On a recent weekend, whales stranded themselves on 4 different beaches around the world. Why do they do this?
Listen to my BBC radio chat with Malcolm (actually, with Nick Piercey this time. We discuss whale strandings starting at 25 seconds into this clip:
[audio http://www.brookes.ac.uk/lifesci/runions/DrMolecule/20120904 – Beached whales and tree diseases.mp3]
Stories about animals committing mass suicide are just not true – whether the animals in question be lemmings or whales. On of the central tenets of evolutionary biology is that the gene is selfish. Mass suicide is not a biological imperative – animals seek to reproduce and raise their young in all cases. While it is true that evolutionary theory talks of a ‘struggle for survival’ and ‘survival of the fittest’, there is no indication that whales that strand themselves are at less than peak fitness. So, although the reasons for whale strandings are unclear, the thing that you can be sure of is that whales are not ‘committing suicide.’
Why then do these terrible tragedies occur? It is easy to point the finger and say that man is to blame. That might be partially true, and I will explain why in a bit, but there have been documented whale strandings since well before we filled the ocean with human technology that might confuse whales.
What is usually true is that it is the ‘toothed’ whales like pilot and sperm whales that beach themselves. These are the whales who hunt and eat meat like fish and seals. Larger whales such as Blue whales who filter zooplankton (the baleen whales) much less commonly beach themselves. The suggestion here is that it is the act of hunting in packs for animals that shelter in shallower waters that contributes to whale strandings. I hear you say, ‘But whales aren’t stupid!.’ Far from it, I think I made the point already that they wouldn’t have survived this long if something as common as a bit of shallow water was going to confuse them so much that they died.
My contention is that shallow waters have probably resulted in confused and stranded whales throughout evolutionary history but that it is exactly this that should have selected for whales that are exquisitely able to survive in these environments. Orcas have been observed to beach themselves as a hunting strategy. They catch seals on the beach and wait for the next wave to re-float them. This is a learned / evolved behaviour that not all whale species have acquired. Evolution is a slow process – especially for large, long-lived organisms that take years to produce successive generations. That’s why deer haven’t ‘evolved’ the ability to avoid cars. No predators with which they have evolved move as fast as a car, and cars have only been around for a few decades. I can imagine that if we kept driving cars at deer for many thousands of years that the ability to avoid them would evolve in deer.
OK, so whales have evolved to survive in shallow water environments and should be able to avoid being beached. It still happens – and it happens much more commonly on some beaches than on others. Scientists postulate that when pods of hunting whales stray into unfamiliar territory, they can become confused. This confusion is most prevalent in areas where the angle between beach and sea bottom is very shallow – it does not, therefore, reflect the sound that whales make as a navigation aid back to them and they remain unaware that they are dangerously close to the beach. This situation combined with strong current or tides is why whales end up high on beaches.
Whale rescue agencies have been set up around the world and are staffed, generally by volunteers, so that they can be mobilized quickly when whales beach or, even better, when a pod of whales gets close to a stranding site. Whales have not evolved to support their own weight for very long (I even find it a bit uncomfortable when I lay on the couch for more than a few hours watching TV!) and they quickly become very ill when beached. Even when rescued, they commonly remain confused and often re-beach themselves.
I mentioned at the top that human human activity might play a role in the increasing number of whale strandings that are being observed. In early September this year (2012), pilot whales beached themselves at 4 different locations around the world. That just seems like too much of a coincidence. One factor that seems to cause a great deal of confusion in whales is noise. And man do we fill the oceans with noise. Not only the din caused by ships engines but, increasingly, SONAR from military exercises. These sounds can be louder (240 decibels) than any sounds on land including jet engines and rock concerts. Think about whales who have evolved to navigate using sound – hear for yourselves the plaintive whale sounds that some species can detect from hundreds of miles away. The incidence of whales beaching increases after military exercises involving SONAR, and scientists have observed that many of the whales involved in these beachings have acoustically-induced hemorrhages around the ears.
We must consider the natural environment of these animals before we deploy things like load SONAR for military purposes – is it worth it?
While I’m on the topic of what we do to the whales environment, watch this amazing video of a humpback whale being rescued from a fishing net. I just about didn’t watch it because I was concentrating on whale strandings when I found it. It leaves me with mixed emotions. On the one hand, the majesty of the animal and it’s apparent cooperation with the humans. On the other, what if they hadn’t encountered the whale. It would just be a statistic on its way to extinction.
In this case it seems that the less “fit” (those with less acute acoustic sensitivity) may be the ones less inclined to beach themselves.
I can think of evolutionary arguments that would go either way on this issue. Does lower acoustic sensitivity mean that a whale is more sensitive to water depth changes than others might be? We don’t know. The fact that up to dozens of whales ina pod will strand themselves at one time suggests that stranding isn’t strongly correlated with individual fitness anyway.