an animal’s neurobiological development into consideration. In the early stages of development, the nervous system might be too immature to support “consciousness.” An animal might only gain consciousness after they have reached a certain developmental milestone. Finally, the author notes that if an animal is sentient, they can communicate with others and interact with their environment.

Keeping these key features of welfare-aligned sentience in mind can help us understand which animals need our particular consideration.

Still, the author points out, humans are prone to anthropomorphize, which means that we like to project human-like traits onto non-human beings and things. In the past, animal sentience critics have flagged this: Do we only see sentience in animals because we want to see it? Current scientific knowledge largely stands against this argument. Regardless, the author argues, we still need to use caution when making sentience judgments.

While the sentience of vertebrates has been widely researched in the past, invertebrates have so far been neglected. But there is increasing evidence for the sentience of some invertebrates such as crayfish, slugs, sea crabs, bees, and flies. It will be interesting to follow the development of this research field.

All in all, there’s an increasing evidence base that supports animal sentience. Of course, we still need more research to give us even more insight into the topic, but in the meantime, animal advocates have a good base of knowledge and theory to work from.