Bonnethead Shark Head-Shape Change Is Not Driven by Diet, University of Miami Study Finds

Bonnethead sharks are born with pointed heads, but as they mature, females develop dramatically rounder snouts—a transformation long thought to run the other way, with the point emerging in males at maturity. New research from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science shows the opposite: both sexes start life with pointed heads, and the snout rounds out with growth, most dramatically in females.

Bonnetheads are the smallest hammerheads and the only shark species known to show a clear difference in head shape between males and females, called sexual dimorphism. Their flattened head, or cephalofoil, supports sensory organs. "Younger males and females both had more pointed heads, but the head became rounder as the sharks matured. That change was much greater in females," said Kathy Liu, lead author of the study published in the journal Integrative Organismal Biology, who conducted the research while earning her master's degree at the Rosenstiel School.

To test whether diet explained the difference, researchers analyzed carbon and nitrogen isotope signatures, which reveal an animal's diet history, in muscle samples from 137 bonnetheads. Bonnetheads eat mostly crabs and other invertebrates but also consume and digest seagrass, making them the first shark species shown to be omnivorous. Diets differed between Biscayne Bay and Tampa Bay and shifted as sharks grew, but males and females in the same bay drew on the same local food sources. "Our findings provided no evidence that differences in diet or foraging were driving the development of the sharks' sex-specific head shapes," Liu said.

"This study helps narrow the drivers of a biological trait that has puzzled scientists," said Catherine Macdonald, a co-author and director of the Shark Research and Conservation Program at the Rosenstiel School. Between May 2022 and May 2023, the team sampled 105 bonnetheads in Biscayne Bay and 39 near Tampa Bay, measuring, sampling and photographing each head against a grid before release, then compared head curvature using ImageJ software.