Winter Feature

Where do frogs go in Winter?

By Jim Foster - Conservation Director

As the days get shorter, we’re often asked “Where do frogs go in the winter?” Like all amphibians, common frogs are “ectothermic” – that is, they can’t generate their own body heat and instead their body temperature depends on that of their surroundings. Because of their physiology, they become less active as the temperature reduces. Winter poses a particular problem – it’s too cold for frogs to be active, so they need to find somewhere to sit it out.

In Britain, common frogs largely hibernate on land. They usually find somewhere underground, or tucked inside a structure that sits on the ground surface. Hence, frogs might overwinter in a mammal burrow, or inside a compost heap. The important thing is that it’s a place where the frog will be buffered against extreme cold, and won’t lose too much water. Having permeable skins, frogs are at risk of drying out if they spend long in a place with no moisture.

Some adult male common frogs spend the winter in ponds, secreted among leaves and mud at the bottom. This is a risky strategy, as smaller ponds can freeze over and frogs can sometimes die through a lack of oxygen.

But clearly for some frogs the benefit of being first in the pond when females arrive in spring, means the risk is worth taking. Outside gardens, common frogs often hibernate in larger ponds and those with some inflow, which means freezing is less likely.

Hibernation ends when the temperatures start to rise again. For adult common frogs this triggers a rapid exodus from their hibernation quarters to breeding ponds. That’s normally February to March across much of Britain, although there is wide variation depending on local weather conditions.

All British amphibians and reptiles hibernate, though there are differences in where, when and how they spend this challenging winter period. Indeed, “hibernate” may be a slightly misleading term. Some scientists prefer the term “brumation”, to indicate that the animals simply enter a period of low metabolic function and activity, but not in

the way that mammals overwinter in a very deep sleep. Hence reptiles and amphibians may in fact be active intermittently during hibernation –studies show they adjust their position slightly through winter, and on warmer days they can emerge from hibernation for brief periods of activity. In some areas, newts and frogs spend the winter in ponds and simply lead a slow-moving existence before spring speeds things up.

As our climate changes, amphibian and reptile hibernation will be affected. There is evidence that the hibernation period is shortening, and animals may be more

active too, expending precious energy reserves. Wetter winters and resultant floods may create a higher risk of drowning for animals that overwinter in normally flood-free areas. We have a lot more to learn about the net effect of all this though. It’s possible that having a longer active season could be beneficial in some respects.

If you have a garden or manage a nature reserve, you can help by ensuring there are suitable sheltered places where animals can spend the winter undisturbed. Our Habitat Management Handbooks give advice for reserve managers, and our “Dragons in your Garden” leaflet, free to download from our website, should be useful for those with an outdoor space or garden. If you are lucky enough to have amphibians or reptiles in your garden or allotment why not take part in our Garden Dragon Watch survey. You can find answers to other frequently asked questions on our website FAQs section.

Amphibians in icy weather

With the freezing temperatures around the country early in 2021 we have had a lot of enquires about ponds freezing and how this will affect our amphibians. Here we’ll try to answer some of your most frequently asked questions and put your mind at rest.

In Britain, amphibians largely hibernate on land but some common frogs may lie dormant at the bottom of ponds in winter. Occasionally, in particularly icy spells, frogs can die of 'winterkill', where toxic gases (released in the pond through natural decomposition of dead leaves) cannot escape from the pond due to the layer of ice. Though this can be upsetting to pond owners this phenomenon is largely natural and will only affect a very small percentage of the local frog population.

Amphibians can breathe through their skin. Providing that there is sufficient oxygen in the water, they can survive for long periods beneath the ice. A traditional solution has been to create a hole in the ice to allow gas exchange with the air. Recent research suggests that this may be ineffective, and growth of plants and green algae may be more helpful, as these oxygenate the water, even under ice. A hole in the ice probably won't make much difference to the oxygen level in the pond but it may help air breathing creatures which can swim to the surface for air. It will also give other wildlife somewhere to drink.

Clearing snow from the surface of a frozen pond may help, allowing more light to enter, hence increasing oxygen production from submerged plants and algae. Never pour hot water on to the ice or use chemicals or salt. Similarly, do not be tempted to smash the ice as this can damage pond liners and plants. The best way to create a hole in the ice is to leave a ball or other floating object in the pond which can be removed to leave a hole after it freezes.

Finally please do be very careful around frozen ponds. Although it looks solid the ice can be deceptively thin - it won’t hold much weight. Cleaning snow from the ice will make the pond more visible to anyone nearby.

Our native amphibians are well adapted for our weather conditions and although this very cold weather can be concerning they will undoubtedly muddle through and will soon be taking advantage of spring sunshine and spawning once more. Page 10 - Photo top right: Common frog © Chris Dresh, Middle left: Compost heaps are often used for hibernation © Richard Sharp, Bottom right: Log piles make great shelter © Chris Dresh. Page 11 - Photo top right: Sweeping snow from frozen pond © Kim Boughey, Middle left: Frosty garden © Paul Kirkland.