Nature, nurture and randomness

Author: Kate Laskowski is a behavioral ecologist at the University of California, Davis.

More than genes and upbringing determine animal personalities: There’s a good dose of chance in the mix, too.

Fish such as the Amazon molly (right) have individual personality quirks from birth. Photo: Manfred Schartl

In the age-old debate about nature versus nurture — whether our characteristics are forged by our genes or our upbringing — I have an answer for you. It is both. And it is neither.

I’m a behavioral ecologist who seeks to answer this question by studying a particular kind of fish. The Amazon molly (Poecilia formosa) is an experimental goldmine for these types of questions. She naturally clones herself by giving birth to offspring with identical genomes to her own and to each other’s. A second quirk of this little fish is that her offspring are born live and are completely independent from birth. This means I can control their experiences from the earliest possible age. Essentially, this fish gives me and my colleagues the opportunity to perform “twin studies” to understand how and why individuality develops. And what we’ve found may surprise you.

As humans, we know the critical importance of our personalities. These persistent differences among us shape how we navigate our worlds and respond to major life events; whether we are bold or shy; whether we ask someone on a second date or not.

Given the obvious importance of personality, it’s perhaps a bit surprising that scientists generally overlooked these kinds of differences in other species for a long time. Up until about 30 years ago, these differences (what I prefer to call “individuality,” as it avoids the human connotation of “personality”) were typically viewed as cute anecdotes with little evolutionary importance.

Instead, researchers focused on the typical behavior of a given population. With guppies, for example — a classic workhorse of behavioral ecology research — researchers found that fish will, on average, swim more tightly together if they live among lots of predatory fish, whereas fish from areas with fewer predators spend less time schooling and more time fighting one another, as they don’t have to worry so much about being eaten.

We now realize that there is at least as much variation between individual fish in the same population as there is between populations of the same species. It’s like height in humans: While men are on average taller than women, some women are taller than some men. These individual differences can matter a lot for an animal’s success. In those guppies, for example, bold behavior increases a male’s chance of mating, but also increases his chance of finding himself in the jaws of a larger fish. Which behavioral tactic is best can change depending on the circumstances. Aggressive mice are very successful in resource-rich environments, but if resources are harder to find, less aggressive mice do better by spending more time finding elusive food.

So what shapes these consistent behavioral differences between individuals? A whole sub-field of behavioral ecology has now spent more than 20 years trying to figure out when, how and why individuality emerges. This is where our cloned fish come in.

When we rear our genetically identical Amazon mollies in identical environments, we might expect them to behave the same. But they don’t. These little clones show individuality on the very day they are born — some are highly active and social, going far afield for food; others prefer to hunker down and snatch drifting food as it approaches them. Something else is guiding their individuality.

Our genetic code is just one component of the instructions we inherit from our parents. We also inherit myriad other molecules that interact with our genes to tell our cells which genes to activate when, among other things. These so-called “epigenetic” mechanisms might be playing a larger role in the development of our behaviors than scientists previously thought.

Our cells are always subject to some level of randomness. In a developing fish, the connections among the neurons in her brain would never be exactly the same, for example, if we could somehow ask her to develop twice. Given the same recipe and the same kitchen, even the same baker will produce slightly different cakes on different days because of the seemingly minuscule vagaries involved in measurement or stirring technique. While all creatures are subject to such noise, its impacts are usually masked by the effects of larger variation in genetics and environment. It’s only with our mollies that we’re getting a handle on how big a role randomness may play in the development of individuality.

Where does this leave us? As humans, we spend considerable time and money trying to nail down why we are the way we are. Obviously, the biology we inherit from our parents and our experiences will always play major roles in our lives and our personalities. But my mollies have also taught me that it’s not all down to that; there’s a good dose of randomness in there, too.

For me, this idea of random chance playing such a big role in our personalities is liberating. We are who we are, sometimes for little reason at all. I take this as a call to embrace the diversity of ourselves. Each of us is truly unique; we are inherently impossible to replicate.

This article originally appeared on June 6th 2023 in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. doi: 10.1146/knowable-060623-2. Sign up for the newsletter.

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