Scientists Overturn Theory Behind the Bonnethead Shark's Strange Head Shape

A new study of bonnethead sharks (Sphyrna tiburo) in Florida is challenging an earlier explanation for how males and females develop different head shapes. Researchers from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science found that the sharks' shovel-shaped heads become more rounded as both sexes grow older, with the change especially pronounced in females. Earlier research had suggested that males instead grew a more pointed snout once they reached sexual maturity.

Bonnetheads are small relatives of hammerhead sharks and the only shark species known to show a clear sex-based difference in head shape, a pattern scientists call sexual dimorphism: males usually have a noticeable point along the front edge of the head, while females tend to have a smoother, rounder outline. "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," said Kathy Liu, the study's lead author.

The team sampled 105 bonnethead sharks in Biscayne Bay and 39 in Tampa Bay, photographing each shark's head to measure its curvature, and analyzed muscle tissue from 137 sharks for chemical signatures that reveal what an animal has been eating. Males and females within the same bay relied on many of the same local food sources, and the researchers found no evidence that diet or foraging differences were driving the different head shapes between sexes.

"This study helps narrow the drivers of a biological trait that has puzzled scientists," said Catherine Macdonald, a co-author and director of the Shark Research and Conservation Program at the Rosenstiel School. Scientists still do not know what advantage the bonnethead's pointed head provides; one possibility raised is that it helps younger, smaller sharks swim more efficiently, a benefit that may matter less once females grow larger to support reproduction.

The study, titled "A Head of the Curve: Bonnethead Shark (Sphyrna tiburo) Cephalofoil Morphology and Trophic Ecology," was published in the journal Integrative Organismal Biology.