Florida study examines how bonnethead sharks’ head shape changes with age
Researchers at the University of Miami and other institutions found that in Florida, the heads of bonnethead sharks, Sphyrna tiburo, become rounder with age in individuals of both sexes. This change is more pronounced in females. The findings challenge the previous explanation that males develop a more pointed front part of the head only after reaching sexual maturity, ScienceDaily reports.
Bonnethead sharks are small relatives of hammerhead sharks. They are the only known shark species with pronounced sex differences in head shape: males usually have a more noticeable projection on the front edge of the head, while females have a smoother and rounder outline. The study showed that young males and females had more pointed heads, and their shape became rounder as they aged.
Diet did not explain the differences
The scientists also tested the assumption that different head shapes could be related to diet or methods of finding food. They studied sharks from Biscayne Bay and the Tampa Bay area, populations on opposite sides of Florida. Sharks from these areas belonged to different food webs, and their diets changed as they grew. At the same time, males and females within the same bay used many of the same local food sources.
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An analysis of stable carbon and nitrogen isotopes in the muscle tissue of 137 sharks showed that the isotope indicators of animals from Biscayne Bay barely overlapped with those of sharks from Tampa Bay. This indicates feeding in ecosystems with different ecological characteristics. The researchers cautioned that such differences do not necessarily mean the consumption of entirely different prey species, as the same or similar prey in different ecosystems may have different isotope indicators.
How the study was conducted
From May 2022 to May 2023, the team sampled 105 sharks in Biscayne Bay and another 39 in the Tampa Bay area. Each animal was measured, a small muscle tissue sample was taken, and both sides of its head were photographed on a special grid panel before it was quickly released. The scientists used ImageJ software to compare head curvature.
What advantage a pointed head shape may provide remains unknown. The authors suggest it may help younger and smaller sharks swim more efficiently. Further research into swimming performance and embryonic development is needed to test this hypothesis. The article was published in the journal Integrative Organismal Biology.
