11. ORGANISMS AND THERMAL ENERGY
493
on evaporation of water to prevent overheating. Those which cannot
evaporate water freely, due to scaly or horny skins, must avoid the heating effects of direct sunlight (cf. reference 37).
It must not be concluded that because the poikilotherms have little
or no ability to regulate their internal temperatures their metabolic
processes are completely at the mercy of the external temperature. There
is ample evidence for adaptation of these organisms to warmer or cooler
temperatures in such a way as to result in much the same level of activity regardless of the external temperature. As Barcroft (38) stated,
"It is clear that nature has learned so to exploit the biochemical situation so as to escape from the tyranny of a simple application of the
Arrhenius equation. She can manipulate living processes in such a way
as to rule, and not be ruled by, the obvious chemical situation."
Although many examples of biochemical and physiological change in
response to temperature are known, the molecular basis of these adaptations is still little understood. The generalization may be made that an
organism adapted to low temperatures has a higher metabolism at a
particular temperature than one acclimated to high temperatures. This
holds for adaptations of members of the same population in the laboratory and for adaptations to different climatic conditions among different
populations of the same species in nature. A large number of examples
of such adaptations has recently been discussed by Bullock (37).
Adaptive regulation may occur at the molecular, cellular, organ, or
behavioral level. While the underlying biochemical mechanisms are not
well known, it is evident that these must include (a) a mechanism for
perception of absolute temperature, as distinguished from temperature
differences, and (b) a compensation system. The temperature-sensing
mechanism may lie in the rate of discharge of nerve fibers, although
this is not definitely established (12, 39). The compensatory mechanisms
appear to be varied. In reptiles, which show little capacity for physiological adaptation (37), compensation is accomplished on the behavioral
level, by moving into and out of the sunshine, for example. Hormonal
control is undoubtedly a factor. Thyroxine has been shown to affect the
temperature adaptation of frog's heart (38, 40, 41). The metabolism of
isolated organs is frequently found to be adapted in the same way as
that of the intact animal from which they were derived (42, 43, 44).
However, in some cases certain organs may show temperature adaptation and others not (43, 44).
The activity of various enzymes in cell or tissue homogenates has
been found to vary with the temperature to which the organism was
adapted. To cite a few examples, there was an increase in the dehydrogenase activity of breis from goldfish liver tissue, snail salivary and
493
on evaporation of water to prevent overheating. Those which cannot
evaporate water freely, due to scaly or horny skins, must avoid the heating effects of direct sunlight (cf. reference 37).
It must not be concluded that because the poikilotherms have little
or no ability to regulate their internal temperatures their metabolic
processes are completely at the mercy of the external temperature. There
is ample evidence for adaptation of these organisms to warmer or cooler
temperatures in such a way as to result in much the same level of activity regardless of the external temperature. As Barcroft (38) stated,
"It is clear that nature has learned so to exploit the biochemical situation so as to escape from the tyranny of a simple application of the
Arrhenius equation. She can manipulate living processes in such a way
as to rule, and not be ruled by, the obvious chemical situation."
Although many examples of biochemical and physiological change in
response to temperature are known, the molecular basis of these adaptations is still little understood. The generalization may be made that an
organism adapted to low temperatures has a higher metabolism at a
particular temperature than one acclimated to high temperatures. This
holds for adaptations of members of the same population in the laboratory and for adaptations to different climatic conditions among different
populations of the same species in nature. A large number of examples
of such adaptations has recently been discussed by Bullock (37).
Adaptive regulation may occur at the molecular, cellular, organ, or
behavioral level. While the underlying biochemical mechanisms are not
well known, it is evident that these must include (a) a mechanism for
perception of absolute temperature, as distinguished from temperature
differences, and (b) a compensation system. The temperature-sensing
mechanism may lie in the rate of discharge of nerve fibers, although
this is not definitely established (12, 39). The compensatory mechanisms
appear to be varied. In reptiles, which show little capacity for physiological adaptation (37), compensation is accomplished on the behavioral
level, by moving into and out of the sunshine, for example. Hormonal
control is undoubtedly a factor. Thyroxine has been shown to affect the
temperature adaptation of frog's heart (38, 40, 41). The metabolism of
isolated organs is frequently found to be adapted in the same way as
that of the intact animal from which they were derived (42, 43, 44).
However, in some cases certain organs may show temperature adaptation and others not (43, 44).
The activity of various enzymes in cell or tissue homogenates has
been found to vary with the temperature to which the organism was
adapted. To cite a few examples, there was an increase in the dehydrogenase activity of breis from goldfish liver tissue, snail salivary and