15.10.2018 – Apnoe artists

It was an unexpectedly spectacular day out on the Atlantic. Our spotter lead us to a herd of Bottlenose dolphins (Tursiops truncatus) that contained tiny little calves, as well as to a large, dispersed group of logging Short-finned pilot whales (Globicephala macrorhnychus) accompanied by larger, more interactive oceanic-type Bottlenose dolphins before the grand finale; Sperm whales (Physeter macrocephalus). After 5 of the giants lifted their flukes to dive into the depths, a mother and a calf approached our boat carefully before we met yet another group of logging pilot whales and bottlenose dolphins.

Everybody knows that Sperm whales can dive to extreme cold, crushing depths and are able to withstand immense changes in pressure. To do so, these and several other deep-diving cetacean species boast a number of anatomical and physiological adaptations that enable them to reach these incredible depths without attaining internal injuries or embolisms.

The most fundamental of such adaptions is a flexible thorax and elastic tissue in the lungs and trachea that essentially allow both the rib cage and the lungs to collapse. Sperm whales have ribs that are bound by loose, bendable cartilage allowing the bones to fold into themselves rather than snap at these pressures. In order for this to happen, the lung must be completely emptied before a the animals take the plunge, meaning the oxygen used during a dive must be obtained elsewhere. Myoglobin, an oxygen-binding protein in the muscles of mammals, is ten times more concentrated in cetaceans and binds 3 times more oxygen than haemoglobin, the pigment responsible for oxygen transport in our circulation system. The blood stream of cetaceans is also finely equipped for adventures in the dark ocean. Large elastic veins and a unique capillary network known as the reta mirabile helps prevent air embolisms from occurring, which are especially dangerous for humans attempting to dive to greater depths.

Of course our „greater depths“ aren’t comparable to the incredible 2500m reached by the enormous male Sperm whales (and for all we know they probably can dive even deeper) but nonetheless there are universal strategies for air-breathing to remain underwater for some time, that are used by Spem whales as well as human freedivers. Like Sperm whales, freedivers avoid unnecessary movement underwater to minimize oxygen consumption. Sperm whales use their large flukes to propel their bodies downwards before simply gliding headfirst into the dark ocean. Beyond a certain depth, the body becomes negatively buoyant and sinks without effort. Deep diving cetaceans also drastically reduce their heartbeat to an extremely low rate of 4 beats per minute. While humans are not able to reach such low heart rates, breathing exercises before freediving help reduce heart rate and oxygen consumption.

Well, I’m confident there were people who held their breath today during these incredible sightings. In the midst of all this surface action during the sightings, it was humbling to imagine what all these artists of the ocean are experiencing deep below the surface in the vastly unknown dark ocean.

By Paula Thake

Sightings of the day

Ribeira Brava 

14:30 Bottlenose dolphins, Short-finned pilot whales, Sperm whales