240 Comparative Animal Physiology 



tion. Among fish, too, oxygen uptake is correlated with activity and efficiency 

 of the respiratory mechanism as well as with the availability of oxygen. 



Oxygen Consumption in Relation to Seasons. Seasonal variations in oxygen 

 uptake are not easy to distinguish from variations produced by such factors as 

 temperature, nutrition, activity, and the like, with which they are bound. 

 Rather, the seasonal variations represent the balance of these individual factors. 

 along with intrinsic influences of the organism itself, such as endocrine control 

 of dfurnal and reproductive cycles. Changes in the metabolic rate at oestrus 

 have been noted in birds. Some evidence is available, however, for seasonal 

 oxygen consumption adjustment to temperature changes. Whereas poikilo- 

 thermic marine animals. for the most part respond to seasonal decreases in 

 temperature bv a proportional reduction in oxygen consumption, some regula- 

 tion has been demonstrated in the cunner,'"'^' Fiinditliis/-*'*^ and the sand crab, 

 Emerita talpoida.'^^'' The adaptation tends to ofl:'set the effect of temperature 

 and to maintain a stable metabolic rate from season to season. Determination 

 of the metabolism of Ftindtiliis at 15° C. in February gave higher values than 

 measurement at the same temperature in July. The cunner, Taiitogolahrus, 

 when investigated for oxygen consumption in summer and winter at tempera- 

 tures ranging from 0° to 30° C, yielded slightly higher values in winter than 

 in summer for temperatures below 15°. The sand crab shows the best regula- 

 tion of all and, according to Edwards, behaves somewhat like homoiotherms, 

 making an internal adjustment in metabolic rate to oft'set the temperatures 

 experienced during the winter. The oxygen uptake at all temperatures belov\ 

 20° C. was greater in winter than in summer, the rate of winter metaboHsm 

 at 3° being about four times the summer rate at this temperature. It would 

 be of interest to know whether this seasonal temperature regulation might be 

 reproduced in part by short term acclimatization— i.e., would one of these 

 animals acclimatized to low temperature in the summer show increased 

 metabolism? 



Oxygen Consumption in Relation to Temperature. The responses of oxygen 

 consumption to temperature changes ha\'e been recognized for many years, and 

 valuable surveys of the basic literature are available.^''- '""• -'^ A general 

 distinction can be made between the poikilothermic animals whose oxygen 

 consumption increases with rising temperatures and the homoiothermic ani- 

 mals whose oxygen consumption increases as the temperature decreases. The 

 difference is mainly due to the presence in the latter of a temperature-regu- 

 lating mechanism b\ which external temperature changes arc offset to a 

 large extent by metabolic and physiological reactions, \'asomotor control jilaying 

 a predominant role. The frequently used expression, Qio, is a convenient 

 measure of the effect of changing temperature on respiratory processes, but 

 the variation in O,,, values over the biological temperature range makes its 

 use and significance somewhat limited (see Chapter 10). Much higher Qio 

 values than the exjicctcd 2-3 arc obtained at low temperatures; Krogh s experi- 

 ments on poikilothermic \ertebrates includes values from O,,)r3l0.9, based 

 on measurements o\cr the live degree range from to 5 C^, to Ok, ^2. 2 at 

 20-27.5° C.-'-' 



A more precise expression to describe temperature characteristics is based 

 on the Arrhenius formula and is constant over the biological temperature 

 range for each given set of conditions (Ch. 10). The equation reflects the 



