Nanachi is an anthropomorphic animal character in the manga and anime series Made in Abyss. They have pupils shaped like horizontal rectangles. Rabbits, the Earth species that Nanachi most obviously resembles, have round pupils– but they also lack the pawpads and long tail prominent in Nanachi’s design. What animals was Nanachi’s creator thinking of when he added this feature, and what does it tell us about Nanachi’s sensory experience and most likely ecological niche?
The most prominent animals with horizontal pupils are ungulates: horses, goats, deer, sheep. Their pupils give them a wide field of vision that makes it easier to keep watch for predators given that they spend so much time grazing with their heads close to the ground.
There’s also a diverse collection of carnivores with horizontal pupils. Some mustelids (weasels, martens, fishers, and wolverines) have them, while others (badgers, ferret-badgers, and otters) have standard mammalian round pupils. Mongooses and meerkats may look like mustelids but are more closely related to cats; nevertheless, they have distinctly horizontal pupils. My initial impression was that horizontal pupils help these short-legged predators track horizontal motion as they hunt on flat expanses of ground. Then a friend of mine pointed that martens and wolverines also climb to hunt arboreal prey like birds and squirrels, so I’m not sure what’s going on here– maybe martens and wolverines spend enough time on the ground to make horizontal pupils worth it on net, or it’s a conserved trait among that branch of mustelids, or some mysterious other thing.
We can assume that Nanachi has better peripheral vision than their round-eyed companions, which is useful for a cute fluffy thing in a hostile environment like the Abyss. Nanachi is frequently depicted eating meat, but their pupils would be of little help in hunting given that the Abyss’s vertical orientation ensures that most animals are adept fliers or climbers (unless Nanachi’s tapped into the mysterious factor that also aids martens and wolverines in arboreal hunting).
The real question is: when they turn their head, do their pupils stay parallel to the ground?
For ungulates, the answer is “yes”. Having horizontal pupils to watch for predators isn’t particularly useful unless you can rely on them to stay in position regardless of where your head is. For carnivores, the answer is less clear: they move their heads more frequently than grazing animals do, so there’s less disadvantage to briefly having their pupils in a suboptimal orientation. However…
Despite the furry bits, the animal that Nanachi most obviously resembles is a human. And when humans rotate their heads, their eyes rotate to preserve their orientation! You can go to a mirror right now and test it out!
So yeah, Nanachi’s eyes would look weird if they tilted their head to the side.