The Diving Superheroes (The Sulidae family of seabirds - the plunge-diving Boobies and Gannets)
Image capture from BBC Coverage Olympics 2024
Once again the Olympic divers are on my mind. Today the major competition in Olympic diving was the women’s 10m platform event. China has won every gold medal in this event since 2008 and in the last two Olympics, they have won silver as well. Today, once again it was the Chinese athletes who won the gold and silver medals. Watching these women, who at 17 and 18 years of age look as though they never make a mistake even when diving from a handstand 10 meters from the water or while completing multiple rotations in the air, I am yet again in awe of what the human body can do. But, I also noticed that each of the divers had physio-tape on several parts of her body. The most common areas were the wrists, the upper arms and the lower back. In the last blog (follow this link) I wrote about the fact that diving is a sport that has its own special risks and that Olympic divers can suffer a range of injuries when diving from a 10-meter platform.
Let me now move the discussion to the special adaptations of a group of seabirds that need to make high dives into the water just to get a meal. What special adaptations allow Boobies and Gannets, members of the Sulidae family of seabirds, to dive into the ocean from much higher elevations several times per day while avoiding injury?
Northern Gannets and Brown Boobies, for example, may plunge-dive into the water from 30 meters above the water surface, three times higher than the highest platform in the Olympics. From this height, they enter the water at a speed of 87 km/hr (54 mph). Some gannets have been recorded to dive from even higher elevations on a regular basis and they always do it head first. How can they not only survive this but repeat it over and over again without regularly getting hurt?
There are a number of interesting anatomical and physiological adaptations that make this group of seabirds the masters of plunge diving. The first and most obvious feature is the shape of the beak and head. Boobies and Gannets have an arrowhead-shaped beak, with a narrow tip that gradually widens to the exact width of their skull. The beak tip breaks the water surface tension and the rest of the beak then pushes the dense water out of the way. This protects all the soft parts, such as eyes and eardrums, from the impact of the rapidly increasing pressure, when entering the much denser medium at this high speed. In addition, the nostrils do not open to the exterior of the beak, like in petrels, shearwaters and albatrosses, but open internally on the roof of the palate. This prevents water from being forced into the nasal passages as the beak enters the water at high velocity.
From: Smithsonian Insider: Study shows how birds dive safely at high speeds
“Smithsonian ornithologists Lorian Straker and Carla Dove and their co-authors investigate the biomechanics of gannets’ dives. They found that the birds’ head shape, neck length and musculature, and diving speeds work in concert to ensure that the force of the water doesn’t buckle their slim necks……..
The primary force acting on the gannet’s head as it plunges beneath the water is drag, which increases with speed. To analyze what other parameters affect the force the bird experiences, the researchers created a simplified model from a 3-D printed cone on a flexible rubber “neck,” and plunged this system into a basin of water, varying the cone angle, neck length, and impact speed. High-speed video showed whether the neck buckled…..’What we found is that the gannet has a certain head shape, which reduces the drag compared to other birds in the same family,’ Jung said. The researchers also discovered that the birds further reduce the risk of buckling by contracting their neck muscles before impact, straightening the S-shaped neck.”
For the purpose of comparison, let's consider how a human body would fare when hitting the water surface after a jump or dive from this height. Remember the highest Olympic divers are jumping from 10 meters although some very strong people can sustain the forces of a 20-meter dive without getting injured. The height of a regular gannet dive (30 meters) would be impossible for a human being trying to enter the water head first. It would even be challenging to land with a feet-first entry without sustaining injuries.
Boobies and Gannets, on the other hand, do this every day. So what is different about them? As mentioned above, they always enter the water head first and they have a beak that takes the first hit when they hit the water. However, they also have other behavioral and anatomical adaptations to this lifestyle. Being birds, their front limbs are wings rather than arms, but the wings are mostly composed of the same sets of bones as the human arm. Both human arms and bird wings have the same bones (Humerus, Radius & Ulna). A major difference between our arms and birds’ wings is the density of the bones. While human arm bones are filled with marrow and dense tissue, bird’s wing bones are hollow.
Upon entering the water, these birds fold their wings behind them to form a torpedo shape. This also serves to protect the wing bones from the forces of entering the water at high speed. Once underwater, they open their wings and use them to move around. Because they have air-filled lungs they will be positively buoyant. In addition, bird lungs are completely different from mammal lungs, they include a hard lung and multiple small and large air sacks.
The Sulidae family of seabirds also have a unique evolutionary adaptation, specifically cushioning their plunge-diving lifestyle, as I found out when discussing this group of birds with Linda Elliot, the director of the Hawaiʻi Wildlife Center. They have a large number of small air bubbles, distributed all over their breast, under their skin (subcutaneously), like a lining of bubble wrap. Some of these air bubbles are connected to the bird's air sacks. These serve as a cushion, dampening the impact with the water surface.
What this means for the diving bird moving underwater is that it is filled with both large and small pockets of air. That means that its body is very buoyant, like a balloon underwater, and
it will have to fight to stay underwater and not pop up to the surface. In fact, the initial dive is as deep as a booby or gannet can go. From that point onward the bird will be moving towards the surface. So, to capture their prey they need to be approaching from underneath and catch the fish or squid on the way upwards. As this also means that the fish will be silhouetted against the brighter surface, it works fine as long as the bird has enough velocity as it enters the water to get below the school of fish or squid.
The fact that boobies and gannets can not swim down any deeper because of their air-filled bodies helps explain why these birds will start their dive 30 meters above the surface or more, forming their body like a torpedo, hitting the water at almost 90 km/hr. Think of the contrast, the human Olympic divers are unconcerned with how deep underwater they can go and instead, they are focused on the amazing turns and twists that they can make in their few seconds in the air and then when they hit the surface they are focused on a body formation that will reduce the water splash to a minimum.
As a point of interest, some people have been known to cliff dive head-first from a height of 35 meters (115 feet). From that height, you will hit the water at a speed of about 87 km/hr (55 mph). This is extremely dangerous for a number of reasons. Bodily impact from high platform, or cliff diving can lead to soft tissue damage, compression fractures, neurologic injuries including spinal cord shock, even concussion due to the brain bumping up against the interior of the skull. And yet, our desire to be birdlike, weightless in the air still draws many of us to experience extreme sports activities. Perhaps more admirable, is the amazing feats of aerial displays of the Olympic athletes, who are able to attain the brief moments of suspension mid-air due to their rigorous and consistent training. And the rest of us, have the privilege to share in those feats of joyful movement.