Requiem Threats: agonistic displays
in the Grey Reef and other sharks
The deep sense of peace I usually feel while diving was not there. November 1982: I am 27 m (90 feet) below the surface on a ledge at the northern tip of Osprey Reef – some 230 km (140 miles) off the Queensland coast. And I am about to ‘attack’ one of the fastest and best-armed creatures that swim.
Feeling the need for reassurance, I concentrate on the heft of my DPV (Diver Propulsion Vehicle); it doesn’t seem nearly as solid as it did on deck. The time is 11:01. Any minute now, the big female Silvertip Shark (Carcharhinus albimarginatus) I know as “Notchfin” will be cruising by this spot, heading south at a leisurely 1.5 knots – just as I have seen her do a half-dozen times before. Although it is obscenely hot in the glaring sun topside, down here it is surprisingly chill; I begin to shiver.
11:03 – right on schedule – a flicker of movement in the diffuse half-light betrays her approach. With terrifying suddenness, the spectral blur resolves into crisp focus, a deep V-shaped cut on the forward edge of the first dorsal confirming her identity. Notchfin is about 1.7 m long, probably not yet mature; except for her injured fin, her lines are clean with no signs of mating scars. She moves with the liquid grace of a creature that is perfectly comfortable with its role in the universe. Her presence is nothing short of electric. My awareness of the viscous cold evaporates and for a moment we are suspended in an incredible blue nothingness. She is a marvel: sleek and solid and – I remind myself – potentially lethal.
This realization snaps the spell. Less than 15 m (50 feet) of water separate us; my breathing seems awfully loud. With exaggerated precision, I raise the DPV and point it toward the oncoming shark. How will this wild animal react to my charging toward it? Swallowing dryly, I tighten my grip on the DPV and flick the switch to “ON” …
Conventional wisdom holds that sharks are “unpredictable”. However, a certain type of Indo-Pacific shark has long been known to consistently signal its readiness to attack by a complex rolling display. When persistently approached by divers, the Grey Reef Shark (Carcharhinus amblyrhynchos) raises its snout, depresses its pectoral fins, arches its back, and flexes and holds its tail sideways. This posture is often accompanied by laterally exaggerated swimming in a yawing “figure-eight” pattern. This display increases in intensity when the animal’s escape route is restricted. If a diver continues to approach, this shark launches a lightning-fast, slashing attack followed by a rapid retreat. From the results of their classic study in Micronesia, researchers Richard H. Johnson and Donald R. Nelson concluded that the display of the Grey Reef Shark “appeared ritualized in nature and is likely to be of value in normal social encounters”. Every time either researcher approached a Grey Reef Shark, the same display was elicited. Here we have a case where at least one species of shark, when presented with a specific (admittedly artificial) situation, reacts predictably.
As a diving naturalist particularly interested in the behavior of sharks, I just had to see this display for myself. Since we had the very same species of shark off the Queensland coast, I decided to attempt to provoke one of ‘our’ Grey Reef Sharks into displaying, following the methods of Johnson and Nelson. Much to my disappointment, our Australian Grey Reefs didn’t display – they merely dropped their pectoral fins and accelerated away for a distance of about 12 m (40 feet), then proceeded on their aristocratic way like a James Bond martini: shaken, not stirred.
Undaunted, I decided to try the same technique on another, closely related species of shark. I knew when and where to intercept a certain individual Silvertip Shark, identifiable by her damaged first dorsal fin. But I made one modification: the addition of a DPV. Since no one could have told me what to expect, I wanted something solid between me and the shark. Having stationed myself at a strategic site along Notchfin’s route, I planned to wait motionless until she approached to within about 15 m (50 feet). I decided to use the DPV only to pursue the test subject and to switch it “OFF” as soon as the animal began to display.
Notchfin didn’t seem to notice my motorized approach until I was about four m (13 feet) away. Her immediate reaction was disappointingly familiar: she dropped her pectoral fins and accelerated away until she was about 20 m (60 feet) distant, then assumed her former cruising speed. I changed direction and charged after her again. When the approach distance was 3.5 m (11.5 feet) – about two body lengths – Notchfin dropped her pectoral fins, darted some five or sic body lengths away … and then came charging toward me with her pectorals strongly depressed. As my mind snapped violently into Dreamtime, I remember thinking – quite calmly – “This is it, mate: you’re about to snuff it.”
Then a very curious thing happened. When she got within about two body lengths, Notchfin turned broadside to me and greatly slowed her forward movement. The posterior two-thirds of her body were lowered and held tense (in what I interpreted as a threat posture, akin to an arch-backed Halloween cat) and began to “shiver”. Her form was reduced to a blue-grey blur; the white apices of pectoral, first dorsal, and upper caudal fins (which inspired the vernacular name for her species) fairly glowed in the darkling light. She continued to display like this until her mid-body had more-or-less passed by me (Perhaps when I was within her “blind spot”?). She then straightened her body and shot away with her pectoral fins still strongly depressed …
When my adrenaline levels subsided enough to afford something resembling rational thought, several points about the interaction struck me as particularly interesting. For openers, there was Notchfin’s apparent reluctance to confront or challenge me. Of course, her repertoire of instinctive and learned responses probably included precious little about “What to do if attacked by a crazy Banana-bender driving a DPV”, so my charge must have had a certain “shock value”. But what I found most intriguing was that Notchfin showed little or no signs of being threatened until I approached within two body lengths’ distance and did not bring into play the “shiver” element of the display until she approached within two body lengths. It seemed as if the shark was punctuating the fact that a clearly defined (in her mind, at least) boundary had been violated.
Over a 10-year period, I had been able to elicit this display in 8 out of 28 trials, involving at least five individual Silvertips. This strongly suggests that Notchfin’s display was typical of Silvertip Sharks when threatened in this manner. The threat display of the Silvertip Shark shares certain features with that of the Grey Reef Sharks studied by Johnson and Nelson, yet includes some totally different elements. Pectoral fin depression, flank displaying, and an overall increase in muscular tension are common to the displays of both species, but vertical depression of the posterior portion of the body and the “shiver” are unique to the Silvertip threat displays. I have since expanded my study of shark threat displays to 25 species of sharks, representing a total of eight families. Each shark species has a display with a unique combination of features, often including at least one element not exhibited by other species. However, for all their variety, the threat displays of all sharks studied to date feature: sustained pectoral fin depression, exaggerated swimming movements, an overall increase in muscular tension, and are terminated by rapid withdrawal.
Animals have evolved many complex and subtle ways to communicate with one another, including specific scents, patterns of sounds, colors and markings. Often, these features are accentuated by combining them with behavioral elements to more clearly communicate such things as species identity, readiness to play, fight, or mate. A “display” may be defined as a stereotyped behavior, or series of behaviors, modified by evolution to communicate. Displays are often derived from non-communicative behaviors whose elements are exaggerated, slowed, and otherwise modified (“ritualized”) until they take on a new meaning. Most displays are species-specific and concerned with reproductive or agonistic behaviors. “Agonism” is a convenient blanket term for any behavior arising in competitive contexts. The term embraces a broad spectrum of behaviors, including space and distance regulation, displacements, defensive threats, escape, submissive gestures, and self-assertive behavior.
What are displaying sharks trying to communicate? We may never know for sure, but a few general principles can be deduced. Since most – if not all – displays known to date occur in approach-withdrawal conflict situations, they may properly be termed agonistic in nature. Despite Tennyson’s colorful description of Nature “red in tooth and claw”, actual physical combat is rare among animals. If the most basic and powerful drives among living things are to survive and reproduce, it doesn’t make sense for an individual to risk a tussle which could result in injury and thus interfere with its ability to feed and/or potential for representing its genes in future generations. This is why so many animals posture and display toward one another in competitive situations: the idea is to bluff one’s opponent into backing down without having to engage in actual fighting.
Johnson and Nelson stated that the stereotyped displays they observed in Grey Reef Sharks tended to occur in situations unfamiliar to the shark. The exaggerated, ritualistic nature of these displays marks them as defensive rather than competitive or aggressive. Therefore, a rapidly approaching diver is probably perceived by a shark as a potential threat. Behaviorist George Barlow has suggested that the agonistic displays of sharks may represent a kind of motivational conflict, or “indecision”, between fleeing (escape motivation) and attacking (defense motivation). In my experience, sharks are basically “path of least resistance” types: given the opportunity to flee rather than fight, most sharks will simply swim away.
Scuba has proven itself to be a powerful tool for investigating the behavior of sharks in the context of their natural environment. It is becoming increasingly clear that sharks are far from the unpredictable, socially unsophisticated creatures we had once mistaken them for. We now know that sharks typically adhere to well defined “home ranges” and that their daily and seasonal movements are highly regular and predictable. Although they have a strong sense of home turf, individual sharks will tolerate other sharks – including members of the same species – swimming, and even feeding, within their home range. Thus, sharks cannot be said to be “territorial” – that is, defending a specific geographical area for their exclusive use.
My own research has also revealed that sharks, like people, have a sense of “personal space” (sometimes termed idiosphere*) with well-defined boundaries – a kind of moving territory, which they definitely will defend. This personal space is roughly spherical in shape and its radius is directly proportional to the size of the animal. My work on reef-dwelling whaler sharks (genus Carcharhinus) has borne out a pattern of a critical approach distance of about two body lengths.
This length proportional defended volume strongly supports the idea that sharks have a good awareness of their own body size. While observing sharks under baited conditions, I and other workers, have noticed a definite size-related “pecking order”. In our own species, the personal space defended by individuals is also related to social rank, with higher ranking humans having larger personal spaces. As social vertebrates, we may have more in common with sharks than we realize.