Fitness cost refers to the inherent trade-off in organisms between the ability to survive and reproduce (fitness) and the energy or resources expended in doing so. In the context of evolutionary biology, fitness cost is often studied in relation to adaptations or traits that confer a benefit to an organism in certain environments, but also come with a cost in terms of energy, time, or other resources. Researchers may investigate how organisms balance these costs and benefits, the mechanisms by which fitness costs are incurred, and how they can impact evolution and adaptation. Understanding fitness costs can provide insights into the evolutionary dynamics of populations and how organisms respond to changing environments.