The bleak midwinter

As we descend the mountains or move south from the Arctic tundra, the conditions experienced by organisms through the year become less extreme overall and we encounter temperate conditions. The most noticeable change is that this allows the growth of large trees. Trees need at least 30 days in the year when temperatures are at 10 °C or higher and, when there is sufficient light, in order to develop and construct their trunks. Conditions at other times of the year, however, can be just as harsh as those experienced on the Arctic tundra, with temperatures falling below — 40 °C and the ground covered with a thick blanket of snow. This not only threatens to freeze organisms' cells but slows growth and denies organisms access to liquid water. Organisms which live in temperate regions must survive these bleak winter conditions.

The forests just below or south of the treeline consist solely or largely of coniferous (cone-bearing) trees. The leaves of conifers are reduced to thin needles. Snow does not easily settle on such leaves and this helps prevent branches being broken by the weight of snow. The spines contain only a little sap, which is sugary, making freezing less likely. The tree is protected against water loss by the size, shape and thick waxy covering of their needles. Pine needles have relatively few stomata and these are protected at the bottom of pits which lie in grooves along the length of the needle, reducing water loss. When the ground is frozen, cutting off water to the tree, the stomata close.

Most conifers are evergreen, retaining their needles throughout the year. The larch, however, grows in areas which are dry, as well as cold in winter, and cannot afford to lose any water during winter. It sheds its leaves and becomes dormant. This is a much more efficient and reliable way for a tree to survive the winter. Where the summers are long enough for them to grow and set seed in one season, the forest is dominated by deciduous broad-leafed trees. Their leaves are much more efficient at gathering light than are those of conifers, but they are too susceptible to water loss, freezing and wind damage during winter to remain on the tree. As winter approaches, nutrients and chlorophyll are withdrawn from the leaves, this process being triggered by the shortening days and falling temperatures of autumn. This reveals the waste products of photosynthesis that remain and the leaves turn red, yellow or brown. A blockage forms at the base of the leaf stalk, sealing it off until it withers and detaches from the tree. The tree then sits out the winter in a dormant state. Shedding and regrowing leaves each year is demanding of resources and deciduous trees can only grow where there is a sufficient supply of nutrients and a reasonably long summer.

Many animals also survive the winter in a dormant state. A few insects remain active throughout the winter but become inactive (quiescent) during periods of particularly low temperatures. Some show longer periods of dormancy. Perhaps the most sophisticated response to winter conditions is to enter a period of diapause. The insect anticipates the onset of winter by entering a long-term dormant state (diapause) in which it remains over winter before resuming activity in the spring. This usually only occurs in one stage of the life cycle, but, for different species, this may be the egg, larva, pupa or adult. The key feature of diapause is that it is not triggered by the harsh conditions themselves but by changes which indicate that the harsh conditions are approaching. One of the most important cues that trigger the entry of the insect into a state of diapause is photoperiod (the relative length of day and night). During the autumn, the days get shorter and the nights get longer. This change in photoperiod is a more reliable indicator of the onset of winter than changes in temperature, which may be only temporary. The change in photoperiod acts directly on the brain of the insect, triggering the production of hormones which control the changes that result in diapause. Both diapausing and non-diapausing insects have mechanisms which allow them to survive subzero temperatures. These will be explored in Chapter 5.

Fish and amphibians have little ability to regulate their internal temperature while reptiles can only do so by behavioural mechanisms, such as basking in the sun. These ectotherms are generally at the same temperature as their environment and, when temperatures fall, they lose heat and become inactive. The survival of most species over winter depends on choosing overwintering sites where they are protected against freezing, such as deep within the ground and at the bottom of deep lakes and ponds. Some can survive freezing (see Chapter 5).

Many birds and some large mammals migrate to warmer climates to avoid a harsh winter. Small mammals, however, must remain and survive. They cannot conserve their body heat as well as a large mammal can, given their larger surface area in relation to their volume. Their ability to develop a thick coat as insulation is also limited by their size, since a thick coat would limit the movement of a small animal. If the fur of a mouse was as thick as that of a musk ox, its feet would not reach the ground. Small mammals can compensate, however, for their limited insulation by burrowing beneath the snow and by forming lairs, burrows, dens and other forms of shelter which insulate them from the external environment. Most mammals maintain their body temperature at about 37-38 °C. They do this by generating heat metabolically and by measures for conserving heat. To generate heat, the animal needs to consume food and many mammals remain active throughout the winter if they can find sufficient food to do so. They will, however, undergo periods of dormancy of varying degrees and duration. The function of dormancy is related to the need to conserve food during periods when it is scarce, rather than the survival of low temperatures per se.

Many birds and mammals, including ourselves, undergo periods of dormancy in which they become inactive and there is a drop in body temperature. Birds and mammals active during the day sleep at night. In humans, oxygen consumption drops by about 10 per cent and body temperature by 1-2 °C. Even these small changes result in considerable savings in energy for an endothermic animal, of about 7-15 per cent, which conserves their food resources. Animals are easily aroused from sleep by disturbance, but some enter deeper periods of dormancy. During torpor, a more profound dormancy, body temperature drops lower than it does in sleep (to between 10 °C and 30 °C). The animal has to restore its body temperature before it can become active and thus takes longer to arouse (a few hours). Torpor may occur on a daily basis, in humming birds for example, or for longer periods.

Hibernation (winter sleep) is certainly a way of avoiding harsh winter conditions, but it is much less widespread than most people think. Grey and red squirrels are active throughout the winter and maintain a high body temperature. They rely on stored food reserves and spend most of their time in their nests and are thus seen less during winter. Few Arctic mammals hibernate and lemmings and Arctic rodents remain active beneath the snow. Hibernation requires the build-up of food reserves and Arctic mammals cannot eat enough food during summer to achieve this. Some bears (such as brown, grizzly and Himalayan bears) spend the winter in dens. These may be natural caves, holes dug out of hills or beneath the roots of large trees. In winter, they often become covered by snow, which improves their insulation. Female polar bears dig large snow caves, but only when they are to produce cubs. Brown bears may spend 3-5 months in their dens, even giving birth to cubs before emerging in spring. The bears are, however, easily aroused from their dormancy within the den, as some investigators have found to their cost, and it is a form of sleep or torpor rather than a deep hibernation.

Deep hibernation (sometimes called true or classic hibernation) involves a decline in temperature to within a few degrees of the ambient temperature. Body temperatures as low as — 2.9 °C have been recorded from hibernating ground squirrels. A wide range of small mammals (and a few birds) exhibit deep hibernation. They prepare for hibernation by eating in excess of their daily requirements and storing food in their bodies in the form of fat. This fat is the main source of energy while they are hibernating, although some species store seeds and nuts to eat during periods of arousal. During entry into hibernation, the animal retreats to its burrow and its body temperature falls over a period of several days. The hibernating site is usually sealed and this promotes a build-up of carbon dioxide which helps depress the metabolism of the animal. Hibernating animals have a low level of metabolism, low temperatures, the heart rate falls and they do not drink, defecate or urinate. Hibernation results in considerable energy savings. The energy consumption of hibernating hedgehogs (at 5.2°C) is 96 per cent below that when they are active. Some hibernating animals have periods of arousal, when they raise their body temperature and resume activity for a brief period. For those which store food supplies, this enables them to feed, but others do not store food and the reason for arousals in these species is something of a mystery.

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