Field of Vision
The gerbil's field of vision is wide and is mainly due to the positioning of their eyes on the head. A human's field of vision is narrower than this because the eyes are positioned on the front of the head, which is dissimilar to that of the gerbil. The human's field of vision is around 180 degrees of a circle if measured laterally i.e., from side to side of the head. A gerbil's view of the world is much higher than this, being around 215 degrees. So the gerbil can see much more of its world at any given moment. This type of vision has evolved so that the gerbil can see any potential predators as they come into its field of vision. It doesn't need to see the predator clearly or know how close it is, it just needs to see it the moment it comes into view. This type of vision is different to that of the carnivore or omnivore because they need to see how close an object is and be able to judge its movement, so they can make accurate judgements based on both of those parameters. To do this they need binocular vision, or to be able to view the object with both eyes, which mean the eyes are positioned at the front of the head so that both eyes can then view the object.
The field of vision of both eyes overlap because the image from each eye is slightly different. These slightly differing images from each eye are then interpreted by the brain to build up a 3D image of the world. This type of vision allows them to judge how far away an object is and also how fast it is moving. It does however reduce the total field of vision. In humans', binocular vision is around 140 degrees out of a possible 180 degrees. The gerbil does have very slight binocular vision but the overlap from the two eye images is restricted to peripheral vision only and makes up a small percentage of its view of the world. This is the exact opposite of primates that have true binocular vision, where they have only a very small percentage of monocular vision around the edges of their field of vision.
The pupil and its role in gerbil vision
A gerbil's pupil is positioned vertically in the eye. This is consistent for animals that live in desert or open steppe environments. Having vertical pupils gives the gerbil greater depth perception in the horizontal plain. This enables them to see predators both on the ground and in the air. Horizontal pupils such as those found in sheep, goat, deer etc, give greater depth perception in the vertical plane, which allows them to watch predators approaching on the horizon.
Can a gerbil see in colour?
Gerbils have some form of colour vision, but it is quite dissimilar to human vision. The signals that the brain interprets from the eyes come from cells called rods and cones on the retina, human cones are sensitive to red, green and blue light, and the signals from these cells when interpreted by the brain produce the colours we see in everyday life.
Gerbils like humans also have rods and cones, but a far greater proportion of rods, ( 87% rods 12-14% cones) this is to be expected in rodents that are largely active at dawn and dusk and explains why gerbils eyes look huge compared to more nocturnal species such as mice and hamsters. The retinas of nocturnal and diurnal mammals differ quite a bit in their photoreceptors. Cones are regarded as the receptors for day vision where as rods are the receptors for night vision. Day vision is coloured and sharp and night vision is colourless and blurred. The gerbil which shows active periods during day and night uses both rod- and cone-based vision, and the retina of the diurnal gerbil contains around 12-14% cones as mentioned above, whereas the retinas of truly nocturnal species such as nocturnal flying squirrels contain only 1% cones and are rod dominated for night time vision. It has been established that gerbil's retinas have a maximum cone density of around 45,000-50,000 cones/mm2, and are regarded as having a well-developed cone system. The types of cones present on the retina determine what colours the gerbil can see in. The most abundant cone type (11-13% of photoreceptors) is the middle-to-long-wave sensitive cone visual pigment, which are green sensitive. A minor cone population (around 2-5%) are blue sensitive cones. So it's highly probable that the gerbil may possess dichromatic green-blue colour vision.
In addition to this, the cone types present on the gerbil's retina that are sensitive to shorter wavelengths also has the ability to be sensitive to UV light. These cones make up around 1% of the total cone population present on the gerbil's retina. Apart from the gerbil there are only three other rodent species capable of seeing UV light. These are the house mouse (Mus musculus), brown rat (Rattus norvegicus), and Botta's pocket gopher (Thomomys bottae). They are unique among mammals in being able to do this. These UV wavelengths are well beyond the spectrum visible to humans. The ability to see UV light could have many useful functions, for e.g. certain staple foods, scent markers or even predators or predators faeces may appear to the gerbil to fluoresce because they emit radiation at a frequency that can be picked up by these cone pigments.
It has been noted that scent marks of small rodents are known to be visible in Ultra Violet light due to fluorescence (Desjardins et al. 1973) because the scent markers absorb parts of UV radiation and emit the absorbed energy as visible light. Predators with this ability to detect UV light use scent markers to find and assess prey densities. Birds of prey are known to differentiate between different species of animals by the fluorescence that their scents marks gave off, and this is how they identify their preferred food.
How do gerbils use their eyesight with their other senses?
In humans the central part of the eye is known as the Fovea. It is packed with cones and is the point where vision is most acute. It allows us to examine things closely, for e.g. when we read, or to recognise a familiar face, etc. The human devotes much more of the brain to analysing the visual information from the fovea than it does to the rest of the retina. Humans rely very heavily on the fovea than the rest of the visual system as it provides precise information that we need in every day life. The gerbil and other animals are not physiologically equipped to resolve fine detail like the human. The brain has evolved different kinds of problem solving strategies in different species, presumably because in each species the problems for continued survival are different, so the brains path of evolution has been different too.
A gerbil's visual acuity is very different to humans, and they depend on several other prime senses combined to assess information presented to them. For e.g. with gerbil eyesight, if you held two fingers out approximately three feet away from them, they would find it impossible to distinguish the two fingers and would see them as a single object. Recognition of other members of a colony is also not based on vision alone, and such senses like smell come into play. A gerbil can often gather a good deal of information about an object by simply running over it, using their sense of touch.
As mentioned earlier, everyday problem solving in the gerbil is very different from the human. In the wild it has to deal with several predators, and every time it leaves the confines of its burrow it risks exposure to predators. Because of this the gerbil has to be able to detect a threat instantly to be able to escape back into its burrow. To be successful in this the gerbil has to be able to define and compute distances very quickly and accurately. It also has to know how far to jump to reach safety. When a human faces the problem of negotiating and navigating itself through a space they tend to use the peripheral areas of the retina to achieve this. Although a gerbil doesn't possess the focussing physiology to achieve specificity in what it sees they are remarkable at navigation, at estimating speeds and also distances.
Because gerbils lack the precision and sharpness of vision to accurately distinguish objects like humans can, they have evolved different strategies in achieving this. An example of this is if a human is sat at the dining table and was asked to pass the saltcellar, one strategy that could be used for discriminating the salt from the pepper would be if the two containers looked different. A gerbil cannot use this strategy because it lacks the precision to distinguish the differences between the two objects. However, if the salt was always at one side of the table, and the pepper always at the other side, it's most likely that you could reach the salt without having to look at the object very carefully. A gerbil would most likely use spatial strategies for recognition similar to this in its natural environment.
A gerbil doesn't necessarily have to recognise a predator on what it looks like, it only needs to know where it is. So if there is a sudden movement overhead it will simply dart to the nearest tunnel entrance, without the need to check if it's a predator or non-predator. In familiar territories it will act very differently to its actions in an unfamiliar territory, but its overall reaction is based on where the potential predator may be located, than with what it actually sees in its field of vision.
To determine gerbils responses with predators and also to analyse different special strategies undertaken by gerbils, many differing tests have been conducted to find out how and why they respond in various situations.
In predator situation tests, gerbils are released into a large area and various objects are passed along them at ceiling height, and their responses are filmed. Whenever any object passes across their upper visual space, the gerbil will react very rapidly. The object in itself need not look like a bird of prey; it can just as easily be a black piece of cardboard to initiate a fleeing response in the animal. This basically means that the gerbil will react in a given situation when conditions are appropriate, but it doesn't base its actions on the basis of visual details. It will flee to the nearest burrow depending on how it perceives the positioning of the predator; its position in the sky and how the predator is moving.
In these tests an interesting thing was discovered which was that when fleeing from an aerial predator a lone gerbil will react very differently to two or more gerbils in the same situation. A lone gerbil will simply flee into the burrow when a threat is perceived, however if two gerbils are in the same situation, they will never queue up to go into the same burrow, the second gerbil will always seek out an adjacent burrow. Their reaction to a predator is modified in the company of other gerbils because they have learned over countless generations that the best way of assuring their survival is to go to the two nearest places of refuge.
If several gerbils are placed into this situation, where the burrow has several openings they have worked efficient methods of avoiding the problem of queuing. This is a problem that would very much tax human's capabilities, but gerbils are able to decide instantly how to react under these given circumstances, and can take off without looking, because the gerbil has evolved a remarkable ability to accurately map their whereabouts in a given space. To do this they don't have to entirely depend on their vision alone.
Article by Eddie Cope
Thanks to Liz Arblaster for the Red-eyed gerbil photos & Loz Whitmore for the close-up eye main picture