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The Strange Case Of The Scarred Sirenians

By Doug Perrine

 

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Dugong Feeding
Dugong or Sea Cow, Dugong dugon, feeding on sea grass, Halophila ovalis, accompanied by Pilot Jacks, Gnathanodon speciosus, Western Australia Image #: 003806

Parents of the current crop of tattooed, pierced, and branded teenagers may take scant comfort in the realization that assisted self-mutilation has been practiced in various human cultures for thousands of years. In fact the practice may have arisen well before the appearance of humans on the planet.

Consider the beaked whales. These mysterious creatures are large enough to have few, if any, natural enemies. Most, or all, species are believed to feed by vacuuming up soft-bodied squid in the deep sea, a seemingly low-risk endeavor. Teeth are needed neither for feeding, nor for defense, so they have been mostly dispensed with through evolutionary atrophication. In spite of the lack of predators or strongly-resisting prey, the bodies of many of these whales are covered with scars. The accepted explanation is combat between males for mating rights, using tusks which generally erupt only in males.

Explaining the appearance of Risso’s dolphins, Grampus griseus, is equally challenging. Like some members of the Jackson clan, they are born dark and become progressively whiter as they age. In this case, however, the whitening is a result of the accumulation of scar tissue, rather than the other way around. Adults are often so completely covered by white scars that they appear as snowy white as belugas.

An informal survey of educational web sites concerned with cetaceans revealed that some ignored the scarring phenomenon altogether – one even going so far as to state that Rissos become darker with age. One mentioned the scars, but offered no explanation. Of the remainder, a couple attributed the scars to the hooked tentacles and beaks of the squid that form the greater part of the diet of G. griseus. A majority stated confidently that the scars were inflicted by fighting between members of their own species, with one refining that assessment to conclude that the scars are inflicted by the lower teeth of males.

Risso’s dolphins of both sexes have teeth (up to seven) in the lower jaw only, so it’s a sure bet that if intra-specific combat is to blame, it is the lower teeth that are used. In contrast, bottlenose and spotted dolphins appear to use their upper teeth, or both sets, to rake each other. Also, especially in bottlenose dolphins, it is mature males which are most heavily scarred. In Rissos, the scars occur on both sexes. Mature animals appear to have more scars merely as a result of steady accumulation over time. The few teeth that Risso’s dolphins do have are peg-like and often badly worn in older individuals, which is another curious thing, as you wouldn’t expect soft-bodied squid to cause much wear and tear on the teeth.

One might suppose that evenly-arrayed sucker claws would leave characteristic round scars, or that bite scars could be matched to beak sizes of preferred prey, but I’ve yet to see any analysis of this sort. To my untrained eye, the scars don’t appear to support the uncooperative lunch hypothesis. Older animals are so covered with scars and whitened that it is difficult to discern any pattern. However in younger darker animals, it can clearly be seen that many of the scars occur in parallel tracks that look very much like the rake marks on other species of dolphins. This lends some credence to the alternative hypothesis advanced by British biologist Colin MacLeod that, in his words, "unpigmented instraspecific scarring acts as an indicator of male ‘quality’ (dominance and/or fitness) in aggressive social interactions."

In other words, according to MacLeod, the scars on Risso’s dolphins (and 17 other species of toothed whales that he lists) serve a primarily decorative function. They make the scarred animals look "tough," which discourages less dominant animals from picking a fight with them, reducing the risk of unnecessary injury to both parties. He proposes that this is true in most of the toothed whales that feed on squid, and therefore do not need their teeth for feeding. In these species, including sperm whales, narwhals, and many of the beaked whales, the teeth have been modified to serve as weapons for fighting amongst their own kind.

Interestingly, in many of these species, scars do not regain normal skin pigment as they heal, and therefore remain visible and accumulate as the animals ages. Generally in marine mammals fresh scars are white, but pigment cells migrate into them, obliterating them over time. In bottlenose dolphins, which use their teeth primarily for feeding, scars inflicted by other dolphins usually fade in a period a little less or more than a year. In Risso’s dolphins, on the other hand, such scars did not re-pigment during the course of a five-year study. Just as members of some African tribes rub ashes into self-inflicted wounds to produce permanent decorative scars, some toothed whales may permanently retain scars for use in social signaling.

In Risso’s dolphins there is also the possibility that some of the wounds are inflicted by other cetaceans which Risso’s dolphins harass, in apparent acts of klepto-parasitism (stealing meals). I’ve observed Rissos mercilessly badgering false killer whales and pilot whales, probably in an attempt to force them to release or regurgitate recently-taken prey. If these larger and better-armed whales ever retaliate or forcibly defend themselves, again, you would expect patterned scars matching their tooth spacing. As I recall, in my intermediate school, the bully that was always stealing other kids’ lunch money did end up with a few more scars than most of us.

Cetacean specialist Todd Pusser, while unable to offer any definitive explanation of the scarring phenomenon, did tender the tantalizing tidbit that scarring patterns on Risso’s show a distinct correlation with latitude. In both the North Atlantic and the North Pacific, Grampus living close to the equator show little scarring, with the amount of scarring increasing in proportion to distance north of the equator.

However perplexing the uneven complexion of Risso’s dolphins may be, it is merely the most extreme case of a skin problem that is fairly widespread among the toothed whales. This is not the case, however, in the sirenia (sea cows). Scars on West Indian manatees are almost always directly attributable to interactions with motor craft, and not to natural causes. This is not too surprising in vegetarian animals that have only flat plant-grinding teeth, set far back in the mouth.

Dugongs are also plant-eaters, but a pair of short tusks erupts in mature males (and sometimes in very old females). Male dugongs also engage in much fiercer competition for females than do manatees. Therefore I should not have been too surprised when I first viewed an assemblage of dugongs, in Australia’s Shark Bay, to see that many had a considerable collection of long meandering scars decorating their backs, even though I had never seen this phenomenon mentioned in any of the papers I read on the species. As pointed out to me by Dr. Paul Anderson, one of the leading authorities on dugongs, many of the scars occur in parallel tracks, just about the same distance apart as the tusks on a male dugong. Still, there were perplexing questions. If the scars are inflicted by competing males, why did they appear to be nearly as prevalent on the backs of females as males, and why were they also present on juveniles?

A fisherman who had observed the courtship and mating activities of dugongs had an interesting idea. He suggested that scars are acquired when dugongs crash into the coral as a pack of males pursues a female in heat. This he had observed with his own eyes. There was considerable damage to the coral as well, he noted. This seemed entirely plausible, and could explain the presence of scratches on the backs of females, but not those on juveniles. It also fails to explain the presence of scars on dugongs that live in areas where there are few corals and where mating herds have never been observed.

After pondering, contemplating, and pondering some more (all useful ways to pass the time when bad weather, bad tides, or other circumstances make it impractical to get out looking for dugongs), I came up with a set of hypotheses, which I supposed to mutually exclusive: 1) The scars are inflicted by tusked males, as a display of dominance over subordinate animals, including rivals in courtship; 2) The scars result from inadvertently scraping against coral or other sharp objects during social interactions, including courtship; 3) The scars are self-inflicted when the animals scratch themselves against hard objects to relieve an itch. Being a realist, and with only a few weeks available to observe these enigmatic animals, should they choose to show themselves to me, I held little hope of determining which of these possibilities was the true explanation. Much to my astonishment I was able to gather evidence that all three hypotheses are correct.

To the dismay of Dr. Anderson, much of what is known about manatees is often assumed to also be true of dugongs. "Dugongs are not just salt-water manatees," he says, "[they are] about as alike as camels and giraffes." In terms of gross external morphology, dugongs are smaller and sleeker than manatees, have dolphin-like flukes rather than the round tail paddles of manatees, and lack nails on the flippers (present in some manatee species, but not others.) Unfortunately, while manatees have been relatively well studied, as a result of the U.S. Endangered Species Act, very little is known about dugongs. This is baffling and disturbing, given the wide distribution of dugongs throughout the Indo-Pacific region, and the probability that they are critically endangered in most parts of their range.

Necessity, therefore, dictates that what is known of manatees serve as a starting point for considering the behavior and natural history of dugongs, even though Anderson considers manatees to be "aberrant sirenians" comprising "an odd little side-branch that adapted to riverine environments." Manatees are considered "semi-social sirenians." They may travel in groups or alone. They aggregate, but this may often be a result of attraction to a food source or other environmental feature rather than to each other. They do interact with each other in seemingly affectionate ways, but the only long-term bonds that have been documented are between mother and calf.

What little is known about the social lives of dugongs suggest that there may be radical differences from manatees. Anderson found that male dugongs in Shark Bay stake out temporary courtship territories, which they defend against other males, during mating season. The territories contain little to no food, and females come there only to mate. This is a classic example of a lek, a reproductive strategy used by several types of terrestrial mammals, but unheard of in manatees. In other places, dense aggregations (for unknown purposes) of hundreds of dugongs have been photographed from the air, whereas manatee aggregations rarely exceed a few dozen, and then only in the presence of a strong environmental attractant, such as a warm water discharge from a power plant in winter.

My own limited observations have convinced me that dugongs are more social than manatees. They (at least sometimes) travel in herds, and engage in social interactions that are indicative of a social structure. On one occasion I was able to observe and photograph a male raking his tusks down the back of a presumably-subordinate animal. This was outside of the mating season, but could presumably occur in the context of sexual rivalry as well. Chalk up one point for hypothesis number one. In some mammals, such as lions, males are known to kill infants in order to bring their mothers back into heat and mate with them. A fisherman reported to Dr. Anderson that he had seen male dugongs drive calves away from their mothers. A calf could conceivably be scarred during such an attack, but be reunited with the mother and survive. Anderson also suggests that young animals could be scarred merely by getting in the way of tusked older males when the males interact with females and each other. Females could be scarred during attempts to manipulate them for mating.

On another occasion, I observed two animals come together in a face-to-face embrace. It looked like a prelude to mating, but again, it was outside of the mating season. Once they had ahold of each other, swimming became awkward, and they crashed down into the coral, possibly scraping against it as they extricated themselves and swam back out. Chalk up one point for hypothesis number two. Dr. Anderson has also observed "fighting" and "cavorting" behavior among juvenile dugongs. A collision with coral, rocks, branches, etc. during either of these activities could certainly be a possibility wherever such obstacles are present.

Manatees, like most animals, seem to frequently get itches that they can’t reach with their own appendages. Some of them have favorite "scratching posts" consisting of rocks or sunken tree branches, against which they rub themselves regularly. Having observed this behavior in manatees on a number of occasions, I expected that dugongs might exhibit similar behaviors. It was no surprise then when I saw (and photographed) dugongs rolling on their backs in the bottom sediments, and one that repeatedly returned to a particular rock and rubbed its back along it. All it would take would be a small protuberance on the surface of the rock to leave a nice long scratch on the back of the dugong. Chalk up one point for hypothesis number three.

Much of what is known about dugongs has to do with their feeding behavior. Like manatees, they use their lips to manipulate sea grasses, algae, and other plant material into their mouths. The closest living relatives of the sirenians are elephants, so it is reasonable to think of the snout as sort of a short trunk. However dugongs use this apparatus in a very different way from manatees when feeding. The lips move in the opposite direction while feeding, and the downward-directed snouts of dugongs make them obligate bottom feeders, while manatees are able to raise their heads to feed on vegetation floating on the surface, growing on river banks, or overhanging rivers.

Depending upon what type of grass they are feeding upon, dugongs often uproot the entire plant to get at the energy-rich rhizomes. Tusks can be useful for uprooting plants, but females and subadults seem able to feed just fine without them, raising the question of why tusks evolved (more about this later). Feeding dugongs create meandering trails, leaving small patches of ungrazed grass, which spread out to regenerate the pasture after the dugongs have moved. Studies by Drs. Helene Marsh, Anthony Preen, and Ivan Lawler, at James Cook University in Australia, have shown that dugongs have definite preferences among various species of seagrass, and also that they prefer young shoots to mature blades. Not surprisingly, the researchers found young shoots to have more available nutrients than older plants. The researchers found that dugongs are able to "cultivate" meadows of the preferred species with a high proportion of the preferred fresh shoots by grazing heavily in one area, then moving on to another and leaving the first to recover for a period before returning.

In the more temperate parts of their range, however, dugongs may be excluded from their preferred foodstuffs by seasonal temperature changes. In Shark Bay, for example, during October - April, dugongs are heavily concentrated in the eastern part of the bay, where they feed on Halodule – a preferred seagrass which is both tender and nutritious. In May-June, water temperature drops precipitously, and most of the animals move out to the western side of the bay, where they feed primarily on Amphibolis antarctica, an apparently acceptable, if not ideal, alternative. On really cold days, when temperatures in the bay plummet, they will seek refuge in the warmer sea waters outside the bay. Out of desperation, they sometimes snack on the marine algae that grow on the shallow reefs outside the bay, but with a disdain that indicates they are merely putting something into an empty stomach, and not really feeding themselves. At such times, they are often seen right in the surf zone. They bob around in massive breakers with apparent immunity, but Anderson points out that they may sometimes get caught and rolled by a big wave, and end up with a few new additions to their scar pattern.

Shark Bay has also gained fame as the home of large numbers of tiger sharks. Shark researcher Mike Heithaus found dugong flesh in the stomachs of half of the tiger sharks he examined, and in all of the stomachs of large tiger sharks from which he was able to retrieve complete gut contents. He suggests that tiger sharks may be a major factor controlling the size of the dugong population in Shark Bay. A few years ago, a film crew happened upon a several tiger sharks attacking a sick dugong in Shark Bay, and videotaped the animal being reduced, over the course of a couple of hours, from a live animal weighing hundreds of kilograms to a few scraps of floating intestine weighing perhaps a kilogram or so.

Nothing in the video, nor in the appearance of the longitudinal scratches down the backs of dugongs, gives me any indication that these marks might be the result of unsuccessful predation attempts. Shark bites, like dolphin rakes, are fairly distinctive, and nothing I’ve seen on a live dugong looks to me like the work of a hungry shark. However, Dr. Anderson reports occasionally encountering dugongs with recognizable shark bite scars. Predation on dugongs by orcas has also been observed in Shark Bay. Many North Pacific humpback whales bear the marks of unsuccessful orca predation attempts, but I have seen nothing resembling these on a dugong. It may be that when tiger sharks or orcas attack a dugong, they are generally successful, and do not leave a scarred survivor.

I believe the scarification is primarily something the dugongs are doing to themselves, and/or each other. Dr. Anderson concurs, and believes that male tusks are the primary source of the scars. However he also considers that certain scars found primarily on older animals do not appear to match this pattern, and may be caused by sunburn.

Dr. Daryl Domning, an expert on the evolution of sirenians, considers the tusks to be primarily digging tools for uprooting seagrasses and has related the widely varying sizes of tusks of fossil dugongs to their diet. Anderson, however, argues that the tusks have played a strongly social role throughout the evolution of dugongs. Dugongs may have tusks primarily for purposes of fighting and mating, inflicting scars on each other in the process. If the dugongs don’t believe the scars enhance their beauty, at least they don’t seem terribly concerned about them. And, as long as they don’t start dying their bristles purple and green or inserting pins through their nipples, I don’t suppose we should be unduly concerned either.

However, indications of declines in dugong populations worldwide should concern anyone who cares about the oceans. A report released in July 2001 by a task force headed by Dr. Marsh provided evidence of a 97% decline in dugongs along many areas of the urban Queensland coast over a 38 year period. In a perhaps belated attempt to stem the decline, dugong sanctuaries have been established where commercial fishing with nets is banned. Fishermen have mounted strident protests, even though compensation for lost income was figured into the plan. They have even petitioned to have the dugong declared a "common" animal in the region, and stripped of all legal protection.

Outside of Australia, the situation is much more dire. Even though official protection has been granted in a number of countries, the lack of enforcement means that poachers are continuing to decimate local populations, and at the same time displace them from preferred areas for feeding, resting, and socializing. Even if not directly targeted, dugongs may be killed in nets set to catch fish, or for shark control. Coastal development also displaces dugongs while destroying seagrass beds required for feeding, and introducing pollutants into their habitat. In Palau, fishermen have even used explosives to kill dugongs, and the population is dropping precipitously toward zero. Dugongs are also believed to be headed towards imminent extinction all along the coast of East Africa, and are probably already extinct in the Maldives, the Seychelles, Taiwan, and Mauritius. Once widespread throughout the Western Pacific, Indian Ocean, Red Sea, and Arabian Gulf, and numbering at least in the hundreds of thousands, if not millions, the dugong has been reduced to fragmentary populations in most of those areas from which it has not already been extirpated.

What are we losing as the dugong disappears from our planet? We are losing a large enigmatic marine mammal that is often credited with the origin of the mermaid myth. We are losing the last survivor of a group of animals that were once the dominant grazers on seagrass beds throughout the tropics (including the Caribbean, which was once home to as many as six species of dugong simultaneously). We are losing the only truly marine mammal that is an herbivore (manatees are mostly estuarine or riverine, not full-time sea dwellers). We are losing one of the two important grazers on sea grass beds. Dugongs exert a controlling influence over sea grass ecosystems. As the other major seagrass grazer, the green sea turtle, is also endangered, significant changes in the ecology of sea grass habitats can be expected. We are losing an inoffensive vegetarian that poses little or no threat to humans unless provoked. The few reported attacks on humans by dugongs consist mostly of cases where underwater photographers approached lonely male dugongs, and were nearly drowned when the flattered dugong responded positively to the perceived solicitation and attempted to mount the photographer. We are losing a large and important sea creature that has been little studied and rarely filmed or photographed. And we are losing another link in the web of ocean life, which is gradually being reduced from a vital intact ecosystem to scattered and tattered remnants of the depredations of a terrestrial invader which has reproduced out of control and violated the boundaries of its own environment.

More information about dugongs and manatees, and their conservation, can be found at http://www.sirenian.org.