Yasuhiko Naito

Yasuhiko Naito

Penguins and seals respire using lungs. Why do they dive and how do they act in water? An array of researches conducted over the past twenty-odd years is shedding light on the ecology of marine life.

Unveiling Ecology in the Sea

What sort of a world is the sea in which penguins and seals spend most of their lives?

First of all, water generates a buoyant force. Penguins and seals can therefore move in water using less energy because they are freed from gravity thanks to this buoyant force. On the other hand, reptiles, birds, and mammals all have lungs, and the air in their lungs makes them float. Thus, they need to dash into water to swiftly reach the depth at which gravity and the buoyant force are balanced with each other.

Secondly, water has high thermal conductivity, a property that siphons the heat from their bodies. For this reason, seals and whales are protected by a thick layer of subcutaneous fat. However, it has yet to be determined why small animals like penguins can be active in water even when their body temperatures fall.

Thirdly, water has a great specific gravity, about 800 times as great as that of air; while aquatic animals are freed from gravity in water, they need more energy to move through the heavier water. It is thus efficient to swim at constant velocities rather than at accelerated velocities.

Surveys using data loggers are intended for studying the ecology of these animals in water. The devices are attached to penguins and seals when they are out of water for breeding. Once attached to the subjects, all that is required is to wait until they return to land. As they molt every season, the devices come off naturally even if someone fails to retrieve them. It is a method that puts the least burden on the subjects.

This particular survey method became widespread in the 1980s, but the problem in those days was the size of the recorders. However, with the assistance of makers producing weather devices, recording needles, and paper, we managed to downsize the recorders: about 2.5 cm in diameter and 8 cm in length. These are continuous recorders that can record both time and depth for 3 straight weeks.

Figure 1 shows the movements of seals in the sea for a period of 2.5-3.0 months, namely, an interval between breeding periods. What is amazing about this record is that the seals kept swimming almost without a break throughout the period we surveyed. They dived for 24-28 minutes at a time to the depth of 400-500 meters (up to 1,260 meters). Don¡Çt they suffer submarine sickness? When do they sleep? How do they capture their prey? Questions arise anew from this record. But a closer look at it reveals that the depth of their diving is very consistent. At a certain depth, there must be some places swarming with animals on which they prey. As for sleep, perhaps the brief periods of natural descent or ascent (enabled by the slight imbalance in specific gravities) would be sufficient for them.

During the period between the 19th century and the beginning of the 20th century, elephant seals had been hunted to the verge of extinction for their skins. After the ban on hunting on land, however, they rapidly recovered in number ? a situation that could be attributed to their feeding grounds, which lie deep in the sea, free from any environmental pollution. The spectacle of elephant seals swimming along the surface of the sea had never been witnessed. No wonder: it turns out that they spend as much as 90% of their life in the sea under the water.

As far as we know, emperor penguins can dive for 20 minutes to the depth of 543 meters, and south elephant seals, for as long as 2 hours to the depth of 1,500 meters. These records are truly surprising, considering that they are pulmonary respiration animals.

To Understand the Total Ecology of Life

Studies on the ecology of the marine life took a great leap forward in the 1990s with the introduction of downsized, multifunctional digital recorders. Specifically, it became possible to gather a variety of data (e.g., changes in speed and water temperatures per hour) through the use of micro digital loggers.

Figure 2 shows the relations among the depth of diving, speed of diving, and fluttering of Adelie penguins. When diving, they flutter a lot to accelerate; upon reaching a certain depth, they begin to swim at a low speed (less than 1.5 m/s). They stop fluttering before surfacing, but their swimming speed increases nevertheless. This is because the lungs, which have been compressed to control buoyant force, become inflated as they ascend due to the reduced water pressure ? a phenomenon that can be compared to a balloon ascending in water. However, the reason why they slow down just before reaching the surface of the sea has yet to be determined.

In addition to the information mentioned above, there was a need to know what and how much they prey on in water. We therefore invented a method by which pictures are taken by underwater cameras attached to penguins and seals (please see Photo 01). Another method uses thermometers to measure temperatures inside their stomach, which decreases as they prey on cold feed in the sea. When they feed little, the temperatures recover quickly, and when they feed a lot, the temperatures recover slowly. There is also an improved method that measures temperatures inside their esophagus; this method is more sensitive, and hence it can detect temperature changes caused by a smaller amount of feed.

Through these kinds of surveys, we found out that penguins do not feed while they are diving. They start to feed upon reaching a certain depth, but do not chase after their prey at high speed in order to conserve energy. Instead, they prey on something that comes across them occasionally. Moreover, they sometimes slow down to feed, but start swimming again right after the feeding. It is becoming clear that they repeat this series of movements.

It seems that penguins and seals have an ecology that is in harmony with their prey, while taking advantage of the environmental constraints of the Antarctic and Arctic Circles. As shown in Photo 02, penguins form groups. Why do they need to form groups in the first place? How is each individual related to its group? There are still many unknown areas. As the majority of wildlife lives in groups, we need to protect not only individuals but also the groups themselves. Specifically, there is a need to study the relations among individuals, groups, and the environment.

We are now keeping track of 200-300 subjects, each of which is attached with recorders, in order to study both individual and group actions. We can trace basic actions such as lying, standing, sliding, and walking second by second (please see Fig. 03). With these data available, it may be possible to shed light on the logic of an individual, mutual relations within a group, and how each individual and group is related to its surrounding environment.

A study that once only measured time and depth has made a dramatic leap forward within the past 20 years, and it is now necessary to analyze the complicated data in a multi-lateral manner. We therefore need to address a variety of studies in the future in collaboration with researchers of information science.