Physiological and ecological aspects of low-temperature tolerance in embryos of the wood frog, Rana sylvatica

Wood frogs (Rana sylvatica) breed in late winter in temporary ponds, where eggs are deposited in communal surface rafts. Subsequently, developing embryos may face subzero temperatures. In the laboratory, individual embryos supercooled moderately (to –4.2°C for embryos in their jelly capsules; to –5.0°C for embryos removed from the jelly capsule) when chilled in the absence of external ice nuclei, but did not resist inoculative freezing when chilled in contact with external ice. They survived acute episodes (1–18 h) of supercooling to temperatures between ca. –0.5 and –2.0°C. Stage-12 embryos (six of six) survived freezing for 1 h at –2.0°C, but only one of six survived a 3.5-h freezing episode at the same temperature. Stage-13 and -14 embryos survived freezing episodes of 4.5 h at –1.0°C (six of six survived) and 18 h at –0.5°C (six of six survived), respectively. In the field, temperatures were monitored every 5 min for 21 d in embryo masses of wood frogs and Jefferson salamanders (Ambystoma jeffersonianum) at a breeding pond in central Kentucky. Wood frog embryos experienced higher daily maximum and lower daily minimum temperatures than salamander embryos, which were deeper in the water column. Wood frog embryos spent more time at risk of cryoinjury (i.e., at temperatures [Formula: see text]1°C), but the two species accumulated similar degree-days over the 21-d observation period. Their position in the water column may permit wood frogs to garner a developmental advantage in milder years, when daily minimum temperatures are similar between the two species. Wood frog embryos appear to offset some of the risks associated with this more exposed position by tolerating somatic freezing at high subzero temperatures.