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10 Overwintering in Northern Cricket Frogs (Acris crepitans)
- University of California Press
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55 Although winter weather in north-temperate regions may dominate 6–9 months of the year, this season has received relatively little attention in studies of amphibian life history. While some aspects of behavioral and physiological responses to cold have been elucidated, we generally have not applied this understanding to the management and conservation of amphibian populations. To begin to address this concern, I provide a basic description of the various overwintering methods used by amphibians and of the physiological responses that accompany these methods. Then I describe the unique overwintering method of northern cricket frogs (Acris crepitans) and consider how their physiology and winter habitat use may be contributing to their recent severe declines, especially in the northern portions of their range (Gray and Brown, this volume). Overwintering Methods Employed by Amphibians Amphibians have colonized temperate regions from tropical centers of diversity (Duellman, 1999), so it is not surprising that the most commonly used method for surviving cold climates is simply to avoid the cold altogether. To do so, many species overwinter in aquatic sites that do not freeze completely . The most familiar examples are species in the family Ranidae, which typically overwinter on the bottom of ponds, lakes, and streams (e.g., Bohnsack, 1951; Bradford, 1983). Even if the surface of the water freezes, liquid water is most dense (and thus heaviest) at 4 C (Marchand, 1991), so a frog resting on the bottom remains relatively warm and free of ice. However, hibernating in permanent bodies of water is not without challenges. Predation can be a factor. Fishes are known to eat hibernating frogs (e.g., 20% of the trout captured during winter had frogs in their stomachs; Emery et al., 1972). In response , some ranid tadpoles (e.g., American bullfrogs, Rana catesbeiana) are toxic, presumably to protect themselves from predation (Kruse and Francis, 1977). Hypoxia (lack of oxygen) is also a concern, particularly when ponds freeze over and the oxygen exchange between water and air is prevented. The metabolism of living organisms and the organic decomposition in the pond consumes oxygen, eventually lowering oxygen concentrations to levels that are lethal to amphibians (e.g., Barica and Mathias, 1979; Lannoo, 1998a). Species that typically overwinter in such sites have evolved mechanisms to enhance their tolerance of hypoxic (and even anoxic) conditions (Christiansen and Penney, 1973; Boutilier et al., 1997; Holden and Storey, 1997), but even with such mechanisms, winter kills of amphibians are well known (Barica and Mathias, 1979; Bradford , 1983). Rather than hibernating in water, other amphibians escape freezing by moving underground, below the frost line. True toads (Bufonidae) and spadefoot toads (Pelobatidae) are actively fossorial, burrowing more than 1.1 m underground as the frost penetrates the soil (Breckenridge and Tester, 1961; Ruibal et al., 1969; Seymour, 1973; Kuyt, 1991). Animals in the mole salamander family (Ambystomatidae) also dig underground or exploit existing burrows to get beneath the frost line. The soft passageways of ant mounds allow underground access for both lungless salamanders (Plethodontidae) and chorus frogs (Hylidae; Caldwell, 1973). Although one might expect that oxygen would be limiting for burrowers, this is not the case, at least not in the desert regions where oxygen levels have been measured (Seymour, 1973). The costs associated with burrowing include morphological adaptations for digging, the energy required to burrow into the soil, and mechanisms to prevent or tolerate water loss to the surrounding soil (Pinder et al., 1992). Not all amphibians protect themselves from the cold. Some North American anuran species hibernate on the forest floor under only a thin layer of fallen leaves. In such shallow sites, frogs are within the frost zone, when the soil and leaves surrounding the frog freeze, the frog also freezes. These species are said to be “freeze tolerant” and include striped chorus frogs (Pseudacris triseriata; Storey and Storey, 1986), spring peepers (P. crucifer; Schmid, 1982), gray treefrogs (both eastern, Hyla versicolor, and Cope’s, H. chrysoscelis; Schmid, 1982; Costanzo et al., 1992, respectively), and wood frogs (R. sylvatica; Schmid, 1982). Remarkably, these species have evolved physiological mechanisms that enable them to survive freezing of up to 70% of their body water. Adaptations supporting freeze tolerance include glucose production upon freezing (glucose acts as a substrate for anaerobic metabolism and reduces cellular dehydration), tolerance of dehydration (losing water to ice is similar in a physiological sense to being dehydrated), and a TE N Overwintering in Northern Cricket Frogs (Acris crepitans) JASON T. IRWIN 56 OVERWINTERING IN ACRIS CREPITANS suite of other more subtle...