Costanzo J P, Litzgus J D, Iverson J B, Lee R E
Department of Zoology, Miami University, Oxford, OH 45056, USA.
J Exp Biol. 1998 Nov;201(Pt 22):3105-12. doi: 10.1242/jeb.201.22.3105.
Hatchling painted turtles (Chrysemys picta) hibernate in their shallow natal nests where temperatures occasionally fall below -10 C during cold winters. Because the thermal limit of freeze tolerance in this species is approximately -4 C, hatchlings rely on supercooling to survive exposure to extreme cold. We investigated the influence of environmental ice nuclei on susceptibility to inoculative freezing in hatchling C. picta indigenous to the Sandhills of west-central Nebraska. In the absence of external ice nuclei, hatchlings cooled to -14.6 1.9 C (mean s.e.m.; N=5) before spontaneously freezing. Supercooling capacity varied markedly among turtles cooled in physical contact with sandy soil collected from nesting locales or samples of the native soil to which water-binding agents (clay or peat) had been added, despite the fact that all substrata contained the same amount of moisture (7.5 % moisture, w/w). The temperature of crystallization (Tc) of turtles exposed to frozen native soil was -1.6 0.4 C (N=5), whereas turtles exposed to frozen soil/clay and soil/peat mixtures supercooled extensively (mean Tc values approximately -13 C). Hatchlings cooled in contact with drier (less than or equal to 4 % moisture) native soil also supercooled extensively. Thus, inoculative freezing is promoted by exposure to sandy soils containing abundant moisture and little clay or organic matter. Soil collected at turtle nesting locales in mid and late winter contained variable amounts of moisture (4-15 % w/w) and organic matter (1-3 % w/w). In addition to ice, the soil at turtle nesting locales may harbor inorganic and organic ice nuclei that may also seed the freezing of hatchlings. Bulk samples of native soil, which were autoclaved to destroy any organic nuclei, nucleated aqueous solutions at approximately -7 C (Tc range -6.1 to -8.2 C). Non-autoclaved samples contained water-extractable, presumably organic, ice nuclei (Tc range -4.4 to -5.3 C). Ice nuclei of both classes varied in potency among turtle nesting locales. Interaction with ice nuclei in the winter microenvironment determines whether hatchling C. picta remain supercooled or freeze and may ultimately account for differential mortality in nests at a given locale and for variation in winter survival rates among populations.
刚孵化的彩龟（锦龟）在它们出生时的浅巢穴中冬眠，在寒冷的冬天，那里的温度偶尔会降至零下10摄氏度以下。由于该物种的耐冻热极限约为零下4摄氏度，刚孵化的幼龟依靠过冷却来在极端寒冷中存活。我们研究了环境冰核对内布拉斯加州中西部沙丘地区原产的刚孵化的锦龟接种性结冰易感性的影响。在没有外部冰核的情况下，幼龟在自发结冰前冷却至零下14.6±1.9摄氏度（平均值±标准误；N = 5）。尽管所有基质的含水量相同（7.5% 水分，重量/重量），但与从筑巢地点收集的沙质土壤或添加了保水剂（粘土或泥炭）的原生土壤样本物理接触冷却的海龟之间，过冷却能力差异显著。暴露于冷冻原生土壤的海龟的结晶温度（Tc）为零下1.6±0.4摄氏度（N = 5），而暴露于冷冻土壤/粘土和土壤/泥炭混合物的海龟则广泛过冷却（平均Tc值约为零下13摄氏度）。与较干燥（含水量小于或等于4%）的原生土壤接触冷却的幼龟也广泛过冷却。因此，暴露于含有大量水分且几乎没有粘土或有机物的沙质土壤会促进接种性结冰。在冬中和冬末在海龟筑巢地点采集的土壤含水量（4 - 15% 重量/重量）和有机物含量（1 - 3% 重量/重量）各不相同。除了冰之外，海龟筑巢地点的土壤可能含有无机和有机冰核，这些冰核也可能引发幼龟结冰。经过高压灭菌以破坏任何有机核的原生土壤大量样本，在约零下7摄氏度（Tc范围为零下6.1至零下8.2摄氏度）时使水溶液成核。未高压灭菌的样本含有可水提取的、大概是有机的冰核（Tc范围为零下4.4至零下5.3摄氏度）。这两类冰核在海龟筑巢地点的效力各不相同。在冬季微环境中与冰核的相互作用决定了刚孵化的锦龟是保持过冷却状态还是结冰，最终可能解释了给定地点巢穴中不同的死亡率以及不同种群冬季存活率的差异。
