The Croak #11 - Night Vision: The liminal world of frog eyesight

(he) "wears yet a precious jewel in his head" 

-William Shakespeare

Frogs are the most numerous of the amphibians, with almost 7500 species known globally. They are exceptionally diverse, exhibiting an extremely wide array of life histories, behaviours, breeding strategies, shapes, colours and sizes. This diversity of adaptations is reflective of the diverse habitats that frogs occupy – from waterways, wetlands, trees, grasslands, mountain seeps, forests and even deserts, where tunnel-dwelling species have evolved particularly interesting survival strategies to cope with arid conditions.

These adaptations to different habitats are reflected in the anatomy of different frog species, with burrowers having digging tubercles, swimmers having varying degrees of webbing and climbing frogs having sticky toepads or in the most extreme of cases, even being able to glide. When it comes to the features of frog eyes, these are also variable and related to the lifestyle and micro-habitat of the species. Most frogs are nocturnal, and are supreme predators of their invertebrate prey, so eye position and vision are of extreme importance. 

Eyes are among the most charismatic features of frogs and toads and are some of the most spectacular eyes of any animal. People have long marvelled at the animals’ many iris colours and pupil shapes. Yet little is known about the anatomical basis of that diversity. Frog eyes come in a stunning range of colours and patterns. Most frogs see well only at a distance, but they have excellent night vision and are very sensitive to movement. Most species have large eyes that protrude above the top of their head, giving them extremely wide fields of vision and excellent depth perception. The bulging eyes of most frogs allow them to see in front, to the sides, and partially behind them. Another interesting function of the eyes of a frog is their aid in eating. When a frog swallows food, it pulls its eyes down into the roof of its mouth. The eyes help push the food down its throat. Although a frog has teeth and a tongue like humans do, they use their teeth to keep the victim in the mouth and not for chewing. A frog’s tongue is attached at the front of the mouth as opposed to the back, like ours. This is why it is difficult for a frog to swallow without the help of their eyes.

Each eye positioned atop the head gives frogs a field of vision of up to 360 degrees, which helps for a species that can’t turn its head. This peripheral vision helps them spot predators and prey.  Frogs would starve to death surrounded by food if it was not moving. Evolution has favoured vision in amphibians that focuses on active and mobile objects.

Unlike humans, frogs have three distinct eyelids – an upper and lower eyelid, and a third known as the nictitating membrane, which is clear and helps protect the eye from damage and drying out. For species that spend time under water, this membrane acts like goggles, allowing the eye to remain open so that the frog can see underwater without the eye coming into direct contact. The nictitating membrane of the red-eyed tree frog, Agalychnis callidryas, has a spectacular tiger-stripe design, which camouflages the bright red colour of the eyeball without compromising the frog’s vision. 

Figure 1: Frogs have nictitating membranes – a third, clear eyelid, that helps protect the eye

The shape and orientation of the pupil of is also highly variable, with a study in 2021 identifying seven main shapes: vertical slits, horizontal slits, diamonds, circles, triangles, fans and inverted fans. The most common shape, horizontal slits, appeared in 78 percent of studied species. Though uncommon in other vertebrates, horizontal pupils seem to have given rise to most of the other shapes in frogs and toads. Pupil shape affects the amount of light that reaches the retina and its light-receiving cells. These photoreceptor cells come in two types – rods and cones. Rods are generally used in low-light conditions and cones in bright light. Nocturnal species tend to have more rods, while diurnal species have more cones. For diurnal species, the risk of damage to their retinas from UV light, has prompted adaptations such as UV-blocking pigments in their lenses or protective lenses. The Maluti River Frog, Amietia vertebralis, is a great example of this, with an outgrowth above the pupil called an umbraculum, that helps shield the eye from UV radiation – an important feature for a water-dwelling frog in a country with extremely high incidence of cloudless days at high elevation. 

Figure 2: The distinctive 'umbraculum' of the Maluti River Frog, Amietia vertebralis, that helps shield the eye from UV radiation at high altitudes

Frogs that live in low-light conditions, such as forests or leaf-litter, or that are largely nocturnal have very sensitive eyes, allowing them to navigate these dark places and still be able to catch prey and avoid predators. The South African endemic genus of Ghost Frogs, the Heleophryne, get their name from the ‘spooky’, dark gorges in which they are found, and have some of the most intricate eye patterns of any species in the country. The Table Mountain Ghost Frog, Heleophryne rosei, named for Skeleton Gorge where it was first discovered in 1925. These frogs live in small crevices in rocks and streams alongside fast-flowing streams. This complex environment may be the reason for the beautiful eye patterns. 

Figure 3: Table Mountain Ghost Frogs, Heleophryne rosei, have incredible patterning on their eyes – Photo credit: Joshua Weeber

Frogs’ eyes can also come in a dazzling array of colour, from red, to gold, to yellow, to green or blue, and all sorts of combinations of colour and pattern. The bright eye colours, for example the bright red of well, Red-eyed Tree frogs, Agalychnis callidryas, from Central America, is thought to play a role in predator avoidance. These frogs spend most of their time trying to blend in. With their legs pulled in tight and their eyes closed, they look like little more than a vague, green blotch. But should a predator still notice them, these frogs can use their shocking colour scheme to their advantage. When disturbed, they flash their bulging red eyes and reveal their huge, webbed orange feet and bright blue-and-yellow flanks. This technique, called startle coloration, may give a bird or snake pause, offering a precious instant for the frog to spring to safety. While the red-eyed tree frog is not toxic, that flash of colour is enough to startle a predator or make it second guess its choice, giving the frog that split-second it needs to escape. The same principle may apply to our local Natal Tree Frog, Leptopelis natalensis, with its large, orange eyes with cat-like vertical pupils. 

Figure 4: Natal Tree frogs, Leptopelis natalensis, have large, protruding orange eyes, with the distinctive vertical pupil. Photo credit: Kyle Smith

Another amazing fact about frog eyes is that they can regenerate structures of the eye after damage, and this serves as a scientific model to study this process. Current research aimed at blindness prevention in humans involves the chemical induction of cell regeneration. Frog eyes also change as frogs develop from larval phase to adults. For species that start life as a tadpole underwater, their eyes must change when they metamorphosize into a frog. This happens because the speed at which light travels is slower in water than in air. As tadpoles adjust their sight to live in the terrestrial environment, the lens changes from spherical to a flatter shape to maintain vision when light is moving faster through air.

Diversity is life, and for the frogs, this is reflected down to their retinas. No pun intended. Yet another fascinating aspect of amphibian biology demonstrating the incredible evolutionary journey that has led to this group being able to successfully colonise most ecosystems through their 300-million-year existence.