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Given that animals can't tell us about their experiences, what forms of evidence are used to suggest whether an animal is conscious?
This subject is important, since one argument for adopting vegetarianism as a lifestyle is a belief in animal rights. For example, some veg*ns argue along these lines:
All animals have the ability to suffer in the same way and to the same degree that humans do. They feel pain, pleasure, fear, frustration, loneliness, and motherly love.
Animal consciousness is a very interesting topic and has been in focus for quite a while. So, one should not be surprised that Wikipedia has a large article for this subject.
Before going into animal consciousness, we should mention some parts of the consciousness (source). Since artificial intelligence also deals with artificial consciousness, I used it as a reference, as it states more clearly the components.
Scientific approaches for animal consciousness assessment:
Mirror test - used to test self-awareness is passed by various animals such as some apes, elephants and magpies.
Pain or suffering - there is evidence that many animals feel pain or suffering (source)
Many animals also exhibit more complex behavioural and physiological changes indicative of the ability to experience pain: they eat less food, their normal behaviour is disrupted, their social behaviour is suppressed, they may adopt unusual behaviour patterns, they may emit characteristic distress calls, experience respiratory and cardiovascular changes, as well as inflammation and release of stress hormone
cognitive bias and emotion
Cognitive bias in animals is a pattern of deviation in judgment, whereby inferences about other animals and situations may be drawn in an illogical fashion.[82] Individuals create their own "subjective social reality" from their perception of the input. It refers to the question "Is the glass half empty or half full?", used as an indicator of optimism or pessimism. Cognitive biases have been shown in a wide range of species including rats, dogs, rhesus macaques, sheep, chicks, starlings and honeybees.
metacognition
Metacognition is defined as "cognition about cognition", or "knowing about knowing [...] There is evidence that rhesus monkeys and apes can make accurate judgments about the strengths of their memories of fact and monitor their own uncertainty, while attempts to demonstrate metacognition in birds have been inconclusive. A 2007 study has provided some evidence for metacognition in rats, but further analysis suggested that they may have been following simple operant conditioning principles, or a behavioral economic model.
This article from the Stanford Encyclopedia of Philosophy deals extensively with animal consciousness issues such as pain and suffering, emotions, perceptual phenomenology, mental time travel, self-consciousness and metacognition. It also deals with epistemological and metaphysical issues related to this topic (first paragraphs).
Disclaimer: There is no conclusion to this question as of now and given the trouble we have with defining what exactly is conscioussness in humans, not even speaking of animals, I doubt there will be a definite answer to this question anytime soon. My answer will therefore ignore certain philosophical nuances and will work with assumptions that are based on facts, instead of working with pure facts themselves.
This will be a rather long (and I mean long) introduction concerning the composition of brains in humans and animals and their comparison. It is not crucial for getting the gist of it and it will have its own TL;DR at the end so feel free to skip to there.
Cerebral cortex is a part of mammalian brain to which things like memory, attention, perception, awareness, consciousness or thoughts are linked. The biggest part of cerebral cortex in humans is the neocortex - its evolutionary most recent part.
There can be many things to be looked at when looking at a brain as whole. We can look at the total number of neurons, at the size of brain relative to the body size, at the number of neurons in certain areas (such as the ratio of neurons in neocortex compared to total number of neurons in the brain), but also at things that may be less obvious at the first sight such as the ratio of neural and non-neural cells in the brain, the encephalisation quotient or the neural density of the brain. I will try to address each one first and then draw a conclusion about human brain afterwards.
Looking at the total number of neurons, elephants have much bigger brain in terms of total number of neurons than humans with ~257 billions of neurons in elephant's brain compared to ~87 billions of neurons in human brain. We do not see elephants solving quadratic equations with a swing of their trunk though and so it is clear that simply the total number of neurons alone, while indicative of computational power, is not a good measure of cognitive abilities.
Should we take a look at the brain and body size relationship instead? Not really. While we as humans do score rather highly for this criteria (brain size is 2% of total body mass), even better so do marmosets with their 2,7%. Marmosets are small primates and they do not belong to the brightest of primates. It does not seem likely brain and body size ratio is the answer.
Could we explain this with encephalization quotient instead? Encephalisation quotient is a ratio between actual brain mass and a predicted brain mass for an animal of given size. It takes into account allometric effects. In this measure, humans do, finally, come first. There are problems with this approach, however. If we were to use this measure, rhesus monkeys should be smarter than gorrilas which, based on their behaviour, they are not.
Or is it that the size does not matter in this case? Probably. For example because it does not account for things such as neural density. Neural density, for example, may be one of the reasons why birds are acting intelligently while having such small brains. This study determines that birds are able to pack twice as many neurons as primates to the same brain size.
If size is out of the picture, what else could we look at? Where human and elephant brains differ notably is for example where these neurons are located. For humans, the ratio of cerebral cortex neurons is much higher, with ~16.3 billions of neurons in their cerebral cortex, compared to only ~5.6 billions of neural cells in the cerebral cortex of elephants. Is this the defining characteristic for human like cognition? And if so, what about great apes whose brains are similarly constructed with rather high numbers of neurons in the cerebral cortex? Although human brain was for long believed to be unique and somewhat outlier amongst animal brains, this notion is gradually being debunked as can be seen for example here or here and the studies show that human brain is simply a scaled-up version of primate brain. For gorillas and chimpanzees for example, the aforementioned ratio is even higher than in humans, since of the total number of ~33 billions and ~22 billions of neural cells respectively, gorillas have ~9.1 billions of these in their cerebral cortex and chimpanzees ~6 billions.
As recent research shows, there are some differences among the brains of other primates and ours. One such measure is the neuropil space.
The neuropil is defined as the space between neuronal and glial cell bodies that is comprised of dendrites, axons, synapses, glial cell processes, and microvasculature. Accordingly, the proportion of cortical gray matter that is composed of neuropil may serve as a proxy for the total interconnectedness among neurons from local intrinsic circuits and extrinsic projections within a region.
Just as an intersting fact, the optimal proportion has been calculated to be 3/5 of the gray matter.
To continue
.. it has been reported that there is a significantly greater fraction of neuropil space in layer III of the frontopolar cortex (area 10) and Broca’s area (areas 44 and 45) of humans relative to apes, while there are no phylogenetic differences in the neuropil fraction among primary sensory and motor cortical areas
Broca's area is an area related to speech production and Area 10, while not very well understood yet, is indicated ot be relevant in strategic thinking, memory recall and other executive functions.
The study concludes that
.. our findings suggest that significant modifications of neuropil within particular prefrontal cortical areas accompanied the evolution of the human brain. Such anatomical specializations might potentially provide increased capacity for enhanced neuronal integration to support executive cognitive functions.
Since our knowledge about the functions of the cerebral cortex shows that it is the centre of abilities we usually associate with consciousness and higher cognitive processes, we could simply stop here and conclude that what is needed to achieve human-like cognition is a certain number of neurons (with there being a non-linear relationship between the number of neurons and cognitive abilities, hence our much better cognitive abilities comapred to other priamtes) with a certain proportion of these neurons located in cerebral cortex (this being the reason why elephants are not an intellectual master race) and with a certain proportion of cortical gray matter being composed of neuropil (giving us advantage over other primates).
However, things are not so simple still. This conclusion would exclude animals which we know pass some tests related to consciousness (I will be addressing this later) such as some types of birds or even invertebrates such as octopuses, but which do not belong to the Mammalia class and do not have cerebral cortex at all. To explain this, we could look for similar structures in non-mammalian brains, perhaps based on the genetical similarities.
Whether particular brain structures in different species are related to each other (i.e., whether they diverged from a single structure in a common ancestor) is often a subject of debate and controversy. It can be difficult if not impossible to determine based solely on location, anatomical organization or functional similarity. This is because it is relatively easy for these parameters to change over the course of evolution – they can be affected by changes to one or two genes, which means there is plenty of variation in these phenotypes within the population – the raw material for evolution by natural selection.
But luckily:
If the final phenotypic end-point of any particular region is quite variable, the opposite is true of the genetic pathways that specify the identity of the region at earlier stages of development. These involve master regulatory genes, whose expression is turned on or off in various parts of the embryo...
...These genes interact in a complicated network of feedforward and feedback loops to orchestrate this complicated sequence. The networks in which they operate are so interlocked and involved in so many different parts of the embryo that mutations to any one gene tend to have very drastic consequences and will be rapidly selected against. These early regulatory systems are thus far less variable and tend to be highly conserved across evolution...
It is thus possible to tell whether a brain region in one species is homologous to one in another species (which may look quite different in its mature characteristics) by looking at how those regions were specified.
And after the introduction, the interesting part:
Similar patterns of gene expression argue that the cortex of mammals and the “pallium” of birds and reptiles are indeed related to each other.
And even more interestingly, the study mentioned later on in the article has found out that the commonalities go as far as mushroom bodies of a certain annelid worm, called Platynereis.
It is therefore not only possible, but highly plausible, that even species which do not possess cerebral cortex may be capable of the same or similar things as those who do (and as we shall see later, they do seem to be capable of these).
If you have read my answer up until here, you may wonder what all this has to do with the presence of consciousness. I have included much of the previous information for two reasons. It is, I believe, insanely interesting and, more importantly, it shows just how much there is we have to take into account if we want to try and biologically determine the likelihood of a certain animal having consciousness.
Now, does the presence of cerebral cortex really shows that other animals are conscious? As I mentioned in the disclaimer, we cannot know. We can suppose though. And based on what we know about cerebral cortex, based on what we know about brain structure and based on what we know about the genetic origin of similar structures in other classes of animals than mammals, I would venture to claim that we can assume that many animals possess at least some level of consciousness.
Studies have shown that cerebral cortex, found in mammal's brains, is heavily involved in higher cognitive processes which we usually associate with consciousness. Comparing human brain with that of other mammals, there is no single defining factor that would make ourselves cognizant - and, by its lack, render other animals incognizant - that we know of. This goes, however, beyond the Mammalia class, based on genetical similarities in brain formation etc. it is highly possible that even animals without cerebral cortex (and therefore - if we just had a simple look at its functions and assuemd its lack will signify the lack of these functions - without higher cognitive abilities) do have some similarly performing structures in their brains which may indicate consciousness.
And to add some further credibility to my conclusions, here is the conclusion from the "Consciousness in Humans and Non-human animals" conference:
Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.
Attendees of which also said that:
The absence of a neocortex does not appear to preclude an organism from experiencing affective states,
Since this answer is already long enough and there are others who talked about this point before me (great answer was given by Alexei), I will just shortly describe this.
Another way we can determine if an animal is conscious or not is to try and define what we think conscious means and devise tests for animals to pass in order to peruade us of their consciousness. This has been done many times with many animals, with really interesting results.
I may change this later but as of now, I will simply link to some interesting findings and studies for some animals (I really love the octopus ones). I will also add some more pointers later.
Birds:
Making connections between phenomena
Octopuses:
As a matter of fact, only the creature in question knows whether it is conscious or not. This is because there's a subjective first person side to consciousness in addition to the objective third person side. No amount of third person insights (including scientific) can establish whether a creature is conscious or not. No amount of the latter can be a substitue for the former, as far as the dichotomy is concerned.
However, as far as the animals are concerned, there's no more reason for you to doubt the consciousness of a cow or elephant than that of a fellow human. We understand the evolution of the brain, from reptilian to mammalian to human, and we also know from observations of anaesthetic patients to personal experiences that thinking, logic or any other human brain (neocortical) function (e.g. language) is not a requisite ingredient to consciousness or subjective experience. Or at least the emotion of pain conveyed by these animals when they're being mercilessly slaughtered is one of the most foolproof intuitive ways I know to claim that a creature is conscious.
So to conclude, I don't think that butchers or anyone in those awful factory farms can really defend any claim to the contrary, no more than they could defend if the same treatment was delivered to one of their loved ones. In other words, animals are conscious, or at least the kind of animals relevant to human lives. But yes, with that said, I do think however that only a cricket (say) knows for sure whether it is conscious or not. And we'll face the same problem with advanced AI even if they claim to be conscious.
