Disney's Zootopia imagined a modern, animal-focused Garden of Eden: predator and prey living together in peace. The movie, which celebrates its fifth anniversary this week, masquerades as a story about cute animals while smuggling in a message about prejudice and living together with our differences.
Conflict arises when the predators lose their civility, reverting to their baser primal urges when injected with a serum. The foundations of their civilization crack under worries that lion and lamb may not be capable of living together. Of course, the truth comes out. The situation has been manufactured by powerful individuals intent on maintaining and growing their power. The people — er, animals — are pawns in a larger game and their biases played upon to sow discord. Heavy stuff for a movie starring a bunny.
It all hinges on the idea that behaviors like aggression can be artificially induced or enhanced via the introduction of an external stimulant.
But that's not true. Or is it?
A cursory look at the animal kingdom shows a spectrum of violent aggression ranging from highly aggressive species and individuals to more docile ones. Ferocious as a lion, timid as a mouse, and all that. Understanding what drives aggression is an ongoing field of study, but there seem to be at least a few key elements at play.
Aggression almost always relates to the acquisition of resources, either tangible or abstract. Food and water, mating choices, and space top that list. A lack of any one of these can spark acts of violence. Still, different species have variable reactions to their pursuit.
In "When Violence Pays," published in Evolutionary Psychology, the authors describe an optimization analysis to explain violence. Faced with an evolutionary need, individuals undertake a cost-benefit analysis bumping the need against the consequences of its pursuit. In short, a calculation is made: does the potential cost of violence outweigh the potential gain? For some species and some individuals, the answer is yes.
Humans and other primates in particular have a high frequency of violence and the authors suggest this is due to the relatively low cost of violence. Other animals, those we'd consider to be more docile, find the cost of violence — energy expenditure, potential risk of injury, or death — too high. So, they employ other strategies.
The level of aggression seen in a species can be and is impacted by external forces in this way. Species are seen to exhibit higher levels of aggression when their environment shifts into a situation where one of these crucial resources is in short supply.
In 2018, after tropical storms hit the Gulf of Mexico, a group of researchers followed the activities of 240 communal spider colonies (because that's a terrifying thing that exists) and found that they were becoming more aggressive. The introduction of extreme weather pushed the collective aggression levels higher, resulting in the deaths of some members of the colony, and an overall higher level of aggression in later generations. In some cases, infighting became so severe the colonies disbanded.
We see similar phenomena in laboratory mice. In nature, mice stratify into social hierarchies. A dominant male mouse will live with several females and their offspring. Other males will leave to find their own digs. Aggression is avoided by way of distinct territories. Space is a vital resource for the health of mouse populations. In a laboratory setting, male mice have no such option. Often housed together in close quarters, there is no opportunity to leave. That same cost-benefit analysis may take place but the variables have changed. With limited space, the benefit of aggression overwhelms the cost and violence ensues.
It's clear that external stimuli can impact how aggressive an individual animal might become. The potential for violence may be innate, but actualizing it comes from outside. Still, it's not quite as on-the-nose as a serum-filled dart. Maybe we can get closer.
BODY AND BRAIN
In humans, aggression is often linked to anger or fear. There's an emotional experience that corresponds to the lashing out. It's unclear if other animals experience similar emotional states. After all, we can't know what they're feeling, especially when it comes to animals like fruit flies.
We do know, however, that similar mechanisms are at play in fruit flies and humans alike. It's believed that serotonin and a certain neuropeptide are responsible for regulating aggression in animals. Fruit fly studies show that an increase in serotonin production correlates with increased aggression. Conversely, increased neuropeptide F moderates aggression. We see similar results in humans with serotonin and neuropeptide Y (the vertebrate equivalent of F).
The ubiquity of this relationship suggests a deep evolutionary origin. Apparently, life on Earth has been angry for a very long time.
It isn't just the brain that impacts your level of aggression. Some of that responsibility falls on your gut, and not just when you're hangry. Studies indicate there may be a relationship between the dominant biota in your digestive system and the level of aggression you exhibit. Research involving aggressive and non-aggressive dogs found different dominant species in their microbiomes.
This alone isn't enough to suggest that bacteria have a measurable impact on our behavior. It could be true that innately aggressive dogs seek out different foods. The microbiota could be a result of the aggression, not the other way around, but that's not what further research has borne out.
Mounting evidence suggests lines of communication between the gut and the nervous system, and it's not a one-way street. This is clearly demonstrated in yet another study involving mice.
Two populations of mice, one docile and one less so, were raised in a "germ-free" environment. Researchers then colonized the mice with gut bacteria from the opposite population and their behavior changed. Those mice who were previously docile became less so, and vice-versa.
Behaviors that we typically attribute to our minds appear to be much more complex, arising from a confluence of external and internal stimuli, much of which is outside our control.
A conspiring government official with sufficient knowledge of the factors at play, could, we suppose, feasibly manufacture anger and aggression within a population, Zootopia-style, if they knew what buttons to push. You wouldn't need a secret serum, just an understanding of resources, social structures, and an endless supply of antibiotics and yogurt.