Chelonians in a changing climate: can nest site selection prevent sex ratio skews?
Article first published online: 22 OCT 2013
© 2013 The Zoological Society of London
Volume 16, Issue 5, pages 491–492, October 2013
How to Cite
Rödder, D. and Ihlow, F. (2013), Chelonians in a changing climate: can nest site selection prevent sex ratio skews?. Animal Conservation, 16: 491–492. doi: 10.1111/acv.12079
- Issue published online: 22 OCT 2013
- Article first published online: 22 OCT 2013
Within the next decades, global environmental conditions are predicted to change dramatically with a magnitude exceeding changes experienced during the last millennia. In most areas, these changes will severely affect annual mean temperatures, especially in higher latitudes, and frequency of extreme weather events (Parry et al., 2007). Global biodiversity will likely be affected across multiple scales, including potential extinctions, as has been suggested for the Monteverde golden toad (Incilius periglenes Anchukaitis & Evans, 2010) and the tuatara (Sphenodon guentheri Mitchell et al., 2008), population declines and loss of genetic diversity (Bálint et al., 2011; Habel et al., 2011). Poikilothermic taxa, especially those species exhibiting temperature-dependent sex determination (TSD), are considered to be particularly vulnerable (Deutsch et al., 2008; Hulin et al., 2009; Huey, Losos & Moritz, 2010; Kallimanis, 2010; Mitchell & Janzen, 2010). Among these, taxa exhibiting FM-TSD, yielding males at higher temperatures, are suggested to be profoundly affected (Mitchell & Janzen, 2010), while species exhibiting inter-population variation in pivotal temperatures or transitional range of temperatures possess a larger potential to adapt and compensate increasing ambient temperatures (Hulin et al., 2009).
The evolution of TSD in Chelonians has been subject to numerous scientific studies suggesting multiple independent evolution events (Janzen & Krenz, 2004; Valenzuela & Adams, 2011). By affecting gender ratios, TSD might be an evolutionary dead end in terms of climate change (Mitchell & Janzen, 2010), while also providing potential advantages under specific circumstances (Warner & Shine, 2008; Silber, Geisler & Bolortsetseg, 2011). An increasing number of scientific studies highlight the potential global chelonian diversity decline caused through climate change by screwing up sex ratios (e.g. Mitchell & Janzen, 2010; Ihlow et al., 2012). Recent analyses of natural history traits regarding species exhibiting TSD suggest potential behavioral or phonological adaptions to compensate ambient fluctuations and therefore prevent skews in gender ratio (e.g. Doody et al., 2006; Telemeco, Elphick & Shine, 2009; Refsnider & Janzen, 2010). However, previous studies revealed the magnitude of adjustments regarding nest depth that would be necessary to compensate for the effects of climate change to be unfeasible for some species (Mitchell et al., 2008; Telemeco et al., 2009).
In the current issue of Animal Conservation, Refsnider et al. (2013) test whether adjustments of nest depth may sufficiently compensate for gender ratio skews caused by rising ambient temperatures. The authors use nest depth measurements of more than 2300 nests of painted turtles Chrysemys picta, a frequently used model species in ecological studies (e.g. Valenzuela, 2009). Based on this dataset, they precisely describe the phenology of natural nests and experimentally manipulate nest depths to determine whether hatchlings emerging from shallow, medium or deep nests differ significantly in terms of fitness. Despite a rather low sample size of 21 experimentally modified nests included in their analyses, the authors detected statistically significant differences in offspring performance in 165 specimens among nest depth classes. Interestingly, they were not able to detect differences in incubation regime, survival, body size or sex ratio of hatchlings. A reasonable explanation is the physiological limitation of nest depth restricted by nesting females’ body sizes, yielding a comparably narrow temperature range across all nest depth classes (table 1, Refsnider et al., 2013). As temperature differences were only 1.2°C across nest depth classes, the potential of C. picta to compensate for the effects of increasing ambient temperatures related to climate change by adjusting nest depths appears to be rather limited, also suggesting low adaption potential for other small-sized chelonian species exhibiting TSD. Previous studies revealed reptiles to select nest sites based on solar radiation or substrate temperature, which directly refers to canopy cover (Doody et al., 2006). Refsnider et al. (2013) suggest shade cover to be a better predictor for gender ratios than nest depth. Despite fairly restricted sample sizes of eight nests designated as ‘shallow’ nine nests assigned to ‘mean’ and only four ‘deep’, this result is in concordance with previous studies suggesting shade cover as an important factor to compensate thermal stress related to climate change (Refsnider & Janzen, 2012).
The study provides valuable insights in potential strategies of C. picta as a model organism to compensate for the effects of climate change. This kind of study is on the front line of research documenting the response of species facing environmental stress, which are urgently needed to estimate potential future losses of biodiversity. Future studies could also control for potential parental effects on hatchlings fitness by means of splitting clutches among treatments, consider a larger sample size and selecting artificial nesting sites in close proximity to reduce potential differences in humidity, precipitation and solar radiation. Documenting this type of fundamental ecological responses in a broad range of species will ultimately provide the baseline to understand interactions between biodiversity and climate change.
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