Brett's hypothesis
Brett's hypothesis[1][2] also known as the heat-invariant hypothesis[3][4] or Brett's heat-invariant hypothesis[5] proposes that upper thermal tolerance limits are less variable geographically than lower thermal tolerance limits. This hypothesis was originally proposed for fish[1] but lately has been supported by studies with reptiles,[6] amphibians,[5] and aquatic insects.[7] Three different mechanisms are proposed for the existence of this large-scale pattern of thermal tolerance limits variation:
- A constrained evolutionary potential of upper thermal tolerance limits
- The buffering effects of thermoregulatory behaviour has greater potential to face heat rather than cold stress
- Resolution of thermal data used
Global versus local scales in Brett's hypothesis
While Brett's hypothesis has been strongly supported at global scales, heat tolerance seems to respond differently to smaller-scale climatic and habitat factors. For instance, lizards from the
at local scales.