Wednesday, February 2, 2011

Orcas With a Whale of a Tooth Problem

[by Alexander Simolaris]

According to a report released in Nature last month, Orcas (Orcinus orca) of the Pacific U.S. have been actively predating on sleeper sharks (Somniosus pacificus). This development is contrary to the popular notion that Killer whales feed almost exclusively on marine mammals and large schools of small fish. However, the catch for the whales is severe damage—and often times complete destruction—of their teeth.

While interesting, the find is reminiscent of the discovery that some whale pods feed on sea-otters—a sign of desperation stemming from a crippling reduction in biomass. If the cost of predating sharks is equally high, then the results of the study suggest that the the whales' situation is more alarming than previously imagined—or is it?

Interestingly enough, seals pose a more direct risk to whales than sharks. Seals, among other prey, are more maneuverable and defensive during an attack; they are more likely to bite an oncoming whale than any other prey item. As counter-intuitive as this may be, sleeper-sharks—during the actual hunt, anyway—are rather innocuous targets. Incurring accelerated tooth decay may simply be a worth-while trade-off that has no significant, net effect on the whales' fitness.

Under this paradigm, there is an argument in the marine community to classify the shark-eating orcas, collectively known as “residents,” as a separate species from the “transients,” which predate almost exclusively marine mammals. These populations do not exchange vocalizations or genetic material despite being fully capable of doing so, indicating the potential development of incipient species. Should the designation be made official, conservation efforts in the region may be, at the very least, administratively affected by the change.

While these possibilities are both interesting and timely, the results of the study are difficult to interpret with any degree of certainty. Populations of pacific whales were discovered in the 70s, specific designations to residents and transients classified in the 80s while dietary habits have only been analyzed in the last decade. Despite years of work, scientists really know very little about the nuances of regional whale behavior. This problem, in addition to the notable absence of a second longitudinal study, forms only one line in a laundry list of variables that makes the data so difficult to interpret. As the pace of research is dictated by ever dwindling numbers of slowly reproducing whale pods, the important implications of the article's results may take decades before coming to fruition.


  1. I find it very interesting that these offshore killer whales are not considered a separate species from the "resident" whales since they have extremely differing behaviors. They have different genetics, one hunts in small packs and sneaks up on the prey while the other is loud and more vocal during a hunt. On top of this, they don't even mate with each other. They're isolating themselves to make it even clearer that they are not the same. I wonder if they live in similar environments or if that is the reason why they predate different members of the food chain. It's a shame that we can't see everything these animals are doing because then it would be much easier to classify them.

    Posted by Kevin McLaughlin

  2. Although the studies on the dietary habits of the killer whales are still young, I think it would be interesting and helpful to determine the type of food these specific orcas hunt that has gotten so low in numbers that they are now resorting to the sleeper sharks. For example, if scientists were able to connect the decline of a certain population of sea lions due to pollution and the shift in diet of this population of orca, this information could be used to better protect these amazing animals. I cannot imagine that these orcas are deliberately choosing this diet because their teeth are simply not cut out for this type of food and it would cause a drastic decline in the lifespan and population size. Or, if this dietary decision is deliberate, perhaps we will see a classic example of Darwin’s natural selection when the sleeper-shark-eating population declines. This would be sad, but still a very interesting find.

  3. The above article was posted by Marlena Grasso

  4. To clear up Kevin McLaughlin's confusion about biological species, the reason resident and transient whales are considered to be the same species has nothing to do with their behavior, but the fact that, even though they may have stopped reproducing with one another, they still have that ability to reproduce and make viable, fertile offspring. That is the (one of the many) definition of species. However, if they remain this way and never reproduce in the future, they could diverge through evolution and this could create either a pre-or-post-zygotic barrier in their reproduction, which would in turn cause these groups of whales to be labeled as different species. What is truly a shame is that this research will take so long to generate an answer to why these whales began to predate sleeper sharks over seals. The fact that the whales have changed their eating patterns that have been stable for so many years definitely suggests some sort of pressure or endangerment on this species. Yes, sleeper sharks put up less of a fight, but they cause the whales’ teeth to slowly decay, effectively killing these whales over many years, as they need their teeth to continue putting up a fight against seals, chewing sleeper sharks, and even to begin digestion of small fish. Seals put up a fight, but this tussle is worth it since the whales’ teeth will remain unharmed. Maybe, if whales only prey on sleeper sharks, they will get sloppy in their fighting skills while great whites, for instance, stay tough. And in the future, great whites could easily take on these whales, just a theory of one of the many future implications this may have.

    Posted by Derek Melzar

    Your point on the whales falling under the same species as a result of their potential to reproduce is certainly warranted. At the moment, there is some argument over exactly how to define a species in the scientific community. The best examples include the "officially" named species Canis lupus (wolves,) Canis latrans (coyotes) and Canis familiaris (common dogs). These three are technically different species but can actually interbreed, resulting in "Canis soupus" offspring that, as we speak, is still undefined.

    There is some crossover between Canines and Whales: both are threatened groups, and so how we categorize them as species has implications for their level of protection within the multinational community.

    Posted by Alexander Simolaris

    NOTE: the above response was directed to Derek Melzar.

    [RESPONSE] to Marlena Grasso


    I'd agree that scientists working on this project should try to get a better understanding of the pressures resulting in the whales' predation of sharks. The behavior, from a purely "selection gradient" point of view seems to be very bad for the whales' long term health. From an intuitive view, I can't imagine it would be even remotely tolerable for a predator to lose its teeth. Even if a whale with destroyed teeth can live off sharks, they are permanently excluded from almost any other major predation.

    Posted by Alexander Simolaris

  7. [RESPONSE] to Kevin Mclaughlin,


    Your question on the Residents' environment is interesting. Predating on sharks seems like somewhat of a choice, as indicated by nearby transient whales predating on marine mammals. That said, perhaps the cost of attacking seals is simply greater than predating sleeper sharps. The fact that killer-whales have a huge range, and have the potential to be migratory, also raises more questions about their specific predatory behavior.

    Posted by Alexander Simolaris