HEART AND RESPIRATORY RATES AND THEIR SIGNIFICANCE FOR CONVECTIVE OXYGEN-TRANSPORT RATES IN THE SMALLEST MAMMAL, THE ETRUSCAN SHREW SUNCUS-ETRUSCUS

Heart and respiratory rates of the smallest mammal (mean adult body ma ss 2g), the Etruscan shrew Suncus etruscus, were determined at rest an d under stress conditions. Heart rate was obtained from electrocardiog rams (ECGs), recorded via foot electrodes, The mean +/- S.D. heart rat e of resting animals (ambient temperature 22 degrees C) was 835+/-107 min(-1), the mean maximal rate amounted to 1093+/-235 min(-1). The hig hest single value recorded was 1511 min(-1), which is the highest hear t rate reported so far for an endotherm. The respiratory rate was also obtained from ECG recordings, which showed the electrical activity of the breathing muscles during inhalation, and additionally by recordin g the movements of the thoracic wall with a laser autofocus system. Th e mean resting respiratory rate was 661+/-93 min(-1), the mean maximal rate was 758+/-109 min(-1) and the highest single value recorded was 894 min(-1). At 22 degrees C, the specific oxygen consumption rate is 67 times higher in resting S. etruscus than in resting humans, Under t hese conditions, the respiratory rate of the shrew is 47 times higher but the heart rate only 12 times higher than in man, Therefore, to ach ieve an adequate circulatory oxygen transport rate, the product of rel ative stroke volume and arterio-venous O-2 difference has to be 5.6 ti mes higher in the shrew than in man, whereas for an appropriate ventil atory oxygen transport rate the product of relative tidal volume and o xygen extraction has to be only 1.4 times higher in this small insecti vore than in man, The maximal possible oxygen transport rates of the v entilatory and the circulatory system have been estimated and compared with the diffusional transport capacity of the lung, These rates amou nt to approximately 1000 ml O-2 kg(-1) min(-1). According to our resul ts and data in the literature, an aerobic scope of 7-10 seems to be re alistic for the Etruscan shrew.