Cetacean Protection Behaviors by Sean Chamberlin
While cetaceans have few predators, humans, sharks and other cetaceans (i.e. killer whales) represent a threat, especially to the young. While cetaceans may rely on speed, stealth or physical strength to avoid or confront predators, some exhibit group behaviors designed to thwart or discourage predators.
Behaviors to avoid predators typically involve detection, which may be visual or acoustic. Many cetaceans are known for spyhopping, a behavior where they raise their head out of the water and look around. Cetaceans have been shown to have excellent vision above water as well as below, a trait often exemplified by their skills with balls and other objects in captivity. Spyhopping has been suggested to function for prey finding, surveillance (danger avoidance) and orientation (e.g. Tyack 2000; after Mobley and Helwig 1990). In noisy environments (i.e., when waves, rain and wind create surface disturbances), spyhopping may assume greater importance for detection. Passive detection of sounds must certainly play some role in predator avoidance, as evidenced in the stealth hunting of transient killer whales. Underwater playbacks of recorded killer whale sounds cause gray whales to alter their path. Many cetaceans, including sperm whales, belugas, narwhals and various species of dolphins and porpoises have been observed fleeing from killer whales. In many cases, the fleeing individuals remained silent, suggesting that they were avoiding detection or tracking by the predator. Dolphins may avoid certain species of sharks, especially great whites, bulls and hammerheads. Attempts have been made to use acoustic signals on tuna gillfishing nets to discourage dolphins from coming near the boats but these techniques, while useful for small cetaceans, have limitations for large ones (e.g. Stone et al., 2000; Cox et al., 2004). Human-produced sounds do not appear to alter the behavior of whales, as the frequent collisions of ships with whales demonstrates. Thus, the type of sound and perhaps other cues may mitigate the particular response observed to avoid predation or danger.
Some cetaceans employ concealment to avoid predation. Dusky dolphins have been observed hugging the shore near the surf zone, perhaps to use the sound of breaking waves to mask their movements (e.g., Wells et al. 1999, in Reynolds and Rommel). Dwarf and pygmy sperm whales have been observed to defecate and disperse the resultant cloud with their flukes in response to great white sharks. Gray whales may attempt to hide in kelp beds when killer whales are nearby (e.g. Tyack 2002). Young dolphins have been observed to herd next to a research vessel while the adults gave chase to sharks (e.g. Heithaus 2001). Given that there are few opportunities for concealment in an aquatic environment, this method of predator avoidance is likely not a primary behavior.
Physical confrontation and combat with predators occurs when detection or avoidance efforts fail. Early whalers were well aware of the strong bond between mother and calf and often exploited this relationship to their gain. By injuring the calf, they were assured that the mother would try to protect it, thus offering a larger prey and a greater reward in whale oil. Modern observations of gray whales, as noted above, confirm the attempts of adult females to use their body and other physical means to protect their calf. Combat between sharks and dolphins gained popularity in the 1960s television series, Flipper, but observations of dolphins attacking sharks may be the stuff of fiction. Both direct and indirect observations support predation of sharks on juvenile dolphins, as evidence by shark bite scars. Bite marks on adult females in Shark Bay, Australia, suggest that mothers attempt to protect their young. Bottlenose dolphins have been observed to chase away a tiger shark following a successful kill on a young calf (Mann and Barnett 1999). Analyses of stomach contents of sharks provide direct evidence that several species of sharks, including white sharks, tiger sharks, bull sharks, sixgill sharks and sevengill sharks, are frequent predators on odontocetes (Heithaus 2001). A variety of other sharks may be occasional or scavenging predators. Though not fatal, the cookie-cutter shark, with its suction lips and cookie-cutter teeth, will take a plug of flesh from unsuspecting cetaceans. Captive sharks have been trained to repel non-predatory sharks but refused to obey commands when a bull shark was introduced. This behavior suggests that direct repulsion of predators may be a last resort.
The most successful form of predator avoidance involves group behaviors. Group formation—the aggregation of multiple individuals in a particular region—may be partly an evolutionary response to predation. The fission-fusion societies of whales and dolphins—the breaking apart and coming together of multiple groups—may occur, in some instances, to reduce risk from predators. Bottlenose dolphins in Sarasota Bay, Florida, may form groups in response to bull sharks (e.g., Heithaus 2001). Free-ranging open ocean cetaceans, like the Atlantic spotted dolphin, may benefit from traveling in groups, as this species is often seen in clusters of up to 100 individuals. As with fishes, formation of groups or schools may overwhelm predators, cause confusion or reduce the risk of a given individual. In cetaceans, group formation may provide a ready means to act cooperatively if and when danger occurs. The most direct observation of group deterrence of predators comes from sperm whales. In response to harpooning or the presence of predators, sperm whales form what has been termed the marguerite formation, the circular arrangement of individuals into a formation that resembles a marguerite flower. In this formation, injured or vulnerable individuals, like young sperm whales, are placed at the center and surrounded head-on by several other larger adult sperm whales (Wells et al. 1999). Group protection may also offer refuge for females giving birth or in caretaking of sick or injured individuals, a behavior known as epimeletic behavior. Most likely, group formation serves a number of functions in addition to protection. The closely knit killer whale pods that inhabit Puget Sound attest to group behavior that serves a function besides protection, as these animals are not known to have any predators.
Whatever the ultimate function, group behavior in cetaceans is often accompanied by specific acoustic signals. These signals identify members of specific group ands likely function to maintain group integrity and cohesion. The best known example comes from multi-decadal studies of resident populations of killer whales in British Columbia and Washington. These animals exhibit well-defined but dynamic social structures comprised of social units ranging from mother and offspring to entire communities. Scientists who study killer whales define at least four important social units with killer whale societies:
- matriline—individuals linked by descent from a common mother, i.e. male and female offspring and/or grandsons and granddaughters
- pod—a group of related matrilines, anywhere from 5-50 individuals
- clan—related pods, defined on the basis of shared vocalization patterns; separate clans do not share calls
- community—groups of pods found within a geographic range and which rarely interact
The matriline is the heart of the killer whale community. Offspring never leave their matriline although matrilines will mingle as a pod and males from one pod will breed with females from another pod. However, individuals in different communities have never been observed to breed. Unlike most mammals where male offspring disperse and form new groups, male killer whales remain with their community their entire life.
A good example of the ways in which behavior and communication act to maintain the social structure of killer whale societies is provided by the two communities—dubbed northern and southern—that reside in the coastal waters of the Pacific Northwest. The northern resident community inhabits a region from the northern half of Vancouver Island to southeastern Alaska. It consists of three clans, 16 pods and 34 matrilines among least 216 individuals. The southern resident community ranges primarily from southern Vancouver Island to southern Puget Sound. This community is composed of at least 83 individuals (as of October 2003) in 19 matrilines distributed among three pods in one clan. Each clan, pod and matriline are designated by letters and/or numbers in a manner probably only understood by the scientists who name them (but see Ford et al., 2000 for a detailed explanation).
Like most toothed whales, killer whales produce a number of vocalizations that are best described as whistles, pulsed calls, squeals, squawks and screams, among others. These acoustic signals occur in addition to and at times simultaneously with the buzzing (rapid series of clicks) used in echolocation. The pulsed vocalizations of killer whales are quite distinct and can be separated into what are known as discrete calls, vocalizations that sound the same every time they are produced. On average, resident populations produce about a dozen or so discrete calls. These discrete calls form what is known as a dialect. Pods represent the basic social unit of a particular dialect; clans may share related dialects, i.e., their discrete calls are similar but not identical. Separate clans exhibit no similarity in dialects. Thus, given that there are four clans among the two Pacific Northwest communities, there are at least four distinct dialects “spoken” by these whales.
Interpretation of the meaning and purpose of discrete calls and dialects is largely speculative but scientists hypothesize that these vocalizations provide a means for genetically related groups to identify each other and maintain contact. In the grand scheme of evolution, there are advantages to helping related individuals as their genomes most closely match your own. By promoting the survival of your kin, you increase the change of survival of your most closely related genes, a concept known as kin selection. It is believed that dialects are learned by calves from their mother during their early development. Related dialects imply similar lineages. However, analysis of dialects suggests that learning may take place between different matrilines in a form of cultural transmission. Thus, the evolution of this behavior and its bearing on the social structure of killer whales may not be completely straightforward (e.g. Deecke et al., 2000). Continuing studies of killer whale societies, their vocalizations and their culture may provide important insights into the evolutionary development of other society-forming animals, including primates and humans.
Note: Sperm whales also share vocalizations known as codas to maintain groups of individuals. However, unlike killer whales, sperm whale groups may be unrelated and temporary. While matrilines maintain some associations, males disperse from their natal groups. These behaviors have different implications for the transmission, sharing and modification of codas over the lifetime of an individual. For an excellent review, see Rendell and Whitehead 2004.