On the Ethology of Reef Sharks
Tags & Taxonomy
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. This is especially hard to achieve with sharks.
Fortunately, in the wide lagoon near my home in French Polynesia, which averages a depth of two meters, it was possible to observe the sharks easily for long periods and collect a large amount of data on their movements and behaviour. The blackfin reef shark, (Carcharhinus melanopterus) was especially easy to observe. Over the years, I identified six hundred individuals and could recognize three hundred on sight.
Ethology is a field of zoology, and one of its major principles is to “know your animal” by observing its behaviour over an extended period of time. Such observations take so long in today’s rapid pace of science that it’s far easier to place tags on them.
There are excellent advantages in accomplishing studies of numerous animals by such means, and the field of behavioural science can well use such information. Its popularity is without question, but it cannot often explain, except in the broadest sense, why animals do what they do, since individuals often don’t do what others around them do. That’s why the work of ethologists will always be valuable.
Following the ideas of Konrad Lorenz, Nikolaas Tinbergen, Arthur Myrberg, Donald Griffin, and others, I identified the sharks by drawing the markings on both sides of the dorsal fin, and found that observing them as individuals revealed a whole new dimension of behaviour. One of the most surprising things was the degree and variety of individual differences that they displayed.
Each shark’s behaviour was unique and seemed flexible from day to day. Not only was their behaviour towards me unique, but so was their use of time and their patterns of roaming.
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 the same time, then the following day she was roaming 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 it was filled with visiting neighbors.
Once a week I brought them some scraps from local fishers so that I could identify the sharks in the area that evening, and I visited at random times to observe them and to accompany them when possible while they roamed. Eventually, they became so used to my presence that they treated me as they would another shark. (One cannot expect an animal to treat one in a brand new way because one is a different species). Thus I was able to witness their intimate social and emotional behaviour as if no one was watching them. For all the years I knew them, I kept seeing new behaviour patterns, some which would have been unbelievable to me in the beginning.
The study area was seldom visited by other people, so observations could be obtained without disturbance, and the sharks were relatively unaffected by other human encounters.
The lagoon was the domain of the female blackfin reef sharks, each of whom spent much of her time in a favoured region called a home range; the pups were born and matured in those sheltered waters. Beside the resident females, others whose home ranges were farther away came from time to time, and there were many juveniles.
The youngest were two to three years old, since they remain in hiding until they grow enough to mix with the rest of the community. While most of the males lived in the ocean on the outer slope of the barrier reef, a very few ranged through the lagoon. These male sharks were smaller than the females, lithe and muscular, most being less than four feet long. They seemed less attached to a home range and roamed much farther afield during their daily travels.
In the mating season, bands of males from the ocean would arrive at nightfall, to the excitement of the females. The largest females were close to six feet in length, and maternally heavy with a more rounded silhouette.
The residents of the study area soon recognized the sound of my kayak crossing into the lagoon from the adjacent bay, and anchoring at my usual place, and when I slid underwater, they were waiting, no matter what time I came. But the one I had dubbed “Bratworst” would always arrive two minutes after me.
This began to puzzle me so much that I decided to investigate and anchored and slid underwater without a pause on my next visit. Not a shark was visible. So I drifted silently with the current and found Bratworst lurking behind a coral, just beyond visual range of my boat. It was my first clue that sharks listen from beyond the visual range, and come when they decide to.
I had learned years before, when first observing them and learning to move swiftly and silently, that sharks can hear a person swimming long before he or she comes within visual range. The shark usually vanishes before the person sees that it is there, but the possibility that members of such an ancient line of animals could use the limit of visibility as a veil behind which to conceal themselves, was new.
I often observed sharks following each other beyond the visual range. The shark I was with would catch up to the one she was following, and swim nose to tail with her or side by side.
Then, after resuming her arcing path for a time, she often caught up to another shark and briefly swam beside her. The sharks generally roamed in circular or oval paths of various diameters which crossed at the centre and formed rough figure of eights or cloverleafs. Such patterns likely brought them repeatedly into contact with each other’s scent trail, allowing them to keep track of each other while travelling in circling paths, out of visual range of each other much of the time .
Curious sharks also followed me, always remaining hidden behind the veiling light except for an occasional pass into view. Apparently they listened and used their lateral line sense to monitor my actions, instead of eye-sight. I began regularly checking to see who was following me by stopping and drifting with the current, whereon the sneaky sharks behind soon came into view.
These sharks knew each other as individuals, and it was soon clear that they had preferred companions with whom they liked to travel. Some companionships were so strong that I never found one shark without the other. Some friends separated on occasion and travelled with different companions for a time, reappearing months later together again, while some sharks always travelled alone.
At times groups of sharks, all from a particular region, and often accompanied by specific males, ...
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