Amidst the ongoing debate over whether artificial intelligence is really sentient, scientists are still trying to figure out the same thing about non-human animals. Amazingly, naturally-produced painkillers have long been considered a key indicator of possible sentience in animals. America can ban all the opioids it wants, but still won’t stop your brain from making them or seeking the relief from suffering they provide, and so they have been seen as a kind of smoking gun for sentience in non-human animals as well. But a look at the evolutionary role of endorphins really highlights the complexity of conscious experience – especially our own.

The field of animal consciousness studies has grown in leaps and bounds over the past couple of decades. The clearest evidence comes from studies in neuroscience on the neurophysiological basis of behavioral responses to animals’ subjective experiences — such as a dog seeking to relieve its thirst by drinking — definitively overturns the legacy of mid-20th Century behaviorism, in which animals were considered to have a set of automatic behaviors that nevertheless didn’t reflect a sentient mind.

“We are constantly underestimating animals,” David Mellor, a retired professor of applied physiology and bioethics, told Salon from his home in New Zealand. “Every time, we are surprised at how smart animals are. It’s because we have underestimated them, not because they’ve suddenly become smart.”

Indeed, it’s human understanding that seems to have deepened.

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“It’s really only in the last fifteen years, I would say, that there’s been an attempt to integrate consciousness science with comparative psychology or comparative cognition, the study of animal minds, so that now we’re seeing the emergence of a science of animal consciousness,” Jonathan Birch, a philosopher of biological sciences at the London School of Economics, told Salon. Birch was just back in London after co-hosting a conference in April on animal consciousness in New York City that culminated in the release of a pithy but important new declaration on animal sentience by dozens of luminaries in the field.

Over those last fifteen years, the markers we look for as hints of sentience – that within their furry (or scaly, or feathery) little heads there is a quality of “what it’s like for that animal to be itself” – have grown beyond simply what we expect based on our own human or primate, or even mammal experience.

We’ve become willing to consider that other mammals and less familiar animals – crabs, say, or octopuses – really are sentient, not just seeming so.

Importantly, the definition of sentience isn’t a neutral one: it is profoundly political, linked as it is to resource use and granting of rights. Once we understand a creature to be sentient, we recognize ethical imperatives. It is not so long since it was broadly believed that human infants didn’t experience pain the way older children and adults do, with babies rarely receiving anesthetics before painful procedures, their cries considered automatic and their nervous systems considered too........

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