In general, the ground-dwelling squirrel species are social, often living in well-developed colonies, while the tree-dwelling species are more solitary. 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 , while many of the usual food sources have not yet become available.
During these times, squirrels rely heavily on the buds of trees. Squirrels, being primarily herbivores, eat a wide variety of plants, as well as nuts , seeds , conifer cones , fruits , fungi , and green vegetation. Some squirrels, however, also consume meat, especially when faced with hunger. Indeed, some tropical squirrel species have shifted almost entirely to a diet of insects.
At least some of these probably were variants of the oldest basal "protosquirrels" in the sense that they lacked the full range of living squirrels' autapomorphies.
The distribution and diversity of such ancient and ancestral forms suggest the squirrels as a group may have originated in North America. 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. Refer in your answer to the key concepts of the influence of predators, sensory communication, coevolution and niche separation. Answer Tree-dwelling mammals may escape terrestrial predators but they are exposed to others, notably birds of prey. Many tree dwellers are nocturnal and there are advantages for some e.
Since visibility in trees may be limited, communication by smell can be important e. A reciprocal influence on evolution is evident in the intimate relationship between some arboreal species and the trees that sustain them, e. Tree dwellers are unlikely to feed indiscriminately; rather there is niche separation. Different tree squirrels that occupy the same part of the forest may have different food preferences. Available niches within a forest area are likely to be utilised by specialist feeders.
One group of accomplished tree dwellers are the primates - a term you perhaps think of as synonymous with monkeys and apes. Monkeys and some apes display some of the most striking adaptations to tree-living. All share a lengthy list of defining features, mostly related to the following broad categories: Limbs and locomotion. The hands and often the feet are grasping, with mobile fingers and toes, generally with touch-sensitive pads at the tips.
The first digit the thumb or the hallux, i. Try this with your own hand to verify it. Rather than a curved and rigid claw, at least some of the digits of primates have flat nails, making manipulation e.
They have very flexible shoulder joints; hindlimbs are normally dominant in locomotion. The senses. The eyes point forwards and are set close together. The fields of view of the eyes overlap which, together with a distinctive 'wiring' arrangement linking the eyes and the brain, imparts stereoscopic, '3-D' vision.
In general, vision like hearing is more significant than smell, and colour vision is widespread. The face is flattened, with the muzzle i. The brain. This organ is relatively large. The cerebral cortex - in primates, more often called the neocortex - is elaborately folded and complex in structure. This part of the brain is involved with the highly complex processes that include learning, reasoning and memory. In all, up to 30 or so diagnostic features of the type described above have been identified for primates, though not all primates display each trait.
The others include: their distinctive dentition, linked with a generally omnivorous diet; the structure of the ear; and reproductive features, such as small litter sizes and long gestation periods, relative to body size.
Primates have an extended period of juvenile growth and their overall rates of growth and reproduction are generally low, which some biologists interpret as the price paid for a large brain. Brain tissue is metabolically a very expensive tissue to develop and maintain, so for large-brained mammals, less energy is available for growth and reproduction.
The benefits of a large brain are very significant. For example, the elaborate social behaviour of primates is seen by many biologists to be as much a defining feature of primates as the types of anatomical feature just listed. While monkeys and apes and ourselves may be the most familiar primates, they are far from the only members of this order.
I'll focus on types of primates other than apes or monkeys, where again a tree-dwelling lifestyle is especially widespread. Such primates comprise three subgroups, all introduced in LoM: the bush-babies or galagos of Africa Section 4 ; the lorises of Asia and the pottos of Africa Section 4 ; the lemurs of Madagascar Section 5. LoM refers to all such primates as prosimians [p. The term prosimian implies that these primates are in some sense 'forerunners of the monkeys' - historically monkeys and apes were referred to as 'simians'.
More than likely, prosimians do more closely resemble the ancestral primates than do modern-day monkeys and apes, though thinking of them simply as primitive 'pre-monkeys' would be a mistake.
As you'll see, they have complex and rich lifestyles and display sophisticated adaptations to tree dwelling. LoM mentions one type of prosimian that stands apart from its relatives [pp.
Indeed, most taxonomists now favour grouping tarsiers within the primate suborder that comprises the true monkeys and apes and urge the abandonment of the term prosimian.
You're likely to encounter this preferred classification scheme in your wider reading and in the classification scheme featured in the Open University course from which this course originates S , so I'll outline it here.
The newly defined suborder Strepsirhini includes what DA refers to as prosimians, i. The suborder takes its name from the features of the nose, in particular a naked area the rhinarium surrounding what are often referred to as 'comma-shaped' nostrils.
The muzzle of strepsirhines is commonly described as 'prominent, moist and glandular' and, as I've hinted, strepsirhines are thought, for a variety of reasons, to resemble more closely the ancestors of primates. Compared to other primates, they have smaller brains, a better sense of smell and longer snouts. As DA's comments imply, the tarsier's muzzle is different: 'fur grows almost to the edge of them [the nostrils] and surrounds them' [p.
This feature defines the suborder Haplorhini, together with the fact that the muzzle is less prominent. The haplorhines therefore include the tarsiers and the monkeys and apes. The names of the 13 present-day families are on the right. Colour differences reflect degree of divergence.
All these dates are very, very approximate; for example, some primatologists now argue that primates originated as long as 90 million years ago, rather than the 60 million years ago implied here, which comes from direct reading of the fossil record This revised classification is meant more accurately to reflect the presumed evolutionary history of primates. Figure 2 provides such an evolutionary tree, plotting the different lineages that evolved from the presumed common ancestor of primates, stretching back into the Paleocene.
You can see a divergence from this ancestor into the two branches reflecting the suborders just introduced, the differences in colour evident as they split. The evolutionary theory of aging, in its several, nonmutually exclusive forms 3 — 8 , proposes that senescence is the result of late-acting, deleterious mutations that accumulate because of the diminishing effectiveness of selection with increasing age.
Extrinsic mortality is one major factor that contributes to the accumulation of deleterious mutations by limiting exposure of these late-acting mutations to selection; thus, the evolutionary theory of aging predicts that extrinsic mortality will be a principal determinate of the rate of senescence in age-structured populations 9. This theory predicts that populations experiencing high extrinsic mortality rates will accumulate more deleterious mutations, evolve earlier senescence and reproduction, low somatic maintenance, and shorter maximal lifespans.
Conversely, populations subject to low extrinsic mortality rates will eliminate late-acting deleterious mutations more effectively and evolve delayed senescence, late fecundity, durable somas, and greater longevities. These relationships have been demonstrated in laboratory 10 — 12 , wild 13 — 15 , and simulation 16 studies. In the wild, flying birds and bats experience lower rates of extrinsic mortality 17 and greater longevity 18 — 23 than their nonvolant relatives, presumably because of decreased predation.
Although other factors such as hibernation and reproductive rate have been shown to play a role in bat longevity 22 , these factors are accordant with evolutionary theory of aging, and it is clear that the exceptional longevity of Chiroptera as a whole is the result of flight. In addition to flying birds and mammals, gliding mammals are longer-lived than nonvolant, nongliding mammals 21 ,
Keywords: senescence, extrinsic mortality, terrestriality, marsupials, primates Senescence, or aging, is an intrinsic biological phenomenon that limits an organism's maximum potential lifespan, even in the absence of extrinsic sources of mortality such as predation, disease, and environmental hazards. In the winter months, this squirrel exists almost exclusively on a diet of the inner lining of the bark the phloem layer , which is an essential component of the tree's transport system; the resulting damage to the tree is significant. Monkeys and some apes display some of the most striking adaptations to tree-living. Answer The forest provides a variety of resources available for exploitation - e. But this cosy mutuality is not guaranteed. Google Scholar Rasmussen, D.
Frederick Wood Jones proposed that tree climbing lead to selection for a grasping forelimb, and Grafton Elliott Smith that grasping feet and hands were an adaptation for agility in a complex three-dimensional environment. Folia Primatologica, 53, — Google Scholar Bennet-Clark, H. A preliminary study of the western tarsier, Tarsius bancanus Horsfield. Monkeys and some apes display some of the most striking adaptations to tree-living. The seeds of the commercially important West African iroko tree depend on the straw-coloured flying fox for their dispersal.
Google Scholar Cartmill, M. Answer This layer is more protected from sunshine and is generally more stable. LoM refers to all such primates as prosimians [p.
Google Scholar Sussman, R. The hands and feet of the potto in Figure 4c and h show all these features; those of the loris, which DA describes in LoM p. The term prosimian implies that these primates are in some sense 'forerunners of the monkeys' - historically monkeys and apes were referred to as 'simians'. I've already drawn attention to the distinctive hands and feet of primates. You can see a divergence from this ancestor into the two branches reflecting the suborders just introduced, the differences in colour evident as they split. Flying foxes also help in the recolonisation of deforested areas and in the establishment of plants on land newly formed or recently devastated by volcanic eruption.
Indeed, most taxonomists now favour grouping tarsiers within the primate suborder that comprises the true monkeys and apes and urge the abandonment of the term prosimian. Pedley Ed. Huddling together in groups should in theory reduce the rate of heat loss in cooler conditions, and decrease the rate of warming when it's very hot. But DA also stresses that they can maintain 'the fertility of the rain forest'.
Generally, the thumb and often the big toe are enlarged, diverge strongly and are to some degree opposable on the hand or foot. Doctoral dissertation, University of London. This organ is relatively large. Answer This layer is more protected from sunshine and is generally more stable. Philosophical Transactions of the Royal Society, , — Behavior Young squirrels 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 jump as a fast mode of locomotion in arboreal and terrestrial biotopes. The tassel-eared squirrel, for example, depends on the Ponderosa pine for food and shelter. Google Scholar Cartmill, M. Edinburgh: Edinburgh University Press. Many such baby squirrels have been rescued and fostered by a professional wildlife rehabilitator until they could 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 rehabilitators are willing to spend their time doing this and such animals are routinely euthanized instead.
Age Differences in Locomotion of 2 Subtropical Galaginae. London: Chapman and Hall. The haplorhines therefore include the tarsiers and the monkeys and apes.