Researchers at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science and their collaborators found that the sharks' shovel-shaped heads become rounder as both sexes mature — particularly in females. Previous research had suggested that the more pointed snout seen in males developed when they reached sexual maturity.
The study is the first in-depth assessment of bonnethead head shape and diet in Biscayne Bay. Sharks from Tampa Bay were included to determine whether the patterns were consistent across two populations on opposite coasts of Florida.
Bonnetheads are small members of the hammerhead shark family and the only shark known to have clear sex-based differences in head shape. Males typically have a more prominent point along the front of the head, while females have a smoother, more rounded profile. Scientists refer to this difference between the sexes as sexual dimorphism.
"Our findings show that the pointed head shape is not a feature that develops in males when they reach maturity," said Kathy Liu, lead author of the study, which she conducted 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."
Researchers also examined whether differences in diet could help explain the sharks' contrasting head shapes. Their analysis found that bonnetheads in Biscayne Bay and Tampa Bay were feeding within distinct food webs and showed different dietary patterns as they grew. Within each bay, males and females relied on many of the same local food resources.
"Our findings provided no evidence that differences in diet or foraging were driving the development of the sharks' sex-specific head shapes," said Liu.
"This study helps narrow the drivers of a biological trait that has puzzled scientists," said Catherine Macdonald , a co-author of the study and an associate professor in the Department of Environmental Science and Policy and the director of the Shark Research and Conservation Program at the Rosenstiel School. "By examining body shape and diet in the same animals, we were able to assess whether diet and morphological changes co-occur. Our results suggest there is another cause behind the differences in head shape between male and female bonnetheads."
From May 2022 to May 2023, researchers sampled 105 bonnetheads in Biscayne Bay and 39 in the Tampa Bay area. The sharks were captured using scientific longlines in Biscayne Bay and primarily scientific gillnets in Tampa Bay.
Researchers measured each shark and collected a small muscle sample before photographing both sides of its head on a specially designed grid board. The animals were then promptly released.
The photographs were analyzed using ImageJ software, which allowed the researchers to measure and compare the curvature of each shark's head. The field method provided precise measurements while minimizing the time the animals were handled.
Muscle samples from 137 sharks were analyzed for carbon and nitrogen isotope signatures, which provide information about where an animal has been feeding within a food web. This approach gave researchers a broader picture of diet than examining recently consumed prey alone.
The analysis showed little overlap between the isotope signatures of sharks from Biscayne Bay and Tampa Bay, indicating that the two populations relied on food webs with distinct environmental signatures. The researchers noted that this does not necessarily mean the sharks consumed entirely different prey because similar prey species can have different isotope signatures in different ecosystems.
The function of the bonnethead's pointed head remains unknown. The researchers propose that the shape could provide a swimming benefit to younger, smaller sharks. As females mature and develop larger bodies needed for reproduction and gestation, that potential advantage may diminish. Additional studies of swimming performance and embryonic development will be needed to test that hypothesis and determine when the differences between males and females first emerge.
The researchers said applying the same photographic method to bonnetheads in other regions could help reveal how head shape varies across the species' range. Such comparisons could also improve understanding of regional populations and support conservation strategies tailored to local conditions.
Combining physical measurements with information about diet allows researchers to test possible explanations for how unusual traits evolve. Studying bonnetheads across different regions can also reveal biological and ecological differences among populations that might otherwise be overlooked.
The study titled: A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology , was published on July 27, 2026, in the journal Integrative Organismal Biology.
The authors are Kathy Liu; University of Miami Rosenstiel School and Field School, Jasmin Graham; Minorities in Shark Science, H Ro and S L Kim; University of California, Merced, T. R. Wiley and J.M. Gardiner; Havenworth Coastal Conservation, L.J. Baker; California State University, Monterey Bay, C. Macdonald; Rosenstiel School and Field School.
The research was supported by Field School and the University of Miami Shark Research and Conservation Program. Tampa Bay fieldwork was supported by the Tampa Bay Environmental Restoration Fund and Disney Conservation Fund. Stable isotope analysis was supported by the University of California, Merced, and partial project support was provided by the Maxwell/Hanrahan Foundation.
About the University of Miami and Rosenstiel School of Marine, Atmospheric, and Earth Science
The University of Miami is a private research university and academic health system with a distinct geographic capacity to connect institutions, individuals, and ideas across the hemisphere and around the world. The University's vibrant academic community comprises 12 schools and colleges serving more than 19,000 undergraduate and graduate students in more than 180 majors and programs. Located within one of the most dynamic and multicultural cities in the world, the University is building new bridges across geographic, cultural, and intellectual borders, bringing a passion for scholarly excellence, a spirit of innovation, and a commitment to tackling the challenges facing our world. The University of Miami is a member of the prestigious Association of American Universities (AAU).
Founded in 1943, the Rosenstiel School of Marine, Atmospheric, and Earth Science is one of the world's premier research institutions in the continental United States.
The school's basic and applied research programs seek to improve understanding and prediction of Earth's geological, oceanic, and atmospheric systems. by focusing on four key pillars:
*Saving lives through better forecasting of extreme weather and seismic events.
*Feeding the world by developing sustainable wild fisheries and aquaculture programs.
*Unlocking ocean secrets through research on climate, weather, energy, and medicine.
*Preserving marine species, including endangered sharks and other fish, as well as protecting and restoring threatened coral reefs. www.earth.miami.edu .
