Swedish grass snakes are the northernmost egg-laying reptiles in the world. A 68-year data sampling shows that grass snakes are benefiting from longer and warmer summers, but their winter survival has actually decreased. A lack of insulating snow cover in winter is a likely explanation.
Carl Edelstam was a curator at the Swedish Museum of Natural History and collected data on grass snakes in a nature reserve outside Stockholm from 1942 to 2009.
“When Carl passed away in 2016, his sons contacted us asking if we were interested in the recorded data on grass snakes. It was amazing for us to have access to such a data set, consistently sampled over so many years. Such long-term data are very rare, especially for vertebrates," says Simon Kärvemo, a researcher at SLU who led the study.
The researchers' analyses show a mixed picture of the effects of climate change on grass snakes. On the positive side, their summer survival has increased. They have also grown larger on average over 60 years, with males gaining five centimetres and females seven centimetres. The snakes have also increased their body mass. This is probably due to a longer active summer period in a warmer climate. Grass snakes emerge from hibernation on average 10 days earlier in the spring and have more time to feed.
On the other hand, winter survival has decreased, especially in recent decades. This may be linked to milder winters with less insulating snow cover - in the study, snow cover of less than 15 centimetres seemed to have a negative effect on grass snake survival.
"It is alarming that winter survival has decreased over the years. Especially as the grass snakes were generally in better condition after the summer and should potentially have a better chance of surviving the winter," says Simon Kärvemo.
When Carl Edelstam started his study in the 1940s, climate change was not a relevant issue. The new findings on grass snakes in Sweden show the value of long-term data sets, also for answering new questions as they arise.
"As far as we know, there is no previous study of a reptile in a temperate climate that shows such a clear link between survival and hibernation, let alone a study that shows a critical threshold for shielding snow depth," says Simon Kärvemo.
The study is a collaboration between SLU, Johan Elmberg at Kristianstad University and Ludvig Palmheden at the Swedish Museum of Natural History