October 28, 1922] 



NA TURE 



567 



artificial night (period of darkness) and day (period of 

 artificial illumination) on birds confined in a darkened 

 room the temperature rhythm was altered. After 

 the second day the diurnal birds adapted themselves 

 to the changed conditions so that the maximum 

 temperature occurred at night and the minimum during 

 the day. When the experiment was ended and birds 

 again led a normal life in relation to daylight, the 

 diurnal rhythm quickly returned. A similar change of 

 rhythm has been produced artificially in the monkey. 

 This bears on the question as to the cause of the 

 diurnal temperature rhythm in animals. Some believe 

 that there exists in the body a fixed periodicity of 

 which the temperature rhythm is an expression, and 

 that this periodicity persists under all conditions, 

 and is, to a large extent, independent of outside 

 influences. Others are inclined to question the exist- 

 ence of this mysterious periodicity, and look upon the 

 diurnal variation as being due entirely to the action 

 on the body of the various outside influences which 

 aSect body temperature, notably, muscular activit) 

 and sleep. The fact that the rhythm may be altered 

 by changing the daily routine appears to give support 

 to the latter view. 



Although a distinct diurnal body temperature 

 rhythm is found in birds with a wider range in many 

 cases than in mammals, there is little evidence of a 

 seasonal variation. This is all the more interesting, 

 since no class of non - hibernating homoiothermal 

 animals show greater evidence of cyclical bodily 

 changes than do birds. During moulting time, in the 

 late summer and autumn, they shed their feathers and 

 show other signs of depressed vitality; while in the 

 spring, in preparation for the mating and breeding 

 seasons, they put on fresh plumage and become ex- 

 tremely active. However, heat production, if not 

 greatly increased in a short time, has no relation to 

 body temperature. 



As in the case of mammals, the temperature of the 

 female was found to be slightly higher than that of the 

 male of the same species and under the same conditions, 

 in most cases, but in certain groups the opposite was 

 found. For example, in the herons (Ardeidse), in three 

 species we have the following averages : Great blue 

 heron (Ardea herodias), male, io4°-8 F., female, io3°-7 F. 

 snowy heron (Egretta candidissima), male, io4°-8 F., 

 female, io4°-o F. ; and the black-crowned night heron 

 (Nycticorax ncevins), male, io3°-5 F., female, io2°-6 F. 

 Here there is a very pronounced difference in favour 

 of the male, and the same is found in certain other 

 shore birds. 



Among other factors that influence the body tempera- 

 ture of birds it was observed that large masses of food, 

 if cold, will frequently cause a sudden fall in tempera- 

 NO. 2765, VOL. I io] 



ture in a bird of small size, while bathing may produce 

 a slight fall. 



As in the case of mammals, nestlings and immature 

 birds show a lower temperature and a wider variation 

 than adults, due to the fact that the temperature 

 control is less perfect. In a black-necked stilt (Himan- 

 topus mexicanus), one day old, a temperature as low 

 as 95°'3 F. was recorded. Apparently this applies 

 only to species with altricial offspring ; it is not found 

 in birds with precocial young, where the mechanism 

 of temperature control is well organised at birth. 



In considering the method of temperature control 

 in birds, Mr. Wetmore believes, with Soum, that the 

 air-spaces play an important role in the regulation of 

 heat loss. On account of the feather covering and 

 the absence of cutaneous glands, little heat is lost by 

 radiation and evaporation from the skin. This throws 

 an increased burden on the respiratory system, supple- 

 mented by the ostial spaces, and the regulation of heat 

 loss through this channel is the chief factor in avian 

 temperature control. The author brings forward some 

 first-hand evidence in favour of this belief. 



Discussing the significance of temperature control 

 in general, the statement is made that " In the bird, 

 the regulation of body temperature has reached its 

 highest point, though birds stand second to mammals 

 from an evolutionary point of view. Proof of this 

 is found in the fact that birds have the highest body 

 temperatures known, and that none of them hibernate." 

 This conclusion scarcely seems justifiable on the 

 evidence at hand. The degree of heat control of any 

 species is not to be measured by the actual height 

 of the body temperature, but rather by" its diurnal 

 variation, and according to this standard the regula- 

 tion of body temperature appears to have reached its 

 highest point in Homo sapiens, since the diurnal range 

 in him is less than in any other so-called homoiothermal 

 animal so far investigated, although the actual body 

 temperature is among the lowest for mammals and far 

 below that of any of the birds. 



It is generally held that the higher the bird in the 

 zoological scale, the greater is the body temperature. 

 The author agrees with this statement, as a rule, but 

 points out many discrepancies. If the appended tables 

 be examined it will be noticed that, when arranged by 

 families, the highest temperatures are found in pigeons, 

 cuckoos, woodpeckers, and in the great passerine order 

 beginning with the Tyrannidse and ending with the 

 Turdids. In five species of the former the average 

 body temperature for male or female was rio J F. or 

 more. The highest average temperature for both sexes 

 w as found in the western pewee (Myiochanes richardsoni) 

 with a mean of no : -2 ¥.. the greatest single individual 

 reading being ii2°-7 F. Contrary to popular belief, 



