Effects of Taste Quality and Hardness of Food on the Feeding Behavior of Axolotls

Abstract

The behavioral responses of axolotls to chemical stimuli representing the four basic taste qualities (sweetness, saltiness, sourness, and bitterness) were quantified by measuring the ratio of rejection and the number of bites of gel pellets that contained various concentrations of sucrose, NaCl, citric acid, or quinine hydrochloride. A simplified method to measure the hardness of gel (gel strength) was devised and the feeding responses towards various hardnesses of gel pellets were also quantified by measuring the rejection ratios. Rejection ratios towards chemical stimuli other than sweet gels tended to increase with concentration and the degree of rejection was different depending on the quality of taste. These results showed good correspondence to those of a previous study, suggesting that axolotls distinguish the salty, sour, and bitter tastes, but not sweet tastes. The number of bites was dependent on the concentrations as well as the taste qualities, and bites on sour and bitter gels showed a different type of biphasic concentration dependency. When the hardness of gel pellets was changed at a constant concentration of chemical stimuli, the rejection ratios tended to increase with gel strength. The potential of salty taste to induce swallowing (a preference for salty taste) was suggested by reductions in the rejection ratios when gel pellets included NaCl but not sucrose.