Figure 5: Trade-offs and complementarity are distinct presentations of the same phenomena. In complementarity, species respond to changes in temperature via relatively small but coordinated changes in both size and shape. In trade-offs, the response to temperature is highly constrained in one functional trait (size in this case), so all morphological thermal response occurs through the other functional trait (shape in this case). The size and shape lines are interchangeable, such that species could respond via changes in size instead of shape.
Abstract
Context. Ecogeographical rules such as Bergmann's and Allen's have long guided expectations about how organismal size and shape vary across temperature gradients. Yet these rules are typically tested independently despite sharing a common thermoregulatory basis. I synthesised recent empirical literature to evaluate how often Bergmann's and Allen's rules are analysed jointly and assess what is lost when size and shape are studied in isolation. A review of 86 studies published since 2021 reveals that less than half examine both rules together, and far fewer test for co-dependence between them.
Linking Size and Shape: Isolated rule testing obscures several processes structuring thermoregulatory adaptation. These processes can generally be thought of as interactions—where temperature effects on one trait (size or shape) depend on the state of another, producing context-dependent or even inverse clines. One important class of interactions can be described broadly as trait × ‘strength of constraint’ interactions, providing a common conceptual foundation for the theories of trade-offs and complementarity. Both theories propose that stronger adherence to Bergmann's rule is linked with weaker adherence to Allen's rule, and vice versa, and that functional constraints are likely an important determinant in partitioning morphological change across size-shape axes. I argue that trade-offs are a boundary case of complementarity, rather than representing categorically different phenomena.
Thermo-Morpho Framework: I propose a Thermo-Morpho framework that treats size and shape as complementary axes of thermoregulatory adaptation, explicitly linking ecogeographical rules to their shared biophysical basis.
Beyond Morphology: These same principles extend to physiology, coloration, phenology, and behaviour, which frequently interact with morphology to shape thermoregulation. Focusing narrowly on single traits risks mischaracterising adaptive pathways and underestimating organismal flexibility under climate change.
Conclusion: We must move beyond single-rule analyses toward integrative tests of organismal response to climatic warming.