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Two different approaches to the calibration of inter-species SDR performance differences that have emerged over the past decade are discussed. One, the discontinuity hypothesis, proposes a qualitative dimensional shift across vertebrates, with a two-category scale based upon the presence or absence of error reduction on SDR tasks. The other approach, the continuity hypothesis, argues for a quantitative dimensional shift across species, with a scaling of inter-species performance differences in terms of absolute scores. Important directions for future research are suggested.

There is general agreement that birds and mammals, but not fish, can display error reduction on successive discrimination reversal (SDR) tasks. Reptiles, however, show error reduction on some but not other tasks. To provide further sampling of reptilian SDR performance, two species of crocodilians, the American alligator and the American crocodile, were tested on a spatial discrimination reversal task. Both species displayed error reduction, the alligator being appreciably inferior to the crocodile.

A number of experiments with SDR methodology have yielded inter-species performance differences that are consistent with taxonomic rankings. However, one troublesome problem that plagues such comparisons is the question of whether or not such differences might be due to uncontrolled variation in either drive (D) or incentive (K) levels across species. As an initial step in the evaluation of the problem a series of experiments designed to study the effect of variation of both variables, within and across species, upon SDR measures are reviewed. While it is clear that both Dand Kdo effect SDR measures, previously obtained inter-species performance differences are recovered when species are equated on these variables.

Attention is directed to the isolation of the dimensions of SDR methodology which make it sensitive to inter-species performance differences that have systematic taxonomical significance. Two dimensions in particular are indicated, inconstancy of reinforcement and the maximum opportunity for the generation of negative transfer. To determine if variation in magnitude of negative transfer is diagnostic of phyletic level, indices were studied across different phyletic levels of birds. More “primitive” birds developed greater magnitudes of negative transfer than more “advanced” birds. The relevance of these data to the retention decrement and the differential extinction hypotheses is examined.