3.2 Natural dives
The physiology of the diving response can be studied in the laboratory, but investigating the behaviour of a diving mammal in its natural environment can be more of a problem. However, modern physiological techniques have made it possible to record continuously physiological variables (such as heart rate) and information on depth and position during the spontaneous dives in the wild that are part of the animal's normal behaviour. Most such findings show that the majority of an animal's dives don't approach the 'Olympic record' limits that represent maximum effort. For example, in the Weddell seal most dives were shorter than 20 minutes and there is little evidence of profound adjustments in the heart and circulatory system during this time, or of the presence of lactic acid in the blood of these animals as they surface. Lactic acid is an important indicator of the onset of anaerobic metabolism (i.e. metabolism that doesn't use oxygen). The production of lactic acid is a relatively inefficient way of releasing energy by the breakdown of simple sugars obtained from the diet. (In humans, lactic acid appears in the bloodstream during intense activity such as sprinting, when the supply of oxygen to the muscle cells isn't sufficient to allow the oxidation of sugars in the 'conventional' way.)
What do these observations suggest about the physiological events that accompany natural dives?
They suggest that a full-scale diving response is not typical of these shorter dives. If tissues were starved of oxygen, substantial levels of lactic acid would probably be evident in the blood, as anaerobic metabolism became established.
The implication is that in shorter dives, the amounts of oxygen carried by the Weddell seal (mainly in the blood, as we've seen) are sufficient to tide the animal over, without a sustained or profound diving response. But with natural dives of 45 minutes or so, anaerobic metabolism becomes much more significant and the selective re-routing of blood (together with bradycardia) becomes much more important.
What about the animal's hunting behaviour? The Weddell seal, for example, spends most of its time hunting cod and other fish out of sight beneath the Antarctic ice.
Marine biologist Terrie Williams and her colleagues at the University of California have studied the hunting behaviour of Weddell seals by attaching recording equipment to animals captured near their feeding holes. The seals are fitted with video and audio recorders, plus monitors that note the animals' depth, speed, compass bearing and flipper strokes. The seals are then released back into the sea. The information that the scientists receive allows them to construct a three-dimensional map of the dive that is tied to specific pieces of behaviour recorded by the video camera. The seals hunt in three dimensions in low levels of light and have to find, stalk and catch their prey while holding their breath.
Figure 4 shows a dive by an adult female Weddell seal. The dive lasted for 10 minutes 30 seconds, and the animal travelled a total of 760 m. About mid-way through the dive, at point C, the video footage revealed that the seal attacked a large Antarctic cod, approaching from behind and below its tail. Let's look at what happened in more detail.
The seal started the dive with a few powerful strokes of its flippers. Water pressure would have quickly collapsed its lungs, making the seal denser than the surrounding water. The negatively buoyant animal could then have continued its descent with little effort. (The team have discovered that in pinnipeds and cetaceans this effortless gliding minimises the effort expended by the animal while submerged.) The seal sank slowly to a depth of 51 m (right arrow), rose slightly to a depth of 33 m (left arrow), and then started a new descent, maintaining roughly the same bearing and speed. This course was taking it in the general direction of the fish.
At point A, some 23 m from the fish, the seal used a few large flipper strokes to change direction and speed, accelerating away from the fish at an angle of 58 degrees to its previous course. It then used a looping turn (point B) to bring it back towards - but beneath - the fish. It accelerated again towards the target but slowed just before contact was made. After the attack had failed, the seal did not pursue the fish but continued its descent for a while, before turning left (point D) and proceeding quickly back to the ice hole.
The data gathered tell us a lot about what happened but little about why. We are still left with the problem of interpreting the sequence. Try this for yourself. For example, why might the seal have moved away from the fish at point A?
Here is my interpretation (and it is difficult, if not impossible, to prove). I suspect that the initial descents and ascents on a constant bearing were a searching strategy. At point A, the seal spotted the cod a short distance away (remember the poor light). If it approached the fish head-on it would be seen quite quickly. The turns at points A and B allowed the seal to attack the fish from behind and below.
After accelerating out of the second turn, it pulled its flippers to its side and coasted towards its prey, which accounts for the deceleration recorded just before contact. The seal would be camouflaged against the darkness of the deep water and the fish would be silhouetted against the ice.
If you find this topic as interesting as I do, you have the opportunity to find out more from Reading 2, which reproduces in full the original scientific paper by Terrie Williams and her colleagues that reported these finding to the scientific world. The language used in the paper is sometimes technical and assumes some familiarity on the part of the reader with specialist terminology and the use of recording techniques. But you don't need to understand it all to get something from the paper. You'll see that more information is provided than was possible in the much abbreviated description of the research I've just presented. The reasons the research work was undertaken are outlined, and a fuller account provided of the three brief encounters - from 57.4 hours of recording! The broader significance of these findings is discussed, with an emphasis on what was new and unexpected about these observations and the potential of the technique for revealing more about the hunting behaviour of diving mammals.
Click 'View document' to open Reading 2
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Part of the enjoyment of study is the opportunity to read more about a subject of particular interest, and the two papers linked with Section 3 reflect two very different types of publication available. The first reading, 'Into the abyss' from the popular science magazine New Scientist, is easy to read in that it assumes little specialist knowledge from the reader. Its style is dramatic and involving - look at the opening sentence: 'It's deathly dark, wet and you're chilled right through'. It makes you want to read on. Publications such as New Scientist or BBC Wildlife provide an excellent means of developing your interest in topics that have caught your attention in this course. But relying solely on articles written by science journalists as your source of information can cause problems, despite the many excellent articles produced, especially in the publications just named. Assertions made may not always be backed by evidence and sometimes accuracy and caution are sacrificed for the journalistic purposes of a particular angle to capture the reader's attention. And dealing with so many snippets of journalistic information may prove difficult. The over-arching concepts and ideas - the scaffolding that binds bits of information together - can be lost sight of, which is where the formal study of the subject, perhaps through OU courses, will help.
Reading 2, 'Hunting behaviour of a marine mammal beneath the Antarctic fast ice', is an example of a research paper, reprinted from the American journal Science. Such a publication is geared much more towards the interests and specialist knowledge of fellow scientists. It is included here to give you an idea of how research in this field is undertaken and the form in which it is published. The writing is much more measured and cautious in tone, describing the results in detail and stressing how the findings relate to overall knowledge of the field. For a fellow researcher in the field, the technical detail provided here, for example in footnote 6 on page 236, would be invaluable, but for most of us the details are more than we need to know! If you study more science, research papers will assume a greater importance, but at this level it's more important to know what potential benefits they offer, rather than to spend precious time struggling with the details. Most original research papers are even less easy to read than this example!