New Study Challenges Established Theory on Bonnethead Sharks’ Head Shape Development

A recent study of bonnethead sharks (Sphyrna tiburo) in Florida is challenging prior assumptions regarding the development of their distinctively different head shapes between sexes.

Researchers from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, along with collaborating institutions, discovered that the sharks’ shovel-shaped heads become increasingly rounded as both males and females age—a transformation that is especially pronounced in females. Previous theories had suggested that males developed their more pointed snouts upon reaching sexual maturity.

Tracking Bonnethead Head Shape

This investigation represents the first detailed analysis of both head morphology and diet in bonnethead sharks from Biscayne Bay. The team also examined sharks from Tampa Bay to determine whether these developmental patterns were consistent across populations residing on opposite sides of Florida.

Bonnetheads are small relatives of hammerhead sharks and are the only shark species known to exhibit clear sex-based differences in head shape. Males typically display a more prominent point along the anterior edge of the head, whereas females tend to possess a smoother, more rounded outline. Scientists classify this physical disparity between sexes as sexual dimorphism.

“Our findings demonstrate that the pointed head shape does not develop in males upon reaching maturity,” said Kathy Liu, the study’s lead author, who conducted the research while earning her master’s degree at the Rosenstiel School. “Younger males and females both had more pointed heads, but the cranium became rounder as the sharks matured, with this change being significantly more pronounced in females.”

Diet Does Not Explain the Difference

The researchers also investigated whether males and females developed different head shapes due to consuming different foods or foraging in distinct ways. Their findings indicated that bonnetheads in Biscayne Bay and Tampa Bay occupied separate food webs and experienced dietary shifts as they aged. However, males and females living within the same bay relied on many of the same local food sources.

“Our findings provided no evidence that dietary differences or foraging variations were driving the development of the sharks’ sex-specific head shapes,” Liu stated.

“This study helps narrow down the drivers behind a biological trait that has long puzzled scientists,” said Catherine Macdonald, co-author of the study and associate professor in the Department of Environmental Science and Policy, as well as director of the Shark Research and Conservation Program at the Rosenstiel School. “By examining body shape and diet within the same animals, we were able to assess whether diet and morphological changes co-occur. Our results indicate that another factor is responsible for the differences in head shape between male and female bonnetheads.”

Measuring Sharks Across Florida

Between May 2022 and May 2023, the team sampled 105 bonnethead sharks in Biscayne Bay and 39 in the Tampa Bay region. Research longlines were used to capture sharks in Biscayne Bay, while Tampa Bay specimens were primarily collected using scientific gillnets.

Each shark was meticulously measured, and a small muscle sample was extracted. Researchers subsequently photographed both sides of the animal’s head against a specialized grid board before rapidly releasing it.

The team utilized ImageJ software to analyze the photographs and precisely compare the curvature of each shark’s head. This methodology enabled researchers to gather detailed measurements while minimizing the sharks’ handling time.

Chemical Clues Reveal Different Food Webs

Scientists also analyzed muscle tissue from 137 sharks for carbon and nitrogen isotope signatures. These chemical markers can indicate where an animal has been feeding within a food web, offering a broader perspective on long-term diet than merely examining recently consumed prey.

The isotope signatures of Biscayne Bay sharks showed minimal overlap with those from Tampa Bay, suggesting the two populations were feeding within ecosystems characterized by different environmental signatures.

The researchers cautioned that this does not necessarily mean the sharks were consuming entirely different prey species. The same or similar prey can carry varying isotope signatures depending on the ecosystem in which they reside.

Why Do Bonnetheads Have Pointed Heads?

Scientists still do not understand the advantage provided by the bonnethead’s pointed head.

One possibility proposed by the researchers is that the shape may assist younger and smaller sharks in swimming more efficiently. As females grow larger to support reproduction and gestation, this potential benefit may diminish.

Future research on swimming performance and embryonic development will be necessary to test this hypothesis. Such work could also reveal when the differences between male and female head shapes first emerge.

Expanding the Research Beyond Florida

The researchers noted that the same photographic technique could be employed to study bonnethead sharks in other parts of their range. Comparing populations across different regions could reveal additional variations in head shape and help scientists better comprehend how local populations differ.

These comparisons could also hold conservation value by assisting researchers in identifying biological and ecological differences that might be crucial when developing region-specific management strategies.

By combining measurements of body shape with dietary information, scientists can test competing explanations for how unusual biological traits develop and evolve. Studying bonnetheads across a wider geographic range could also reveal population differences that might otherwise remain concealed.

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