Background

Recent rapid advances

The last ten years have been an exciting time for the science of animal cognition and behavior. Striking new results have hinted at surprisingly rich inner lives in a very wide range of other animals, including many invertebrates, driving renewed debate about animal consciousness. To give just ten key examples:

Crows can be trained to report what they see. In a 2020 study published in Science, Andreas Nieder and colleagues trained crows to report their visual perceptions using head gestures. The crows were shown either a bright stimulus, a dim stimulus, or no stimulus. The crows generally reported accurately whether they had been shown a stimulus, though they sometimes made mistakes, especially when the stimulus was very dim. Throughout the experiment, researchers measured activity in a brain region thought to be associated with high-level cognition in birds (the NCL). They found that NCL activity tracked whether or not the birds reported seeing a stimulus, not whether or not a stimulus was presented. In other words, the results suggest that brain activity in the NCL is a neuronal correlate of visual experience in crows.

Octopuses avoid pain and value pain relief in the conditioned place preference test. The “conditioned place preference” test was developed to assess pain in lab rats. In 2021, cephalopod expert Robyn Crook tried it with octopuses. First, Crook allowed octopuses to choose between two chambers within a tank. Next, some octopuses experienced the effects of an injection of acetic acid while in their preferred chamber. These octopuses (but not controls injected with saline solution) developed a lasting aversion to that chamber. Then, octopuses injected with acid experienced the effects of a local anesthetic (lidocaine) in the chamber they initially disfavored. These octopuses (but not controls) developed a lasting preference for the chamber where they experienced the effects of lidocaine. In a rat or a human, we would infer from this pattern that the acid injection caused pain that the lidocaine relieved, so we should be ready to draw the same conclusions about an octopus.

Cuttlefish remember details of specific past events, including how they experienced them. Many animals can recall past events, including what happened, where it happened, and when it happened. A 2020 study went further, showing that cuttlefish can remember how they experienced an item—for instance, whether they saw it or smelled it—a capacity known as “source memory.” Researchers exposed cuttlefish to either the sight or scent of a crab, fish, or shrimp. They trained the cuttlefish to indicate whether they had seen or smelled each prey animal after a three-hour delay. After the training, the cuttlefish were able to complete the same task with new prey animals, like mussels or snails.

Cleaner wrasse fish appear to pass a version of the mirror-mark test. Questions of self awareness in animals have long been explored using the “mirror-mark test,” which tests whether an animal, upon seeing a mark on their own body in a mirror, will try to remove that mark. In a surprising series of studies between 2019 and 2023, researchers showed that cleaner wrasse fish can pass the four phases of the test. First, when exposed to a mirror, the fish react aggressively as though they believe they see a rival fish. Second, the aggression fades and the fish begin performing unusual behaviors in front of the mirror, such as swimming upside down. Third, the fish seem to study themselves in the mirror. Finally, after the experimenters place a colored mark on the fish, the fish, on seeing the mark in the mirror, attempt to remove it by scraping against an available surface.

Garter snakes pass a scent-based version of the mirror-mark test. Determining whether animals can recognize themselves in a mirror may not be an appropriate test of self awareness for all species. Some animals, like snakes, rely primarily on scent or other non-visual cues to navigate their environment. A 2024 study tested self-recognition in snakes by measuring their reactions to cotton pads soaked in various different scents: (1) their own scent, (2) their own scent with a “mark” of a different scent, (3) the “mark” scent alone, (4) the scent of an unknown snake, and (5) the scent of an unknown snake with a “mark.” Garter snakes investigated their own marked scents longer than any other scent. This suggests that the snakes recognize their own scents and notice when their scent has changed.

Zebrafish show signs of curiosity. Many animal species—including raptors, tortoises, and honeybees—show signs of a desire to seek new information. In 2023, researchers tested for these signs in zebrafish. They found that zebrafish show sustained interest in new objects, but that their interest fades more quickly with the number of new objects they observe. Since zebrafish explore new objects voluntarily and in the absence of any additional reward, they seem to find learning new information intrinsically rewarding. 

Bees show apparent play behavior. While much of the existing research on animal consciousness is focused on pain, researchers are increasingly looking for signs of positive experiences. In a 2022 study, researchers found that bumblebees roll wooden balls around in a manner consistent with five characteristics of play. First, bees rolled the balls because they found it intrinsically rewarding, rather than as a means to an end. Second, the behavior did not serve an apparent function. Third, the bees were not rehearsing a behavior they use for another purpose, like foraging or mating. Fourth, bees rolled balls repeatedly but not in exactly the same way each time. Finally, the behavior increased when the bees were relaxed, indicating that it was a pleasant experience, not a stress-induced one. 

Crayfish display “anxiety-like” states, altered by anti-anxiety drugs. A series of studies between 2014 and 2017 investigated how crayfish respond to stress, exploring the possibility that they might be a useful model of anxiety. Researchers placed crayfish in a maze with both bright and dark pathways. Crayfish have a natural tendency to explore new environments, but they prefer dark to light. When researchers increased stress in the crayfish by administering electrical shocks, the crayfish became significantly more averse to the bright areas of the maze. Benzodiazepines are used in humans to alleviate anxiety, and crayfish given these drugs were once again willing to explore the bright parts of the maze.

Crabs balance competing motivations to make flexible decisions. A long-term research program by Robert Elwood and colleagues has investigated how hermit crabs and shore crabs make decisions in the face of risk. A 2024 study looked at how shore crabs balance their aversion to bright light against their aversion to electric shock. The crabs normally enter a shelter to escape bright light but may choose the bright light over the shelter if they experienced a shock in that shelter in the past—and their decision depends on how intense the shock was and how bright the light is. Other animals, like rats, iguanas, and bees, also make subtle, memory-dependent trade-offs between competing priorities. These trade-offs suggest the animal has a “common currency” for weighing needs of very different kinds, a currency that does for them what pleasure and pain do for us.

Fruit flies have active and quiet sleep—and social isolation disrupts their sleep patterns. Drosophila fruit flies have been known for many years to have a form of sleep. Now a new study has found ways of inducing two different kinds of sleep: “quiet” sleep, involving significantly decreased brain activity, and “active” sleep, where brain activity persists despite a lack of outward behavior. Just as slow-wave sleep and REM sleep serve different functions in humans, the researchers hypothesized that quiet and active sleep serve different functions in fruit flies. Quiet sleep seems to slow metabolism and regulate stress, while active sleep seems to support cognitive function. Meanwhile, a 2021 study published in Nature showed that sleep in fruit flies is disrupted by social isolation; flies sleep best when in the presence of other flies.

An emerging new picture of animal consciousness

What is consciousness? The term has a variety of meanings. The New York Declaration on Animal Consciousness focuses on one important meaning, sometimes called “phenomenal consciousness” or “sentience.” The question here is which animals can have subjective experiences. This can include sensory experiences (say, the experience of a particular touch, taste, sight, or smell) as well as experiences that feel good or bad (say, the experience of pleasure, pain, hope, or fear). This sense of the term “consciousness” is what Thomas Nagel had in mind when he famously asked “What is it like to be a bat?”.

Subjective experience requires more than the mere ability to detect stimuli. However, it does not require sophisticated capacities such as human-like language or reason. Phenomenal consciousness is raw feeling—immediate felt experience, be it sensory or emotional—and this is something that may well be shared between humans and many other animals.  Of course, human-like linguistic and rational capabilities may allow some humans to have forms of experience that other animals lack (e.g. a linguistic “inner monologue”). Likewise, many other animals may have forms of experience that we lack. 

Which animals are conscious in this sense? The advances just described, taken together, are sending a clear message: we need to take seriously the possibility that a very wide range of animals, including all vertebrates and many invertebrates, can have subjective experiences. 

It would be inappropriate to talk about “proof,” “certainty,” or “conclusive evidence” in the search for animal consciousness, because the nature of consciousness is still hotly contested. However, it is entirely appropriate to interpret these remarkable displays of learning, memory, planning, problem-solving, self-awareness, and other such capacities as evidence of consciousness in cases where the same behavior, if found in a human or other mammal, would be well explained by conscious processing. These behaviors make it more likely that these animals have consciousness without proving that they have it, just as the symptoms of a disease make it more likely that you have the disease without proving that you have it. 

With other mammals and birds, we can now say that the evidence establishes strong scientific support for attributions of consciousness—not conclusive evidence, but many lines of evidence all pointing in the same direction. With other vertebrates (reptiles, amphibians, and fishes) and many invertebrates (cephalopod mollusks such as octopuses and cuttlefish, decapod crustaceans such as hermit crabs and crayfish, and insects such as bees and fruit flies), we can now say that the evidence establishes at least a realistic possibility of consciousness. The chance is high enough to warrant further research aimed at addressing questions of consciousness in these animals. The chance is also high enough to warrant serious consideration of their welfare.

About the New York Declaration

The New York Declaration on Animal Consciousness aims to crystallize the message that has emerged from the last ten years of work. Among the 40 initial signatories of the declaration are world-leading experts on human consciousness (Christof Koch, Anil Seth, David Chalmers, Liad Mudrik, Lucia Melloni, Nao Tsuchiya), bats (Yossi Yovel), birds (Nicola Clayton, Irene Pepperberg), reptiles (Gordon Burghardt, Anna Wilkinson), fishes (Culum Brown, Becca Franks, Noam Miller), octopuses (Jennifer Mather, Robyn Crook, Peter Godfrey-Smith, David Edelman), cuttlefish (Alex Schnell), hermit crabs (Robert Elwood), bees (Lars Chittka, MV Srinivasan, Andrew Barron, Martin Giurfa), and fruit flies (Bruno van Swinderen). 

We now encourage anyone with relevant expertise to join our distinguished initial signatories by signing the declaration yourself.

What do we hope the declaration will achieve? One aim is to convey the excitement of the emerging science of animal consciousness and to encourage more work on this topic. This is a pivotal moment: a critical mass of people are daring to study animal consciousness rigorously and systematically. While uncertainty remains both about the nature of consciousness and about which animals are conscious, high-quality research has already reduced our uncertainty about these questions. We hope that scientists, universities, and governments will see that this field is making rapid progress, that it has the potential to make even more (including progress towards better theories of consciousness), and that it deserves your support.

A second aim is to encourage reflection on animal welfare. This Declaration makes no specific policy recommendations, and the signatories have a wide range of views on moral, legal, and political issues. The point of agreement is that certainty about consciousness should not be required for consideration of welfare risks. If there is a realistic possibility that an animal is conscious—for instance, that octopuses can suffer—then this possibility merits consideration in policy contexts—for instance, in decisions about whether to support octopus farming. Policymakers should take reasonable steps to mitigate welfare risks for all vertebrates and many invertebrates as researchers seek to advance our understanding of them.

Note 1: After the April 19 event on “The Emerging Science of Animal Consciousness,” the contributors will produce a detailed Scientific Appendix summarizing the evidence presented at the meeting, plus other relevant lines of evidence. This will appear on the Declaration website in due course. In the meantime, you can find a commentary on the rationale for the declaration by initial signatory Noam Miller in Learning & Behavior here.

Note 2: This background document was prepared by Kristin Andrews, Jonathan Birch, Jeff Sebo, and Toni Sims. It may be cited as: 

Andrews, K., Birch, J., Sebo, J., and Sims, T. (2024) Background to the New York Declaration on Animal Consciousness. nydeclaration.com.

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