Animal Sentience: What the Evidence Can Tell Us

What would count as evidence that an animal can feel something? A withdrawal from a sharp stimulus? A lasting change in behaviour? A preference for relief? These questions matter because an animal cannot describe an experience to us in words.

Sentience means the capacity for subjective experience, including experiences that may feel good or bad. It is different from being clever at a task. A creature does not need to resemble a human, or perform an impressive trick, for its welfare to matter.

Why one observation is not enough

A response to injury can have more than one explanation. Researchers therefore look for patterns across several kinds of evidence rather than treating a single movement as decisive. Behaviour, nervous systems, learning and the effects of pain relief can all contribute to an assessment.

A 2021 review led by Jonathan Birch examined more than 300 studies concerning cephalopods and decapod crustaceans. Cephalopods include octopuses, squid and cuttlefish; decapods include crabs, lobsters and crayfish. The review used eight criteria to assess evidence, including responses to noxious stimuli, learning and trade-offs between avoiding harm and other priorities.

The authors recommended regarding these groups as sentient for animal-welfare policy. This was an assessment of converging evidence, not a claim that every species had been studied equally or that every question about experience had been settled.

A small experiment with a larger question

Pain is not the only experience researchers investigate. In a 2022 study published in Animal Behaviour, bumblebees repeatedly rolled wooden balls without receiving food for doing so. The researchers tested alternative explanations and concluded that the behaviour met established criteria for animal play. Bees also preferred a chamber colour previously associated with ball rolling.

This is evidence of play-like, rewarding behaviour in the study conditions. It does not establish that all insects feel pleasure, or tell us precisely what a bee experiences. Its value lies in opening a question that is easy to dismiss when an animal is small and unfamiliar.

Evidence and interpretation

There are two mistakes to avoid. One is to project an entire human emotional life onto an animal because a behaviour looks familiar. The other is to assume that unfamiliar behaviour cannot be accompanied by any experience at all.

A careful explanation can hold evidence and uncertainty together. It can say what researchers observed, how they tested their explanation and where the inference becomes less certain. Words such as “suggests” and “supports” are useful when they describe the actual strength of a conclusion.

What follows for care?

Research can inform a decision without making the ethical decision for us. Knowing that there is credible evidence of sentience still leaves questions about what risks we should impose, which practices we can avoid and how much uncertainty we are prepared to accept on another being's behalf.

The ethical position of Nest & Grove is that uncertainty deserves care. Where a choice can reasonably reduce the risk of suffering, the absence of a complete account of an animal's inner life is not a compelling reason to ignore that choice.

That is an interpretation of the evidence, not a laboratory finding. Keeping the distinction clear makes the argument stronger: we can be honest about what we do not know while taking seriously what research gives us reason to consider.

Read the research

• Birch and colleagues (2021), Review of the Evidence of Sentience in Cephalopod Molluscs and Decapod Crustaceans, a commissioned evidence review.
• Galpayage Dona and colleagues (2022), Do bumble bees play?, an experimental study in Animal Behaviour.