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Orbit verticality and body size as predictors of vertical pupils in nocturnal mammals

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Nov 05, 2026 version files 1.18 GB

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Abstract

Vertically elongated (vertical) pupils in mammals have long been regarded as adaptations for protecting nocturnal species’ retinas from bright light and, more recently, for enhancing depth perception in ambush predators. However, these explanations are based on limited taxonomic sampling and have not been tested across representative samples of mammals. Here, we used phylogenetic comparative methods to evaluate these hypotheses across 19 terrestrial mammal orders. We found no association between vertical pupils and ambush predation or exposure to bright light in nocturnal species. Instead, vertical pupils were strongly associated with dim-light-adapted species with high orbit verticality and small body size. Optical ray tracing on a schematic domestic cat eye indicates that vertical pupils may enhance peripheral brightness along the horizontal plane under dim light. These findings are consistent with the interpretation that vertical pupils are associated with dim-light vision in nocturnal mammals with specific morphological features. More broadly, our results suggest that interactions between morphology and visual ecology play a central role in shaping pupil form and provide a revised framework for understanding the adaptive significance of vertical pupils in mammals.