These caterpillars of the Large White butterfly (Pieris brassicae) only have to survive a mild frost. However, how do invertebrates survive the harsh winter cold?
There is no doubt that winter is a difficult time if you are an invertebrate. Not only does the cold slow them down making them easier prey for hungry birds, but temperatures below zero can kill. To combat this insects and spiders have evolved lots of strategies to survive cold weather.
In places such as Canada and Central Europe night time temperatures can fall to as low as -40°C, yet insects survive this. We know they do because we see these places full of them again every summer. So how do they do it?
Water is an excellent insulator and, because the water does not lose heat as easily as the land, the temperature of the water stays quite constant. However, invertebrates living in water during winter have two things to worry about.
- Will the water freeze?
- If the water does freeze, will it all freeze or just the surface?
If the water doesn't freeze or only occasionally a thin layer freezes on the surface then you have no problems. Food is often plentiful. For example, many of the leaves that fell off the trees in autumn end up in streams and ponds and they provide an excellent food reserve.
Some invertebrates eat the leaves directly whilst others feed on the fungi and bacteria that rot the leaves down. These animals are called detritivores. Of course, all of these detritivores are food for carnivores so for many aquatic invertebrates, winter is a time of major growth.
Another reason that winter is a time for growth in aquatic invertebrates is oxygen. Aquatic invertebrates still breathe oxygen only they get the oxygen from the water not the air (unlike terrestrial invertebrates or ourselves). The water absorbs oxygen from the air and the colder the water the more oxygen it can absorb. This means invertebrates (and fish) have more oxygen available to them in cold water and can be more active.
However, if the surface of the water freezes over then air is not in contact with the water, so oxygen cannot be absorbed. Thick and persistent ice can mean that oxygen becomes very scarce.
If the pond or stream freezes solid then the invertebrates have the same problems as those on the land. Often though the bottom of a pond will not freeze and even if it does it is usually warmer than the land.
The problems facing invertebrates over winter can be broken down into two categories:
- Cold weather
One problem during winter is a lack of food - most of the trees and shrubs have lost their leaves and there is little if any sap running in twigs. This means herbivorous insects (insects that feed on plants) have to find a way to live without food for months. To do this they enter a dormant (inactive) state like a deep sleep with all their metabolic (body) processes slowed down.
In winter this state is called hibernation. When insects use the same trick to avoid a hot dry period in summer (for example, in Africa or India) it is called aestivation. Of course, no herbivores means no food for the carnivores (animals that eat other animals) so the carnivorous insects and spiders also have to survive winter without food.
Like everything in nature there are exceptions to these rules, and not all insects hibernate over winter. Some exceptions are the moths you occasionally see at night such as the Winter Moth (Operophtera brumata) and the December Moth (Poecilocampa populi) that can both fly at temperatures close to zero. Other exceptions are the many Collembola which don't seem to mind the cold and are often seen hopping around on the surface of snow.
A third exception are some Linyphiid spiders. About 9% of spiders in relatively mild areas remain active throughout winter. These are nearly all Linyphiids who can still make a web at temperatures as low as -1°C. They feed mainly on Collembola, but even they freeze and die if the temperature gets down to -7°C.
How do terrestrial invertebrates survive the freezing conditions? There are several ways:
- Avoid the cold weather
- Protect yourself against the cold weather
Avoid the cold weather
One way to avoid cold weather is to migrate to a warmer climate and return after winter. Perhaps the best example of this is the Monarch butterfly (Danaus plexippus) in North America.
Monarch butterflies migrate south from North America every year and overwinter in Mexico or in California, USA. For some individuals this migration is roughly 3,000 miles. In their overwintering sites the butterflies gather in trees in huge numbers. In spring the butterflies migrate back again.
The second method of avoiding cold weather is to burrow down into the soil or leaf litter. Being in the soil is similar to being in water and is much warmer than being on the surface. Spending winter deep in the soil means that you not only avoid the cold but also the chilly winds and the beaks of hungry birds.
Many invertebrates overwinter in the soil. In areas where it snows regularly the snow can act like a giant blanket keeping away the worst of the winter weather. Another favourite place to avoid both chilling winds and the probing beaks of birds is beneath the bark of trees. Occasionally the cold weather can still reach insects under bark and those in the soil.
Protect yourself against the cold weather
Some invertebrates use their body fluids to protect themselves against the cold. This protection comes in two forms:
- Freeze tolerant invertebrates are those that can survive being frozen solid. Invertebrates survive the freezing by controlling where ice crystals form within their bodies. Although the ice crystals form inside the body of the invertebrate they do not damage the cells and organs of the animal. When the weather gets warmer the crystals melt and the invertebrate becomes active again.
- Freeze intolerant invertebrates are those that use special "anti-freeze" chemicals to stop themselves freezing. These chemicals are called polyhydroxy alcohols and they work with other components of the invertebrate's body fluids (thermal hysteresis proteins) to prevent ice forming inside the insect.
Freeze intolerance is usually a strategy used by invertebrates found in cool to mildly cold climes. Freeze tolerance is used in really cold areas.
The "anti-freeze" chemicals are found in both freeze tolerant and freeze intolerant invertebrates. The freeze tolerant insects use these chemicals to prevent ice crystals forming inside their cells whilst the haemolymph around them freezes. Freeze intolerant insects use the chemicals to try and prevent freezing altogether. However, if the temperature drops too low then they can't stop ice crystals forming inside their cells and this can kill both freeze tolerant and freeze intolerant invertebrates.
By stopping ice from forming an invertebrate can become "supercooled". Supercooling is when a liquid is cooled to a temperature below its freezing point and it does not freeze.
Freeze intolerant insects, those that rely on super cooling themselves without freezing, will freeze solid and die if the temperature drops below their supercooling point. In comparison, freeze tolerant insects still freeze solid but can survive this freezing. The supercooling point is the temperature below which an animal is unable to stop itself freezing despite the chemicals in its body.
A good example of an insect that is both freeze tolerant and has a low supercooling point is the beetle Pytho deplaratus. It can supercool (remain unfrozen) to temperatures as cold as -54°C. If the temperature drops further it can still survive for a while because it is freeze tolerant and ice forms around its cells not within them.
Two more tricks invertebrates use are to reduce the amount of water in their body and also empty their gut of food. Emptying your gut of food works both to reduce your overall water content and to remove a possible source of ice crystal formation.
The strategies used by insects and other invertebrates to survive winter are many and varied. Fortunately the strategies work and, when the weather gets warmer, the insects return in great numbers.