A Bonnethead Shark’s Unusual Head Shape Is Not What Scientists Thought
For decades, the bonnethead shark’s distinctive shovel-shaped head has raised a deceptively simple question: why do males and females develop different profiles? A new study of bonnetheads in Florida suggests that a leading explanation may be wrong. Rather than developing a pointed snout when they reach sexual maturity, male and female bonnetheads appear to begin life with relatively pointed heads that gradually become rounder as they grow. The shift is especially pronounced in females, challenging assumptions about how one of the most recognizable examples of sexual dimorphism in sharks develops.
Bonnetheads, scientifically known as Sphyrna tiburo, are the smallest members of the hammerhead shark family and the only shark species known to display clear sex-based differences in head shape. Adult males generally have a more prominent point along the front edge of the head, while females tend to develop a smoother, broader curve. This head structure, called a cephalofoil, is more than a visual trademark. In hammerhead sharks, the expanded head may influence sensory perception, maneuverability, lift, and the way the animals interact with their environment.
The new investigation, led by Kathy Liu at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, examined bonnetheads from two Florida regions: Biscayne Bay on the state’s southeastern coast and the Tampa Bay area on the Gulf Coast. The researchers sampled 105 sharks in Biscayne Bay and 39 near Tampa Bay between May 2022 and May 2023. Studying animals from opposite coasts allowed the team to determine whether patterns in head shape and feeding ecology were consistent across different environments.
Each shark was measured, photographed from both sides of the head, and released after researchers collected a small muscle sample. The photographs were taken against a specially designed grid board and analyzed with ImageJ software, a digital image-analysis platform commonly used to quantify biological structures. By measuring the curvature and proportions of the cephalofoil, the researchers could compare head shape across body sizes, sexes, and populations while limiting the amount of time each animal spent in handling.
The results contradicted the idea that a pointed head emerges in males at sexual maturity. Younger males and females both tended to have more angular, pointed cephalofoils. As the sharks matured, their heads became rounder, with the change substantially stronger in females. This pattern indicates that the adult difference between the sexes is not necessarily caused by males acquiring a new pointed feature. Instead, it may result from females undergoing a more extensive change in shape as their bodies grow and their reproductive demands increase.
The researchers also investigated whether feeding behavior could explain the contrasting head profiles. Because head shape may affect swimming, turning, or prey capture, scientists have proposed that males and females could experience different ecological pressures if they consume different foods. To test this possibility, the team analyzed carbon and nitrogen isotope signatures in muscle tissue from 137 sharks. These chemical markers provide information about an animal’s position in a food web and can reveal longer-term dietary patterns that may not be captured by examining the contents of a single stomach.
The isotope data showed that sharks from Biscayne Bay and Tampa Bay occupied food webs with notably different environmental signatures. However, within each bay, males and females generally relied on many of the same local resources. The findings therefore offered no evidence that sex-specific diets or foraging strategies were driving the development of the sharks’ different head shapes. The researchers caution that distinct isotope signatures between the bays do not prove the sharks consumed entirely different prey: the same species can carry different chemical signatures when living in separate ecosystems.
The function of the pointed bonnethead head remains unresolved, but the study offers a possible direction for future research. A more pointed cephalofoil could provide a hydrodynamic advantage to younger, smaller sharks by influencing lift, drag, or turning performance. As females mature, however, their bodies become larger and must accommodate reproduction and gestation. Those changing physical demands could reduce the value of a head shape that benefits juvenile swimming. This hypothesis still needs to be tested through controlled studies of swimming performance, body mechanics, and embryonic development.
The work also highlights how combining morphology with ecology can help scientists separate plausible explanations for unusual traits. Measurements alone revealed how bonnethead heads change with growth, while isotope analysis showed whether those changes tracked differences in feeding. Applying the same photographic method to bonnetheads in other parts of their range could reveal whether regional conditions produce additional variation in cephalofoil shape. Such information may help researchers distinguish inherited differences from environmentally influenced traits and improve conservation planning for local shark populations.
The study, titled “A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology,” was published July 27, 2026, in Integrative Organismal Biology. The research was conducted by scientists from the University of Miami, Field School, Minorities in Shark Sciences, the University of California, Merced, Havenworth Coastal Conservation, and California State University, Monterey Bay. Supported by several conservation and research organizations, the study narrows the search for the biological forces shaping the bonnethead’s remarkable head and leaves scientists with a sharper question: if diet is not responsible, what causes males and females to follow different developmental paths?
Subject of Research: Animals
Article Title: A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology
News Publication Date: 27 July 2026
Web References: https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obag024/8711149; https://sharkresearch.earth.miami.edu/
References: Liu, K. et al., “A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology,” Integrative Organismal Biology. DOI: 10.1093/iob/obag024
Image Credits: University of Miami Shark Research and Conservation Lab
Keywords: bonnethead shark, Sphyrna tiburo, hammerhead sharks, cephalofoil, shark morphology, sexual dimorphism, trophic ecology, stable isotopes, Biscayne Bay, Tampa Bay, marine biology, shark conservation
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