[Subtitle.] Rather than focusing on which animals are conscious, this article argues that it’s far more relevant to animal welfare to determine how animals experience consciousness.
This is a crosspost for What If We Assumed That All Animals Are Conscious? by Jo Quaid, which was originally published on Faunalytics' website on 21 July 2025. It summarises the article “All animals are conscious”: Shifting the null hypothesis in consciousness science by Kristin Andrews, which was published on Mind & Language on 4 January 2024.
Investigating Animal Consciousness
The common methods that researchers use to determine which animals are likely to be conscious fall into two schools of thought. The first involves using theories of consciousness. However, according to the author, this method has major issues related to the theories themselves. Because they’re based on the idea that human consciousness is the only case example, they exclude most animals more distantly related to humans.
The second method tries to find observable features — markers of consciousness — instead of defining consciousness itself. While this is considered best practice, the author believes that it leads to bias towards more highly developed animals rather than equally applying to all animals. In its current use, the marker method only reinforces the consciousness of animals who humans have decided are worth looking into further.
Therefore, the author suggests developing a new theory of consciousness based on the idea that all animals are conscious.
An Example Of The Marker Method
The author cites a hallmark review of the evidence of sentience in cephalopods and crustaceans as a case study of the marker method’s use. Sentience can be considered as one kind of consciousness.
The review uses pain-related behaviors and biological structures as markers, concluding that there’s very strong evidence of sentience in octopuses and strong evidence of sentience in true crabs. Applying these same markers to Caenorhabditis elegans, a type of nematode with fewer than 350 neurons [adults have 302 neurons], the author suggests that there’s substantial evidence of sentience in these simple animals as well. However, as nematodes have long been presumed to be unconscious, this raises the question of whether the marker system can accurately be used to measure animal consciousness.
[Below is what Andrews says about nematodes (it is not part of Jo´s summary).]
Given the determinate development of their nervous systems, 30-some years ago it was taken as given that C. elegans are too simple to learn. However, once researchers turned to examine learning and memory in these tiny animals, they found an incredible amount of flexible behavior and sensitivity to experience. C. elegans have short-term and long-term memory, they can learn through habituation (Rankin et al., 1990), association (Wen et al., 1997), and imprinting (Remy & Hobert, 2005). They pass associative learning tasks using a variety of sensory modalities, including taste, smell, sensitivity to temperature, and sensitivity to oxygen (Ardiel & Rankin, 2010). They also integrate information from different sensory modalities, and respond differently to different levels of intoxicating substances, “support[ing] the view that worms can associate a physiological state with a specific experience” (Rankin, 2004, p. R618). There is also behavioral evidence that C. elegans engage in motivational trade-offs. These worms will flexibly choose to head through a noxious environment to gain access to a nutritious substance when hungry enough (Ghosh et al., 2016)—though Birch and colleagues are not convinced this behavior satisfies the marker of motivational trade-offs because it appears that one reflex is merely inhibiting another (Birch et al., 2021, p. 31).
C. elegans are a model organism for the study of nociceptors, and much of what we now know about the mechanisms of nociception comes from studies on this species (Smith & Lewin, 2009). Behavioral responses to noxious stimuli are modulated by opiates, as demonstrated by a study finding that administration of morphine has a dose-dependent effect on the latency of response to heat (Pryor et al., 2007). And, perhaps surprisingly, when the nerve ring that comprises the C. elegans brain was recently mapped, researchers found that different regions of the brain support different circuits that route sensory information to another location where they are integrated, leading to action (Brittin et al., 2021).
Even if we grant the author's low confidence in nematodes' having marker five (motivational trade-offs), current science provides ample confidence that nematodes have markers one (nociceptors), two (integrated brain regions), four (responsiveness to analgesics), and seven (sophisticated associative learning). Given high confidence that nematodes have even three of these markers, the report's methodology [Birch et al. (2021)] would have us conclude that there is “substantial evidence” of sentience in nematodes.
Markers Can Be A Slippery Slope
Next, the author goes on to discuss how markers of consciousness are used to find other markers of consciousness and, together, these marker “clusters” increase confidence in an animal’s consciousness. For example, markers of happiness like smiling and laughing in humans are linked to the ultrasonic laugh-like chirps in rats that are thought to indicate a similar emotion. Likewise, the brain chemical oxytocin, which is associated with social bonding and learning in humans, has a correlate in Caenorhabditis elegans. These nematodes possess nemotocin, a neuropeptide that’s involved in learning and mating — thus suggesting similar functions in very distantly related species.
For the author, this reinforces their issue with the marker system: that it doesn’t really provide a cutoff of consciousness within the animal kingdom.
Using Markers To Answer The “How” Rather Than The “Who”
According to the author, a lack of markers of consciousness doesn’t necessarily mean that an animal is unconscious either. Negative markers, such as lacking a central nervous system or possessing too few neurons, rest on the assumption that we know what kind of neurobiology is necessary for consciousness, which we don’t.
Meanwhile, new research on animal biology is constantly occurring. There’s a strong possibility that a marker of consciousness will be discovered that will implicate more and more of the animal kingdom until all animals are included in it.
Oftentimes, researchers will dismiss markers of consciousness that show up in simpler animals like Caenorhabditis elegans who’ve been deemed unconscious. However, the author points to the fact that one of the most classic markers, the ability to hold a conversation, is practiced by an unconscious entity now: large language models. This doesn’t make the marker of language invalid, but rather highlights the need for other markers of consciousness to be present.
Due to the fact that there are no negative markers and new markers are constantly being discovered, and the idea that older markers tend to be downgraded rather than discarded as new discoveries are made, the author argues that all animals can be found to be sentient through the marker system, regardless of how simple or complex they are.
Thus, they believe that markers are far more useful in telling us about an animal’s experience of consciousness than whether or not they’re conscious.
Finally, the author concludes their argument for why all animals should be considered conscious by highlighting the potential ethical concerns of assuming that they aren’t. Beyond introducing bias and reducing the quality of scientific research, doing so ignores animals’ needs and wants, limits our understanding of them, and ultimately impacts how we treat them. When we assume that all animals are conscious by default, the focus of the consciousness question can then shift from the who to the how.
The original article by Kristin Andrews makes a distinction between the question of the distribution of consciousness (which animals are conscious?) and the question of the dimension of consciousness in a given animal (How is the animal conscious? What does its mental life look like?). It then argues that we should first, as a kind of working hypothesis, assume that all animals are conscious, and instead study the dimension of consciousness question, to only then develop a theory of consciousness that is capable of answering the distribution question.
In my view, you should 'go all the way' with this and go directly to the conclusion that the 'distribution question' is ill-formed (or redundant), and only questions regarding the way mental lives are structured are meaningful. Any process or activity can semantically be labeled as a 'mental life' and the 'dimension question' is then ultimately a proxy for how similar you think these mental lives are to human mental lives (or some kind of idealized extrapolation of human mental life, the question then of course becoming what this extrapolation should look like). So if you want, you can assign processes (e.g. behavioral and neural processes) that are structured very differently from human mental lives a low score of 'consciousness' and processes very similar to human mental lives a very high score, without supposing an additional on-off property of sentience. In that sense, the 'dimension of consciousness' question just answers the reformulated 'distribution' question with it, if you then label processes with a very low score as 'unconscious'.
In my view, 'sentience' in the sense of the 'hard problem' is a red herring (and a philosophically ill-formed concept). The ethically relevant question is whether a mental life is structured to include interests that our moral values demand we respect.
As an aside, the summary rightfully notes that unreflectively assuming that something is unconcious can lead to ethical concerns. So it is a bit jarring how the text just seemingly states as a fact that modern LLMs are unconscious without giving any reasoning for this (though the text might also be interpreted to assume this for the sake of answering a hypothetical objection).
Thanks for the great point, PSR! Strongly upvoted, and agreed.