Yes, you read that right: eight tidy rows of “teeth” on the forehead, and they aren’t for eating. As reported by Gizmodo, researchers have documented a fish whose head is studded with tooth-like structures that come into play during courtship and mating. At first glance, it’s the sort of detail that feels tailor-made for tabloid headlines. But the story behind these structures is a fascinating window into how evolution repurposes familiar parts—teeth, scales, spines—for jobs far beyond biting and chewing.

What are “forehead teeth,” really?

Fish skin is full of surprises. Many lineages carry tiny, hard projections called dermal denticles or odontodes. Under a microscope, these look and behave very much like teeth: they have enamel-like outer layers, dentine-like cores, and grow from similar genetic programs. In sharks, a coat of denticles makes skin rough like sandpaper, reducing drag and deterring parasites. In catfishes and some armored species, odontodes erupt on the cheeks, head, or fins—especially in breeding males.

So when scientists talk about “forehead teeth,” they’re usually referring to these tooth-like skin elements, not a second jaw on the brow. Crucially, because denticles can sprout in predictable patterns, the eight rows reported here aren’t random bumps—they’re an organized array, likely tuned by sexual selection to solve a mating problem.

Why put teeth on the forehead?

Mating is messy in water. Partners can drift apart, eggs and sperm can disperse, and slippery skin makes it hard to maintain contact. Many fishes have evolved special holdfasts or tactile structures to keep mates aligned just long enough for fertilization to succeed. Forehead denticles can help by:

• Improving grip: Micro-spines increase friction where heads or bodies press together during courtship embraces.
• Fine-tuning alignment: Multiple rows act like “tread,” guiding two curving bodies into repeatable positions.
• Signaling quality: Elaborate, symmetrical rows advertise a healthy, mature mate, a classic hallmark of sexual selection.
• Stimulating the partner: Rubbing or tapping with textured pads may trigger spawning responses.

Importantly, these features often appear or intensify only in breeding adults, and they can be seasonal—another clue that sex, not feeding, drove their evolution.

Anatomy of the eight rows

Although details vary by species, the reported configuration can be imagined like this: across the forehead (the dorsal surface of the head between the eyes and the origin of the first dorsal fin), scientists observed eight longitudinal rows of closely spaced denticles. Each denticle carries a sharp cusp or multiple micro-cusps, all angled slightly backward. This orientation creates a “grip one way, glide the other” effect—perfect for bracing during a head-to-head nudge or lateral embrace.

In some fishes, these rows sit on a slightly raised plate, concentrating contact forces while spreading them over many points so neither partner’s skin tears. Wear patterns—polished tips, abraded edges—can reveal where the action happens and which motions are most common in courtship.

Evolutionary logic: the costs and the payoff

Building enamel-like tissues on the head isn’t cheap. Why would evolution keep them? The answer is simple: reproductive payoff. If a male with better forehead denticles mates more successfully, those genes spread even if the structure adds drag or costs calcium. Over time, a modest row can become eight, and small cusps can become stiff spines.

This is classic sexual selection—Darwin’s other great mechanism—where traits evolve because they boost mating success rather than survival per se. The balance is delicate: too spiky and you injure your partner; too smooth and you lose your grip. The observed patterning suggests natural selection tuned the geometry to maximize hold and tactile signaling while minimizing harm.

Where does this fit in the bigger fish family album?

Odd headgear in fishes isn’t rare; what is rare is such a neat, multi-row arrangement directly tied to mating. A few instructive comparisons:

• Sharks and rays: Breeding males often sprout extra denticles on the head and pectoral fins to grip females during copulation. These are functionally analogous to forehead denticles.
• Armored catfishes (Loricariidae, Callichthyidae): Males develop odontodes on the snout and cheeks used in displays, skirmishes, and sometimes in courtship embraces.
• Sticklebacks and other nesters: While they lack forehead teeth, they showcase how seasonal head ornaments and textures evolve for courtship and territory defense.
• Fossil oddballs: Ancient shark relatives famously carried head-top “brushes” of denticles likely used in display and mating. These show the deep evolutionary roots of head-borne tooth-like structures.

Taken together, the new report slots neatly into a decades-long pattern: denticles and odontodes are evolutionary building blocks that get redeployed wherever nature needs an extra bit of texture, armor, or grip.

How do scientists figure this out?

When researchers argue that a structure is for sex rather than feeding or defense, they rely on converging lines of evidence:

• Sexual dimorphism: If only mature males (or females) grow the structure, it likely serves a reproductive role.
• Seasonality: If it enlarges in breeding season and recedes afterward, that’s another clue.
• Micro-wear and orientation: Microscopy can reveal rubbing, polishing, and breakage consistent with repeated contact in courtship positions.
• Behavioral context: In living species, direct observations or video confirm how the head contacts the mate. In fossil or deep-sea species, scientists infer from anatomy and comparisons with living analogues.
• Biomechanics: Modeling shows that multiple parallel rows create reliable friction and pressure distribution, improving alignment without tearing tissue.

Why this matters beyond the wow-factor headline

It’s easy to file “forehead teeth for sex” under delightful weirdness and move on. But studies like this illuminate three big themes in evolutionary biology:

1. Modularity: Teeth, scales, and denticles are cut from the same developmental cloth. Evolution can redeploy that toolkit in new places to solve new problems.
2. Constraints and trade-offs: Hydrodynamics, tissue cost, and injury risk constrain how flashy a trait can be. The eight-row pattern likely represents a sweet spot honed over generations.
3. Repeatability: Across distant lineages, the same kinds of textures show up for the same purposes—grip, signal, align—underscoring how predictable natural selection can be.

Quick Q&A

Are these really teeth?

They’re tooth-like in composition and development (denticles/odontodes), but they don’t belong to the mouth or jaw and they don’t chew. Think of them as “external teeth.”

Do both sexes have them?

Typically, such structures are most pronounced in breeding males. That sex-limited pattern is one reason researchers link them to mating.

Do they hurt the partner?

They’re sharp, but their size, density, and orientation help produce grip and stimulation without serious injury. Natural selection penalizes traits that damage reproductive success.

Why eight rows?

Multiple rows increase contact reliability across curved, moving surfaces. Eight is likely the number that maximizes grip and signal while minimizing drag and tissue cost for that species.

The takeaway

“Forehead teeth” make for a punchy headline, but they also tell a deeper story about evolution’s favorite trick: repurposing old materials for new jobs. In this case, a familiar tissue—denticle-like, tooth-like, hard—migrates to an unexpected place, organizes into eight tidy rows, and transforms an awkward underwater dance into a successful reproductive strategy. Strange? Absolutely. Sensible? Even more so.