Labor of Love: Female Nurse Sharks Swim Less but Expend More Energy Than Males During the Mating Season

Sharks exhibit a broad range of reproductive strategies that often include complex behaviors and varying levels of multiple paternity within broods. Understanding these strategies is important for conservation and management, it is inherently difficult to measure the behavioral and physiological forces driving each strategy. We used acceleration data loggers (ADLs) to quantify fine-scale behaviors and activity-based energy expenditure in nurse sharks (Ginglymostoma cirratum) during their mating season in the Florida Keys. Females of this species show shallow-water refuging behavior during the mating season, as well as a high frequency of multiple paternity. Accelerometer data showed that even though females spent a lower percentage of their time swimming than males (59% vs 81% respectively), they still expended the same or more energy than males, as quantified by Overall Dynamic Body Acceleration (ODBA). This result was due to a higher mean tailbeat amplitude (1.47 m/s²) in females compared to males (0.78 m/s²), and was likely caused by the heavier weight of females. Nurse sharks are ovoviviparous, meaning that the females carrying ripe ova are heavier during the mating season than at any other point in their reproductive cycle. The extra energy required for swimming likely makes it difficult for females to escape male advances, and may be the driving force behind female refuging behavior as well as multiple paternity in this species.