Invertebrate Sentience: How Should We Study And Value It?

Researchers from Rethink Priorities, a project focusing on bringing awareness to and conducting research in neglected areas, has recently published a review of the literature available on invertebrate sentience and pain perception. The full table can be found here, in which 50 categories of behaviors and biological facts are examined as they relate to 13 categories of invertebrates. In this summary, we’ll look at the researchers’ methodology and explain their process of selecting behaviors and invertebrates to study.

First, the researchers explain why they chose to focus on invertebrates instead of more complex animals. Put simply, we already know that mammals, birds, reptiles, amphibians, and even fish have some degree of sentience and pain perception. The science is mostly settled when it comes to these animals, but comparatively little research has been done on insects, arachnids, crustaceans, worms, and other arthropods. The researchers make clear that they do not expect to come to a strong conclusion one way or the other at the end of this project. The existing research is too scant to make such a claim.

By selecting properties that are relevant to consciousness and sentience, the researchers hope to draw clearer links between animals known to be sentient, like cows, and invertebrates. If both can be shown to have the same sentience-linked property, it follows that the invertebrate might be sentient. The researchers note that the current literature is overly broad. The best table currently available only refers to animals by taxon, which each encompass millions of species. If even one species in the taxon has the property in question, the table shows a check mark in its slot. This ignores the fact that species within taxa display wide variation in capabilities. Furthermore, the researchers believe that the seventeen properties used by this table aren’t enough to make a determination about sentience. For this reason, the Rethink Priorities team examined fewer animals, but more properties.

The included properties range from biological features like nociceptors (pain receptors) and opioid receptors, to behaviors like avoidance of noxious stimuli and tool use, to cognitive capacities like the ability to be conditioned, and motivational tradeoffs – the ability to pay a cost to avoid a harm/receive a reward. The researchers also looked at how invertebrates reacted to painkillers in a similar way to humans, and if they will self-administer painkillers when provided. All of these factors are linked to sentience in mammals, birds, and other vertebrates, so invertebrates that display many of them are more likely to have some degree of sentience and/or consciousness. The researchers also looked at whether certain behaviors can be done unconsciously by humans, as this would mean that they are not necessarily associated with consciousness in animals.

The researchers chose animals to study based on the availability of data, likelihood of sentience, level of exploitation by humans, and how numerous they are in the world. Fruit flies and California sea hares are used as model organisms by many scientists, meaning a wealth of data exists. Honey bees, crabs, and crawfish are extensively used by humans for food, meaning they would have the most to gain from our acknowledgment of their sentience. Ants were examined for their sheer numbers: almost 20% of terrestrial biomass is in ants alone. Octopuses were included due to their high intelligence. The authors also wanted to include other varieties of invertebrates, so earthworms and moon jellyfish were examined. Finally, spiders and cockroaches were included, possibly due to their common interactions with humans.

The researchers make note of two principles that should influence the reading of their results. The first, Morgan’s Canon, states that we should not ascribe an animal’s behavior to higher cognitive processes if it can be explained by lower processes. It doesn’t preclude us from ascribing sentience to animals, but cautions us against ignoring the role of lower cognitive functions. The second principle is simply called the “Precautionary Principle.” It states that we should err on the side of caution when interacting with an animal if there is even limited evidence of its sentience. If there’s a reasonable chance that an animal can suffer, we should avoid causing them harm. These principles do not have to be in conflict; we should not be too quick to ascribe higher intelligence to animals based on limited evidence, but we also should give the animal the benefit of the doubt based on this limited evidence. We can act as if an animal is sentient even without knowing for certain that it is.

For animal advocates, this insight into Rethink Priorities’ research methods and decision-making are quite valuable. They bring up several good points to consider in future research, such as the benefits of looking at smaller groups of animals over larger ones, as well as cautioning against reading too much into certain behaviors or biological features. Simply raising the issue of invertebrate sentience is an important step, too, given the relative lack of information available. In order to understand the extent to which insects and other invertebrates are capable of thought, emotion, and suffering, research needs to be done specifically regarding these phenomena.