AMPHIBIANS AND REPTILES

Patrick T. Gregory and Linda A. Gregory

ECOLOGICAL CHALLENGES FACED BY NORTHERN AMPHIBIANS AND REPTILES

Despite the genealogical distance between them, amphibians and reptiles do share ancestral traits that influence their ecology and provide other, non-phylogenetic, reasons to consider them together. Perhaps the most important of these is that they are ectotherms. Ectotherms have a low metabolic rate because they do not generate significant amounts of body heat metabolically (a few very large reptiles are exceptions under some circumstances). Rather, they maintain an appropriate body temperature by behavioural means. This economy of metabolism means that ectotherms, in general, are more tolerant of shortages of essential resources (e.g. food and oxygen) than are endotherms, which need more food and a higher metabolic rate to maintain high body temperatures. On the other hand, ectotherms are able to regulate their body temperature over only fairly modest ranges of ambient conditions. Where it is very cold, amphibians and reptiles either will not occur, or they somehow must avoid exposure to winter weather. Identifying the specific habitat types used by amphibians and reptiles to survive the cold winter months is essential to the maintenance of their diversity.

Winter poses one of the greatest challenges to temperate-zone amphibians and reptiles, including those in the Montane Cordillera (Gregory, 1982; Pinder et al., 1992). Above-ground winter temperatures are simply too low for ectotherms to survive. Therefore, if amphibians and reptiles are to persist in a region with cold winters, they must evolve ways to avoid the coldest conditions. Amphibians and reptiles lack the ability of birds to migrate to warmer climes for the winter, although they often make relatively short-distance migrations between summer and winter habitats. Consequently, the only way for them to avoid the full force of winter is to hibernate in insulated microhabitats underground or underwater. Because sites that are suitable for hibernation may be limited, communal hibernation may occur; this has been best documented for snakes (Fig. 3), but occurs in other reptiles and in amphibians as well. Most of the species of amphibians and reptiles in the Montane Cordillera probably hibernate underground, either finding pre-existing cavities (e.g. snakes and lizards) or digging themselves into soft soil (e.g. toads). Some terrestrially hibernating frogs can survive extracellular freezing of up to 65% of their body water at modest subfreezing temperatures for relatively brief periods (Pinder et al., 1992). The best known example is the Wood Frog (Fig. 4), which is Canada’s most widely and most northerly distributed amphibian. Presumably, freeze-tolerant amphibians are able to hibernate in shallower sites than those that cannot tolerate freezing; however, little is known about actual hibernation sites chosen by these animals in the wild, so it is difficult to determine the ecological significance of freeze-tolerance. In contrast to terrestrial hibernators, some aquatic frogs and turtles avoid lethal temperatures in winter by hibernating under the ice in water, where they require either water currents to provide oxygen (e.g. Leopard Frog) or an ability to tolerate the high levels of lactic acid produced by anaerobic respiration (e.g. Painted Turtles, Fig. 5). In contrast to juvenile and adult Painted Turtles, hatchlings overwinter in the relatively shallow, terrestrial nest in which they hatched; survival in winter depends on their ability to tolerate freezing, like Wood Frogs, or to supercool to low temperatures and thus avoid freezing (Storey and Storey, 1996).

Northern climates can be a challenge not only in winter, but also in summer, which can be cool and short (Larsen and Gregory, 1988). For ectothermic animals such as reptiles with long embryonic developmental periods, summer thermal conditions might limit the ability to reproduce successfully. One apparent response to this in lizards and snakes has been the evolution of viviparity, which is especially well represented in species that occur in cool climates (Shine, 1985). For example, 6 of 11 species of lizards and snakes recorded in the Montane Cordillera are viviparous. Viviparity is thought to be advantageous in cool climates because the pregnant female, by regulating her body temperature, can provide the best possible thermal conditions for development of her offspring, allowing them to be born before the onset of winter. Egg-laying, or oviparous, species, on the other hand, leave their eggs much more to the mercy of the vagaries of the weather by depositing them in a nest in the ground, carefully selected as the nest site may be. Recent studies have shown that pregnant females of viviparous species are particularly careful thermoregulators, supporting this hypothesis. Additional support comes from distributional data. Within the Montane Cordillera, the only species whose ranges extend any significant distance north are viviparous; by contrast, most of our oviparous species are confined to the hottest parts of the extreme south.