The Shape of a Snake's Pupil:
- fundacionvivarium
- Aug 2
- 4 min read
Updated: Aug 4
A Biological Clock or a Map of Its Life?
Have you ever wondered what you can discover by looking into a snake's eyes? Beyond their mysterious aura, scientists have long settled on the concept that vertical pupils belong to nocturnal species, while round pupils indicate a snake with diurnal habits. The reality, however, is much more complex, fascinating, and surprising.
Beyond the Myth: What Science Teaches Us
The widespread belief directly links pupil shape to a snake's biological clock, but recent studies have challenged that idea. Research like the one published in Science Advances (Banks et al., 2015) analyzed hundreds of species and found that the pupil's shape is more related to hunting strategy and microhabitat than to the simple difference between day and night activity.
“Snakes with vertical pupils are usually nocturnal… However, this is not a strict rule, as several species defy this pattern and can change their activity based on environmental, ecological, or behavioral conditions.” (Natera, Esqueda, & Castelain. Atlas Serpientes de Venezuela.)
“…Liu et al. (2012) found that the absolute and relative eye diameter is greater in snakes with diurnal habits than nocturnal ones, suggesting that such variations in eye size may be related to eye use under different light intensities. However, this is not a strict rule, as several species defy this pattern and can change their activity based on environmental, ecological, or behavioral conditions.” (Natera, Esqueda, & Castelain. Atlas Serpientes de Venezuela.)
"Boa constrictor linnaeus, 1758: A predator of a large number of vertebrates, it can be diurnal-nocturnal and terrestrial-arboreal, inhabiting a wide variety of ecosystems from savannas and xerophytic zones to mountain cloud forests, including agricultural and urban areas." (Natera, Esqueda, & Castelain. Atlas Serpientes de Venezuela.)
What Does the Scientific Evidence Say?
Recent studies have challenged the traditional idea that the vertical pupil is simply an adaptation for night vision:
Brischoux et al. (2010) demonstrated that the vertical shape helps avoid daytime glare and improves visual acuity on the horizontal plane, an advantage for ambush predators. However, they also showed that there are nocturnal snakes with round pupils and diurnal snakes with vertical pupils. Foraging mode (ambush vs. active search) and "polyphasic" activity (species that alternate between day and night activity) better explain the variability.
Ecological niche studies show that variation in pupil shape is associated with both evolutionary history and ecological role, not just photoperiodism. The vertical pupil can aid in the detection of prey on the horizontal axis and camouflage the eye, making it useful for ambush predators, even if they are active during the day.
So, What Does a Snake's Pupil Really Reveal?
The shape of a snake's pupil isn't just an anatomical curiosity; it offers valuable clues about its behavior, hunting strategies, and adaptation to its environment. However, these clues are not infallible rules. Let's see what the science truly tells us.
Vertical Pupils: Precision for Ambush
Snakes with elliptical or vertical pupils are typically predators that hunt by ambush, especially from the ground. This pupil shape gives them greater visual acuity on the horizontal axis, which is ideal for detecting subtle movements on a low plane. It also allows them to regulate the entry of light with great precision, which is useful in low-light conditions like dusk or night.
But be careful: not all snakes with vertical pupils are exclusively nocturnal. Many are crepuscular or even diurnal. The pupil's shape says more about the hunting style than the time of day.
Round Pupils: Constant Vision for Active Foraging
Species with round pupils, on the other hand, tend to be active foragers. These snakes travel through their environment, whether on the ground or in vegetation, in search of prey, and they need a more stable and wide field of vision. The round pupil offers a constant and effective visual response in well-lit conditions.
However, there are no absolutes here either: some round-pupiled species show nocturnal activity, demonstrating that the evolution of this trait responds to multiple ecological pressures and not just a single factor.
A Look Beyond the Pupil
While there's a strong correlation between pupil shape, hunting habits, and periods of activity, we can't use this feature as a fixed rule to predict a snake's behavior. There are numerous exceptions, which is why it's important to analyze the pupil's shape along with other factors such as:
Habitat (terrestrial, arboreal, subterranean)
Diet (fast or slow, visual or olfactory prey)
Hunting strategy (ambush vs. active search)
Activity schedule (diurnal, nocturnal, crepuscular, mixed)
The Feline Analogy
To illustrate this concept more simply, think about felines. A domestic cat, a small ambush hunter, has vertical pupils. In contrast, a lion or a tiger, large predators that hunt actively, have round pupils. Both can hunt at night, but their way of doing so is radically different, and their pupils have adapted accordingly. Snakes follow the same evolutionary logic.
Conclusión
The next time you look into a snake's eyes, its pupil will tell you a story, a much deeper story than the time of day. It will reveal if it's a patient hunter waiting from the ground or a tireless explorer searching for its next meal.
References:
Banks, P. T., et al. (2015). "Pupil shape and ecological niche in vertebrates. Science Advances, 1(7): e1500391. DOI: 10.1126/sciadv.1500391 https://www.science.org/doi/10.1126/sciadv.1500391
F. Brischoux, L. Pizzatto, R. Shine. "Insights into the adaptive significance of vertical pupil shape in snakes" https://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2010.02046.x
https://www.newscientist.com/article/dn19273-slit-pupils-help-snakes-ambush-their-prey/
Atlas Serpientes de Venezuela (Natera, et al.. 2014)
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