Of all the creatures that stressed out Newt Scamander in Fantastic Beasts and Where to Find Them, the size-shifting snake-bird hybrid he called an occamy raised the most hell. The occamy’s defining trait is its ability to grow to fit the space available to it — a characteristic, J.K. Rowling revealed in a recent interview, that was inspired by a common misconception about pet fish. Rowling used to believe that goldfish would grow to fit their tanks, but her deep dive into real-life biology showed she was very much mistaken.
“I had been told in my youth that fish only grow to the size that’s available,” Rowling told Entertainment Weekly. “But then I looked it all up and found, to my horror, that fish remain stunted if the water quality is poor so they don’t have that quality at all.” This error led her to coin the new term choranaptyxic, defined as “a creature that can shrink or grow to fit available space.”
Scientists used to debate whether a version of this trait existed in nature. Big animals exist and have existed in nature, sure, but do they evolve to be so big because their “available space” was “all the space”? If so, then why do small animals in the wild exist — and persist? In 2008, researchers publishing in Science used computer modeling to show that the general rule of thumb in nature is that big is better, until it’s not: They found that, while it’s true that evolution favors bigger mammals, there’s another force that prevents growth from freewheeling out of control — extinction.
“The tendency for evolution to create larger species is counterbalanced by the tendency of extinction to kill them off,” Santa Fe Institute computer scientist and lead author Aaron Clauset, Ph.D. told LiveScience. “The distribution of sizes over time is stabilized because these processes balance out.” While bigger animals may seem to be at an advantage because they can overpower predators, travel longer distances for food, and store more nutrients, it’s also harder for them to fill up their bellies and find shelter. Over the course of evolutionary time, the interplay of these factors — with each other and with the environment — led to most animals becoming a lot smaller than we’d expect, given the amount of space available.
Follow-up studies on this topic revealed more details about why animals don’t grow to fit their available size. In 2011, McGill University ant researchers publishing in Nature Communications found that body size in the species they studied was largely controlled by a single gene called EGFR. This gene’s activity was toggled through methylation, a natural process that lets the body turn gene function up and down as if it were controlled by a dial. The study provides evidence that there was reason for this species to evolve a control mechanism for size, which in turn suggests that growth isn’t a process that will continue unchecked just because an individual has all the space and nutrients it needs.
Taken together, the existing research on body size suggests that Scamander’s choranaptyxics, if they existed IRL, would fare very well indeed; their ability to become excessively large or fantastically small would allow them to skirt both the pressures of extinction and evolution. Of course, while it’s entirely possible that Rowling might have seen one especially large fish in an aquarium in her youth, leading her to believe that the magical occamy could exist, it’s pretty likely that it was just a case of overfeeding — and very unlikely that it would have grown that big outside of its tank.