Simple observation of squirrels chasing through the tree tops suggests that they are equipped with good eyesight. Morphological examination of the squirrel eye suggests excellent, wide-angle vision with exceptional focussing power across the whole retina. We humans, by contrast, have a single point on the retina, called the fovea centralis (or simply fovea), where our colour and detail sensing cone cells are most densely packed and hence our vision is sharpest. The result is that while we can only focus on a single (central) area at a time, the periphery remaining blurred, a squirrel can clearly see what's next to, and above, it without moving its head. Squirrels, like humans, also have a blind spot caused by the optic nerve passing through the retina and, in The Eurasian Red Squirrel, Bosch and Lurz note that it forms a small strip at the top of their field of view. Squirrel vision is based heavily on movement. They will approach an observer to within a couple of feet, if she/he is standing very still, although the slightest movement can have them dashing for cover.
The concept of colour vision in sciurids has been studied, with some mixed results. Early retinal studies suggested that it was composed predominantly (if not entirely) of cones, implying good visual acuity and the ability to see colours. Studies of light-sensing chemicals in the retina called photopigments implied the presence of pigments sensitive to long (red) and medium (green), but not short (blue), wavelengths of light. Work on squirrel spectral sensitivities by Barbara Blakeslee, Gerald Jacobs and Jay Neitz during the late 1980s, however, found that Greys had a peak rod sensitivity of 502nm (note: rods only sense light intensity, not colour) and peak colour sensitivities at 444nm (blue) and 543 nm (green). This implies that squirrels are sensitive to blue and green wavelengths, but cannot decipher red.
More recently, electroretinographic studies suggest that squirrels do possess dichromatic (two-colour) vision, with colour discrepancy similar to a human protanope (i.e. someone who is red-green colour blind). Grey squirrels (and probably Reds) have a two-tiered retina: one tier in the centre of the retina is composed of rod cells that are used in low light and provide monochrome vision, and the outer tier is composed of colour-sensitive cone cells – the rod to cone ratio in Greys is about 2:3. Overall, this implies that squirrels have good vision and the ability to see colours, albeit that their pallet is reduced over ours and red objects will have a yellow-green hue. The comparatively sparse rod cells also suggests that these animals have relatively poor scotopic (dusk) and nocturnal (night-time) vision compared to, for example, a wood mouse or fox, both of which have more rods.
The foregoing does, however, raise the question of how, if Greys are protanopic, they managed to discriminate between the red and green biscuits offered to them by the University of Exeter’s Ian MacDonald? MacDonald found that not only could his subjects discriminate between red and green in a foraging situation, but they could also discriminate colour hue (intensity), which seems unlikely given the retinographic and photopigment data. The reason for this contrariety is unknown, but it has been suggested that the squirrels may have been responding to the smell or brightness of the biscuits, rather than to colour.
Tree squirrels also have a pale yellow pigment in their lenses that absorbs ultraviolet light. Originally, biologists thought that the yellow pigment served simply to reduce chromatic aberration and provide better contrast to the image; i.e. that it serves a function similar to sunglasses, reducing glare. It has been suggested, however, that its role is much more important than simply offering clearer vision in bright conditions; the pigment may offer valuable protection to the squirrel's retina. In a paper to the journal Investigative Ophthalmology and Visual Science during 1989, Robert Collier and colleagues at the University of Rochester, New York, described how aphakic squirrels (i.e. those without a lens in their eye) exposed to UV light showed significant lesions on their retina, while those that had normal lenses showed no such damage. The anatomists concluded:
“The results of this study prove that the yellow lens of the gray squirrel is a near-UV radiant energy filter that is capable of protecting the retina from physical damage that would result from exposure to environmental levels of long-wavelength UV energy.”
There are very few studies on the hearing thresholds of squirrels, although several authors have commented upon how these animals seem sensitive to even relatively quiet disturbance. Moreover, alarm calls and other vocalisations are an important part of squirrel society, which suggests that sound is important to these animals. I am not aware of any studies on either Red or Grey squirrels. Indeed, the majority of studies have been conducted on Fox squirrels (Sciurus niger), a relative of the Grey found in North America. In one series of experiments, psychologists at the University of Toledo, led by Laura Boester, investigated the hearing ability of this species. Boester and her colleagues found that their subjects could hear sounds of between 113 Hz and 49 kHz when played at 60 dB (about the volume of normal human conversation), with a particular sensitivity to 1 dB sounds of 8 kHz. By comparison, humans typically hear in the range of 20 Hz to 20 kHz, with a peak sensitivity between 1 and 4 kHz. Thus, squirrels hear over a range that is roughly two-and-a-half times greater than ours, but shifted towards higher frequencies.
The study also assessed how well the squirrels could locate sounds played to them; this is done by measuring how good the animal is at telling a sound has moved from directly in front of them (zero degrees), and is called the minimum audible angle (MAA). Under ideal conditions, we’re pretty good at noticing when a sound has moved as little as a single degree either side. Ground squirrels, by contrast, can’t tell that a sound is no longer dead ahead until it has moved about 30 degrees to either side. Boester and her co-workers found that Fox squirrels were twice as good at telling a sound had moved than ground squirrels, with an MAA of 14 degrees. In their 2006 Squirrels: The Animal Answer Guide, Richard Thorington Jr. and Katie Ferrell give a slightly wider hearing range for Fox squirrels: 63 Hz to 56 kHz (close to that of domestic dogs), but they don't cite their source.
Smell & Touch
I have previously mentioned that squirrels appear to use their sense of smell in the final stages of cache retrieval and observations of squirrels scent-marking trees and recently-stripped bark suggests that scent plays an important role in squirrel social structure. Indeed, males appear to locate females in oestrus by scent and, in her book Squirrels, Jessica Holm describes how both Red and Grey squirrels have been observed to make very direct journeys of more than a kilometre to a female on heat. Certainly, while watching squirrels in a mating chase, the males appear to follow a scent trail left by the female leading the suitors, stopping periodically to sniff the bark over which she has run and on which she has sat. Holm suggests that smell may be important in helping squirrels distinguish each other.
Squirrels also have a compliment of highly sensitive hairs, called vibrissae. The long whiskers located are perhaps the most obvious, arranged in five parallel rows on either side of the nose. The whiskers furthest from the nose are the longest, growing to just over 7cm (2.7 in.), and they decline in size moving towards the nose. The head is also adorned with smaller vibrissae, situated around the eyes, the ears, the cheeks and under the chin. Short vibrissae, ranging from 2-4cm (0.8-1.6 in.), are located on the underside (among the white belly fur), on the wrists, on the elbow (a hair unique to squirrels), on the feet and at the base of the tail. In their 1977 paper to the Journal of Zoology, Heikki Kangasperko and Raija Peura at the University of Oulu in Finland refer to the wrist vibrissae as forelimb sinus hairs and describe pairs of vibrissae on the squirrel’s belly: one pair of thoracic sinus hairs on the chest; one pair of anterior abdominal sinus hairs at the midriff; and two pairs of posterior abdominal sinus hairs around the groin.
The vibrissae are thicker than the main pelage and embedded in follicles that are densely innervated, making them highly sensitive to the hair’s movement and, in particular, to it bending. In other words, these hairs are very sensitive to touch. Vibrissae allow their owner to build a ‘mental map’ of its terrain that helps it assess rough terrain, orientate itself, find its way around in low light, detect vibrations that might signal danger, help them avoid obstacles and judge the size of gaps. They can also help squirrels in their choice of food and, in her Shire Natural History booklet, The Red Squirrel, Jessica Holm notes that:
“Red squirrels also display a remarkable sense of touch, being able to distinguish between the weights of good and poor seed kernels, while manipulating them inside their thick outer casings.”