Preference and aversion for deterrent chemicals in two species of Peromyscus mouse

Abstract

Deterrent chemicals such as quinine hydrochloride (QHCl) are generally considered to be aversive to mammals at all detectable concentrations. However, several species contain individuals that drink solutions containing low concentrations of deterrents in preference to plain water. The present study examines this paradoxical preference in two species of mouse, Peromyscus melanotis and P. aztecus. Preliminary findings had suggested that whereas some P. aztecus prefer low concentrations of QHCl, no P. melanotis prefer any concentration of QHCl. Experimen 1 tested the hypothesis that individual mice that prefer low concentrations of QHCl would respond similarly to four other deterrents described by humans as bitter and/or astringent (ouabain, hop extract, sucrose octaacetate, and tannic acid) in 48-h, two-bottle choice tests. Peromyscus aztecus displayed a large amount of intraspecific variation in response to all five deterrents. Those P. aztecus that drank low concentrations of QHCl in preference to plain water were significantly more likely to respond similarly to low concentrations of the other deterrents. No P. melanotis displayed a preference for any concentration of either deterrent. Experiment 2 examined the temporal stability of the response to 0.1 m M QHCl in P. aztecus over six consecutive choice tests. Mice were divided into three groups based on their initial response to the QHCl solution (preference, no response, or rejection) and then subjected to the 12-day test. The response of mice within each of the groups did not change significantly over time. Because the preference for low concentrations of the deterrent chemicals is reminiscent of the preference many humans show for the taste of QHCl in Schweppes Tonic Water ™, this phenomenon is referred to as the Schweppes effect. Based on the results of this study and others, it appears that the Schweppes effect is: (i) widespread in the animal kingdom, (ii) species specific, (iii) polymorphic within a species, (iv) apparent during the forst night of exposure, (v) gender nonspecific, and (vi) elicited by chemically unrelated deterrents. While the physiological mechanism(s) underlying the Schweppes effect and its ecological relevance remain obscure, several possibilities are discussed.