Electrophysiological recordings show that water is not tasteless to cats.
Also, unlike most mammals, cats appear indifferent to sucrose, but this may be because the taste of the sucrose is masked by the taste of the water in which it is dissolved. When the water taste is suppressed by the addition of small amounts of sodium chloride [NaCl, table salt], cats take sucrose avidly. [as in 1951]
…The study reported here resolves this apparent discrepancy. Cats do have some fibers sensitive to sucrose3, 10, but they also have water sensitive fibers that under normal salivary conditions may mask the sucrose responses and thereby interfere with taste discrimination. It is possible, however, to suppress the water responses and thereby render sucrose highly acceptable.
…Even though the water-after-NaCl response is only one of those recorded in the cat, it is particularly important in the present study because NaCl is a major constituent of saliva. When the cat drinks water, the NaCl in the cat’s own saliva is an adapting stimulus, and the fibers sensitive to water-after-NaCl respond. When the cat drinks sucrose, both the fibers sensitive to water-after-NaCl and the fibers sensitive to sucrose respond. Most fibers that respond to water-after-NaCl will not respond to NaCl12 (see also Figure 1). This observation suggests that the water taste in a sucrose solution can be suppressed by the addition of the right amount of NaCl. This amount depends on the adapting concentration (that is, saliva). Fibers sensitive to NaCl fire to concentrations higher than the adapting concentration, whereas fibers sensitive to water-after-NaCl fire to concentrations lower than the adapting concentration. The electrophysiological data suggest that 0.03M NaCl suppresses water responses without stimulating NaCl responses, given any of a wide range of possible adapting concentrations3.
…Figure 2A shows that the cats ingested nearly equal amounts of water and sucrose solution at every concentration tested. (That the two curves rise as a function of sucrose concentration is probably due to loss of liquid from slight diarrhea caused by the accidental intake of sucrose.) A very different picture emerges (Figure 2B) when weak NaCl solution is used as the solvent instead of water; in these cases the animals strongly prefer the sucrose to the weak NaCl solution.
…1951’s finding that sucrose in dilute milk (one part milk to 4 parts water) is preferred by cats fits in well with the result presented here. Mean sodium and chlorine content for whole milk so diluted would approximate 0.006M NaCl17. The exact whole-mouth salivary NaCl concentration for the cat is not known, but it must fall between 0.01M and 0.16M NaCl18. For adapting concentrations in this range, electrophysiological data3 suggest that the 0.006M NaCl in the milk used by Frings would at least partially suppress the water-after-NaCl response.
The taste of water has been widely ignored in behavioral testing. It is now clear that water should be regarded not as a neutral solvent but rather as a taste stimulus itself.