While the Southern Resident orcas appear to co-exist with other species of dolphins and porpoises, their interactions with harbor porpoises do not always seem to be so benign. From time to time, researchers have observed members of J, K, and L pods chasing the tiny porpoises and/or pushing dead harbor porpoises around on the surface. At first glance this seems just bizarre – these orcas eat only fish and invertebrates, and even the type of orcas that do eat marine mammals are seldom known to kill what they don’t then consume. An exception to that is when the young are being taught how to hunt, but even in those cases it is often reported that the victimized seals etc are let go apparently unharmed.
Recently, as the reports came in of humpback whales swimming undisturbed among Southern Resident orcas off the shore of San Juan Island, I again wondered why it is that only harbor porpoises seem to run into trouble with these otherwise peaceful orcas.
Exploring this behavior brought several questions to mind:
Do the orcas actually kill the porpoises? From what I have been able to learn, it does look as though the porpoises can die as a result of interacting with the whales. Robin Baird, of Cascadia Research, has documented several aggressive encounters between the two species and in at least one of them the porpoise died as a result.
Is it intentional? At this point, there is no way to know, for several reasons. First, figuring out what an animal intends is difficult. When an animal behaviorist talks about “intent” in describing an animal’s actions, they are referring to a set of behaviors with a predictable outcome – for instance, think of a lion stalking its prey, crouched, focused and ready to pounce. In that case you can safely say that the lion “intended” to catch what it was hunting. But even in that case, you can’t say that the lion “intended” to kill the prey – the lion’s intent is to catch a meal (resulting in the death), but its intent was not to kill, the prey just has to die in order to be eaten. It is a subtle, but important distinction.
In the case of the orcas and porpoises, the researchers describe behavior that shows the orcas were intent on chasing the porpoises, resulting in the occasional death of a harbor porpoise, but there is no way to know if the orcas were intent on killing them. The lack of broken bones and tooth marks on the dead animals argues against that idea, and in an article in the Vancouver Sun, it was reported that:
John Ford, a whale expert with the federal fisheries department, said from Nanaimo that because it is female killer whales that tend to engage in the behaviour, it is possible they are trying to prop up the porpoises as they might their own young. The porpoises can ultimately succumb to shock, exhaustion, injury or drowning.
“It could be a maternal-driven behaviour that is misdirected towards another species,” said Ford, noting southern residents seem more likely to exhibit the behaviour than northern resident killer whales.
“These animals [porpoises] are often sort of carried about on their backs or heads, pushed around. It’s almost like a behaviour you’d see with a distressed or dead calf of a killer whale. We’ve seen a still-born calf pushed along or carried along by the mother.”
Ford said biologists observe killer whales kill porpoises locally about twice a year, but confirms what they see must be only a portion of the total number of porpoises killed this way.
Why harbor porpoises?
The other local species of dolphins and porpoises that are known to harass, or be harassed by, the Southern Resident orcas are similar in size to the harbor porpoise, but they tend to aggregate in larger communities and are more wide-ranging. The harbor porpoise tends to be more localized and solitary, leaving it more vulnerable. It is possible that the harbor porpoises are more susceptible and succumb to the same treatment that leaves the other species unharmed.
There may be other things about the harbor porpoise that cause it to elicit aggression from other dolphins (orcas are the largest dolphin species, even though because of their size they are referred to as whales). Bottlenose dolphins are known to kill harbor porpoises, and it is thought that the porpoises compete for increasingly scarce food in those cases. Another theory is that the dolphins mistake the porpoises for young bottlenose dolphins, and accidentally kill the porpoises instead of babies of their own species. (The dolphins are thought to kill the babies of their own species, though no one knows for sure why this occurs. You can find more information on this phenomenon here. This seems unlikely, given the fact that dolphins have sophisticated sonar that allows them to even tell different types of fish apart. And given the dolphins’ promiscuous lifestyle, how would the males know they are not killing their own offspring?).
Harbor porpoises are also known to interbreed with Dall’s porpoises, at least locally, and these hybrid animals are occasionally seen swimming with groups of Dall’s. The fact that these hybrids have not been seen with harbor porpoise females implies that it is the male harbor porpoises which mate with the female Dall’s, and not the other way around, although we can’t be certain of that at this point. At any rate, these two species, with divergent lifestyles, clearly mingle, and it may be the case that at least the male harbor porpoise’s behavior may be tied into the aggression with some dolphin species.
Perhaps it is lack of communication. Going back to the recent discovery that various different species may be able to communicate on some level with each other (earlier post) it may turn out that there is just a communication breakdown somewhere along the line. Harbor porpoises don’t communicate with whistles like most other dolphins and porpoises, but instead use only echolocation clicks:
Our results provide strong evidence that porpoises communicate acoustically using specific patterns of clicks with source properties comparable to normal echolocation clicks, and
that they employ stereotyped aggressive click patterns, exposing conspecifics to received levels of up to 180 dB re 1 Pa (pp). The measured source properties render estimated active spaces of less than 1000 meters for porpoises’ communication sounds. Compared to other cetaceans, porpoises must therefore remain much closer to be able to
A disparate selection of toothed whales (Odontoceti) share striking features of their acoustic repertoires including the absence of whistles and high frequency but weak (low peak-to-peak source level) clicks that have a relatively long duration and a narrow bandwidth. The non-whistling, high frequency click species include members of the family Phocoenidae, members of one genus of delphinids, Cephalorhynchus, the pygmy sperm whale, Kogia breviceps, and apparently the sole member of the family Pontoporiidae. Our review supports the ‘acoustic crypsis’ hypothesis that killer whale predation risk was the primary selective factor favouring an echolocation and communication system in cephalorhynchids, phocoenids and possibly Pontoporiidae and Kogiidae restricted to sounds that killer whales hear poorly or not at all (< 2 and > 100 kHz).
What this all means is that harbor porpoises use relatively intense sound, they have to be fairly close when they communicate, and they use sound aggressively. They can use sound that the orcas can hear, or outside the orcas’ hearing range, and that this evolved specifically as a response to predation by the orcas.
Given that the harbor porpoises don’t use the whistles that identify orcas and most other dolphins and porpoises to each other, chances are they can’t tell the fish-eating Southern Resident orcas from the porpoise-eating Transient orcas. If that is so, the porpoises may occasionally be taken by surprise when the Resident orcas are nearby, and launch themselves into a defense strategy designed to distract what they perceive to be a predator (and one that is thought to have shaped the evolutionary biology of the porpoise at that) at close range…with unfortunate consequences.
The problem may be that, coupled with the harbor porpoises’ basic biology, they just don’t understand the language.