New research challenges long-held ideas about bonnethead shark head shape (Sphyrna tiburo). Researchers in Florida have found that the distinctive shovel-shaped heads of male and female bonnethead sharks become rounder with age, challenging previous explanations for their sexual differences in head shape.
Scientists from the Rosenstiel School of Marine, Atmospheric, and Earth Science at the University of Miami and collaborating institutions studied bonnethead sharks from Biscayne Bay and Tampa Bay. Their findings show that both males and females begin life with relatively pointed heads, but their cephalofoils—the broad, flattened head structures common to hammerhead sharks—become rounder as they grow. This change is especially pronounced in females.
Earlier research suggested that male bonnethead sharks developed a more pointed head after reaching sexual maturity. The new study, however, found no evidence that a pointed head is a mature male characteristic.
Tracking bonnethead shark head shape
This is the first detailed study to examine bonnethead head shape and diet together. Researchers analyzed sharks from Biscayne Bay in South Florida and Tampa Bay on Florida’s west coast to determine whether the same developmental pattern occurred in different populations.
Bonnetheads are small members of the hammerhead shark family and are the only shark species known to display clear sex-based differences in head shape. Males generally have a more pronounced indentation along the front edge of the head, while females typically develop a smoother and rounder outline. Scientists refer to these physical differences between males and females as sexual dimorphism.
“Our findings indicate that a pointed head shape is not a characteristic that appears in males after they reach maturity,” said Kathy Liu, lead author of the study, which she conducted while earning her master’s degree at the Rosenstiel School. “Both young males and females had pointed heads, but their heads became rounder as they grew. The change was substantially greater in females.”
Diet does not explain the difference
The research team also investigated whether males and females developed different head shapes because they ate different prey or used different foraging strategies. The results showed that bonnetheads from Biscayne Bay and Tampa Bay occupied different food webs and experienced dietary changes as they aged.
However, males and females living in the same bay consumed many of the same local food resources. This suggests that dietary differences alone cannot explain why adult male and female bonnetheads have different head shapes.
“Our findings provide no evidence that differences in diet or foraging behavior drive the development of sex-specific head shapes in bonnethead sharks,” Liu said.
Study co-author Katherine MacDonald, an associate professor in the Department of Environmental Science and Policy and director of the Rosenstiel School’s Shark Research and Conservation Program, said the research helps narrow down the possible causes of this unusual biological trait.
“By examining body shape and diet in the same animals, we were able to determine whether dietary and morphological changes occurred at the same time,” MacDonald said. “Our results suggest that another factor is responsible for the differences in head shape between male and female bonnethead sharks.”
Measuring bonnethead sharks across Florida
Between May 2022 and May 2023, researchers sampled 105 bonnethead sharks in Biscayne Bay and another 39 sharks in the Tampa Bay region. The team used research longlines to capture sharks in Biscayne Bay, while most Tampa Bay sharks were collected with scientific gillnets.
Scientists measured each shark and collected a small muscle sample for dietary analysis. They also photographed both sides of each shark’s head against a specially designed grid before releasing the animal.
The researchers used ImageJ software to analyze the photographs and measure the curvature of each bonnethead’s head. This method allowed them to collect precise information about head shape while reducing the amount of time each shark spent being handled.
Chemical clues reveal different food webs
The team analyzed muscle tissue from 137 bonnethead sharks for carbon and nitrogen isotope signatures. These chemical markers provide information about an animal’s position in the food web and offer a broader view of its long-term diet than an examination of recently consumed prey.
The isotope signatures of sharks from Biscayne Bay showed little overlap with those of sharks from Tampa Bay. This finding suggests that the two populations were feeding within ecosystems with different environmental conditions and food-web structures.
Researchers cautioned that the result does not necessarily mean the sharks were eating entirely different prey species. The same or similar prey can have different isotope signatures depending on the ecosystem where they live.
Why do young bonnethead sharks have pointed heads?
Scientists still do not know what advantage a pointed head provides to young bonnethead sharks. One possibility is that the shape helps smaller sharks swim more efficiently. Its benefits may become less important as females grow larger and invest more energy in reproduction and pregnancy.
Future research measuring swimming performance could test whether head shape affects the movement and maneuverability of bonnetheads at different ages. Studies of embryonic development may also help scientists determine when differences between male and female head shapes begin to appear.
Expanding bonnethead shark research beyond Florida
The photographic technique used in this study could be applied to bonnethead sharks in other parts of their range. Comparing populations from different regions may reveal additional variations in head shape and improve scientists’ understanding of how local populations develop.
These comparisons could also identify biological and ecological differences that are important for region-specific management and conservation strategies. Studying bonnethead sharks across a broader geographic range may reveal population patterns that remain hidden when research is limited to one location.
By combining body measurements with dietary information, scientists can test competing explanations for how unusual physical traits develop and evolve. The findings from Florida suggest that diet and foraging behavior do not fully account for the sexual dimorphism seen in bonnethead shark head shape.
The study, titled “Top of the Curve: Morphology and Nutritional Ecology of Cephalofoil Development in Bonnethead Sharks,” was published in the journal Integrative Biology.
The study was led by Cathy Liu, with contributions from researchers affiliated with the University of Miami’s Rosenstiel School and Shark Research and Conservation Program, the Field School, Minorities in Shark Sciences, the University of California, Merced, Havenworth Coastal Protection, and California State University Monterey Bay.
The research was supported by the Field School and the University of Miami Shark Research and Conservation Program. Tampa Bay fieldwork received support from the Tampa Bay Environmental Restoration Fund and the Disney Conservation Fund. Stable isotope analysis was supported by the University of California, Merced, and additional project funding came from the Maxwell/Hanrahan Foundation.
Source: www.sciencedaily.com