Exciting new research is investigating how boobies (birds of the genus Sula) may be able to reduce the potentially lethal impact of their high-speed vertical dives by creating a cushion of "supercavitation" bubbles upon impact with the water.
"It is believed that the bodies of boobies, which can dive at speeds close to 100 km/h to catch their prey, possess certain adaptations to protect themselves from the intense impacts," says Dr Yoshinobu Inada, a professor in the Department of Aeronautics and Astronautics at Tokai University Japan. "The desire to uncover and understand these adaptations inspired us to begin this study."
Cavitation occurs when an object moves through water at high speed and the pressure around it drops below the water's vapor pressure, causing bubbles to form. When a large number of these bubbles appear at high enough speed and cover the whole object, it's called "supercavitation". This phenomenon is known to reduce drag, among other effects.
"What we're interested in, though, isn't drag reduction — it's impact mitigation," says Dr Inada. "We think that when boobies dive into the water, supercavitation might be occurring, and the bubbles could work like a cushion to soften the impact."
Dr Inada and his team suggest that the boobies could be diving at high speed on purpose to trigger this protective cavitation effect. "Without this cushioning, hitting the water at 100 km/h would cause a massive impact — likely strong enough to break bones or even be fatal," says Dr Inada.
To test this hypothesis, Dr Inada and his team accurately recreated the head of a booby by using an existing CT scan of their skull and building a 3D model. "We then printed the model using a 3D printer to make a physical replica, and installed an accelerometer inside it to measure impact forces during collisions," says Dr Inada. To launch the model at speeds approaching 100 km/h, they also needed to build a customised launcher, and then filmed the model descending rapidly into the water with a high-speed camera.
A promising preliminary finding of this project is that although the impact force of hitting the water generally went up with dive speed, Dr Inada and his team observed repeated occasions where impact force dropped after reaching very high speeds, which were associated with the formation of a large amount of bubbles.
Analysis of the video footage led Dr Inada and his team to wonder if these bubbles were helping to cushion the crushing forces of impact through cavitation. "We suspect this drop in impact might be due to supercavitation, so we're currently preparing more experiments to see if the effect holds up under different high speed conditions," says Dr Inada.
Cavitation is more often used in the animal kingdom by marine predators as a means of disabling or killing prey, as seen in the rapid-attack claws of pistol shrimps or the thresher shark's debilitating "tail slaps".
Supercavitation is already used in aquatic engineering to achieve higher speeds of submersibles through drag reduction, but there could also be applications in mitigating impact forces. "If we can confirm that supercavitation actually helps to reduce this impact, it could be really useful for human-made objects entering water at high speeds," says Dr Inada. "For example, it might even be applied to the design of spacecraft that splash down in the sea when returning to Earth."
While these cavitation bubbles have been observed over repeated experiments, these findings are still preliminary and Dr Inada would like to continue conducting these experiments at even higher diving speeds of 100 km/h, which is the speed that Northern gannets have been known to dive at.
This research is being presented at the Society for Experimental Biology Annual Conference in Antwerp, Belgium on the 10th July 2025.
