Breathtaking feats

Maiorca was an elite athlete, an outlier. Most of us can hold our breath voluntarily only for a few minutes and can comfortably freedive only a few metres. When marine biologists turned their gaze deeper into the ocean using new technologies, they were astonished by creatures that effortlessly achieved what humans, like Maiorca, strained to do. The elusive Cuvier's beaked whale (Ziphius cavirostris) routinely dives to depths exceeding 2,000 metres, sometimes holding its breath for over two hours. This whale's capabilities hold secrets that illuminate our understanding of both marine and human physiology.

At the heart of any diving organism's capability is the management of oxygen: its storage, delivery, and efficient use. Humans and whales share fundamental physiological mechanisms, yet whales like the Cuvier's whale have evolved adaptations that far surpass our own.

Mammals store oxygen primarily in their blood, muscles, and lungs. While humans have modest amounts of oxygen stored mostly in the lungs and, to a lesser extent, in blood, Cuvier's whales have adapted to rely extensively on their muscles and blood for oxygen reserves, significantly minimising lung use during deep dives. This adaptation reduces buoyancy and avoids nitrogen absorption, which can cause decompression sickness ("the bends") in humans.

The whale's remarkable oxygen-carrying capacity stems largely from its haemoglobin and myoglobin. Haemoglobin, the oxygen-transporting protein in red blood cells, is abundant in whales, allowing their blood to carry much more oxygen compared to human blood. Moreover, whales exhibit extraordinarily high levels of myoglobin in their muscle tissue – up to 10 times more than that found in humans. Myoglobin binds oxygen more efficiently than haemoglobin and provides a robust oxygen reserve directly within muscles. This extensive muscle-bound oxygen allows whales to maintain muscular activity even when oxygen from the lungs or bloodstream is depleted.

Metabolism is another crucial factor. Deep-diving whales can dramatically reduce their metabolic rate, essentially slowing down their bodily functions, to conserve oxygen. During dives, the heart rate plummets, and blood flow becomes selectively restricted, prioritised towards essential organs such as the brain and the heart. This adaptation, known as the mammalian dive reflex, is observed in humans, too – although less dramatically.

These whales also employ what is called 'anaerobic metabolism' when oxygen stores are depleted. During breathing, mammals use aerobic metabolism (the use of oxygen) to generate energy. The anaerobic metabolic pathway generates energy without oxygen, though at the expense of producing lactic acid, which must later be cleared when breathing resumes at the surface. Cuvier's whales manage this metabolic shift, tolerating lactic acid accumulation far beyond human capabilities.

The Cuvier's whale belongs to the family Ziphiidae (beaked whales), whose evolutionary lineage dates back roughly 20-25 million years, originating in the Miocene epoch. Beaked whales evolved specialised adaptations for deep-diving, including a streamlined body, reduced limbs, and specialised feeding anatomy, reflecting their adaptation to deep-water habitats and prey. Cuvier's whales have an exceptionally broad range, inhabiting deep oceanic waters worldwide, from tropical to temperate seas. They are commonly found in offshore waters near continental slopes and submarine canyons, particularly in regions such as the Mediterranean Sea, Pacific Ocean (notably around Hawaii and the U.S. west coast), and the Atlantic Ocean, from the Caribbean to northern Europe. Adult Cuvier's whales typically measure between 5.5 and 7 metres in length. They weigh between 2,500 and 3,500 kilograms. Females tend to be slightly larger than males. Their primary natural predator is the killer whale (Orcinus orca). However, predation events are rarely observed due to the deep-water habitats of Cuvier's beaked whales. Humans pose indirect threats through noise pollution (naval sonar), entanglement in fishing gear, and marine debris ingestion. Cuvier's whales primarily feed on deep-water squid and, occasionally, fish. They forage at extreme depths, typically around 1,000 to 2,000 metres, employing echolocation to hunt their prey in the darkness of the deep ocean.

Deep-water squids have exceptionally large eyes, among the largest relative to body size in the animal kingdom. A prominent example is the colossal squid (Mesonychoteuthis hamiltoni), whose eyes can measure up to 30 cm in diameter. At depths where there is no light at all, why do deep-water squids have such enormous eyes? What can they possibly see in the absolute darkness? These large eyes are highly adaptive in the deep ocean, despite the darkness. Many marine organisms emit their own light (bioluminescence). Large eyes help squids spot prey, predators, or potential mates by picking up faint flashes and glows emitted by other creatures. Large eyes help squids detect approaching predators, such as Cuvier's whales, by sensing the faint silhouette or bioluminescent signals. A war of sonar versus low-light detection.

The discovery of the Cuvier's beaked whale and the understanding of its extreme adaptation is as fascinating as the whale itself. The Cuvier's whale was first scientifically described by French naturalist Georges Cuvier in 1823. Cuvier based his description primarily on a skull discovered on the Mediterranean coast of France. Early knowledge came from stranded specimens. For nearly two centuries after its initial discovery, the Cuvier's beaked whale remained largely enigmatic due to its deep-ocean habitat and elusive behaviour.

Recent advances in technology, such as the Satellite and Digital Acoustic Tags (DTAGs), revealed surprising insights. Scientists attach these sophisticated instruments to whales using suction cups or minimally invasive darts. DTAGs record crucial data like depth, dive duration, temperature, and acoustic environment, providing direct evidence of diving depth and duration. A landmark 2014 study revealed that a tagged Cuvier's whale off the coast of California dove to an astonishing 2,992 metres and remained submerged for 137.5 minutes, setting the record as the deepest and longest dive recorded for any mammal. Passive Acoustic Monitoring (Hydrophones) is another method used to study these whales. Stationary underwater microphones record echolocation clicks, enabling scientists to estimate whale location and depth based on sound propagation through water.

Initially, knowledge of the diet of Cuvier's whales came from examining the stomach contents of stranded animals. More recently, studies have used advanced tools to refine these insights. Analysis of stomach contents of stranded or incidentally caught whales showed squid beaks, indicating squid as their primary diet. Fish remains were found occasionally.

A technique called stable isotope analysis has been valuable in deciphering the depths at which Cuvier's whales catch prey. Stable isotope analysis is a powerful tool for inferring the foraging depths and ecology of deep-diving animals like Cuvier's beaked whales, which are difficult to observe directly.

Here's how it's used: Elements such as carbon and nitrogen can come in two variants: (¹³C/¹²C) and (¹⁵N/¹⁴N), respectively. The proportion of each form accumulates in the tissues of animals based on the proportion in what they eat and where they forage. In other words, you are, at the elemental level, what you eat. ¹³C values vary with depth and habitat: near-surface producers have different ¹³C signatures than deep-sea organisms. Generally, ¹³C becomes more negative with increasing depth, due to carbon sources and food web structure. ¹⁵N values increase with 'trophic' level. Predators accumulate more ¹⁵N relative to their prey. This gives insight into the animal's position in the food chain, and indirectly, the depth, since trophic structure differs with depth. Researchers sample tissues such as skin, blubber, or even bone collagen from Cuvier's whales. These tissues reflect the isotopic composition of the whale's prey over different timescales. Skin shows diet over weeks to months, and bone collagen reflects a multi-year dietary signal.

The isotope profiles of known mesopelagic (200–1,000 m) and bathypelagic (>1,000 m) prey species (like squid and deep-sea fish) are used as a reference. By comparing these profiles to the whale's tissue signatures, scientists can infer the typical foraging depths. For example, if a whale's tissue isotopes match those of bathypelagic squid, it suggests foraging at depths well over 1,000 metres.

Isotope analysis is often combined with depth-logging tags, which show how deep the whale dives. This dual approach confirms isotopic interpretations and helps track seasonal or individual variation in foraging strategies.

Satellite telemetry tags track movement patterns, dive profiles, and geographic distribution over extended periods while DTAGs record depth, speed, movement, and acoustic data. Remotely operated vehicles (ROVs) provide visual confirmation of deep-sea habitats and prey availability. Finally, genetic analysis confirms species identification and relationships within beaked whales.

These advanced tools have revolutionised our understanding of marine mammal biology and conservation, highlighted the whale's exceptional adaptations, and enhanced our understanding of mammalian physiology under extreme conditions. Insights from these discoveries inform conservation policies and expand the possibilities for human medical research related to extreme physiology.

India's deep ocean mission, now well underway, aims to have a human-carrying submersible to explore the ocean depths. Matsya 6000, being developed under the Samudrayaan project of the Deep Ocean Mission, is designed to reach depths of up to 6,000 metres beneath the ocean surface. The submersible features a 2.1-m diameter titanium alloy personnel sphere engineered to withstand the immense pressure at 6,000 metres, maintaining an internal atmosphere of 1 atm for crew safety. It is equipped to carry three individuals and is designed for 12 hours of operational endurance, with provisions for up to 96 hours in emergency situations. Initial trials have been conducted at depths of up to 500 metres, with plans to achieve the full operational depth of 6,000 metres by the end of 2026. The Matsya 6000 is a significant step in efforts to explore deep-sea resources and enhance capabilities in oceanographic research. New and even more wonderful discoveries will undoubtedly come from this mission.

Also Read/Watch

Schorr, G.S., Falcone, E.A., Moretti, D.J., Andrews, R.D. First Long-Term Behavioral Records from Cuvier's Beaked Whales (Ziphius cavirostris) Reveal Record-Breaking Dives. PLOS ONE (2014). bit.ly/cuvier-record

Quick, N., Cioffi, W., Shearer, J., Fahlman, A., Read, A. Extreme diving in mammals: first estimates of behavioural aerobic dive limits in Cuvier's beaked whales. Journal of Experimental Biology, Vol 223, Issue 18 (2020). bit.ly/deep-dive-mammal

Extreme diving in mammals: a Cuvier's beaked whale dived for 3hrs 47mins. bit.ly/duke-deep-dive