Sharks : As Animals and Individuals

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Sharks : As Animals and Individuals

March 23, 2016 - 05:08
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The ancestors of modern sharks were already roaming the oceans 455 million years ago, when green plants were first transforming the land. And now, their presence in the form of more than 470 species split across 14 orders, is woven throughout the marine environment. Sharks are considered vital to oceanic ecology, yet humanity has branded them as mindless killers, and is hunting them into extinction. Yet no one knows what they are really like.

A difficulty in obtaining information about the natural behaviour of wild animals is that detailed observations of the activities of different individuals is necessary over long periods of time, and this is especially hard to achieve with sharks. In today's rapid pace of science, it’s far easier to place tags on them. There are excellent advantages in accomplishing studies of numerous animals through tagging, and the popularity of that method is without question, especially in fisheries. Yet, tagging sharks cannot explain, except in the broadest sense, why they do what they do. And individual animals often don't do what others around them do.

In the shallow fringe lagoons of French Polynesia, it was possible to observe individual sharks for long periods without the encumbrance of scuba gear, and without the problem of the shark disappearing into the depths. Though mainstream science considered them to be practically mindless, they did not behave as if they were operating on instinct alone, so I studied them intensively for seven years to learn what they were like as animals and individuals. The blacktip reef shark was most common, and in time I identified 600 individuals, and could recognize about 300 on sight.

Female domain

The lagoons were the domain of the female blacktip sharks, each of whom spent much of her time in a favoured region called a home range. Young sharks, too, matured in those sheltered waters. Pups were born alive, in shallow areas on the barrier reef that were used as nurseries, and after about six weeks, they began to move away, remaining in hiding in safe refuges of thick coral until they were big enough, at the age of between two and three years, to mix with the general population. Many, at this time, began to travel widely prior to settling down just before maturity, in a home range.

Most of the males lived in the ocean on the outer slope of the barrier reef, yet a very few remained in the lagoon. The males seemed less attached to a home range and travelled much farther afield during their daily circuits. In the mating season, bands of males from the ocean would arrive in the lagoon at nightfall, to the excitement of the females. Male blacktips were smaller than the females, lithe and muscular, while the females were maternally heavy with a more rounded silhouette.

Large differences

There were large individual differences among the sharks, and they varied greatly in their responses to different situations. Each shark had a unique pattern of roaming. Some were nearly always present in their home ranges, while others travelled for months at a time. Individual sharks demonstrated different rates of learning, and their behaviour showed a flexible intelligence.

They remembered the events in their lives, and responded flexibly and appropriately to circumstances unfolding around them. This ability to refer mentally to things that happened in another place and time, is thought by many researchers to indicate consciousness. They were curious, but cautious, and learned quickly. The speed with which they would suddenly move underlined their high level of alertness, and capacity for swift decisions and actions.

Sometimes a shark passed the same coral formation at almost exactly the same time each evening for several nights in a row, then disappeared from the area for a year. Sometimes, day after day, a shark could be found in exactly the same place in her home range at a particular time, then the following day she was hunting on the outer slope of the reef at that time. There were some days that all the females left the study area, and others when they were circling excitedly with visiting neighbours.

Preferred companions.

When the sharks left their home ranges, they often travelled with preferred companions. These sharks knew each other as individuals, and had preferred companions with whom they liked to travel, of the same gender and age. Some companionships were so strong that I never found one shark without the other. Others separated on occasion and travelled with different companions for a time, reappearing months later together again, while some sharks always travelled alone.

Socializing was important to them. Resident sharks would join the travellers moving through their ranges. They would follow each other around and swim side by side for long periods. At such times, they became excited and enjoyed soaring fast through the area together, often touching. An old lady shark who normally never accelerated would suddenly shoot vertical, shake off her remora, and streak away out of sight so fast that the eye could scarcely follow her. Then she would rocket through the scene again, with many others soaring with her, out of view in the opposite direction.

They tended to roam with the lunar cycle, and the thrilled shark tornadoes that formed as the full moon rose, when visiting sharks were present, were incredibly dramatic. Yet though the sharks formed companionships, they associated freely together, and I never saw them fighting; they seemed to have companions, but no enemies.

The blacktip reef sharks tended to roam in circular or oval pathways of various diameters, which crossed at the centre and formed rough figure-eights or cloverleafs. Such patterns seemed to facilitate them coming repeatedly into contact with each others' scent trails, allowing them to keep track of each other, though they were out of sight of each other much of the time.

On occasions in which I was travelling with a shark, she would at times catch up to another individual, and swim nose-to-tail with her, or side by side. Then, after resuming her arcing trajectory for a time, she would catch up to another, different shark and briefly swim with her. Had she been targeting the other shark through the vibrations she made, it is unlikely that she would have approached her each time from behind.

The sharks were used to perceiving things from beyond visual range, and were highly sensitive to when they were visible, and when they were not. In many ways, they used this awareness to their advantage. Their tendency to follow things of interest, and watch and wait while remaining out of view, was basic to their behaviour.

(c) Ila France Porcher
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