Marbled salamander embryos hatch in water if the environmental oxygen pressure is 86 torr or less, but do not hatch if the environmental oxygen pressure is equivalent to that of air. Under hypoxic conditions, embryos hatch in aqueous and nonaqueous media with equal success. Increasing carbon dioxide pressure does not induce hatching, but does decrease the time to hatching by altering environmental pH.
The echidna (Tachyglossus aculeatus) maintained a body temperature of 30.7"C f 1.03 s.d. ( N = 23) at ambient temperatures (TA) between 0 and 25°C.It may, however, also become hypothermic at low TA. At T*=3O0C or above the echidna became hyperthermic. The thermoneutral range was about 20-30°C. At low TA the metabolic rate might be increased several fold. The thermal conductance was at a minimum at TA = 20"C, and was not further reduced at lower TA. At higher TA the thermal conductance increased up to five-fold. The evaporation showed little change with increasing TA. At the highest TA we used (33°C) the evaporation on the average accounted for the dissipation of only about one-third of the metabolic heat produced. These findings suggest that the echidna, although it can maintain its body temperature at low ambient temperature, cannot rely upon evaporation as the major avenue for heat loss at high ambient temperature.The egg-laying mammals, echidna and platypus, are usually described as physiologically primitive in regard to their temperature regulation. This concept is founded upon early measurements which showed that their body temperature is lower than the usual range of placental mammals, and they were therefore considered as intermediate between "warmblooded and "cold-blooded animals. For example, occasional measurements made in the last century on echidna yielded a body temperature of some 28" to 32°C. It was therefore concluded that echidna follows its primitive anatomy by placing itself between the high temperature level of "real" mammals (about 38°C) and the "cold-blooded reptiles.The interesting study of thermal adjustment in monotremes and marsupials published by C. J. Martin in 1903 contributed the first detailed account of respiratory metabolism and thermoregulation in these mammals. It concluded: "Echidna is the lowest in the scale of warm-blooded animals. Its attempts at homoeothermism fail to the extent of 10°C when the environment varies from 5" to 35°C." This conclusion has been quoted, simplified, and rephrased in numerous texts and papers. Further support of the concept of an "ascending evolutionary order" was expressed by Robinson ('54) who observed the reactions to high temperature of the J. CELL. PHYSIOL., 67: 63-72. two extant monotremes as well as six marsupials. However, as far as we know, since Martin's paper there have been no studies of the temperature regulation in the echidna over a wide range of ambient temperatures, and we therefore found it desirable to study three individuals available to us. The results show that the echidna in some respects is an excellent temperature regulator, in other respects less capable, but probably no more so than many "higher" mammals. MATERIALS AND METHODS Animals.Three echidnas (spiny anteater, Tachyglossus aculeatus) were obtained through an Australian animal dealer. The original locality of capture is not known. They weighed between 2.5 and 3.5 kg and were apparently in good condition on arrival. Over a period of two months we determined...
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