Two species of kangaroo rat (Dipodomys), varying in their degree of dietary specialization, were compared in a series of food aversion learning experiments to test the hypothesis that rapid aversion learning is an adaptation of relatively generalist feeders. The more generalist species indeed learned better or more rapidly in certain experiments, but species differences were at least partly a function of the specific test foods. Interpretation of results is complicated by differences between the two species in their initial reactions to particular foods, in the relative efficacy of different foods in supporting learned aversions, and perhaps even in their physiological responses to illness-inducing and control procedures.In the past decade, many psychologists have embraced the notion that animal learning must be viewed comparatively, with attention being given to the ecological specializations and predispositions of subject species. But despite the popularity of such ideas (e.g., Hinde & Stevenson-Hinde, 1973;Seligman & Hager, 1973), there has been surprisingly little explicitly comparative investigation: Few have asked if related animal species that differ in their ecological adaptations respond differently to similar contingencies, and, if so, exactly how.Food aversion learning has been a favorite paradigm of critics of general accounts of learning because of the striking phenomena of long effective delays of reinforcement (Garcia, Ervin, & and cue specificity . The animal that has been most studied is the laboratory rat, a species that associates taste cues and subsequent gastric distress with ease. Rozin (1976) in particular has suggested that the rat's capacity for adaptive modification of feeding behavior is the species-specific adaptation of a dietary "generalist," and that "specialists" might be less talented in this sphere."Specialist-generalist" is clearly not a dichotomy. It may be treated as a dimension along which species may be ranked according to some index of diversity of foodstuffs consumed. A different definition of specialization focuses upon morphological and behavioral adaptations to particular foodstuffs, but we should expect animals that are specialists by the latter criterion to be specialists by the former as well. A clear case of a dietary specialization by either criterion is provided by the chisel-toothed kangaroo rat, Dipodomys microps, in the Owens Valley of California. Most Dipodomys species are predominantly granivorous, consuming seeds of a variety of plant species, but greens are a substantial dietary element in certain seasons. Owens Valley D. microps, by contrast, feeds almost exclusively upon the salty, succulent leaves of chenopod plants, mainly shadscale (Atrip/ex conjertifo/ia), a diet for which it exhibits behavioral and morphological specializations (Kenagy, 1972(Kenagy, , 1973. Cheek pouch and stomach content analyses confirm that D. microps takes a less diverse diet than other Dipodomys, particularly D. merriami, a sympatric species of more typical food habits (e.g., ...
