1. Can moles survive with poor eyesight?
Moles have small eyes and low visual acuity. However, only a few species are completely blind [1]. Their ear holes are also very small or missing entirely [1]. This is likely because underground, there is no need to see far, and light rarely reaches them. Instead, they rely on touch. They use sensory hairs on their faces, legs, and tails, along with sensitive snouts, to sense their surroundings [1]. The star-nosed mole in North America has 22 tentacle-like protrusions arranged in a ring on its nose tip [2]. These help it quickly find prey in the soil. When underwater, it appears they can blow bubbles in and out of their nose tips to smell [1]. They are not alone in this. The naked mole-rat in Africa also has small eyes and low vision but lives in large underground colonies [3]. This suggests that living without relying on sight is not a unique trick. But if eyes are not essential, why do most animals keep them? Is there a cost to having eyes?
2. Why cave-dwelling Mexican tetras lost their eyes
The Mexican tetra, a fish from caves in Mexico, has two types: a surface type with eyes and a cave type without them. Researchers think that in the cave type, a gene stops blood flow around the eyes during development, covering them with skin [4]. They are not born blind; their eyes form and then are covered. Why go this far? A 2015 study calculated the energy cost of eyes and the visual parts of the brain. For a young surface fish weighing about 1 gram, about 15% of the energy used while resting goes to maintaining vision [5]. By the time they reach 8.5 grams, this drops to 5% [5]. In a dark cave, eyes offer little benefit. Researchers think this high energy cost helped drive the evolution of losing eyes. Instead, cave fish rely on the lateral line, a sensor along their body that detects changes in water pressure and flow [4]. This lets them swim by feeling walls, rocks, and other fish. Eyes are not always an advantage.
3. How do scientists measure animal vision?
Human vision is tested with charts showing the letter C. We identify which way the gap faces. This requires language, so it does not work for animals [6]. Instead, researchers use grating patterns—boards with thick and thin stripes. They observe which pattern an animal looks at longer or how their eyes move [6]. As stripes get thinner, they blur into gray. The finest stripe an animal can distinguish determines their visual acuity [6]. This is measured in cycles per degree and converted to human-like numbers [6]. So, animal vision scores are estimates based on stripe detection, not reading. Results can change with lighting or method. Keep this in mind when looking at specific animals.
4. Do eagles really see better than humans?
We often hear eagles see much better than us. A 1985 study on the wedge-tailed eagle in Australia estimated their vision at about twice that of humans, using both stripe tests and cell counts [7]. A 2018 review also cites this figure [8]. However, this is not true for all birds of prey. The American kestrel has 65,000 light-sensitive cells per square millimeter in its central eye area, while humans have about 38,000 [9]. But condors, which eat dead animals, have only about 35,000 cells [9]. Since prey does not flee from them, they do not need high resolution. Many medium and small birds of prey have resolution equal to or lower than humans [9]. A 1986 paper even discussed the limitations of eagle vision [10]. So, "birds see well" depends on the species and lifestyle. Beyond resolution, many birds see more colors. Most have four types of color cells, including ultraviolet, compared to humans' three [9]. Some flower and insect patterns are only visible in UV light. Some birds have sharp eyes, while others have superior color vision. "Good eyes" has multiple measures.
5. Why owls have trouble seeing things up close
Owls have very large eyes relative to their head size. Instead of moving their eyeballs, they turn their necks up to about 270 degrees to look around [11]. Their ability to spot mice in dark forests is due to sensitivity 10 to 100 times greater than humans in low light [11]. However, these eyes have a weakness. Most owls are farsighted and cannot focus clearly on objects closer than a few centimeters from their eyes [12]. This is likely an evolutionary result for catching distant prey. They may use sensory hairs around their beaks to check nearby objects. Strong night vision and poor close-up vision coexist in one eye. How we judge an owl's vision depends on the conditions.
6. Can animals be nearsighted like humans?
Owl farsightedness is due to eye shape. What about nearsightedness? In humans, it often happens because the eyeball grows too long. Researchers have tested this in chicks, monkeys, and other animals [13]. By blurring vision with lenses or blocking light, the eyes grow longer and become nearsighted. This suggests eyes adjust their length based on light stimulation [13]. In fish and birds, this artificial nearsightedness can reverse quickly if conditions return to normal [13]. Nearsightedness is not just a human issue; it is a common reaction in eye development. Even cats show trade-offs. Cat vision is roughly 0.1 to 0.3 on the human scale. An object a cat sees at 6 meters (about 20 feet), a human can see at 20 to 30 meters (about 65 to 100 feet) [14]. But cats can see in light one-sixth as bright as humans need [14]. A reflective layer called the tapetum lucidum bounces light back to the retina [14]. Clear vision versus night vision is a choice made by many animals, from moles to eagles.
7. How to compare animals' eyes at zoos and aquariums
Zoos and aquariums offer chances to compare eye structures. Look at owls and eagles. Note the size and position of their eyes. Animals with large forward-facing eyes see differently than those with small side-facing eyes. If you find an aquarium with Mexican tetras, you can compare surface and cave types [4]. At home, you might see a cat or dog's eyes glow in the dark. This is the tapetum lucidum working [14]. Never shine smartphone lights or flashlights directly into an animal's eyes. For more details, read the original sources, such as the 1985 eagle study or the 2015 cavefish energy paper [7][5].