The Lion Cries Tonight? Emotions and the Animal Brain (Part 1)
“Even insects express anger, terror, jealousy and love, by their stridulation.”— Charles Darwin (1)
Of philosophers who study ethics, 60% rate meat-eating on the ‘immoral’ side of a value scale (compared to 19% of the general population; 2). Such a large discrepancy in beliefs between ethicists and the general population intrigued me. What are the considerations that drive a majority of ethicists to favor vegetarianism? Three main considerations are found across vegetarians: health reasons, environmental reasons, and ethical reasons involving cruelty and animal suffering (3).
The health & environmental reasons are best examined by other fields, but neuroscience can potentially inform the question of whether animals can experience emotions and suffer (although it cannot answer value questions about whether if an animal suffers, does this make it immoral to harm it, etc.). Typically, ask someone whether they believe animals experience emotions and you will get their ‘gut’ answer. Some people believe extremely strongly that animals experience emotions while others see animals as little more than organic automata, and of course others hold a spectrum of beliefs in between these two positions. But we can do far better than go with our ‘gut’ with regard to animal emotion. Neuroscience is far from settling the question, but let us explore the evidence that has been gathered so far.
What are the current theories of emotion?
Before investigating emotions in animals, scientists need a definition and method of studying what an emotion is, which is scientifically difficult. Generally, the word emotion is meant to convey that a person is currently experiencing certain feelings. Sadness feels different from anger which feels different from joy. It is hard to explain what being ‘sad’ is, but every reader right now can imagine exactly what it would feel like to be sad. It should be no surprise that most neuroscientists are very uncomfortable with this terminology.
A simple and currently popular theory among psychologists is to try and quantify emotion based upon theoretical dimensions as illustrated in Figure 1. In the 2-dimensional model (Figure 1A; 10), emotions are thought of in terms of valence (how unpleasant or pleasant something is) and arousal level (either high or low). In this model, an emotion such as surprise has an intense arousal level but is usually valence-neutral. In contrast, happiness is conceived of as having a pleasant valence, but being relatively neutral in terms of arousal. You can add additional dimensions to continue to separate out emotions, such as how long an emotion lasts. Typically joy lasts a shorter time than contentedness, for example (11). One could conceivably continue adding these dimensions until they feel they have captured the entire emotional experience.
The points on these dimensions are what we typically think of when thinking of emotions (sad, happy, surprised) and can be thought of as a constellation of feelings, body states, and behaviors. These constellations are designed to help us deal with stimuli both in the environment or inside the mind. For example, being fearful can keep you safe from a threat or being happy from the memory of a previous success can help push you forward even when you do not want to train anymore. In this way, one can imagine humans having a pool of possible emotions, designed for the challenges the species typically faces.
If animals do have emotions, they likely differ from humans in their emotional capacity because they face different challenges. A fly has less need to be proud or jealous because it is less socially complex compared to a human. Another example would be that species of spiders that lay hundreds of eggs and do not care for their young would have little need for parental attachment but for humans and other animals that raise and invest in their young, this emotional attachment would be a necessary. But how do we begin to understand which emotions an animal might be capable of?
How can you even study emotion in animals?
Scientists have two ways they try to define emotion in animals and relate it back to the points on the emotional dimensions identified in the last section. The first is to ground the study of emotion in terms of observable behavior and physiological changes (4). For example when humans are afraid they display freezing behavior (5). Think about that feeling you get when you cross the street only to have a car bearing down on you; The majority of people would experience a momentary freezing of their body (‘deer in the headlights’). Other more subtle behavioral reactions can be measured as well. Skin conductance (which is a measure of the sympathetic nervous system arousal level) and and respiration increase but blood pressure decreased in people when they were fearful compared to when they are angry (6). In this way, if you can find behavioral and physiological responses that are strongly and reliably correlated with specific emotions across individuals, you can begin to infer what emotions someone is feeling based on what you observe.
A second way to study emotions is to investigate the brain activity during an emotional state. For example, fMRI shows the amygdala, a small almond shaped part of the brain in the temporal lobe shared across most complex vertebrates, is strongly activated when fear is experienced (7). Another measure of brain function is the measuring of neuropeptides and neurotransmitters. One approach is to use radioactive-ligands in PET (much less dangerous than it sounds) to measure chemical activity in the brain. One such study found that patients with a panic disorder had abnormal binding in the benzodiazepine-GABA receptor network, particularly in the right orbitofrontal cortex and right insula, implicating these brain regions as involved in anxiety and its regulation (9). An alternative study approach is to introduce a chemical into subjects’ brains and examine differences in brain activity. A study had subjects ingest a beta-adrenergic antagonist which decreased noradrenaline levels; When viewing arousing images normal noradrenaline levels were associated with increased amygdala activity compared to reduced noradrenaline levels (8). Taken together, it appears that emotions have behavioral, physiological, and chemical signatures which allow them to be studied.
What is the evidence for animal emotion?
Any inference of emotion in animals will have to be through interpretation of the animals’ behavior or brain activity. If an animal’s behavior and/or brain activity matches a human’s during a certain emotion, this is the closest we can get to figuring out if an animal is sad or happy.
The examples from the previous section focused heavily on fear. This is because there is a large overlap in behavior between animals and humans in their displays of this emotion. For example, both rats and humans display freezing behavior when presented with threatening stimuli (12,13). A human who had a bilateral amygdala lesion showed a marked decrease in fear-related behavior and in self-reported emotional experience of fear (5). Lesioning rats’ amygdala leads to a similar decrease in fear related behaviors (12). If lesions in the same locations lead to similar changes in behavior, and a self-reported decrease in emotion in the human, can we assume that the rat was experiencing fear but is not now when missing its amygdala?
Fear is a relatively intense, short-lived emotion. But animal behavior matches human behavior on longer, less acute emotions as well. One of the major impacts of emotion on humans is decision-making. Do you approach the pretty lady/handsome man at the bar? Do you avoid certain streets at night because they make you afraid? In part 2 of my article on pessimism, I discussed a task which probed whether animals become more pessimistic as a result of negative experiences, which turned out to be true for a variety of species including rodents, cows, and bees. Humans also become more pessimistic in response to negative events, mirroring animal behavior.
So far we have mostly been discussing the negative side of emotions. But for more joyful behaviors such as play & love animals match human behavior and neurochemistry as well (14). For example, when rats engage in play with each other, dopamine is heavily involved in initiating play episodes and if you block dopamine, rats are much less likely to play with each other (15). Since dopamine is largely tied to the experience of pleasure in humans (16), this suggests the brains of humans and rodents may function similarly during a period of playing around with members of their species. Similarly, the traditional measure of love is that two individuals (or perhaps more than two) mate with each other exclusively. While numerous animals display this type of behavior (17), perhaps the most famous is the prairie vole (18). Two neuropeptides, vasopressin and oxytocin, have been implicated in the development of mating partner preference in prairie voles (18,19). Both of these neuropeptides are also implicated in a range of human social behaviors (20).
Across a variety of emotions, humans and animals appear to share a similar constellation of behaviors, body states, and brain states. But what is obviously lacking from this exploration is the question of whether animals experience these emotions the same way humans do. This gets into what it means to conscious, a topic I will explore in the next part of this article.
Relating all this back to the question that originally motivated this exploration, what does this have to say about how animals experience their treatment? It appears that there is significant overlap in a number of behaviors and neural activities between humans undergoing specific emotions and animals which are presumably experiencing similar emotions. This is not to say that emotions are universal in the animal kingdom. Which emotions a particular animal will be capable of will depend on the needs of the species. The closer an animal is to humans evolutionarily, the closer their expected emotional capabilities would be to ours.
What this evidence means for the ethics of vegetarianism is that it will require further research and thought. The question of consciousness which I will cover next time will have a large bearing on this question as well. Suppose an animal experiences the feelings of an emotion but is not aware of them in the same way we are. Would it be ethical to treat them kindly to minimize their suffering but to still eat them? Even if they are conscious and suffering, should their suffering be weighted the same as a humans?Understanding the bodily & neural signatures of emotions associated with suffering, and searching for these signatures in the specific animals that are consumed will inform the debate beyond the gut feelings of its participants. But it is a conversation as a society that we will have to have together.
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