The bonnethead is the only shark known to have a head shape that reliably differs between males and females, and for decades marine biologists assumed they knew why: males, the thinking went, grew a more pointed snout once they reached sexual maturity. A new study out of the University of Miami says that story has it backward.
Researchers at the university's Rosenstiel School of Marine, Atmospheric, and Earth Science sampled 105 bonnetheads from Biscayne Bay and another 39 from Tampa Bay over a year of fieldwork, photographing each shark's distinctive shovel-shaped head against a measured grid and analyzing muscle tissue from 137 animals for chemical signatures that reveal diet. The findings, published in the journal Integrative Organismal Biology, show that both male and female bonnetheads start life with pointed heads, which then round out as the sharks mature — a shift that turns out to be far more pronounced in females, not males.
Our findings show that the pointed head shape is not a feature that develops in males when they reach maturity.
Kathy Liu, lead author, University of Miami
Diet doesn't explain it either
The team also tested whether the head-shape divergence between sexes might track with different feeding habits, since a shark's cephalofoil — the flattened, hammer-like head shared by the wider hammerhead family — is thought to help with sensing and maneuvering while hunting. But the isotope analysis, described in the university's own summary of the research, turned up no evidence that male and female bonnetheads eat differently enough to account for it. "By examining body shape and diet in the same animals, we were able to assess whether diet and morphological changes co-occur," said co-author Catherine Macdonald, an associate professor in the department.
They didn't — which leaves the actual function of the pointed juvenile head, and the reason it recedes faster in females, as open questions. One hypothesis the researchers floated is that a more pointed head may offer young sharks of both sexes a swimming advantage before other factors take over as they grow. The team says applying the same photographic method to bonnethead populations elsewhere could help reveal whether the pattern holds across the species' range, from the Gulf of Mexico to South America, or whether Biscayne Bay's sharks are an outlier.
For now, the study stands as a reminder that even a shark studied for decades — one common enough to turn up in touch tanks at aquariums nationwide — can still upend an assumption that has sat unchallenged in the scientific literature since before most of its subjects were born.