I came into work, sat down at my desk, and checked to see if my computer had finished analyzing the footage I had fed it the night before.
I was trying to train a program called SLEAP to look at a video, pick out the four fish that were swimming around, and track their individual movement. I suspected that some of the fish would be consistently more active and others consistently less so.
Like people, animals can have different “personalities.” For example, some dogs like to chase tennis balls while others prefer to nap on the couch. Some cats like to chase laser pointers while others prefer to curl up in your lap. Some fish are active and swim a lot, while others tend to swim less.
I want to know what makes animals behave so differently and why those differences seem to be baked into almost every animal system. By now, we know that personalities are a combination of nature and nurture . Some things we’re born with, some things we pick up along the way.
Take me and my sisters as an example. As children, we were like an identical set of blonde, blue-eyed nesting dolls. Despite our physical similarities, our personalities were quite distinct from an early age. I was bookish and curious. Katherine was competitive. Emma was a leader. Elsa was level-headed. At some level, these personality differences were in our nature. We were born different.
But we are also products of our environments. I think that my curiosity as a kid could have led me down several career paths, but the environment I was in nudged me towards working with animals. A slow accumulation of life experiences at petting zoos, aquariums, my small menagerie of pets, and growing up in a relatively wild part of Los Angeles kept exposing me to the wonders of animal behavior. The environment I was in nurtured a love of animals and started me down the path of studying animal behavior.
I let out a small sigh. The tracking system had failed. Again.
Instead of a set of four nice, tidy points labeling the fish, I had a jumble of points that seemed to follow everything but the fish.
The fish I study are called Amazon mollies (Poecilia formosa). They are small, silvery, and honestly—pretty boring looking. But they have some pretty amazing biology.
They are named after the all-female Amazon warriors of Greek mythology (yes, the same mythical origin of Wonder Woman). Like these warriors, Amazon mollies are all female. They clone themselves! So a mother’s brood of daughters are all genetically identical to their mom, their aunts, and their grandmothers. In fact, they’ve been cloning themselves for almost 300,000 years !
However, they do not behave identically! Despite sharing the exact same set of genes, these fish have distinct “personalities.” Some are more active, aggressive, or social than others.
But how do these differences come to be? This is one of the key questions driving my research. It is possible that tiny differences in their environments begin to nudge these genetically identical fish in different directions, much like my animal experiences nudged me towards a career in animal behavior. But we also know that these behavioral differences are present from birth . So there may be something happening before they are born which causes these differences. There may be subtle changes happening to the offspring while they are still inside of the mother. For example, by changing the amount of steroids her offspring are exposed to, a mother may be able to shape their brain development .
It is also curious that despite being genetically identical and cloning themselves for thousands of generations, each generation has its share of active and less active fish. Why is this behavioral diversity maintained over generations? This is the question behind the second half of my research. I suspect that there may be an advantage to having a more behaviorally diverse population. Maybe having a mix of active and less active fish means that there is less competition for the same food sources. Or maybe having a diverse set of behavioral types is better for detecting predators.
But in order to ask these questions, I have to get our tracking system running smoothly. Once it is working, it will allow me to simultaneously track the behavior of 94 individual fish at a time. I can watch their behavior all day long for weeks at a time and see if their behavior changes as they get older or if they are exposed to predators.
I take a deep breath and settle back into my desk chair for a long session of combing through the videos and correcting the software’s predictions in the hopes that next time, it would be able to identify the fish more accurately. It is tedious work, but nature and nurture has made me both curious and stubborn.
Kirsten Sheehy is a graduate student in the Animal Behavior Graduate Group at UC Davis who is committed to improving diversity, equity, and inclusion in academia. She is exploring animal personality and behavioral plasticity using the Amazon molly, Poecilia formosa—a super rad species of genetically clonal and all female fish!
 Stamps, J., & Groothuis, T. G. G. (2010). The development of animal personality: Relevance, concepts and perspectives. Biological Reviews, 85(2), 301–325. https://doi.org/10.1111/j.1469-185X.2009.00103.x
 Schartl, M., Wilde, B., Schlupp, I., & Parzefall, J. (1995). Evolutionary Origin of a Parthenoform, The Amazon Molly Poecilia formosa, on the Basis of a Molecular Genealogy. Evolution, 49(5), 827–835. https://doi.org/10.2307/2410406Stamps, J., & Groothuis, T. G. G. (2010). The development of animal personality: Relevance, concepts and perspectives. Biological Reviews, 85(2), 301–325. https://doi.org/10.1111/j.1469-185X.2009.00103.x
 Bierbach, D., Laskowski, K. L., & Wolf, M. (2017). Behavioural individuality in clonal fish arises despite near-identical rearing conditions. Nature Communications, 8(1), 15361. https://doi.org/10.1038/ncomms15361
 Mouton, J. C., & Duckworth, R. A. (2021). Maternally derived hormones, neurosteroids and the development of behaviour. Proceedings of the Royal Society B: Biological Sciences, 288(1943), 20202467. https://doi.org/10.1098/rspb.2020.2467