Are powerful females powerful enough? Acceleration in gravid green iguanas (Iguana iguana)

One demand placed exclusively on the musculoskeletal system of females is maintaining locomotor performance with an increasing load over the reproductive cycle. Here, we examine whether gravid (i.e., "pregnant") iguanas can increase their force and power production to support, stabilize, and accelerate the additional mass of a clutch of eggs. At any acceleration, gravid iguanas produced very high mechanical power (average total power = 673 w/kg; total peak power = 1175 w/kg). While the increase in total power was partly a result of greater propulsive power (average propulsive power = 25% higher, peak propulsive power = 38% higher), increased vertical power (roughly 200% increase) was the main contributor. Gravid iguanas were also able to increase peak forces (propulsive = 23%, mediolateral = 44%, vertical = 42%), and step duration (44%) resulting in greater impulses (i.e., the sum of force produced during a step) to accelerate, balance, and support their increased mass. The increase in step duration and smaller increase in peak propulsive force suggests that gravid iguanas may be force-limited in the direction of motion. We discuss how biomechanical constraints due to females' reproductive role may influence the evolution of the female musculoskeletal systems and contribute to the evolution and maintenance of ecological dimorphism in lizards.