Squirrels are members of the biological family ‘Sciuridae’, which includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels, chipmunks, prairie dogs (which are rodents, but named for their habitat and warning call, which sounds similar to a dog’s bark) and flying squirrels. Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by us humans to Australia. The earliest known fossilised squirrels date from the Eocene epoch, a geological period that lasted from about 56 to 33.9 million years ago, and amongst other living rodent families the squirrels are most closely related to the mountain beaver and the dormouse. The word ‘squirrel’, first attested in 1327, comes from the Anglo-Norman ‘esquirel’ which is from the Old French ‘escurel’, the reflex of a Latin word ‘sciurus’, which was taken from the Ancient Greek word ‘skiouros’, from ‘shadow-tailed’, referring to the long bushy tail which many of its members have. A group of squirrels is called a ‘dray’ or a ‘scurry’.
Squirrels are generally small animals, ranging in size from the African pygmy squirrel and least pygmy squirrel at 10 to 14 cm (3.9 to 5.5 in) in total length and just 12 to 26 g (0.42 to 0.92 oz) in weight, to the Bhutan giant flying squirrel at up to 1.27 m (4 ft 2 in) in total length, and several marmot species, which can weigh 8 kg (18 lb) or more. Squirrels typically have slender bodies with very long bushy tails and large eyes. In general, their fur is soft and silky, though much thicker in some species than others. The coat colour of squirrels is highly variable between, often even within, species. In most of the species the hind limbs are longer than the fore limbs, whilst all species have either four or five toes on each foot. The feet, which include an often poorly developed thumb, have soft pads on the undersides and versatile, sturdy claws for grasping and climbing. Tree squirrels, unlike most mammals, can descend a tree head-first. They do so by rotating their ankles 180 degrees, enabling the hind feet to point backward and thus grip the tree bark from the opposite direction. Squirrels live in almost every habitat, from tropical rainforest to semi-arid desert, avoiding only the high polar regions and the driest of deserts. They are predominantly herbivorous, subsisting on seeds and nuts, but many will eat insects and even small vertebrates. As their large eyes indicate, squirrels have an excellent sense of vision, which is especially important for the tree-dwelling species. Many also have a good sense of touch, with ‘vibrissae’
(whiskers, to you and me) on their limbs as well as their heads. These hairs are finely specialised for this purpose, whereas other types of hair are coarser as tactile sensors. Their teeth follow the typical rodent pattern, with large incisors (for gnawing) that grow throughout life, and cheek teeth (for grinding) that are set back behind a wide gap. Many juvenile squirrels die in the first year of life. Adult squirrels can have a lifespan of 5 to 10 years in the wild. Some can survive 10 to 20 years in captivity. Premature death may occur when a nest falls from the tree, in which case the mother may abandon her young if their body temperature is not correct. Many such baby squirrels are rescued and fostered by a professional wildlife carer until they can be safely returned to the wild, although the density of squirrel populations in many places and the constant care required by premature squirrels means that few people are willing to spend their time doing this, so sadly such animals are routinely euthanised instead. A squirrel has a multifunctional use of its tail, including to keep rain, wind, or cold off itself, to cool off when hot, by pumping more blood through its tail, as a counterbalance when jumping about in trees, as a parachute when jumping and to signal with. When a squirrel sits upright, its tail folded up its back may stop predators looking from behind from seeing the characteristic shape of a small mammal.
Squirrels mate either once or twice a year and, following a gestation period of three to six weeks, give birth to a number of offspring that varies by species. The young are ‘altricial’, being born naked, toothless, and blind. In most species of squirrel, the female alone looks after the young, which are weaned at six to ten weeks and become sexually mature by the end of their first year. In general, the ground-dwelling squirrel species are social, often living in well-developed colonies, while the tree-dwelling species are more solitary. In zoology, a crepuscular animal is one that is active primarily during the twilight period. This is distinguished from diurnal and nocturnal behaviour, where an animal is active during the hours of daylight and of darkness respectively. Some crepuscular animals may also be active by moonlight or during an overcast day. Matutinal animals are active only before sunrise and vespertine only after sunset. Hence our ‘mattins’ and ‘vespers’ in the Christian church. Ground squirrels and tree squirrels are usually either diurnal or crepuscular, whilst the flying squirrels tend to be nocturnal—except for lactating flying squirrels and their young, which have a period of diurnality during the summer.
Because squirrels cannot digest cellulose, they must rely on foods rich in protein, carbohydrates and fats. In temperate regions, early spring is the hardest time of year for squirrels because the nuts they buried are beginning to sprout (and thus are no longer available to eat), whilst many of the usual food sources are not yet available. During these times, squirrels rely heavily on tree buds. Squirrels, being primarily herbivores, eat a wide variety of plants, as well as nuts, seeds, conifer cones, fruits, fungi and green vegetation. Some squirrels though do consume meat, especially when faced with hunger. They have been known to eat small birds, young snakes and smaller rodents, as well as birds eggs and insects. Some tropical squirrel species have even shifted almost entirely to a diet of insects. Squirrels, like pigeons and other fauna, are synanthropes in that they benefit and thrive from their interaction in human environments. This gradual process of successful interaction is called ‘synurbanisation’, where squirrels lose their inherent fear of humans in an urban environment. When squirrels were almost completely eradicated during the Industrial Revolution in New York, they were later re-introduced to ‘entertain and remind’ us humans of nature. The squirrel blended into the urban environment so efficiently that when synanthropic behaviour stopped (i.e. people did not leave their rubbish outside during particularly cold winters), the squirrels could become aggressive in their search for food. Aggression and predatory behaviour has been observed in various species of ground squirrels, in particular the thirteen-lined ground squirrel.
Living squirrels are divided into five subfamilies, with about fifty-eight genera and some two hundred and eighty-five species. The oldest squirrel fossil, ‘Hesperopetes’, dates back to the ‘Chadronian’ (late Eocene) about forty to thirty-five million years ago and is similar to the modern flying squirrel. A variety of fossil squirrels, from the latest Eocene to the Miocene, have not been assigned with certainty to any living lineage. Though at least some of these were probably variants of the oldest basal ‘protosquirrels’, in the sense that they lacked the full range of traits of present living squirrels. The distribution and diversity of such ancient and ancestral forms suggest the squirrels as a group may have originated in North America. Apart from these sometimes little-known fossil forms, the evolutionary biology of the living squirrels is fairly straightforward. The three main lineages are the Ratufinae (Oriental giant squirrels), Sciurillinae and all other subfamilies. The Ratufinae contain a mere handful of living species in tropical Asia. The neotropical pygmy squirrel of tropical South America is the sole living member of the Sciurillinae. The third lineage, by far the largest, has a near-cosmopolitan distribution and this further supports the hypothesis that the common ancestor of all squirrels, living and fossil, lived in North America, as these three most ancient lineages seem to have radiated from there. If squirrels had originated in Eurasia for example, one would expect quite ancient lineages in Africa, but African squirrels seem to be of more recent origin. So the main group of squirrels can be split into five subfamilies. The Callosciurinae, sixty species mostly found in South East Asia, the Ratufinae, four cat-sized species found in South and South-East Asia, the Sciurinae, containing the flying squirrels and the tree squirrels, Sciurillinae, a single South American species and Xerinae, including three tribes of mostly terrestrial squirrels, including marmots, chipmunks and [prairie dogs. There are also other Holarctic ground squirrels, African and some Eurasian ground squirrels, and African tree squirrels. It is thought that squirrels are a cause for concern because they often cause electrical disruptions and it has been hypothesised by some that the threat to the internet infrastructure and services posed by squirrels may exceed that posed by cyber-attacks. It makes me wonder who thinks of these ideas. Squirrels have been reported to be ‘successfully trained’ in Chongqing, China to sniff out illicit drugs and in 2023 a team of six Eurasian red squirrels became part of a sub-unit within the Chongqing city police dog brigade. According to Chongqing police department, their small size and agility are beneficial as they are able to help the police detect drugs through tiny spaces in warehouses and storage units that dogs are unable to reach. Yin Jin, a police dog handler, who had been assigned to train these squirrels told ‘The Paper’ that “these squirrels have an acute sense of smell. But in the past, our training problems for small rodents was not developed enough to attempt a program like this” and that her team of squirrels have so far done an “excellent job” in drug detection exercises, but are not yet ready to be deployed. One day, perhaps?
One of the many things my father said to me and I have tried to always remember, is to never promise anything that you are not prepared to carry out. Otherwise folk can and will take advantage. It is a lesson well-learned.
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