Reptile embryos and climate change: Modelling limits of viability to inform translocation decisions

- Western Swamp Turtles in Australia are panned to undergo assisted colonization.
- Turtle embryos may have different requirements to adults for translocation sites.
- A coupled nest-embryo model can be used to select sites for assisted colonization.
- Sites suitable for embryos now will not be suitable under future warmer climates.
- Females need to select warmer nests for eggs to survive now at more poleward sites.

The embryos of many egg-laying species develop under the environmental conditions selected by one or both parents, and these conditions may cease to be optimal under a changed climate. Assisted colonization is an emerging option to relocate species that are threatened by unfavourable changes in the local climate, but the incubation requirements of embryos are rarely considered in conservation translocations despite suggestions that relocation of early life stages could be more effective than relocating older animals. Here we review examples and outcomes of relocations of reptile eggs, and the decision tools currently available for guiding translocation decisions from an embryonic perspective. We then demonstrate a mechanistic approach, using the modelling framework NicheMapR, for assessing the optimal translocation range for the Western Swamp Turtle (Pseudemydura umbrina), a Critically Endangered reptile with an extremely restricted natural distribution. We determine thermal reaction norms and critical thermal limits for embryonic development based on laboratory and field data, and model soil temperatures at typical nest depths, simulate embryonic development at these depths, and map the probability of survival of P. umbrina embryos within the major bioregions of south-western Australia. The same model forced by future climates for 2050-70 demonstrates a southerly shift in the regions where embryos would be viable. However, if P. umbrina is to be translocated in the near future to regions 300-400 km south of its natural range, females will need to nest in relatively unshaded sites to achieve temperatures that are high enough to promote hatchling success.