January 24th, 2022
Written by: Kara McGaughey
As 2022 came to a close, the world’s population surpassed 8 billion. Still, somehow, there’s something about each of us that sets us apart. We have different skill sets, different interests, and different viewpoints, which raises the question: When does this individuality emerge and how does it continue to unfold across the lifespan?
What factors determine individuality?
Our remarkable capacity for individuality is thought to come from differences in both genes and environmental conditions. This interplay between nature (our genetics) and nurture (our environment) and how it influences development is something that has fascinated scientists for decades.
If only genetics were important, people with identical genetic makeup (i.e., identical twins) would develop into identical individuals even if they grew up thousands of miles apart. If only environmental conditions were important, children from the same school or the same neighborhood would develop similarly, having been “nurtured” to behave in certain ways. While we’ve all heard some wild stories of twins separated at birth, we know from experience that despite similar genetics or similar environments people develop into different individuals.
Why turn to fish?
In order to explore how differences in genes and environmental conditions influence behavior and whether individuality can develop without them scientists need to perform rigorous experiments. Ideally, these experiments would use identical subjects (i.e., having all the same genetics) raised in exactly the same environment. The rarity of identical twins (3/1000 births worldwide) combined with the fact that it’s impossible to fully control their environments and lived experiences makes individuality difficult to study in humans .
Fortunately, individuality is not a uniquely human feature. It’s found in most animal species, which means that scientists can use genetically identical animals, like the Amazon molly fish, as subjects. These female, freshwater fish native to Mexico and Texas reproduce by cloning themselves . Not only are all offspring genetically identical and born within minutes of each other (“twins” en masse!!), but it’s infinitely easier to control their environment — tank dimensions, water temperature, lighting, food, etc. — than ours. Moreover, Amazon molly fish require no parental care, making it possible for them to be isolated and placed in this controlled environment immediately after birth.
Last year, using Amazon molly fish, a group of researchers in Berlin, Germany carried out a “twin study,” attempting to explore if and how individuality emerged among a genetically-identical group of fish exposed to the same environment . They separated a large number of fish at birth into isolated, highly-controlled environments and continuously monitored their behavior over 10 weeks for signs of individuality. The group developed a custom recording system, which they positioned above each fish tank and programmed to take photos every 3 seconds during the daylight period for the entirety of the experiment. Using all these photos, the researchers were able to measure and quantify a host of fish behaviors for each fish, including swimming distance, direction, and speed.
What does individuality and its emergence look like?
At the onset of the experiment, researchers identified three possible ways that individuality might emerge among the Amazon molly fish: Individuality could be present at birth (Figure 1, Left), emerge gradually after birth (Figure 1, Center), or emerge all at once after some period of “sameness” (Figure 1, Right). The group used their measures of where, how far, and how fast the fish were swimming to determine which emergence scenario was happening.
Researchers found that even on the first day after birth Amazon molly fish showed large, consistent individual differences across all measures of behavior. While some fish spent nearly all 11 daylight hours in motion, some moved very little, and others moved more or less depending on the time of day. These initial behavioral differences were consistent across the first week of recording, meaning that the fish who tended to swim more did so day after day. Importantly, these differences in swimming distance and speed could not be explained by other factors, like the size or weight of the fish. This suggests that it was differences already present at birth that had the biggest influence on behavior. (Figure 1, Left).
What happens to individuality over the course of development?
Looking beyond the first week of life, it became clear that not only was behavioral individuality present shortly after birth, but that these individual differences were increasing gradually over time (“fanning out” like Figure 1, Center). In other words, individual fish continued to display differences in movement patterns and speeds and these differences got larger as the fish aged (Figure 2). Factors like fish size, weight, and growth rate, couldn’t account for these differences. Instead, it was the uniqueness of behaviors observed early in life that best predicted behaviors later in life. Fish that were most unique on day 1 stayed most unique over the course of the 10 weeks.
The take-home message? Even when genetics and environmental conditions are identical, individual differences persist and increase with age. While the reason for the emergence of these differences remains uncertain, it is clear that they can influence the trajectory of behavioral development, which could have meaningful impacts on later health. While an experiment with fish is not an experiment with humans, these ideas certainly still have the potential to inform future research. There are a host of follow-up studies where the environmental conditions can be systematically varied, testing how nurture and life experience affect the natural emergence of individual differences.
In addition, as the new year rolls around and many of us begin to think seriously about behavioral change, this work invites reflection on the close link between our behavior and ourselves. Perhaps it also offers some comfort to those of us still figuring things out, putting forth that we’ve been unique and forging our own trajectory since the very beginning.
 Hoekstra, C., Zhao, Z. Z., Lambalk, C. B., Willemsen, G., Martin, N. G., Boomsa. D. I., and Montgomery, G. W. (2008). Dizygotic twinning. Human Reproduction Update, 14, 37-47. https://doi.org/10.1093/humupd/dmm036.
 Heubel, K. (2004). Population ecology and sexual preferences in the mating complex of the unisexual Amazon molly Poecilia formosa. [Doctoral dissertation, Hamburg University]. https://ediss.sub.uni-hamburg.de/handle/ediss/736
 Laskowski, K. L., Bierbach, D., Jolles, J. W., Doran, C., and Wolf, M. (2022). The emergence and development of behavioral individuality in clonal fish. Nature Communications, 13, 6418. https://doi.org/10.1038/s41467-022-34113-y
Figure 1: Illustration adapted from Laskowski et al. and made in PowerPoint .
Figure 2: Made in PowerPoint.