Sentience table

There is a research program called Rethink Priorities. It conducts various kinds of empirical research to identify areas that may be relatively undervalued by scholars and activists across fields, and to encourage a reassessment of priorities. As part of this project, researchers examined 53 features related to conscious experience in 18 kinds of organisms, including plants and invertebrates. The results were released in mid-2019. (Detailed introduction: Invertebrate Sentience Table – Rethink Priorities.)

No additional animal experiments were conducted for this research. It is a meta-study reviewing existing studies carried out for various purposes and collecting only the portions relevant to this project. One drawback is that data are scarce for some species.

Nevertheless, I think this is meaningful material, so I have summarized it.

Feature categories

The following are the 53 features, divided into eight categories: responses to noxious stimuli, motivational tradeoffs, learning indicators, cognitive sophistication, responses to drugs, navigational skills, behaviors related to emotional states, and anatomical/evolutionary features.

• Responses to noxious stimuli
  • Physiological responses to nociception or handling
  • Protective behavior
  • Defensive behavior/fighting back
  • Noxious stimuli related vocalizations
  • Movement away from noxious stimuli
• Motivational tradeoffs
  • Paying a cost to receive a reward
  • Paying a cost to avoid a noxious stimulus
  • Self-control
  • Predator avoidance tradeoffs
  • Selective attention to noxious stimuli over other concurrent events
• Learning indicators
  • Classical conditioning
  • Operant conditioning
  • Operant conditioning with unfamiliar action
  • Sensitization
  • Habituation
  • Contextual learning
  • Observational or social learning
  • Taste aversion behavior
  • Pain relief learning
  • Long-term behavior alteration to avoid noxious stimulus (24+ hours)
  • Long-term behavior alteration to avoid noxious stimuli (30+ days)
• Cognitive sophistication
  • Uncertainty monitoring
  • Self-recognition (mirror test)
  • Deception
  • Tool use
  • Flexible tool use
• Responses to drugs
  • Affected by analgesics in a manner similar to humans
  • Self-administers analgesics
  • Affected by recreational drugs in a similar manner to humans
  • Self-administers recreational drugs
  • Affected by antidepressants or anxiolytics in a similar manner to humans
• Navigational skills
  • Navigate known paths/areas
  • Navigate unknown paths/areas
  • Spatial memory
  • Detour behavior
• Behaviors related to emotional states
  • Anhedonia behavior
  • Learned helplessness behavior
  • Displacement behavior
  • Adverse effects of social isolation
  • Stereotypic behavior
  • Fear/anxiety behavior
  • Play behavior
• Anatomical/evolutionary features
  • Neuron count
  • Brain size
  • Nociception
  • Nociception (strict definition)
  • Opioid-like receptors
  • Centralized information processing
  • Vertebrate midbrain-like function
  • Last common ancestor with humans (millions of years)
  • Average Lifespan
  • Motile
  • Distinct sleep-wake states

The study examines whether research exists for each feature in each species and classifies the findings as follows.

• Likely No: 0%–25%
• Lean No: 25%–50%
• Lean Yes: 50%–75%
• Likely Yes: 75%–100%

Animals analyzed

Because the main topic is conscious experience in invertebrates, many of the 18 kinds are invertebrates. Prokaryotes, protists, plants, and humans are also included for comparison.

• Honey bee (genus Apis)
• Cockroach (genus Periplaneta)
• Fruit fly (genus Drosophila)
• Ant (family Formicidae)
• C. elegans
• Crab (infraorder Brachyura)
• Crayfish (family Cambaridae)
• Earthworm (Lumbricus terrestris)
• Sea hare (genus Aplysia)
• Moon jellyfish (genus Aurelia)
• Spider (order Araneae)
• Octopus (family Octopodidae)
• Plant (kingdom Plantae)
• Prokaryotes
• Protists (eukaryotic organisms that are not animals, fungi, or plants)
• Chicken (Gallus gallus domesticus)
• Cow (Bos taurus)
• Human

Results

Ranking by the arithmetic mean of the scores gives the following order.

• Human
• Chicken
• Fruit fly
• Cow
• Octopus
• Honey bee
• Crayfish
• Crab
• Ant
• Cockroach
• C. elegans
• Sea hare
• Spider
• Earthworm
• Protists
• Moon jellyfish
• Prokaryotes
• Plant

Some findings are surprising, such as fruit flies ranking above cattle or octopuses. I suspect this may reflect a bias from the abundance of fruit fly research (in other words, other animals may be underestimated rather than fruit flies being overestimated).

Relevance to the animal movement

Peter Singer, author of Animal Liberation, calls all living beings capable of pleasure or pain (sentience) sentient beings. Because sentient beings have interests, he argues that their interests should receive equal consideration, whether or not they are human.

Tom Regan, author of The Case for Animal Rights, defines all living beings with beliefs, desires, perception, memory, a sense of the future, an emotional life, preferences, the ability to act toward goals, a continuing identity, and so forth as subjects of a life. He argues that all subjects of a life have inherent value and therefore moral rights.

Research like this matters whichever position one supports. Regan’s criteria for being a subject of a life are particularly more demanding than Singer’s. Regan himself therefore conservatively estimates that subjects of a life include many mammals and perhaps some birds. Yet this study suggests that many invertebrates are also likely to have conscious experiences at various levels.

Visualization

Charts visualizing these data can be found in Invertebrate sentience.