A key assumption in ecomorphological studies is that morphology–function relationships are invariant due to underlying biomechanical principles. We tested the hypothesis that morphology–performance relationships are invariant across different seasons by examining how a key performance trait, bite force, and two aspects of morphology (head shape and dewlap size) changed seasonally in the field and in the laboratory in the green anole lizard Anolis carolinensis. We found that not only did bite force change seasonally (up to 80% within the same individual), but relationships between morphology and bite force are highly plastic. Of the three traits examined (bite force, head shape, and dewlap area), only head shape did not change seasonally. We noted opposing trends for how bite force and dewlap area changed seasonally; whereas dewlap areas were large in the spring, and small in the winter, bite forces were low in the spring and high in the winter. This pattern occurred because of a tradeoff at the individual level: individuals in the spring with large dewlaps and high bite forces diminish their dewlaps (but not bite force), whereas individuals with small dewlaps and low bite forces in the spring increase their bite forces (but not dewlap size). We also show that this trend was apparent both in the field (comparing different individuals) and the laboratory (comparing the same set of individuals under standardized conditions). Finally, seasonal changes were not consistent among individuals for either bite force or dewlap area, as individuals changed seasonally in proportion to their initial state. These findings cast doubt on the widely held view of invariant morphology–performance relationships, and offer a cautionary note for eco-morphological studies.