Bonnethead sharks get rounder snouts as they age, not pointier

For years, textbooks explained the bonnethead shark’s unusual head shape with a tidy story: males grow a pointed snout once they reach sexual maturity, while females keep a rounder profile. A new study of sharks pulled from two Florida bays says that story has the direction backwards, and the sharks themselves may be pointing scientists toward a different explanation entirely.

Rewriting the Bonnethead’s Growth Story

Researchers from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, working with several partner institutions, found that young bonnethead sharks of both sexes actually start out with more pointed heads, which grow progressively rounder as the animals mature, with the shift far more pronounced in females. That directly contradicts earlier research suggesting males only develop their pointed snout once they hit sexual maturity. “Our findings show that the pointed head shape is not a feature that develops in males when they reach maturity,” said Kathy Liu, the study’s lead author, who conducted the work while earning her master’s degree at the Rosenstiel School. “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.”

A Hammerhead Relative With a Rare Trait

Bonnetheads are small relatives of hammerhead sharks and stand out as the only shark species known to show a clear, consistent difference in head shape between males and females, a trait scientists call sexual dimorphism. Males typically carry a more noticeable point along the front edge of their shovel-shaped head, while females tend toward a smoother, rounder outline. The new research represents the first detailed look at both head shape and diet in bonnetheads from Biscayne Bay, and the team extended the work to Tampa Bay sharks as well, testing whether the same developmental pattern held up in a population living on the opposite side of Florida.

Ruling Out Diet as the Driver

One of the study’s central questions was whether differing diets between males and females might explain their differing head shapes. To test that, researchers analyzed muscle tissue from 137 sharks for carbon and nitrogen isotope signatures, chemical markers that reveal where an animal has been feeding within a food web over the long term. The isotope data showed that bonnetheads in Biscayne Bay and Tampa Bay were feeding in ecosystems with distinct environmental signatures, and that both populations shifted their diets as they aged. But within each bay, males and females relied on largely the same local food sources, undercutting the idea that diet differences drive the head-shape split. “Our findings provided no evidence that differences in diet or foraging were driving the development of the sharks’ sex-specific head shapes,” Liu said. Catherine Macdonald, a co-author and director of the Rosenstiel School’s Shark Research and Conservation Program, said the work at least narrows the list of possible explanations. “By examining body shape and diet in the same animals, we were able to assess whether diet and morphological changes co-occur,” Macdonald said. “Our results suggest there is another cause behind the differences in head shape between male and female bonnetheads.”

Measuring Heads Without Harming Sharks

Between May 2022 and May 2023, the research team sampled 105 bonnethead sharks in Biscayne Bay, captured using research longlines, and another 39 in the Tampa Bay region, collected primarily with scientific gillnets. Each shark was measured, given a small muscle-tissue sample, and photographed on both sides of its head against a specially designed grid board before being released. Researchers then used ImageJ software to precisely compare the curvature of each shark’s head from the photographs, a method that let the team gather detailed measurements while minimizing how long each animal spent out of the water.

An Open Question About Swimming and Reproduction

Even with diet ruled out, scientists still do not know what functional advantage the bonnethead’s pointed head provides in the first place. One hypothesis the researchers floated is that a more pointed head shape might help smaller, younger sharks swim more efficiently, an advantage that could matter less once females grow larger to support reproduction and gestation. Testing that idea will require future research into swimming performance and embryonic development, work that could also pin down exactly when the differences between male and female head shapes first begin to emerge. The team also suggested the same non-invasive photographic method could be applied to bonnethead populations elsewhere in their range, potentially revealing regional variation that would matter for conservation planning and region-specific management strategies. The study, titled “A Head of the Curve: Bonnethead Shark Cephalofoil Morphology and Trophic Ecology,” was published in the journal Integrative Organismal Biology, with funding support from the Field School, the University of Miami’s own shark research program, the Tampa Bay Environmental Restoration Fund, and the Disney Conservation Fund, among others.

This article was created with the assistance of AI and reviewed by an editor.

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