Level 1 — Absolute Beginner

Bonnethead sharks are small sharks. They are cousins of hammerhead sharks. Their heads are wide and flat, like a shovel.

Male and female bonnethead sharks have different head shapes. Adult males often have a pointed head. Adult females often have a rounder head.

Scientists used to think males grow a pointed head only when they become adults. A team of scientists studied 144 sharks to check this idea. They found something new and different.

Baby bonnethead sharks, both male and female, start with pointed heads. The heads get rounder as the sharks grow older. This change happens faster in females than in males.

shark

A large fish with a strong body that eats other animals.

species

A group of animals that are the same kind and can have babies together.

hammerhead

A type of shark with a head shaped like a hammer.

snout

The front part of an animal's head, including the nose and mouth.

adult

A fully grown animal or person.

researcher

A person who studies something carefully to learn new facts.

seagrass

A plant that grows underwater in the sea.

prey

An animal that another animal hunts and eats.

Level 2 — Elementary

Bonnethead sharks, whose scientific name is Sphyrna tiburo, are small relatives of hammerhead sharks. They are the only shark species known to show a clear difference in head shape between males and females. Adult males usually have a more pointed snout on their shovel shaped head, while adult females usually have a smoother, more rounded outline.

For a long time, scientists believed that males grew this pointed snout only after they became sexually mature, possibly because of what they ate or as a signal to other sharks. This idea had never been carefully tested with a large study that measured both head shape and diet together.

A researcher named Kathy Liu, from the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Science, led a new study to test the old idea. Her team, which also included Jasmin Graham of the group Minorities in Shark Science, studied 144 bonnethead sharks caught in Biscayne Bay and Tampa Bay, two places on opposite coasts of Florida.

The results surprised the researchers. Both male and female bonnethead sharks are actually born with more pointed heads, and their heads become rounder as they grow up, with this rounding happening more strongly in females. The team also found no clear proof that diet explains the difference between male and female heads, even though bonnetheads are known for eating seagrass along with crabs and shrimp.

sexually mature

Old enough to be able to reproduce, or have offspring.

cephalofoil

The flattened, wing shaped head of a hammerhead or bonnethead shark.

outline

The shape or edge of something as seen from the outside.

nonprofit

An organization that works to help people or causes, not to make money.

coast

The land next to the sea.

diet

The usual food that a person or animal eats.

measure

To find the exact size, amount, or shape of something.

sexual dimorphism

A clear physical difference between males and females of the same species.

Level 3 — Intermediate

Bonnethead sharks, small hammerhead relatives formally named Sphyrna tiburo, are unique among sharks for displaying a clear, visible difference in head shape between the sexes. Adult males typically carry a more pointed snout along the front edge of their shovel shaped head, or cephalofoil, while adult females tend toward a smoother, more rounded profile. For years, the leading explanation held that males developed this pointed feature specifically upon reaching sexual maturity, a change some researchers linked to diet or to a reproductive and behavioral signal exchanged between sharks.

That explanation had never been rigorously tested against a large dataset that tracked head shape and feeding habits in the same animals over time. A new study, titled A Head of the Curve, Bonnethead Shark Cephalofoil Morphology and Trophic Ecology, set out to close that gap. Led by Kathy Liu of the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Science, with co-authors including Jasmin Graham of the nonprofit Minorities in Shark Science along with researchers from UC Merced, California State University Monterey Bay, and Havenworth Coastal Conservation, the team examined 144 bonnethead sharks drawn from Biscayne Bay and Tampa Bay, two Florida populations on opposite coasts, chosen deliberately to test whether any pattern held across distinct groups.

The findings ran opposite to the long standing theory. Rather than males acquiring a pointed snout at maturity, both sexes start life with comparatively pointed heads, and those heads round out over time as the sharks age, a shift that proved more pronounced in females than in males. In other words, the adult male's pointed profile is not a trait that newly appears; it is a juvenile trait that females simply outgrow faster than males do.

Just as notably, the study found no clear evidence linking diet to the divergence in head shape between the sexes, undercutting a second long held hypothesis. This matters partly because bonnetheads are unusual among sharks for eating substantial amounts of seagrass alongside animal prey such as crabs and shrimp, making their feeding ecology a natural place researchers had expected an explanation to be found. Instead, the study, the first to assess head shape and diet together in this species in such depth, suggests the real driver of the pattern lies elsewhere, perhaps in growth timing itself rather than in what the sharks eat.

morphology

The study of the form and structure of living things.

trophic ecology

The study of how animals feed and how energy moves through a food web.

hypothesis

An idea proposed as a possible explanation, which can then be tested.

divergence

The process of becoming different or moving apart from a shared starting point.

juvenile

Relating to a young animal that has not yet reached adulthood.

Level 4 — Advanced

For decades, the bonnethead shark, Sphyrna tiburo, has stood out among the roughly 500 known shark species for a trait that seems almost too visible to have gone unexplained for so long: a pronounced, consistent difference in head shape between the sexes. Adult males typically bear a snout that tapers to a point along the leading edge of their shovel shaped cephalofoil, while adult females present a smoother, more rounded silhouette. The prevailing account, repeated across popular and scientific literature alike, held that this divergence emerged at sexual maturity, when males were thought to develop the pointed feature, possibly in connection with diet or as a signal tied to reproductive behavior.

That account has now been overturned by a study published July 27, 2026, in the journal Integrative Organismal Biology, titled A Head of the Curve, Bonnethead Shark Cephalofoil Morphology and Trophic Ecology. Led by Kathy Liu of the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Science, and co-authored by Jasmin Graham of the nonprofit Minorities in Shark Science alongside researchers from UC Merced, California State University Monterey Bay, and Havenworth Coastal Conservation, the study is the first to assess bonnethead cephalofoil morphology and diet in tandem, and it does so across 144 sharks drawn from Biscayne Bay and Tampa Bay, populations on opposite Florida coasts included specifically to guard against a single site's quirks being mistaken for a species wide pattern.

The result inverts the old narrative rather than merely refining it. Rather than a pointed snout appearing anew in males at maturity, both sexes are born with comparatively pointed heads, which round with age in a pattern of ontogenetic change that is markedly more pronounced in females than in males. What had been read as a male trait acquired at adulthood is, on this evidence, better understood as a juvenile trait that females shed more completely and more quickly than males do, an inversion of cause that changes what any future explanation of the pattern must account for.

Equally consequential is what the study did not find. Despite bonnetheads' unusual status among sharks as substantial consumers of seagrass alongside animal prey such as crabs and shrimp, a feeding ecology that made diet a natural candidate explanation, the researchers uncovered no clear evidence that trophic ecology drives the divergence in head shape between the sexes. Taken together, the two findings leave the field with a more accurate description of the phenomenon than it has had before, sexual dimorphism as a difference in the rate of a shared developmental trajectory, and a genuinely open question as to what, if not diet, sets that rate apart between males and females.

silhouette

The outline or shape of something seen against a lighter background.

ontogenetic

Relating to the development of an individual organism from birth to adulthood.

trajectory