Simple SummarySmall island environments can drive rapid changes in animal traits that provide an advantage for individuals attempting to capitalize on scarce resources. One such trait for lizards is bite force—individuals with stronger bites may be better able to access food that is hard to chew and/or guard access to food and territories from competitors. We introduced lizards to five small, uninhabited islets in Greece and tracked changes in the physical features and bite force of the lizards over several generations. We found changes in body size and bite force that suggested that competition was intense on these small islets; however, we did not see a consistent change in diet among these experimental populations. Our results suggest that competition is an important driver of changes in these traits, and changes in diet may be follow-on consequences. It remains to be discovered if diet is also a driver of these changes in other times of the year when food is especially limited. Some species can and do flexibly and rapidly change in response to changes in their environment.AbstractResource-limited environments may drive the rapid evolution of phenotypic traits and ecological preferences optimizing the exploitation of resources. Very small islands are often characterized by reduced food availability, seasonal fluctuations in resources and strong unpredictability. These features may drive the evolution of phenotypic traits such as high bite forces, allowing animals to exploit a wider variety of the available resources. They may also lead to more generalist dietary patterns in response to food scarcity. However, the lack of predators and competitors on such small islands often also leads to high densities and the evolution of strong sexual dimorphism, which may also drive the evolution of bite force. Here, we take advantage of a unique replicated introduction experiment to test whether lizards introduced into very small islands alter their feeding ecology and use different resources, resulting in the evolution of a large body size, large head size and large bite forces. Our results show that three years after their introduction, the island lizards were larger and had greater bite forces and more pronounced sexual dimorphism. However, the diets were only marginally different between animals from the source population on a very large nearby island and those on the islets. Moreover, distinct differences in diet between animals on the different islets were observed, suggesting that the local environment is a strong driver of resource use. Overall, lizards with absolutely and relatively (adjusted for body size) large bite forces did eat larger and harder prey. Taken together, our data suggest that intraspecific competition is an important driver of the rapid evolution of bite force, which may allow these lizards to exploit the scarce and fluctuating resources on the islets. Whether or not lizards will evolve to include other types of food such as plants in their diet, facilitated by their large bite forces, remains to be explored in future studies.