236 Comparative Animal Physiology 



metabolic rates are not easy to detect in lower animals, owing to the difficulty 

 of establishing "basal" or standard conditions, differences in the metabolic 

 rates of men and women have been recognized since the extensive work of 

 Benedict and his co-workers.^^- -"'• -'• Human males have a higher rate of 

 oxygen consumption than females, at all ages, and the difference becomes 

 more marked with increasing age. The difference may be attributed to more 

 "active protoplasmic tissue" in males, as was indicated by Benedict for the 

 correlation of oxygen uptake with age; it may also represent a genetically 

 determined endocrine-controlled state of higher metabolism. A method for 

 the determination of the time of ovulation in women is based on the abrupt 

 increase in body temperature, presumably accompanied by an increased oxygen 

 consumption, which occurs at about the time of rupture of the ovarian follicle. 

 When an appreciable difference in size exists between male and female the 

 analysis of sex difference is rendered more difficult, as in the sand crab, 

 Emerita, in which the male is smaller than the female and on a weight basis 

 has a higher gas exchange.^"^ Sex differences have been indicated in insects, 

 although the data are not concordant or easy to evaluate. Oxygen consumption 

 by Drosophila pupae is about 21 per cent higher in females,-'^ but the differ- 

 ence decreases in later pupal life-^'^ and in fact was shown to be counteracted 

 as the males finally develop a higher rate of metabolism than the females, a 

 condition initially reported by Poulson.-**^ In the bee moth, Galleria, the males 

 show a greater initial gas exchange, the difference vanishing with subsequent 

 development.'^"- -^^^ An identical situation occurs in Daphnia.'-'^-^ One may con- 

 clude with Krogh that these results on immature forms depend on the time of 

 measurement in the period of development, the degree of organized tissue for 

 each sex at that time, and the relative rates of differentiation. A clear-cut case 

 of higher male metabolism has been shown in housefly adults (Musca domesti- 

 ca) measured at 20° C.^°^ Another "sex" distinction in oxygen consumption is 

 the significant difference of 12 per cent in two well established mating types 

 of Paramecium calkinsi demonstrated by means of the Cartesian diver tech- 

 nique. ■*" 



Oxygen Consumption in Relation to Nutrition. The effect of starvation on 

 animal metabolism depends on ability of the animal to live on stored reserves 

 during prolonged periods. The earlier work on man indicated a decrease in 

 the metabolic rate with prolonged fasting,-^*^- ^°'^' '^^'•^ but the extensive investi- 

 gations of Lehmann and co-workers--^ and of Benedict^-^ have demonstrated 

 little change before appearance of severe inanition. Comparable results are 

 indicated by the work on other homoiothermic animals. In special cases, how- 

 ever, as during hibernation, the decrease in food intake is correlated with 

 reduced respiratory exchange, as well as with lowering of body temperature 

 and profound cardiovascular changes. 



Among invertebrates many correlations have been made between oxygen 

 consumption and nutritive intake in insects^-* and Protozoar'^^ The organisms 

 behave as though they ate all they could, and their metabolic levels are deter- 

 mined by the amount of oxygen necessary to utilize the food consumed. 



The nature of the organic food consumed determines the amount of o.xygen 

 required to oxidize it, a principle on which is based the science of biological 

 calorimetry. Lavoisier first demonstrated that the energy content, of the food 

 consumed by an organism very nearly equals the heat produced. The measure- 



