Winter may not seem like the best time to visit woodlands, with bare trees, muddy leaf litter, no birdsong, and many species in a dormant state. Indeed, dormancy and hibernation are common overwintering strategies as plunging temperatures and reduced food availability pose a considerable threat to the survival of trees and animals. Most of our native vertebrate animals remain active in the winter, however, demonstrating an array of behavioural adaptations to help them cope with the harsh conditions. Wildlife spotting in winter can be easier as a result, with large mixed-species flocks of foraging birds, and mammals breaking cover to search for food. This makes winter a great time to get out into our woods and appreciate how the UK’s wildlife has evolved to cope with the seasonal challenges.
The challenges that plant and animal species in temperate climates face in winter are numerous: colder temperatures, lower food availability, frozen drinking water, reduced foraging time due to fewer daylight hours, and the risk of their bodies freezing. These challenges can pose a critical risk to survival for plants, and both endothermic (warm-blooded) and ectothermic (cold-blooded) animals.
There are some underlying physical principles that determine the most effective way for species to adapt to lowering temperatures. The first is that smaller objects lose heat more quickly, or more specifically that animals with a high surface area to volume ratio (e.g. mice) will lose heat more quickly than animals with a low surface area to volume ratio (e.g. badgers). Correspondingly, according to Bergmann’s rule, animals within a taxonomic group that live in colder environments tend to be larger and many small animals reduce their surface area to volume ratio during the winter by huddling in groups. Surface area is also important to trees, and deciduous trees lose all their leaves in the winter partly to reduce the surface area of their leaves and limit water loss.
The second principle is that maintaining a certain core body temperature in colder conditions requires more energy. In response, endothermic animals have evolved varying strategies either to increase their energy intake or reduce their energy requirements. Alternatively, many birds avoid the cold altogether by migrating to warmer climates (e.g. swallows and swifts) or by more localised migrations to warmer coastal areas (e.g. kingfishers). Migration is not only an exodus of our summer visitors in autumn, as many ducks, swans and waders migrate to the UK from colder countries such as Russia and Iceland for the winter, in order to benefit from the plentiful estuary food supply and comparatively milder temperatures.
The fact that frozen water expands in volume constitutes a significant risk of freezing to plants and trees. If water in their living tissues freezes it could burst the cell walls, potentially killing the plant, so they have evolved ways of avoiding or tolerating freezing. The physiological responses of trees to winter demonstrate a finely tuned balance across seasonal cycles, as they need to maximise their growing season but cannot risk delicate new growth being exposed to freezing temperatures.
Deciduous and coniferous trees have different strategies for surviving during the winter months. The most obvious difference is that deciduous trees lose their leaves in the autumn in a process called abscission that shuts down photosynthesis, whereas evergreen conifers photosynthesise all year round. There are also similarities though, with both types of tree going into a dormant state overwinter and stopping new growth aboveground. Conifers are more adapted to extreme low temperatures (down to -40°C) and are found at higher latitudes, forming the immense boreal forest.
Deciduous trees are more efficient at photosynthesis than coniferous trees as their leaves are thinner with a larger surface area. They do not keep the leaves overwinter because there is much less daylight for photosynthesis, the leaves will die if frozen, and keeping them would increase the risk of snow and wind damage to the tree. In the autumn, deciduous trees are triggered to start losing their leaves by shortening daylength, and the abscission process is then accelerated by falling temperatures.
There are two stages to the dormant period in deciduous trees, endodormancy and ecodormancy. The endodormancy period starts in autumn when temperatures start to drop, and the tree undergoes a rapid cold acclimation process that leads to the tree being freeze resistant. Growth stops altogether and the growing shoots are transformed into dormant, freeze-resistant buds, which is why many trees have buds overwinter.
The tree does not progress from endodormancy to ecodormancy until it has reached a threshold of chilling units, a mechanism which slows regrowth and reduces the risk of new leaves being damaged by a late spring freeze. For example, in hazel trees (Corylus avellana), catkins stop developing in November and then start again in January once a chilling threshold has been crossed. During ecodormancy the dormant state is governed by environmental factors, and once the chilling units threshold has been exceeded, the interaction between increasing daylength (photoperiod) and warming temperatures triggers the spring budburst.
Evergreen coniferous trees continue to photosynthesise all year round, even though photosynthesis efficiency is only 30% at 0°C and daylight hours are reduced in northern latitudes. At lower temperatures, the biomass gain from photosynthesis is so negligible that the trees effectively are not growing. They do have a head start in the spring though, and do not have to regrow their entire foliage as deciduous trees do. Coniferous needles are highly adapted to help the tree in surviving lower temperatures. They have a small surface area and a waxy cuticle that reduce water loss, and their dark colour helps to absorb warmth. The conical shape of conifers allows them to avoid gathering snow on their branches, and their thick bark and densely packed forests protect the trees from temperature extremes. Even the shape of pinecones is adapted to help protect the seeds inside from snow.
Both deciduous and coniferous trees in colder climates need to be able to withstand or avoid the water inside their tissues freezing. There are two methods that trees exhibit to circumvent this issue, extracellular freezing (freeze tolerance) and supercooling (freeze avoidance). Extracellular freezing is widely used by trees in cold climates and it happens when the water immediately outside of living cells is allowed to freeze, driven by ice-nucleating proteins. Water within the cells is then drawn out by the external freezing, leaving a more concentrated sugar solution in the cells with a lower freezing point.
Supercooling allows the temperature of water within cells to drop far below 0°C without freezing. It is based on the principle that water needs a seed particle such as dust for ice to form at 0°C, otherwise it can reach temperatures as low as -40°C without freezing. Tropical trees that are exposed to colder temperatures employ supercooling, but trees in temperate climates often combine the strategies, using supercooling within cells and extracellular freezing outside the cells. The buds of most temperate trees have been found to exhibit supercooling, and you can see this in action on a cold day as the bud will not have frozen even if the branch has.
One of the key determinants of overwintering strategy in vertebrate animals is the nature of their food source. Species that are primarily insectivorous have to adjust to the lower food availability by hibernating, changing their diet, or migrating. Invertebrates mostly either overwinter as adults in a state of torpor, or their life cycle is timed to spend the winter as an egg, pupa, or larva. Omnivorous or herbivorous mammals can continue to forage but may adapt behavioural strategies to compensate for lower food availability and colder temperatures.
There are only three mammal groups that hibernate in the UK, bats, hedgehogs, and dormice. Cold-blooded animals (ectotherms) such as reptiles and amphibians also hibernate overwinter, and many invertebrates either enter a dormant state called diapause as eggs and larvae, or go into a state of torpor as adults. Animals in torpor wake to forage intermittently whereas animals that are hibernating do not wake to eat. Hibernating animals reduce their metabolic rate down to an average of 6% of their normal basal metabolic rate, and slow their breathing, causing a reduction in body temperature. Hibernating animals do occasionally wake up and it is thought this could be when the temperature drops too low, to avoid ice crystals forming in their blood.
Endothermic hibernating animals have to survive the whole winter on stored fat, so they go through a period of fattening up in the autumn when they increase their energy intake and lay down reserves of body fat. Ectothermic animals do not have the same energetic requirements in maintaining body temperature and can allow their body temperature to drop. Reptiles have been found to use similar supercooling and extracellular freezing mechanisms to those used by trees in order to avoid or tolerate freezing internally. Hibernating animals have to find a safe and sheltered place to wait out the winter and bats hibernate either in large groups in tunnels and caves, in smaller groups in trees, or singly in crevices such as under roof tiles. Hedgehogs make a nest to hibernate in piles of leaves, under sheds or in log piles. Dormice also make a nest of leaves and grass but hibernate on the floor of woods and forests. Although frogs hibernate on the bottom of ponds, burying themselves into the soft mud, newts hibernate out of the water, in rocky piles or in compost heaps. Reptiles hibernate in burrows, under logs or in rock piles.
Although common shrews do not hibernate, they undergo an incredible physiological transformation to reduce their mass and therefore their energy requirements. They dissolve their own skeleton to shrink their skull by 20% and shorten their spine, reduce their brain size by 30%, and reduce the mass of other organs, reversing the process in the spring.
Invertebrates that overwinter in a state of torpor as adults include ladybirds, some butterflies (e.g. small tortoiseshell, brimstone, peacock), queen wasps and queen bumblebees. Honeybees overwinter as adults but stay in their hive living off the honey reserves that they have stored. Snails find somewhere sheltered, create a mucus door, and enter a state of torpor and slugs bury themselves underground or shelter under logs. Many spiders and larger flies overwinter as adults in a sheltered spot.
Invertebrates demonstrate two strategies to handle freezing, either by using extracellular freezing to control where ice crystals form, or by producing polyhydroxy alcohols which act as antifreeze and prevent freezing. The supercooling antifreeze method tends to be used by invertebrates in milder climates as the animal will die if the temperature drops below the supercooling point. Invertebrates in colder climates incorporate both methods to give them better freezing resistance. Some species such as winter gnats (Trichocera annulata) and winter moths (Operophtera brumata) do remain active throughout the winter.
Animals that stay active during the colder months have an array of behavioural adaptations that enable them to increase food intake or reduce energetic demands. Some animals such as squirrels and jays cache their food (acorns, hazelnuts) during the autumn when it is abundant in a process called scatter hoarding. Spreading their food store over a wide area does reduce the chances of other animals finding it but presents difficulties in remembering where it is hidden. Amazingly squirrels recover about 40 to 80% of their cache, using spatial memory and scent to guide them to buried food. Red squirrels also cache fungi, storing them in tree cavities to dry out. Water shrews (Neomys fodiens) are carnivores and they hoard live food by stunning it with their venomous saliva, creating a larder of prey for later consumption.
Insectivorous birds that do not migrate often have to change their diet during the winter months to cover increased energetic demands. Many bird species such as blue and great tits switch from a diet that is predominantly insectivorous during the breeding season (caterpillars and spiders) to eating more seeds and berries during the winter. Tiny insectivores such as goldcrests change their foraging strategy in the winter, spending more time in lower branches and joining up with large foraging flocks of tits to find insects that larger bird species overlook. Their tiny size means that they cannot go for more than an hour without eating. Many woodland bird species aggregate to form mixed species foraging flocks which doubles their foraging success, contrary to what might be expected. Species that commonly forage as a flock include great tits, blue tits, coal tits, chaffinches, nuthatches, long-tailed tits, treecreepers, and goldcrests.
As well as benefitting from aggregating with other species during the day, many smaller bird species such as long-tailed tits, wrens, and goldcrests roost together at night to conserve warmth. This usually involves small numbers or family groups (long-tailed tits) but there is a record of 61 wrens roosting together in a nest box in Norfolk. It is an effective strategy that works by reducing their surface area to volume ratio and two goldcrests roosting together can reduce their heat loss by a quarter. For species that roost singly such as blue and great tits, they find a sheltered spot such as nest box, fluff up their feathers (ptiloerection) and may reduce their body temperature to conserve energy reserves (hypothermia). Blue tits can lower their body temperature by 5°C, reducing metabolic demands by up to 50% and improving survival by 58%.
Our gardens can be an amazing source of food and shelter if we supply a diverse range of appropriate seeds and nuts and leave overgrown corners for invertebrates and mammals. Woodlands support an even more diverse range of animals during the winter, and native woods in particular provide essential habitat for foraging and hibernating animals. The UK has lost a considerable percentage of its native woodland and it is vital that we restore it by planting new woodlands and managing those we have effectively. Understanding the challenges faced by animals and plants gives us a new appreciation of just how hard winter can be for the wildlife around us and how critically it needs our protection.