HEAT 



105 



fined alternation of wet and dry seasons. 

 This is especially exemplified by the African 

 and South American lungfishes, which re- 

 main in the mud of the drying ponds or 

 marshes and form mucous-lined individual 

 cells in which they stay until the rising 

 water level again frees them. This hfe- 

 history cycle is essentially hke that of fresh- 

 water animals in general in temporary 

 ponds, illustrated among North American 

 fishes by the mud minnow (Umbra limi). 



Overwintering of both frogs and sala- 

 manders frequently involves aggregation in 

 large numbers in a moist terrestrial situa- 

 tion (like a large rotten log), in the 

 bottom mud of marshes and ponds, or in 

 springs. In springs partially torpid frogs 

 may be seen swimming in exaggerated slow 

 motion in midwinter. Toads and the more 

 terrestrial types of frogs may pass the win- 

 ter in solitary burrows on land (Noble, 

 1931). Aestivation of frogs during a dry 

 season is reported. The Central Australian 

 Chiroleptes platycephalus is said to fill its 

 urinary bladder, lymph spaces, and the 

 body cavity with water and to pass the dry 

 season in this condition in a mud cell much 

 like that of a lung fish (Spencer 1896, p. 

 164). Holzapfel (1937), by experiment, 

 demonstrated a relation between the hiber- 

 nation of the common leopard frog in North 

 America and the seasonal cycle, independ- 

 ent of actual temperatures, since the normal 

 dormant condition was readily assumed 

 during the winter months, while frogs sub- 

 jected to low temperatures in the summer 

 months did not become dormant, and died 

 in a relatively short time. The latitudinal 

 gradient of hibernation in frogs has not 

 been studied. The relation between hiber- 

 nation and the sexual cycle varies geo- 

 graphically; thus the leopard frog may 

 breed in the fall in the southwestern United 

 States. 



The more terrestrial reptiles accomplish 

 overwintering mainly by retreat into rock 

 crevices or burrows, often in considerable 

 aggregations; such aggregations may in- 

 clude several species. More aquatic turtles 

 spend the winter buried in mud beneath 

 ponds and stream borders. Hibernation of 

 solitary individuals has been observed in 

 ^n American lizard, the anole. 



Duration of the period of dormancy is 

 proportional to the length of the winter or 

 dry season. It is evident also that there is 

 no period of dormancy in the life cycle of 



fishes, frogs, and reptiles in tropical regions 

 without an extended dry season, and that 

 the latitudinal gradient with respect to win- 

 ter dormancy deserves further examination. 



Hibernation of Mammals 



Hibernation of warm-blooded animals, 

 i.e., of mammals, difi^ers conspicuously from 

 overwintering of poikilotherms. Observa- 

 tion, study, and experiment combined have 

 disclosed further problems and have led to 

 a great diversity of opinion and hypothesis 

 as to the immediate causes of hibernation, 

 i.e., the factors that induce its deathlike 

 torpor in individuals. Such studies fre- 

 quently disregard evolutionary and ecologi- 

 cal causes and relations. The fact that the 

 temperature of the mammal drops below 

 the normal level in aestivation as well as in 

 hibernation points to the functional relation 

 of lowered metabolic rate for surviving an 

 unfavorable season. (Rasmussen, 1916; 

 Johnson, 1931; Benedict and Lee, 1938; 

 "Hamilton, 1939). 



Factors that have been thought to induce 

 the torpor of hibernation are low tempera- 

 ture, especially gradually decreasing tem- 

 peratures; inadequacy of heat-regulating 

 mechanisms; lack of food; dryness of food; 

 concentration of carbon dioxide in hiber- 

 nacula; accumulation of fat; and glandular 

 disturbance. The operation of special hiber- 

 nating glands has been postulated, but the 

 supposed glands (in the marmot) prove to 

 be merely masses of fat. Freedom from ex- 

 ternal stimuli appears to contribute to 

 maintenance of deep torpor, since under 

 experimental conditions activities of the ex- 

 perimenters have been observed to arouse 

 animals from dormancy. The most remark- 

 able feature of hibernation physiologv in 

 mammals is the low internal temperature 

 reached, which approximates that of the 

 environment and falls as low as 1° C: 

 body temperatures of dormant mammals 

 average about 1 degree above that of the 

 environment. Death appears to ensue if 

 temperature falls to freezing; such a fall 

 may arouse the animal and thus save its 

 life, a reaction with obvious survival value, 

 especiallv for mammals that hibernate in 

 relativelv exposed situations. Where the 

 soil is permanently frozen beneath the level 

 reached by summer thaws, hibernation of 

 burrowing mammals can not take place, 

 and hence is absent in polar regions. 



