Past Investigation

  Over the years, one major question has driven researchers to conduct numerous experiments which have taken them to the furthest corners of the Earth:

“What is it that enables certain diving mammals to remain submerged, and even active, for extended periods of time?”

As was noted in Butler and Jones’ (1992) excellent review on the matter, 3 main phases in the evolution of the answer to this question can be highlighted:

1. Data obtained from animals which were forcibly submerged – the “classic” dive response.

The mechanisms which underpin air-breathing mammals’ ability to forage for their prey underwater have been the subject of much investigation for well over 100 years. It should be noted, however, that 2 scientists in particular have had a major influence on our current understanding of diving physiology: Laurence Irving and Per Scholander (pictured).

Laurence “Larry” Irving (left), a US born physiologist, and close friends with Per “Pete” Scholander (right), a Swedish-born physician.

Their experiments helped to reveal that whilst body-oxygen stores were greater in diving mammals and birds than in non-divers, these stores were not large enough to provide for aerobic respiration throughout the duration of some of the recorded dives. As a result, there is an overall reduction in the amount of aerobic respiration during dives so that oxygen can be conserved and therefore the oxygen-requiring tissues (such as the brain and the heart) are not damaged. This reduction in metabolism is achieved by selective peripheral vasoconstriction which leads to reduced blood flow to all parts of the body except the central nervous system (CNS) and the heart.

During these forced-submersion experiments, it was noted that there was a consequent build up of lactic acid which accumulated in the skeletal muscles as a result of the reduced oxygen supply. On top of the vasoconstriction, it was also noted that there was a reduction in heart rate during forced submersions. This phenomenon is known as bradycardia. This process is often used as an indicator of other metabolic processes which occur during dives which form part of the “classic dive response” (peripheral vasoconstriction, reduced aerobic metabolism and increase in anaerobic respiration.) This set of “classic” responses was also known as the ‘Irving-Scholander hypothesis’.

Scholander was not convinced that this hypothesis was the end of the matter. He noted that reduced metabolism could not be possible during ordinary dives, when mammals often do most of their exercise hunting fish and swimming long distances. Scholander and colleagues noted that harbour seals and gentoo penguins almost always dived for durations well within their aerobic dive limit and they became fascinated by the regularity these birds and mammals could perform dives, with only short breaks between each submersion. They realised that these observations were totally at odds with the idea of lactic acid accumulation and long periods of recovery at the surface.

And so we reached the second phase of the evolution of the answer to our question…

2. Data obtained from freely diving birds and mammals.

This phase concluded that most (if not all) natural dives were aerobic. It was one of Scholander’s compatriots, Eliassen, whose paper in 1960 strongly refuted the Irving-Scholander hypothesis. He said that diving would be like normal exercising and blood would be distributed in favour of the working organs. He said that vasoconstriction would, therefore, only take place in the gut. He came under heavy criticism at the time, but it was not until data from freely diving birds and mammals began to surface 13 years later, when it transpired that his proposals had been extremely perceptive.

An Adélie penguin, Pygoscelis adeliae, (left) and a Gentoo penguin, Pygoscelis papua (right)

In 1973, Millard and his colleagues used radiotransmitters to record the heart rates of freely diving and exercising birds: Adélie and Gentoo penguins. They found that the cardiac responses were not comparable to earlier records of cardiac responses in diving. They suggested this was due to the difference in the nature of the dives, with earlier records elicited from restrained birds which had been forcefully dived. They suggested that their results revealed that the true diving response was a composition of the involuntary diving response and the normal mammalian response to exercise. Time-depth recorders (TDRs) used on freely diving Weddell seals by Lapennas in the early 1980s,  alongside further use of radiotransmitters on pochards and tufted ducks by Butler and Woakes, led to the conlusion that most, if not all natural dives were aerobic in nature.

3. Our understanding returns full circle – some dives ARE anaerobic in nature!

Truth be told, it was foolish to make generalisations in the basis of one or two species. Further study conducted on other species including northern and southern elephant seals, gray seals and emperor penguins indicated that, even if only at certain times of the year, some species elicit a very low level of anaerobic metabolism during diving, with lactate shown to accumulate during some dives. This has brought our understanding of the general dive-response round full-circle in some aspects.

More recent studies on species such as the grey seal (pictured) have revealed that some species do elicit low levels of anaerobic metabolism

It is important to note, however, that an increase in anaerobic metabolism during a dive is not necessarily matched by a decrease in aerobic metabolism (and vice versa).

Given the development of our understanding in this field, it is likely that there is still much to be discovered. What is clear, however, is that a ‘one response fits all’ understanding of the dive-response mechanism for diving mammals and birds is incorrect.

**NEXT PAGE** How Low Can You Go? – The record breakers!

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