Adaptations to terrestrial overwintering of hatchling northern map turtles, Graptemys geographica

Abstract

We conducted a 3-year field and laboratory study of winter biology in hatchlings of the northern map turtle (Graptemys geographica). At our study area in northern Indiana, hatchlings routinely overwintered in their natal nests, emerging after the weather warmed in spring. Winter survival was excellent despite the fact that hatchlings were exposed frequently to subfreezing temperatures (to −5.4 °C). In the laboratory, cold-acclimated hatchlings exhibited low rates of evaporative water loss (mean=2.0 mg g⁻¹ day⁻¹), which would enable them to conserve body water during winter. Laboratory-reared hatchlings were intolerant of freezing at −2.5 °C for 24 h, conditions that are readily survived by freeze-tolerant species of turtles. Winter survival of hatchling G. geographica probably depended on their extensive capacity for supercooling (to −14.8 °C) and their well-developed resistance to inoculative freezing, which may occur when hatchlings contact ice and ice-nucleating agents present in nesting soil. Supercooled hatchlings survived a brief exposure to −8 °C. Others, held at −6 °C for 5 days, maintained ATP concentrations at control levels, although they did accumulate lactate and glucose, probably in response to tissue hypoxia. Therefore, anoxia tolerance, as evidenced by the viability of hatchlings exposed to N₂ gas for 8 days, may promote survival during exposure to subfreezing temperatures.