Warm winter not good for toads

Warmer winters are not beneficial, but actually detrimental to amphibians in temperate regions. At higher temperatures they use too much energy during their winter rest. As a result, they are less resistant to sudden frost and they enter spring weakened. This was reported by a European research team in a preprint in January, based on cold experiments with toads. The article still needs to be assessed by independent scientists.

Normally, toads and other amphibians are in deep rest during the winter. They hide in the ground or in the mud of ditches and puddles to avoid the worst frost, but can easily survive freezing temperatures for a while. They then use almost no energy: all processes in their body – from digestion to blood circulation and cell metabolism – are virtually at a standstill. This means they hardly use any fat reserves, there are always enough sugars in their blood as ‘antifreeze’ and they can make a good start in the spring.

Frost resistance

But during warmer winters, this hibernation is incomplete, the researchers write. The animals crawl into their shelters, but their metabolism does not decrease enough. There were already suspicions that this would be at the expense of, among other things, their fat reserves and frost resistance, with negative consequences for survival and even the immune system and fertility, but the physiology behind this had not yet been investigated in detail.

The researchers collected toads in the wild and exposed them to two different experiments. In the first, they kept toads at 4 or 8 degrees Celsius for 48 hours. After two days, the ‘warm’ toads had almost four times as many hormones in their blood that regulate the release of energy. Their oxygen consumption was 27 percent higher and they had lost almost 20 percent of their body weight. The ‘cold’ toads still weighed the same. At that time they could survive five days longer without food than their ‘warm’ brothers. The latter had also become less cold-tolerant: in a behavioral experiment in an ice cream container, they needed higher temperatures to function properly.

Habitat loss

In the second experiment, the biologists examined the genetics of hibernation in more detail. When cooled to 2 degrees below zero, a series of specific genes were found to become active, including genes that code for liver enzymes that help make glucose (the ‘antifreeze’ of amphibians) from body fat.

Toads, the biologists conclude, need their fat reserves during their winter rest to avoid freezing – and to be able to start spring in good condition. Climate change, with warmer winters and more frequent freeze-thaw periods, is therefore unfavorable for them.

The number of toads in the Netherlands has declined by 60 percent since 2008, according to recent figures from CBS and RAVON (Reptile Amphibian Fish Research Netherlands). The cause is a combination of factors, RAVON suspects, including habitat loss and fragmentation, pollution, decline in insects and climate change.

“The high temperatures of recent months do not bode well for the animals in hibernation,” responds Arnold van Vliet of the Nature Calendar (Wageningen University). “If we want to counteract the negative impact, we must do more to protect ourselves. For example, by helping toads cross the road. But especially by providing more habitat and reducing the many other pressure factors.”