If the rule is relaxed to embrace pattern alone, as explicitly advocated by Rensch and Mayr, exceptions can still be found both intra- and interspecifically. Within species, Rensch (1938) reported that 10–30% of the warm-blooded species examined by him were exceptions to Bergmann's rule. Ray (1960) reviewed the literature on body size variation in relation to climate for poikilotherms, and concluded that the rule was supported by 75% of species studied. Nevertheless, these percentages (see also James, 1970; Yom-Tov & Nix, 1986) support Mayr's (1956) contention that the rule would be proved if upheld by the majority of species, although his subsequent definition of a majority as more than 50%(Mayr, 1963) is rather generous in respect of a ‘rule’. Some studies, however, do find that the percentage of species in agreement with the intraspecific rule fails even this criterion (McNab, 1971).

It is the definition of Bergmann's rule, and specifically the taxonomic level at which the rule is considered to act, that has done most to cloud the degree of generality of the effect it describes. Bergmann himself (quoted in James, 1970) stated that ‘(i)f we could find two species of [homeothermic] animals which would only differ from each other with respect to size, . . . (t)he geographical distribution of the two species would have to be determined by their size . . . if there are genera in which the species differ only in size, the smaller species would demand a warmer climate, to the exact extent of the size difference.’ Also: ‘(a)lthough it is not as clear as we would like, it is obvious that on the whole the larger species live farther north and the smaller ones farther south.’Bergmann's formulation was later altered by Rensch (1938), whose revised definition was that ‘within a Rassenkreis [complex of races] of warm-blooded animals the races living in colder climates are generally larger than the races living in warmer regions.’ Rensch considered that the new definition better fitted the rule as then understood, but was quite clear that the revision was his own (‘I myself reduced it to the geographical races of a species’; Rensch, 1938). It was this revision that subsequently became the accepted formulation of Bergmann's rule; later definitions included ‘Races of warm blooded vertebrates from cooler climates tend to be larger than races of the same species from warmer climates’(Mayr, 1956), and ‘The smaller-sized geographic races of a species are found in the warmer parts of the range, the larger-sized races in the cooler districts’(Ray, 1960; see also definitions in Gittleman, 1985; Goudie & Ankney, 1986; Paterson, 1990; McDowall, 1994; Steudel, Porter & Sher, 1994; Smith, Betancourt & Brown, 1995; Atkinson & Sibly, 1997).The notion that the application of Bergmann's rule at the intraspecific level is a derived state was emphasized by James (1970), who noted that it was a considerable modification of Bergmann's original message, although one that fitted well with knowledge of intraspecific body size variation. Quotations from Bergmann (1847; given in translation by James, 1970) imply that he considered the effect to be interspecific, but between closely related species. Whether he intended the example he gave of ‘species within a genus’ to be literal is unclear. Whichever, it is clear that his formulation was not intraspecific, as he thought it ‘paradoxical that the effects of the same rule in races of animals are not very apparent’(Bergmann, 1847, quoted in James, 1970). In this context, it is interesting that Mayr (1956; see also Rensch, 1938) noted that many of the species considered by Bergmann were, when Mayr was writing, afforded only sub-specific status! Nevertheless, since James's paper, Bergmann's rule has been examined at a variety of taxonomic levels, for example within species (Barnett, 1977; Ralls & Harvey, 1985; Yom-Tov & Nix, 1986; Geist, 1987; Graves, 1991; Smith et al., 1995, 1998; Van Voorhies, 1996, 1997; Mousseau, 1997; Partridge & Coyne, 1997), between species within genera (Gittleman, 1985; Taylor & Gotelli, 1994), between functionally related species (Geist, 1987; Cotgreave & Stockley, 1994), and between species within a range of higher taxa (Zeveloff & Boyce, 1988; Cushman et al., 1993; Barlow, 1994; McDowall, 1994; Hawkins, 1995; Hawkins & Lawton, 1995; Poulin, 1995; Poulin & Hamilton, 1995; Blackburn & Gaston, 1996a).