THE VARIATION IN TASTE THRESHOLDS OF RUMINANTS 305 



degree of anorexia can be controlled by manipulating the salivary sodium 

 level by giving or withholding sodium supplement. It would appear that 

 the fall in the sodium level of the extracellular fluid parallels some mech- 

 anism which is controlling food intake. It should be remembered in this 

 regard that the salivary sodium bicarbonate plays an essential role in the 

 digestive process of ruminants. V/hen an experimental calf is allowed 

 continuous access to either 1 per cent NaCl or 2 per cent NaHCOg it will 

 take drink sufficient to maintain its salivary Na/K ratio at normal levels 

 whereas its control twin will take only minimal quantities. 



After a period of depletion the taste thresholds for both members of the 

 twin pair have been examined by the two choice preference technique (Bell 

 and Williams, 1959). The experimental calf shows a much higher taste 

 threshold for sodium chloride for it will take 5 per cent NaCl solution 

 compared to the normal calf which will accept only 0.03 NaCl. Similarly 

 in tests for NaHCOg taste thresholds the sodium depleted calf showed a 

 higher acceptance threshold than its normal twin (Fig. 3). During the 

 discrimination tests the salivary Na/K ratio is restored to normal in the 

 experimental calf when it is imbibing the higher concentrations of either 

 NaCl or NaHCO.. 



TASTE DISCRIMINATION 



•— • EXP. CALF. 

 0---0 CONTROL 



lOO 



SODIUM 

 BICARBONATE 75 

 AS PER CENT OF 

 TOTAL FLUID 

 INTAKE 



50- 



25- 



^-v 



80 40 20 10 0-5 02 5 CH2 006 003 



G/IOOML 



Fig. 3. Two-choice preference test for sodium bicarbonate in monovular twin 

 calves one of which was sodium depleted. (Bell and Williams, unpublished data.) 



When offered falling dilutions of sodium salts the experimental calf at 

 first increased the intake of saline solution so that the Na/K ratio in the 

 saliva remained normal. The animal appeared to be responding to some 

 internal regulatory mechanism of the kind postulated by Richter (1943). 

 When, however, the saline solutions became very dilute the animal failed 

 to take in a sufficient volume of sodium salt and consequently became 

 sodium depleted (Fig. 4). Apparently the self regulatory process collapsed 



