Breaching (marine mammals)

 

 

Breaches are leaps from the water. The breach of a large , – whale is almost certainly the most powerful action peri formed by any animal; that of a dolphin rising many body lengths above the surface is one of the most breathtaking, but breaching is not an immediately functional activity for an aquatic animal. Observers of cetaceans have long pondered the breach, and science has made some inroads into our ignorance of breaching, but important questions remain open, including, “Why breach?”

I. What Is a Breach?

A breach may be defined as an intentional jump from the water in which at least 40% of the animal’s body emerges. It is distinct from “lunging” (often the result of feeding at the surface) when less than 40% emerges and from “porpoising” when small cetaceans (and other marine mammals) reduce drag while swimming fast close to the surface by making horizontal leaps. In breaching, emergence from the water seems an intent rather than a result of another activity. While breaching is used for all cetaceans, the term leaping is more common for dolphins and porpoises.

When breaching, sperm whales (Physeter macrocephalus) and dolphins approach the surface vertically from depth. Other animals swimming in water less than a few body lengths deep, e.g., humpback and right whales (Megaptera novaeangliae and Eubalaena spp.), make a horizontal approach to the breach, gaining speed until, at the last moment, they raise their heads and flukes, pivoting on their flippers so converting horizontal momentum into vertical motion, and thus rising through the surface. To make a full breach, a humpback whale must break the surface at about 15 knots (~8 m/sec), close to its maximum speed. It is likely that some of the most spectacular breaches of other species also represent the full power of the animal.

After breaking the surface, whales and dolphins have many styles of breaches. In the classic breach of a large whale (such as a humpback or right) the animal emerges from the water at about 20-30° from the vertical, twisting so as to land on its back or side, having shown about 90% of its body above the surface at peak emergence (Fig. 1). However, about 20% of the breaches of sperm and humpback whales are “belly flops,” with the animal landing ventrally (Fig. 2), and smaller cetaceans make what are sometimes called “clean-entry leaps” when, after leaping, the animal returns to the water smoothly, beak first (Fig. 3). Young whales and dolphins will frequently completely clear the water with their breaches (Fig. 3), sometimes by several body lengths. Some dolphin breaches, such as the spinning breaches, which give the spinner dolphin (Stenetta longirostris) its name, are remarkably elaborate. Breaching animals produce large splashes upon reentry into the water, which can be visible at many kilometers.

Breaches are often performed in bouts. Extreme is 130 breaches in 75 min, probably all performed by the same humpback whale on Silver Bank, West Indies. As such sequences progress, animals tend to show less and less of their bodies, visibly appearing fatigued.

II. Which Whales and Dolphins Breach?

Quantitative breaching rates are only available for a few species and are usually not comparable. However, it is clear that there are substantial differences between species in the rates of this and other forms of aerial activity. Frequent breachers include the humpback, right, and sperm whales, as well as virtually all offshore dolphins. In contrast, balaenopterids (blue, Bal-aenoptera musculus; fin, B. physalus; sei, B. borealis; Bryde’s, B. edeni; minke, B. acutorostrata, B. bonaerensis), most beaked whales (except the northern bottlenose, Htjperoodon ampulla-tus), porpoises, and river dolphins breach much more rarely.

Breaching rates, then, are not related to size and, at least in the large whales, are inversely related to speed—stouter, slower animals tend to breach more. Instead, interspecifically the best correlate of breaching rate is sociality. Animals found in larger groups, and for whom social structure seems more important, breach more frequently.

Sperm whale breaching off the Galapagos Islands.

Figure 1 Sperm whale breaching off the Galapagos Islands.

Humpback whale belly flopping on Silver Bank, West Indies.

Figure 2 Humpback whale belly flopping on Silver Bank, West Indies.

III. When Do Whales Breach?

The circumstances in which breaches occur can provide important clues as to their purpose or function. In some species, different segments of the population breach more frequently than others. For instance, calves of many species breach more frequently than adults. In sperm whales, the gregarious females breach more often than the much larger, and more solitary, males.

Hector's dolphins (Cephalorhynchus hectori) breaching off South Island, Neiv Zealand.

Figure 3 Hector’s dolphins (Cephalorhynchus hectori) breaching off South Island, Neiv Zealand.

Most baleen whales have pronounced seasonal cycles, feeding in winter at high latitudes and breeding in winter nearer the equator. Humpbacks in the western North Atlantic breach about seven times more frequently on their winter breeding grounds in the West Indies than when feeding off Newfoundland.

In a number of species (including humpback, right, and sperm whales) breaching is observed more frequently when groups are merging or splitting. Male humpback whales may breach when they stop singing on the Hawaiian breeding grounds. Breaching is often observed together with lobtailing and/or flippering, with different animals performing different activities at the same time, or one animal switelling between the different activities. Studies of humpback and right whales have also shown that breaches by one animal seem to trigger breaches by neighbors. On Silver Bank, breaching humpbacks form clusters about 10 km across. One of the most interesting, and in some ways unexpected, findings that has emerged from several studies of different species is that breaching rates of large whales increase with wind speed. There is currently no generally accepted explanation for this widespread pattern.

IV. When Do Dolphins Breach?

Variations in the breaching rates of dolphins seem less consistent across species. For instance, spinner dolphins off Hawaii have maximum breaching rates before a foraging hunt; in dusky dolphins (Lagenorhynchus obscurtis) off Argentina and Peale’s dolphins (Lagenorhynchus australis) off Chile, rates increase during hunts, peaking afterward: bottlenose dolphins (Tursiops spp.) off South Africa also show peak breaching rates after the hunt, but bottlenose dolphins around Coco Island, Costa Rica, breach at roughly the same frequency in all behavioral contexts.

One of the most detailed studies of breaching in dolphins is that of K. S. Norris and coworkers on the very aerobatic spinner dolphins off Hawaii. In this species, breaching was closely related to diurnal activity patterns, being mostly seen from fast-moving active schools in the afternoon.

V. Why Breach?

Few animal activities can have been ascribed so many, and so varied, functions as the breaches of whales and dolphins. Some suggested functions can now be discarded, including breaches resulting from animals being chased by swordfish (Xiphius gladius) or being accidental when animals travel too fast near the surface. However, the available scientific results are consistent with quite a range of other potential reasons for breaching.

A number of authors have suggested that the breach may help cetaceans feed by scaring, stunning, herding, or trapping fish or other prey. Although many, probably the majority, of breaches of most species occur in nonfeeding circumstances, the closely related activity of lobtailing is known to sometimes assist feeding, so a direct benefit to food capture cannot be ruled out as a function for some breaches, perhaps especially the “clean-entry leaps” of dolphins.

Similarly, we cannot completely rule out some rather prosaic potential benefits of breaching such as stretching, looking above the water, or inhaling water-free air in rough weather. However, these too are unlikely to be important functions of most breaches.

One proposed benefit that is more consistent with at least some of the evidence is ectoparasite removal. Among the baleen whales, the more heavily infested species tend to be the most frequent breachers, and in Hawaiian spinner dolphins, 44% of breachers had remoras attached. However, spinners without remoras also breached, and much of the circumstantial evidence points to a very different function—communication.

As noted earlier, the more social species of cetacean breach more and, in most studies, breaching occurs most often when socially important activities are occurring, such as during changes in group composition or group activity. As sociality is based on communication between members of the same species, the strong inference from these results, then, is that breaching is a form of communication.

However, there is a paradox in that while breaches are excellent at conveying information to visually based human observers above the surface, they are far less prominent for the potential or actual social companions of the breacher. Other whales and dolphins cannot generally see the breachers body arcing above the surface, and while the reentry makes a noticeable underwater sound, it seems to be less loud than the natural vocalizations of the animals. The paradox may be resolved by considering the theory of “honest signaling”: signals are especially useful in animal communication if they convey some important attribute of the signaler that cannot be faked. The distinctive underwater sound or bubble pattern produced by a full breach, while not especially prominent in its own right, is an honest signal of the physical abilities of the breacher (which often seems to leave the surface at close to its maximum speed) and its desire to communicate this by using a significant amount of energy (about 1% of a humpback whale’s estimated daily resting metabolic expenditure per breach).

Thus the breach may be a useful signal to nearby potential or actual social companions. What might it be signaling? Suggestions for large whales include aggression, “extreme annoyance” (perhaps with a nearby vessel), an “act of defiance,” courtship, or a display of strength by males. Some scientists have suggested that a breach may be used to add emphasis to some other signal, perhaps a vocalization or visual display. By showing the extent of its physical prowess, and expending a significant amount of energy, the whale accentuates the importance of a companion signal. For dolphins, breaches have mainly been considered signals concerning schooling. For instance, it has been suggested that breaches may be used to define the deployment of a school, to recruit dolphins to a cooperative feeding event, or as social facilitators that reaffirm social bonds.

Finally, there is play. This is probably the most commonly attributed function of breaching by the general public, but it is also seriously considered by scientists who recognize play as a valid, but hard to define, type of behavior. Biological definitions of play usually focus on its lack of immediate biological function, resulting in play becoming a “garbage can” into which activities without an obvious function, such as breaching, get placed in a haphazard fashion. It is likely that many breaches described by human observers as “playful” actually function as important signals. However, it is also likely that some breaches, especially those by young animals, provide no immediate benefit, instead, like other forms of play, helping equip the breacher with abilities that may be beneficial in later life.

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