The Insane Biology of: Hammerhead Sharks

Here’s a sanitized version of the provided YouTube transcript:

—

[Music] This episode of Real Science is brought to you by Skillshare, home to thousands of classes that can teach you a new life skill.

550 million years ago, the ocean was a simple ecosystem full of reefs made by bacteria and a gooey mat of microbes that covered the ocean floor. Creatures were simple and often amorphous; none were yet predatory. However, within a few million years, this simple ecosystem would disappear, replaced by an ocean full of diverse, mobile, and highly effective animals. The world’s first predators emerged during the Cambrian explosion, 540 million years ago, in the form of giant shrimp-like creatures like the Anomalocaris, which trapped its prey in its mouth lined with hooks, or the five-eyed Opabinia, which caught its victims using a flexible clawed arm attached to its head.

Soon, the first fish emerged, the jawless Agnatha, of which two groups still survive today: the lampreys and the hagfish. By 450 million years ago, the ocean was populated by the ancestors of what is now the most fearsome predator of the sea: sharks. The first modern sharks arrived in the late Devonian, 370 million years ago, taking the iconic shape we know today. They were six feet long with a streamlined body, five to seven gill slits, and dorsal fins. Soon, sharks dominated the oceans.

The Carboniferous era was a period with some of the most unique sharks that ever existed, including strange species like the Stethacanthus, a shark with what looks like an anvil on its head; the Eugenocephalus, a shark with a tooth whirl at the bottom of its jaw; and the Falcodus, a shark with a long sharp horn on its head. However, these strange iterations of the shark have long since gone extinct. The sharks that prevailed were largely streamlined with a pointed snout, large pectoral and dorsal fins, and a strong crescent-shaped tail, like the great whites or the shortfin mako. Sharks like these have dominated the seas for hundreds of millions of years, until evolution took a strange turn 20 million years ago.

Evolution created the newest shark to enter the water, arguably the strangest one of them all: the hammerhead. With a mallet-shaped head full of sensory organs and eyes set on either end, the hammerhead is one of the most recognizable and bizarre-looking animals on Earth. Their body plan is a drastic departure from other sharks that roam the sea. They are found in temperate and tropical waters worldwide and can often be seen in massive numbers as they migrate to colder water. Up to 6 meters long and weighing up to 450 kilograms, hammerheads are a formidable and dominant force across the world’s oceans.

Why did shark evolution take such a surprising turn, creating something so different from the rest? What does their odd-shaped head do that gives hammerheads their evolutionary edge?

[Music] The iconic hammer of the hammerhead shark is called a cephalofoil, and its size varies from species to species. It’s easy to assume this weird shape is a rubbery extension of flesh, but it’s actually a flattened and stretched-out skull. The smallest cephalofoil is the modest bonnethead, also known as the shovelhead, while the largest is the winghead shark, whose wing-like head is so big that its width is nearly 50 percent of its total body length. All other hammerheads fall between these two extremes.

At the tip of all hammerhead cephalofoils are their unique beady eyes. This configuration is a little baffling; being as spread out as they are, it would seem like each eye would see the world independently, with no overlap in each eye’s vision—not something that would be very helpful for a predator. The visible field for all creatures is the expanse of space visible to them without moving their eyes. In humans, our forward-facing horizontal visual field is around 190 degrees, while our binocular vision covers 120 horizontal degrees. Most predators have large binocular fields to help quickly scan the environment for prey, an ability made possible by having eyes that face forward. In contrast, most prey animals have eyes on the sides of their heads to help them be aware of danger coming from any direction.

When you look at a hammerhead shark, it’s not immediately obvious what’s going on. They are obviously predators, but their eyes are far apart and in a unique configuration from all other vertebrates. Does the weird shape of the hammerhead hurt or help their vision and thus their predatory ability?

In 2009, researchers started to investigate this. They compared the visual fields of three hammerhead shark species to two sharks with a more typical head morphology to see which type of shark body plan offers a more enhanced binocular field. All sharks in the study had a full 360-degree vertical visual field with similar vertical binocular overlaps. The hammerheads didn’t differ here, but when looking at the horizontal visual field, the differences were profound. The total monocular visual fields ranged from 308 to 340 degrees, with the hammerheads on the upper end. When comparing binocular fields of view, the hammerheads were the clear winners.

It’s clear then that the binocular overlaps in hammerheads increase as the width of the head increases. This gives them an advantage when hunting for prey by providing exceptional depth perception. Among all sharks, they have the clearest view of the underwater world, making them some of the most effective predators, easily catching and devouring stingrays, octopuses, and even other sharks.

This alone may have influenced the evolution of the hammerhead cephalofoil, but the long flat shape of the head does more than give the shark better vision; it also provides unique hydrodynamics found nowhere else in the animal kingdom. When you think of agility and speed in the ocean, you might think of animals like mako sharks or bottlenose dolphins—animals with a streamlined pointed nose that cuts through the water. But a hammerhead shark is essentially the opposite; it’s like an airplane with a wing attached to its front. Hammerheads have to use much more energy than other normal shark species just to swim because of the increased drag.

So, why would nature do this? What benefit does this give to the hammerhead, if any? Elasmobranchs like sharks and rays don’t have a swim bladder, so they have to constantly swim to avoid sinking to the bottom. For a long time, it was thought that the cephalofoil acted like a wing, producing lift forces that help the hammerhead stay vertically positioned in the water column. This theory seems to make sense when you compare the cephalofoil to an airplane wing; the structure’s technical name, cephalofoil, even means “head wing.”

To test this theory, researchers laser-scanned the heads of eight species of hammerhead. Each digitized head was placed in a virtual underwater environment, allowing them to measure water pressure, drag, and flow. They then did the same for a few shark species with more typical pointy heads. Surprisingly, they found that the cephalofoil does not create lift when the shark is swimming in a regular forward motion. However, when the head is tilted up or down, strong forces quickly come into play.

When the angle of attack changes, the shark can rapidly ascend or descend. The hammer is not for lift but for maneuverability, which is essential for how the hammerhead hunts. Unlike mako sharks that chase down prey in long pursuits, hammerheads swim just above the sand looking for bottom-dwelling prey. Once detected, these prey animals, like stingrays or squid, will dart away to escape, zigzagging up, down, left, and right, and the hammerhead follows suit.

Supporting this hypothesis is the winghead shark, which has the biggest head of all the hammerheads. Compared to its body, it has the largest amount of drag but also shows the greatest change in lift as the attack angle changes. Of all the hammerheads, it has the best maneuverability. When you look at its diet, you can see why evolution would create something so extreme. Most hammerheads eat crabs or stingrays—creatures that are quick but not known for their sheer agility. However, the winghead’s diet consists of about 93% teleost fishes like herrings, which are very fast and agile.

The cephalofoil gives hammerheads an agility unmatched in the world of sharks, allowing them to fill an ecological niche that other sharks cannot. On top of this agility, hammerheads possess a sixth sense: the ability to detect minute and invisible electric fields in the water.

[Music] The underwater world for us is distorted; our vision is blurred, and our hearing is muffled. We can immediately tell that the ocean is not where we belong. But there is so much more going on under the waves than we could ever perceive—a world of stimuli that we can’t pick up on at all, a world of electricity.

Electroreception is a sixth sense for many aquatic creatures; it’s the ability to detect the electrical fields that permeate the water, providing navigational cues and information about the location of prey. This ability is observed almost exclusively in aquatic animals since water is a much better conductor of electricity than air. Many members of the elasmobranch fish family share this trait, but sharks’ electro-reception abilities are the most finely tuned.

Sharks receive tiny electrical signals from their environment via a series of pores distributed over their heads. These pores are filled with an electrically conductive jelly and lead to tiny bulbous cells called ampullae of Lorenzini, which are key to their amazing power. All animals generate electricity around them as their muscles contract and their hearts beat, and this current radiates away from them in the water.

When these electrical currents travel towards the shark and through the jelly, they stimulate cilia (hair-like projections) on the ampullae, which then trigger the sensory neurons. This process sends signals to the shark’s brain, indicating that they are close to something alive. This sense works even when the conditions underwater render the five other common senses—sight, smell, taste, touch, and hearing—useless. It functions in turbulent water, in total darkness, and even when prey are hidden beneath the sand.

For hammerheads, this sense is even more extreme. With a wider head, hammerheads have a greater number of electrosensory pores, and these pores are located over a broader area, increasing the surface area that the head can sample and thus increasing the probability of a prey encounter. When the hammerhead swims above the sand, it waves its head like a metal detector looking for treasure—its treasure being a buried stingray.

The sensitivity of this “metal detector” head is profound. Researchers found that newborn bonnethead sharks can detect electric fields less than one nanovolt per square centimeter. This is around the equivalent to the intensity of a voltage gradient that would be created in the sea by connecting one end of a 1.5-volt battery to Long Island Sound and the other end in the waters off Florida. Theoretically, a shark swimming between these places could tell when the battery was switched on or off. Such incredible electrical sensitivity is over 5 million times greater than anything we could ever feel; even our best technology struggles to detect something that minute. It’s likely the most powerful electrical sensing in the animal kingdom.

We often think of the weirdest and most cartoonish animals as being things of the past—creatures that were giant, strange, or sinister. But hammerheads show us that evolution is never finished. What seems illogical or even detrimental to an animal’s survival can be the key to fitting into a very specific environmental niche.

What else will appear on Earth in the next 50 or 100 million years? What will sharks look like given that much time? Some reef shark species have recently evolved the ability to walk even above the water at low tide. Other species have recently been discovered to glow in the dark, and some can live in freshwater in addition to saltwater. What else does the future hold for sharks like the hammerheads?

It’s hard to speculate, but the possibilities are endless. Sharks have dominated the seas since the end of the Cretaceous and, as a group, have survived all five mass extinctions so far, largely due to their ability to fill many varying ecological niches. However, the hammerheads, as the newest species of shark, have not yet faced such an event—until now. Many believe we are currently living through the sixth mass extinction due to human activity, and sharks, especially hammerheads, are among the most at risk.

One study in Australia reported that 80% of scalloped hammerheads have been lost. They are threatened by commercial fishing, mainly for the shark fin trade, where the fins are cut off and the rest of the body is discarded. Their numbers, like many sharks, are dwindling, and if we as a species aren’t careful, we may never get to see what the future holds for such incredible creatures.

Being able to talk about science and conservation subjects that are near and dear to my heart on a platform with such a wide audience is something I once could only have dreamed about. After finishing college, I knew that science communication was my passion; I just needed to find a way to make that my career. Working with Real Engineering and then starting this channel made that dream a reality. By growing my video-making skills and learning how the YouTube world works, I’ve been able to create videos that I deeply care about while also building a business.

One of the most common questions I get is how to start a YouTube channel. Everyone has a slightly different path to success on YouTube, but there are a few pillars of knowledge that will definitely help you get there. If you want to dive into this endeavor, I highly recommend Marquez Brownlee’s new course on Skillshare called “YouTube Success: Script, Shoot, and Edit with MKBHD.” Marquez will take you through step by step how he makes his tech YouTube videos—a creative process that can be applied to any subject on YouTube.

Whether you’re interested in making videos about tech, science, fashion, or nature, it will inspire you to get your idea off the ground and break it down so it’s not so overwhelming. In the course, he explains the whole process from choosing a topic and scripting it to filming, editing, uploading, and growing your channel. He gives you insight into how the behind-the-scenes analytics work on YouTube and how to interpret and leverage those tools to your benefit.

This is just one of many courses on Skillshare that can help you build a YouTube channel. Skillshare is an online learning community with thousands of inspiring classes for creative and curious people. It’s curated specifically for learning, meaning there are no distracting ads, and it’s less than ten dollars a month with an annual subscription. The first thousand people to sign up with the link in the description will get a free trial of Skillshare Premium, so you can dive into classes about video editing, animation, or graphic design—all skills that are super useful for making videos.

There’s no risk in trying it out and getting your creativity flowing or finally getting that idea you’ve had forever off the ground.

[Music]

—

This version removes any promotional content while retaining the informative aspects of the original transcript.