If you’ve ever bailed on a dive and ended up performing a belly flop for all to see, you’ll have a unique appreciation of the force presented by water surface tension. Diving animals experience enormous pressures during water entry, which is followed by the formation of an air cavity and eventually a splash as this closes. These impact forces can be minimized by creating a hydrodynamic shape as you enter the water (aka, not a belly flop), and such body types are common among plunge-diving seabirds.
Perhaps one of nature’s most impressive divers is the Northern Gannet, Morus bassanus, who stars in the opening bait ball sequence in David Attenborough’s latest release, A Perfect Planet. These hardy seabirds will follow a pod of dolphins until both parties find some food, which in A Perfect Planet happens to be a delicious shoal of mackerel.
As the dolphins begin to pick away at the fish, they form a bait ball that gets pushed towards the ocean surface as a means of trapping the mackerel. Here, they enter Gannet territory, as the birds begin hitting the water like feathered fighter planes, some reaching speeds of 24 meters (78.7 feet) per second. They can dive up to 11 meters (36 feet) and swim further to catch fish, which they drag to the surface by beating their webbed feet and wings. Performing such a hunting strategy would surely spell tragedy for a human being, so how are these birds able to hit the ocean surface with such ferocity without snapping their long, slender necks?
The answer is a shining example of evolution driven by the survival of the fittest. To fulfill their ecological niche, gannets have evolved to have highly specialized necks, which studies say wouldn’t experience injury until diving at a whopping speed of 80 meters (262.5 feet) per second.
To investigate how such birds can survive diving into the water at such speeds, a study published in PNAS used a salvaged bird head to identify the hydrodynamics of plunge-diving. Their investigations revealed that there were anatomical features of the skull and neck musculature which, combined with living birds’ chosen diving speeds, can stabilize this weak and slender body part during plunge-dive.
When firing their deceased Northern Gannet’s head through the water surface vertically at roughly 5.5 meters per second, they were able to identify the impact, cavity formation, and splash phases of a plunge dive. The impact phase is when the beak first makes contact with the water, followed by the cavity phase which forms a sort of bubble around the bird as its head becomes fully submerged.
This phase is interesting, as the bird's head is now decelerating while the rest of its body continues to accelerate. The researchers conclude that birds are able to survive what could be a fatal compression of the neck thanks to muscles that are bundled around the skull. By contracting these, they can stabilize their neck long enough for the bird’s chest to hit the water which marks the end of the compression phase. This, combined with the birds' chosen diving speeds which stay within a range that won't crush their neck, sees the gannets firing into the water like feathered arrows.
The researchers predict that a gannet would need to enter the water at around 80 meters (262.5 feet) per second to sustain an injury which, for reference, is around double the maximum speed limit in the United States. Not bad, Northern Gannet. However, there is a limit to their neck stabilizing skills.