A new study from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science has overturned a long-held belief about the head shapes of bonnethead sharks. Researchers found that both male and female sharks start with pointed heads which become rounder with age, a change more pronounced in females. This challenges the previous theory that males developed pointed snouts upon reaching sexual maturity.
The research, the first detailed investigation of head shape and diet in bonnethead sharks from Biscayne Bay and Tampa Bay, Florida, revealed that young bonnethead sharks of both sexes share a more pointed head shape. As they mature, the head becomes rounder, with females exhibiting a greater degree of rounding. Kathy Liu, the lead author of the study who conducted the research while earning her master's degree, stated, “Our findings show that the pointed head shape is not a feature that develops in males when they reach maturity. 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.”
Scientists also investigated whether differences in diet could explain the varying head shapes, examining the food webs of bonnetheads in both Biscayne Bay and Tampa Bay. While they observed dietary changes as the sharks aged and noted distinct food webs between the two bays, they found no correlation between diet and the development of sex-specific head shapes. According to Liu, “Our findings provided no evidence that differences in diet or foraging were driving the development of the sharks' sex-specific head shapes.”
Bonnethead sharks are small relatives of hammerhead sharks and are unique in displaying clear sex-based differences in head shape – males typically have a pointed snout, while females have a smoother, rounded outline. Catherine Macdonald, a co-author of the study, explained that this research “helps narrow the drivers of a biological trait that has puzzled scientists,”
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