i noticed this by accident long time ago and still wonder why?
usually only predators have vertical slit pupil, but degus are fully herbivore by design.
mysterious and enigmatic o_O

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Quick Googling give me this:

The degu and chinchilla have vertically long pupils, similar to those of the nutria. Regarding the role of the vertical slit pupil, the following three hypotheses have been proposed (Malmström and Kröger 2006):

1) a vertically long pupil can reduce the diameter in the horizontal direction more quickly than a spherical pupil, resulting in instantaneous control of the amount of incident light to the eyeball;

2) different curvatures at the pupil edges make it easier for different wavelengths of light to focus on the same location, reducing chromatic aberration and compensating for focus adjustment; and

3) the field of view in the grass is more open in the vertical axis, matching the vertical slit pupil, allowing animals to inhabit grass that is taller than their own body height and objects through gaps in the grass.

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And here's what ChatGPT4 thinks:

While degus are primarily diurnal rodents, they are also active during dawn and dusk, which means they may experience varying light conditions. As such, they have a vertical slit pupil, which is an adaptation to their lifestyle and helps them quickly adjust to changes in light intensity.

Additionally, the vertical slit pupil provides degus with other advantages. It allows them to maintain a sharper focus on objects at different distances, which is important for detecting predators and other potential threats. The narrow pupil also helps to reduce the amount of light entering the eye, which can prevent damage to the retina in bright sunlight.

Overall, even though degus are primarily active during the day, their vertical slit pupil is still an important adaptation that helps them to thrive in their environment.

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