ECOLOGICAL FACTORS 



391 



permits further and more abundant food intake. This cycle proceeds 

 until the optimum body temperature is reached. 50 More heat must be 

 produced by metabolism at lower external temperatures: a canary, at 

 rest in the twilight, produces 319 calories per kilogram per hour at 

 22.3°; at 14° it produces 992 calories. 51 In times of food shortage, 

 stored materials, such as fat and glycogen, must be "burned." In small 

 animals, however, the radiation of heat may become so great at lower 

 temperatures that, in spite of continuous food ingestion, they cannot 



Fig. 113. — Antelope from Tibet (Pantholops hodgsoni), a; and Waller's gazelle 

 (Ldthocranius walleri) from Somaliland, b; After Brehm's Tierleben. 



furnish sufficient oxidizable material for the maintenance of their 

 body temperature. Lapicque 51 found by comparing three birds of dif- 

 ferent size, the domestic pigeon (body weight 390 gm.),. the small 

 pigeon Geopelia striata (weight 48 gm.), and the tiny weaver finch 

 Estrilda astrild (7.5 gm.), that the heat radiation calculated per kilo- 

 gram results in a series of 144, 344, and 1020 calories, and the daily 

 food requirements, similarly calculated, amount to 48, 132, and 390 

 gm. In Estrilda, two-thirds of the ingested food is used in heat pro- 

 duction; with a reduction of temperature to 15°, the shortness of 

 winter days does not offer sufficient time for an adequate food intake 

 for the tiny animal ; in spite of continuous eating it becomes thin and 



