496 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1910. 



The hsemoglobin percentage does not appear to have been deter- 

 mined in the blood of birds, but in view of the greater size of the 

 red blood corpuscles of birds as compared with those of mammals 

 we might expect it to be lower. On the other hand, the fact that the 

 consumption of oxygen and both relative heart weight and pulse 

 frequency are higher in a bird than in a mammal of the same size 

 (e. g.j in the sparrow than in the mouse, in the pigeon than in the 

 guinea pig) may have some bearing on the fact that birds maintain 

 a higher constant temperature than mammals. 



In this connection it is interesting to note that in the lowest mam- 

 mals, the monotremes, and also in the marsupials, in which a lower 

 body temperature is maintained, the heat produced, as measured by 

 the carbon-dioxide output per kilo per hour, is much less than in 

 so-called placental mammals of the same size (10). We know noth- 

 ing at present about relative heart size or pulse rate in these animals. 

 But since the monotremes regulate their temperature by the produc- 

 tion of more heat when required (i. e., in cold surroundings) instead 

 of by always producing a large amount and getting rid of the excess 

 when necessary as the larger at any rate of the higher mammals do, 

 we should expect the pulse rate in them to vary a good deal, and 

 inversely, with the external temperature. The marsupials, utilizing 

 also variations in loss of heat, although to a less extent than placental 

 mammals of the same size, seem to regulate their body temperature 

 extremely well. Of the two monotremes still living. Ornithorhyncus 

 succeeds in doing so quite as well as some of the placental mammals ; 

 and Echidna^ although it fails, makes the attempt for the greater 

 jDart of the year, the oxygen consumption in the individual, at any 

 given external temperature, seeming to some extent to vary inversely 

 with the size according to the determinations made by Dr. Martin 

 of the carbon-dioxide output per unit, weight, and time in three 

 individuals (10). Those placental mammals which do not regulate 

 their temperature the whole year round do not succeed much better 

 than Echidna when they make the attempt, especially on first awak- 

 ing from hibernation. In some of them the temperature seems to 

 remain lower than in other placental mammals. The rectal tem- 

 perature of a bat, for instance, may be only 30° C. when it is wide 

 awake and active (16). The low temperature in such cases seems 

 again to be due to the production of heat being small in comparison 

 with other mammals of the same size. Thus in an active bat weigh- 

 ing about 20 grams, the carbon-dioxide output per kilo per hour 

 was found to be only about 4.5 grams, and therefore considerably 

 less than in a mouse. If we may take this as a measure of the 

 demand for oxygen in an active bat, the heart need not beat with a 

 frequency of more than 250 per minute to supply the demand, seeing 

 that the heart of the but, as we happen to know from two independent 



