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\u2019 most unusual biological traits.<\/p>\n
Researchers at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science and their collaborators found that the sharks\u2019 shovel-shaped heads become rounder as both sexes mature \u2014 particularly in females. Previous research had suggested that the more pointed snout seen in males developed when they reached sexual maturity.<\/p>\n
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.<\/p>\n
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.<\/p>\n
\u201cOur findings show that the pointed head shape is not a feature that develops in males when they reach maturity,\u201d said Kathy Liu, lead author of the study, which she conducted while earning her master\u2019s degree at the Rosenstiel School. \u201cYounger males and females both had more pointed heads, but the head became rounder as the sharks matured. That change was much greater in females.\u201d<\/p>\n
Researchers also examined whether differences in diet could help explain the sharks\u2019 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.<\/p>\n
\u201cOur findings provided no evidence that differences in diet or foraging were driving the development of the sharks\u2019 sex-specific head shapes,\u201d said Liu.<\/p>\n
\u201cThis study helps narrow the drivers of a biological trait that has puzzled scientists,\u201d said Catherine Macdonald<\/a>, a co-author of the study and an associate professor in the Department of Environmental Science and Policy<\/a> and the director of the Shark Research and Conservation Program<\/a> at the Rosenstiel School. \u201cBy 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.\u201d<\/p>\n 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.<\/p>\n 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.<\/p>\n The photographs were analyzed using ImageJ software, which allowed the researchers to measure and compare the curvature of each shark\u2019s head. The field method provided precise measurements while minimizing the time the animals were handled.<\/p>\n 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.<\/p>\n 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.<\/p>\n The function of the bonnethead\u2019s pointed head remains unknown. The researchers propose that the shape could provide a swimming benefit to younger, smaller sharks. As females mature and develop 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.<\/p>\n The researchers said applying the same photographic method to bonnetheads in other regions could help reveal how head shape varies across the species\u2019 range. Such comparisons could also improve understanding of regional populations and support conservation strategies tailored to local conditions.<\/p>\n 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.<\/p>\n