In a plot twist that has scientists re-examining their assumptions and sharks smugly swimming away, a new study of bonnethead sharks (Sphyrna tiburo) in Florida has overturned a long-standing explanation for why males and females sport differently shaped heads. Researchers from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science and collaborating institutions discovered that the sharks' shovel-shaped heads become more rounded with age in both sexes, with the change especially pronounced in females. This contradicts earlier research suggesting males developed their more pointed snouts upon reaching sexual maturity.
The study, the first detailed investigation of both head shape and diet in bonnethead sharks from Biscayne Bay, also included sharks from Tampa Bay to check if the same developmental patterns appeared in populations on opposite sides of Florida. Bonnetheads, small relatives of hammerhead sharks, are the only shark species known to show clear sex-based head shape differences, a phenomenon scientists call sexual dimorphism. Males typically sport a more noticeable point along the front edge of the head, while females tend to have a smoother, more rounded outline.
"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, conducted during 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."
The researchers also tested whether dietary differences or foraging strategies might explain the head shape divergence. Their results showed that bonnetheads in Biscayne Bay and Tampa Bay occupied distinct food webs and experienced dietary changes as they grew, but males and females within the same bay relied on many of the same local food sources. "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, co-author and associate professor in the Department of Environmental Science and Policy and director of the Shark Research and Conservation Program at the Rosenstiel School, noted, "This study helps narrow the drivers of a biological trait that has puzzled scientists. 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."
Between May 2022 and May 2023, the team sampled 105 bonnethead sharks in Biscayne Bay and 39 in the Tampa Bay region. Using research longlines in Biscayne Bay and scientific gillnets in Tampa Bay, they measured each shark, took a small muscle sample, and photographed both sides of the head against a specially designed grid board before quickly releasing the animal.
The team used ImageJ software to analyze the photographs and precisely compare head curvature, a technique that allowed detailed measurements with minimal handling time. They also 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 and provide a broader view of long-term diet than examining recently eaten prey. The isotope signatures showed little overlap between Biscayne Bay and Tampa Bay sharks, suggesting the two populations fed in ecosystems with different environmental signatures - though this doesn't necessarily mean they ate different prey, as the same prey can carry different signatures in different ecosystems.
As for what advantage the bonnethead's pointed head might provide, scientists remain stumped. One possibility proposed by the researchers is that the shape could help younger and smaller sharks swim more efficiently, but as females grow larger to support reproduction and gestation, that benefit may become less important. Future research on swimming performance and embryonic development could test this hypothesis and reveal when the head shape differences first appear.
The same photographic technique could also be applied to bonnethead sharks elsewhere in their range, comparing populations to uncover additional variation and better understand local differences. Such comparisons could have conservation value by identifying biological and ecological differences important for region-specific management strategies.
The study, titled "A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology," was published in the journal Integrative Organismal Biology. Authors include 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), and C. Macdonald (Rosenstiel School and Field School). The research was supported by the Field School, the University of Miami Shark Research and Conservation Program, the Tampa Bay Environmental Restoration Fund, the Disney Conservation Fund, the University of California, Merced, and the Maxwell/Hanrahan Foundation.