Diving physiology of marine mammals and seabirds, by Paul Ponganis

The dive capacities of breath-hold diving endotherms (namely birds and mammals) are an inspiration to scientists and the general public alike. This is best illustrated by the flurry of interest when Cuvier’s beaked whales were recently recorded diving down to nearly 3000 m and for up to 2.5 hours on a single breath. In seeking to understand these abilities, a complicated marriage of biology, physics and chemistry has been necessary. Since the seminal studies by Scholander (1940) and later Kooyman et al. (1980) there has been a growing body of work focusing on various anatomical, behavioural and physiological aspects of diving in mammals and seabirds. Bringing all this work together into a cohesive treatise is not without its challenges, and thus Paul Ponganis’ Diving Physiology of Marine Mammals and Seabirds is a welcome chance to stop and take stock of what has been achieved. As the author states, the goal of his synthesis is to provide an understanding of what has been achieved in the past in order to make progress in the future.

As could be expected the scene is set in Chapter 1, which describes the mammalian and seabird taxa that exhibit diving behaviour. Specific attention is paid to the key parameters i.e., depth and duration that describe diving behaviour. What is of immediate value are the tables of data that have been carefully prepared and I am sure will be of use to future students of dive physiology. Following this chapter, attention is turned to the challenges the aquatic environment imposes on taxa seeking to exploit its abundant resources. Here much of the focus is primarily on humans, due to the greater body of knowledge compared with other mammalian (and seabird) taxa. Nonetheless, discussion is made of the effects of pressure, temperature and reduced light when diving in addition to the various sensory adaptations required. This is particularly interesting when considering those species that must operate in dual air/water environments.

Chapters 3 and 4 describe the various ways in which oxygen is captured, stored and transported within the animal when underwater. These chapters set the trend to follow, where there is a discussion first of mammals followed by seabirds, usually because of the greater body of work performed on mammals thus far. Here as before, the compilation of tables of published results provides a great opportunity to compare and contrast the various strategies observed thus far. One minor criticism I have, however, is that it would have been good to see a discussion of the equations used for calculating respiratory, blood and muscle oxygen stores. For logistical reasons, these equations are often reliant upon a host of assumptions (e.g., oxygen saturation of arterial blood); and allometric calculations of key parameters (e.g., blood volume). Thus, it would have been useful to see these laid out in full to gain an appreciation of the assumptions that are made.

By his own admission Chapter 5 on the physiological dive response (severe bradycardia and peripheral vasoconstriction) is the longest chapter of this book, primarily because the dive response is core to understanding dive physiology in mammalian and seabird taxa. Accordingly this chapter is exhaustive, providing a definitive account of what is known thus far. For those looking to gain an overview of the dive response this will be a slog and they may be better served referring to the review of Butler & Jones (1997), which although dated still provides an apt account of the essentials. Nonetheless, Chapter 5 provides a welcome starting point for future researchers as the author details species by species, study by study, the various cardiac, blood flow and tissue oxygenation states outlined within. What is particularly interesting is the comparison of studies into the earlier highly manipulative, forced submersions; through to the later free diving studies afforded by technological advances. It could be expected that the latter better reflect the actual operating physiology of the study animal when diving.

Following Chapter 5 we are led through specific accounts of the anatomical adaptations of the cardiovascular system (Chapter 6), muscle physiology and anatomy (Chapter 7), thermoregulatory considerations (Chapter 8) and metabolic adjustments (Chapter 9) when diving. As with previous chapters there is a bias towards mammals in the literature, thus providing an opportunity for future researchers to fill in the gaps. Nonetheless, the logic of the arrangement of these chapters is evident when we get to Chapter 10, 'The aerobic dive limit', which represents a synthesis of elements from the preceding chapters. Defined as the dive duration resulting in post dive blood lactate accumulation, the aerobic dive limit (ADL) is difficult to measure in wild unrestrained animals. Accordingly it is often estimated from indirect methods i.e., behaviour, or total oxygen stores/dive metabolic rate. However, the appearance of glycolytic end products in the blood does not necessarily entail exhaustion of body oxygen stores, and Chapter 11 deals with oxygen store depletion and hypoxaemic tolerance. Here the focus is predominantly on pinnipeds and penguins, most likely due to the greater ease with accessing blood vessels. The final biological effect of diving to be covered is pressure tolerance (Chapter 12), particularly with regards to decompression sickness, nitrogen narcosis and high-pressure nervous syndrome. Finally, being both an MD and biologist, Ponganis brings a unique perspective and I personally enjoyed seeing a section on the biomedical applications that all this research has.

I see this as a highly recommended text with broad appeal. It provides a detailed account of the mechanisms that underpin the dive behaviours which are being uncovered with increasing resolution. With the advent of miniaturized dive loggers, coupled with increasingly sophisticated physiological methods, we are entering an exciting phase in dive physiology research. Ponganis’ book provides a much needed collation of what has been achieved thus far and I look forward to future editions that incorporate the work to follow.