Comparison of female (left) and male (right) bonnethead cephalofoil shapes.
Photo Credit: Shark Research and Conservation Program
Scientific Frontline: Extended "At a Glance" Summary: Bonnethead Shark (Sphyrna tiburo) Morphology
The Core Concept: A recent study reveals that the distinctive, shovel-shaped heads of bonnethead sharks (Sphyrna tiburo) become progressively rounder as both sexes mature, dispensing with previous theories regarding the development of their prominent sexual dimorphism.
Key Distinction/Mechanism: Scientists previously hypothesized that male bonnethead sharks developed sharply pointed snouts upon reaching sexual maturity. The new findings establish that both sexes actually begin life with more pointed heads, and the rounding process occurs with age, with females undergoing a significantly more pronounced morphological change than males, entirely independent of dietary habits.
Major Frameworks/Components:
- Morphological Imaging: Researchers utilized ImageJ software and specially designed grid boards to precisely calculate cephalofoil curvature from field photographs, maximizing measurement accuracy while minimizing animal handling time.
- Stable Isotope Analysis: By evaluating carbon and nitrogen isotope signatures within muscle tissue, scientists mapped long-term trophic ecology, proving that distinct local food webs and foraging behaviors do not dictate the structural differences between sexes.
- Ontogenetic Tracking: The study correlated physical body shapes with distinct maturity phases to strictly isolate intrinsic biological development from external environmental influences.
Branch of Science: Marine Biology, Evolutionary Morphology, Trophic Ecology, and Zoology.
Future Application: The standardized, non-invasive photographic methodology developed for this study can be deployed globally to track physical variations across distinct shark populations, thereby supporting highly localized conservation strategies and guiding future biomechanical studies on embryonic development and swimming performance.
Why It Matters: By definitively uncoupling cephalofoil development from foraging habits, this research severely narrows the potential evolutionary drivers behind a unique biological trait, pointing instead toward alternative explanations, such as reproductive physiology or juvenile swimming advantages.
A new study of bonnethead sharks (Sphyrna tiburo) in Florida dispelled a previous hypothesis about how the distinctive head shapes of males and females develop, offering new insight into one of the species’ most unusual biological traits.
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, 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 the 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 noted that 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 the 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.
Funding: 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 the 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.
Published in journal: Integrative Organismal Biology
Title: A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology
Authors: K Liu, J R Graham, H Ro, S L Kim, T R Wiley, J M Gardiner, L J Baker, and C Macdonald
Source/Credit: University of Miami Rosenstiel School | Diana Udel
Edited by: Scientific Frontline
Reference Number: mb080426_01
