DOI: 10.5061/dryad.2z34tmpt4
title: Climate change-induced shifts in survival and size of the worlds’ northernmost oviparous snake: a 68-year study

description: Because of their dependence on ambient temperature ectothermic animals can
 serve as sentinels of conservation problems related to global warming.
 Reptiles in temperate areas are especially well suited to study such
 effects, as their annual and daily activity patterns directly depend on
 ambient temperature. This study is based on annual data spanning 68 years
 from a fringe population of Grass Snakes (Natrix natrix), which is the
 world’s northernmost oviparous (egg-laying) reptile, and known to be
 constrained by temperature for reproduction, morphology, and behavior.
 Mark-recapture analyses showed that survival probability was generally
 higher in males than in females and that it increased with body length.
 Body condition (scaled mass index) and body length increased over time,
 indicative of a longer annual activity period. Monthly survival was
 generally higher during winter (i.e., hibernation) than over the summer
 season. Summer survival increased over time, whilst winter survival
 decreased, especially during recent decades. Winter survival was lower
 when annual maximum snow depth was less than 15 cm, implying a negative
 effect of milder winters with less insulating snow cover. Our study
 demonstrates long-term shifts in body length, body condition and seasonal
 survival associated with a warming climate. Although the seasonal changes
 in survival ran in opposite directions and though changes were small in
 absolute terms, the trends did not cancel out, but total annual survival
 decreased. We conclude that effects of a warming climate can be diverse
 and pose a threat to thermophilic species in temperate regions and that
 future studies should consider survival change by season, preferably in a
 long-term approach.
description: This study is based on data collected by the late Carl Edelstam
 (Fig. 1). From 1942 to 2009 he captured, marked,
 and recaptured Grass snakes <em>Natrix natrix</em> in what is
 today known as the Nacka nature reserve
 (‘<em>Nackareservatet’</em>, 7.3 km<sup>2</sup>,
 WGS84 dec N 59.29, E 18.15, located in the hemi-boreal biotic zone).
 Snakes were captured by hand and kept in cloth bags in the field until
 examination. Each snake was marked by scale-clipping and its natural color
 markings were recorded (i.e., the unique ventral pattern, which can be
 used for individual identification). On each occasion a
 snake was captured (with few exceptions) date, sex, body length (i.e.,
 snout-vent length, SVL), tail length, and body mass were recorded. After
 examination, snakes were returned to the location where they had been
 captured.
description: | Capture data | Explanations | | :--------------------------------------
 |
 :----------------------------------------------------------------------------- | | Capture occasion | *First capture*=1, *first recapture*=2, *third capture*=3, etc. of individuals | | Capture date | The day/month/year the snakes were caught | | Season | *March*=Winter, *April-May*=Spring, *June-Aug*=Summer and *Sep-Oct*=Autumn | | Sex | *Female*=F, *Male*=M, *either juvenile or unidentified sex*=NA | | SVL (cm) | Snout-Vent-Length in centimeter from nose to cloaca, NA=Not Available | | Body mass (g) | Weight of the body in gram, NA=Not Available | | **Snow cover and winter survival data** | | | Year | The year of snow cover and survival | | Season | Winter (as snakes were analysed in spring directly after the winter) | | Survival | Estimated proportional winter survival from the mark-recapture models | | Max snow (cm) | Maximum measured snow cover | | Sex | *Female*=F, *Male*=M, |