New research overturns the assumption that male bonnetheads develop pointed heads at maturity, implying instead that both sexes start out pointed and become rounder as they grow.
Side view showing the distinctive head shape
The unusual shovel-shaped head of the bonnethead shark becomes increasingly different between males and females as they mature. But a new study suggests scientists have had the developmental process backwards.
Bonnetheads (Sphyrna tiburo), the smallest members of the hammerhead family, are unusual among sharks because males and females have visibly different head shapes. Adult females have a broadly rounded cephalofoil—the expanded head characteristic of hammerheads—whereas males retain a more pronounced central point.
Previous research proposed that this point developed in males at sexual maturity, perhaps as a secondary sexual characteristic. Researchers from the University of Miami and collaborating institutions have now found that young bonnetheads of both sexes actually have relatively pointed heads. As they mature, the heads of both males and females become rounder—but the transformation is substantially greater in females.
Measuring shark heads
The researchers studied bonnetheads from Biscayne Bay and Tampa Bay in Florida between May 2022 and May 2023. They photographed the sharks' heads and used geometric analyses to quantify their curvature and shape.
The same developmental pattern manifested in both populations: head shape differed between the sexes, but both became rounder with maturity. Females underwent the greatest change, generating the characteristic rounded adult bonnethead profile.
But why?
That is uncertain.
Because bonnetheads are unusual among sharks in consuming substantial amounts of seagrass as well as animal prey, the researchers studied whether differences in diet might explain the different head shapes. Stable-isotope analysis of muscle tissue revealed dietary differences between locations and life stages, but these did not correlate with the sexual differences in head shape.
The researchers instead propose a hydrodynamic explanation worth investigating. A more pointed cephalofoil might improve swimming performance in small juvenile sharks. As bonnetheads grow, those benefits may become less important. Females also grow larger than males, probably because greater body size provides advantages for carrying developing young.
For now, this is still a hypothesis rather than a demonstrated explanation. What the study shows is that the familiar difference between male and female bonnetheads results from different patterns of growth, rather than males simply developing their distinctive point at puberty.
