Written by Sophie Dunlop, Summer intern 2025
A new record for the deepest free dive with a single breath and no equipment has just been set at 103m depth by Peter Klovar in May 2025[1]. Whilst humans dive primarily for leisure, many air breathing reptiles, birds and mammals dive in search for their next meal and thus have evolved many unique adaptations to overcome the challenges of breath-hold diving such as cold temperatures and metabolic demands. For example, oxygen can be stored in the lungs, in the blood bound to haemoglobin and in muscle tissue bound to myoglobin. However, air in the lungs can be compressed during dives, affecting buoyancy and the absorption of pressurised gas into the blood. Therefore, many breath-holding vertebrates collapse their lungs during dives to reduce this problem.
There are many methods used to investigate the deep-diving physiology of animals. Animals can be tagged with data-logging sensors that can record the physiological functioning of the organism, such as measuring heart and brain function with ECGs and EEGs respectively. Immersion experiments on subjects in the lab can be used to measure metabolic activity during dives, but this is restricted to small species and it is hard to simulate high pressure. Blood samples can be taken before and after diving to study the composition of the blood and comparative genomics can be used to look at which gene families are expanded in diving species and which genes are switched on before and after dives. Traditional anatomical studies are also useful for identifying adaptive structures.
Leatherback turtles are the largest species of sea turtle and they migrate to UK waters in summer to feed on jellyfish blooms. Leatherback turtles can dive to depths of 1,200m for over an hour during migration, but most dives are shallower than 300m[2]. During deep dives they are searching for deep gelatinous zooplankton, then they stay around the area to feed at night when their food migrates to shallower depths. Leatherback turtles were thought to be endothermic meaning they maintain a constant body temperature by generating heat internally through metabolic reactions and this would be how they are able to dive for long periods in cold water. However, studies suggest they are actually ectothermic, meaning their body temperature fluctuates based on their surrounding environment[3]. Therefore, in order to maintain body temperature whilst diving, leatherback turtles have evolved a smaller surface area to volume ratio, insulating heat-exchange blood circulation and fat layers.
Northern gannets are the largest seabird found in the UK, with wingspans of up to 2m. They can dive to depths of over 20m whilst hitting the water at speeds of 60mph to forage on fish shoals, with an extensive network of air sacs between their muscles and in their skull to cushion the impact. The deepest diving bird however, is the emperor penguin, which has been recorded diving to depths of 564m[4] for over 30 minutes[5]. Penguins are able to dive to great depths because their bones lack air spaces leading to reduced buoyancy which reduces the amount of energy needed to dive down and greater structural strength to enhance resistance to bending as water is a denser medium than air6]. Emperor penguins release air bubbles trapped in their plumage before attempting to jump ashore because this reduces drag on the surface of their bodies, enabling them to accelerate and jump out of the water.
The pinnipeds group includes all seals, sea lions and walruses. There are more than 120,000 grey seals in UK waters, representing 40% of the global population. Grey seals can dive to depths of over 300m for up to 20 minutes at a time, however, the deepest diving pinniped is the southern elephant seal which has been recorded at a depth of 2,388m, with dives lasting over 2 hours. Pinnipeds dive to forage for food, to avoid predators and a recent study found that northern elephant seals enter full REM sleep with accompanying paralysis during dives, but only at depths out of view of visual predators[7]. Pinnipeds typically exhale before they dive to reduce buoyancy, then their lungs collapse under pressure, whilst surfactants (molecules with anti-adhesion compounds) line the lungs to enable reinflation at the surface. Their shivering response is suppressed whilst gliding behaviour helps reduce the metabolic demand of dives.
Furthermore, pinnipeds have a large volume of blood for their body size and a high haematocrit (high ratio of red blood cells compared to total volume), so have a higher capacity to store oxygen. The total oxygen store of a hooded seal is 90ml of oxygen per kilogram of body mass, compared to 25ml in humans[8]. Peripheral vasoconstriction (narrowing of blood vessels in the extremities) helps conserve heat whilst prioritising blood flow to the brain and heart. This results in an increase in blood pressure so they slow their heart rate to compensate for this and an elastic chamber called the “Windkessel” in the aorta helps smooth the blood pressure between heart beats[9]. The brain is also cooled by 3oC due to heat exchange with blood returning from the flippers, which further reduces oxygen demand by 15-20%.
Oxygen free radicals (unstable molecules containing oxygen with unpaired electrons) are very reactive and can damage cells, this is known as oxygen toxicity. Breathing oxygen under high pressure increases the effects of oxygen toxicity. Comparative genomics studies found expanded gene families for proteins tackling oxygen toxicity in cetaceans compared to other mammal groups. There are also whale specific mutations in genes coding for antioxidant enzymes[10]. Cetaceans swim by moving their tail flukes up and down, this is known as fluke swimming but this creates high pressure waves in their circulatory system on the downstroke. Therefore, in order to be effective divers they need to reduce these pressure waves, so they possess net-like structures called retia in their circulatory systems, particularly around the brain[11].
There are two main groups of whales, baleen whales which are filter feeders and toothed whales which are active predators. The latter group includes dolphins, porpoises, sperm whales and beaked whales and they can dive for longer and deeper than baleen whales. Cuvier’s beaked whale is the deepest diving vertebrate, with a recorded depth of 2,992m for 137 minutes, however marks on the seafloor at 4,200m are similar to scour marks left by these whales, and anatomical studies suggest the cranial air spaces in Cuvier’s beaked whales could withstand the pressure at 5,000m depth, suggesting they can dive much deeper than recorded[12]. Beaked whales dive deep to feed on mature squid that live deeper than juvenile squid, but are richer in calories and less evasive. Diving deeper also allows them to avoid competition from bigger cetaceans. Sperm whales regularly dive to 1,000m for over an hour, but may dive to 3,000m based on their stomach contents. The Spermaceti is an organ in their head that helps them adjust buoyancy. It contains a wax that solidifies in cold water at the surface becoming denser to help them descend, however, metabolic heat gradually melts the wax so it decreases in density and helps them ascend.
Toothed whales produce sound during dives, for communication and echolocating prey, by passing air stored in nasal cavities (not the lungs which collapse during dives) over phonic lips (similar to the larynx (voice box) in humans)[13]. Echolocation is an extremely useful tool for locating prey at depths where light can no longer penetrate, typically below 200m. The melon is a mass of fatty tissue in the forehead of toothed whales used to focus vocalizations. They use different vocal registers to produce sounds, for example M1 (normal) and M2 (falsetto) for communication, while they use M0 (vocal fry) for echolocation clicks as it allows loud sound generation from a small volume of air so they can dive for longer.
Overall, many air-breathing marine animals, including those found in UK waters, have evolved unique ways to dive deeper and for longer durations than humans, in order to forage more efficiently, feed and avoid predators.
References:
1. Weinman, S. Freediver Klovar breaks Trubridge’s 17-year no-fins reign. Divernet (2025). at <https://divernet.com/scuba-news/freediving/freediver-klovar-breaks-trubridges-17-year-no-fins-reign/>
2. Houghton, J. D. R., Doyle, T. K., Davenport, J., Wilson, R. P. & Hays, G. C. The role of infrequent and extraordinary deep dives in leatherback turtles (Dermochelys coriacea). Journal of Experimental Biology 211, 2566–2575 (2008).
3. Bradshaw, Corey J. A., McMahon, Clive R. & Hays, Graeme C. Behavioral Inference of Diving Metabolic Rate in Free‐Ranging Leatherback Turtles. Physiological and Biochemical Zoology 80, 209–219 (2007).
4. Wienecke, B., Robertson, G., Kirkwood, R. & Lawton, K. Extreme dives by free-ranging emperor penguins. Polar Biology 30, 133–142 (2006).
5. Goetz, K., McDonald, B. & Kooyman, G. Habitat preference and dive behavior of non‑breeding emperor penguins in the eastern Ross Sea, Antarctica. Marine Ecology Progress Series 593, 155–171 (2018).
6. Ksepka, D. T., Werning, S., Sclafani, M. & Boles, Z. M. Bone histology in extant and fossil penguins (Aves: Sphenisciformes). Journal of Anatomy 227, 611–630 (2015).
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9. Blix, A. S., Kuttner, S. & Messelt, E. B. Ascending aorta of hooded seals with particular emphasis on its vasa vasorum. AJP Regulatory Integrative and Comparative Physiology 311, R144–R149 (2016).
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