Saturday, 11 June 2011
How many friends does a dolphin have?
Cetaceans, of course, don't even come ashore to do that (although they aren't quite alone in this respect, even among mammals). These animals are superbly adapted to an aquatic life, even more so than seals, and much, much more so than we decidedly non-aquatic humans. This means that studying cetaceans in their native habitat is a difficult, and rather specialised field - and, depending on what you want to know, studying them in the laboratory may not even be an option.
This means that there are a number of cetaceans about which relatively little is known. This is particularly true of the small to medium species that rarely come close to land, but its also true that we just haven't got round to looking in detail at all the species living in more remote parts of the world, even where they are found in shallow waters.
At no more than five feet long, Commerson's dolphin (Cephalorhynchus commersonii) is one of the smallest of all the cetaceans, and has a very distinctive black-and-white colouration. Apart from the colour, it actually looks more like a porpoise than a dolphin, since it doesn't have the beak or the dome-shaped forehead that most other dolphins have. It lives off the coasts of the Falkland Islands and southern Argentina with, for no particularly obvious reason, a second, smaller, population several thousand miles away in the Indian Ocean.
Within the dolphin family, it is related to other species inhabiting the southern oceans. It, and all of its close relatives prefer shallow water, although they are presumably capable of undertaking much longer journeys when they have to, or they wouldn't be so widespread. Its ancestors appear to have originated off South Africa, where its close relative Haviside's dolphin still lives, before some of them travelled to New Zealand, and then, after leaving behind the population that became the Hector's dolphins, crossing the Pacific to South America.
Commerson's Chilean Hector's
Dolphin Dolphin Dolphin
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| | | Haviside's
| | | Dolphin
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Living close to the coast, it should be relatively easy to study, but, because it's fairly obscure, it hasn't received as much attention as many of its larger cousins, or even of its New Zealand relative. We know that, like many dolphins, they are usually seen together in groups, but what exactly are these groups? Are they more or less permanent pods, with the same animals staying together over a long period of time, or something more fluid? Mariano Coscarella and colleagues, from the Marine Mammal Laboratory at the University of Patagonia conducted a fairly simple study to try and answer this question - and, when you're looking at an animal about which relatively little is known, sometimes a simple study is all you really need to provide genuinely new science.
Basically, they bought tickets for trips on commercial dolphin-watching boats, and photographed as many dolphins as they could. Which, it has to be said, if you are someone who really likes dolphins, does sound suspiciously like your idea of a holiday. At any rate, the idea was to see how often they saw the same dolphins together, and over what sort of time period. There are limitations to this sort of study, of course - a lot of your photographs won't be any good, or some of the individual dolphins might not have scars and so on that make them easy to identify even when you do get a good photo. But, if you do it for long enough, you should get information you can use. And then it's back to the lab for some serious mathematical number-crunching.
So what does all this tell us? The dolphins banded together in groups of around 25 or so, but the membership of these groups changed all the time, suggesting that they are not permanent pods. The area in which the study was taken covered about 40 square miles of water, within a particular bay near the northern edge of the dolphin's range, and individual dolphins would visit the area for around 10 to 20 days at a time, before wandering off to the south, or further out to sea. Some of the dolphins were spotted, mere days later, near the University - 160 km to the south of the main study area - which suggests that they're happy to move long distances, something that's apparently not the case for Hector's or Haviside's dolphins, and so might be considered something of a surprise.
But, while the groups don't have a constant membership, so that all the dolphins aren't travelling together, they aren't completely random either. At least some of the dolphins had fairly constant companions, forming groups of around eight individuals that wandered about between the larger aggregations. So, while they aren't true herd animals, they don't just wander around randomly, but do have some sort of structure to their groups. Unfortunately, the limitations of the study mean that we can't say much more than that; for example, its pretty difficult to tell boy dolphins from girl dolphins without getting very close indeed, so we can't tell whether these longer lasting associations might have something to do with breeding.
All of this suggests that the dolphins are quite friendly animals; they have a few close companions, but they often meet up with relative strangers, and stay with them to feed without obvious signs of conflict. Not all dolphin species behave like this, and it's interesting to ask why. A couple of explanations have been proposed. One theory suggests that larger dolphins tend to group together in more permanent pods than smaller species do. Certainly, the largest dolphins of all - killer whales - do have fairly permanent groupings, while Commerson's dolphins are pretty small, and the medium-sized bottlenose dolphins (the grey ones so familiar from aquaria) are roughly in the middle in terms of their group cohesion.
But why would that be? The suggestion is that it isn't the size itself that matters, but that bigger species tend to live longer, giving them more chance to benefit from long-term companions. Another possibility relates to how the dolphins live. Because they inhabit shallow water, good feeding grounds should tend to remain fairly fixed, be relatively close together, and be easy to find. Indeed, the study area is known as a good place for fishing, and that's presumably as good for the dolphins as it is for humans.
Wandering about in small groups, or even on your own, is perfectly fine where food is readily available. But species that live further out from land will have to travel further to find food wandering about with the currents, which if they travel on their own, they will be much less likely to bump into others of their own species - which has obvious disadvantages. There may also be a benefit to travelling together to avoid predators, hiding behind local bits of terrain not being an option in the deep ocean.
Of course, Commerson's dolphins are both small and shallow water dwelling, so living in small groups, as they seem to, is about what we'd expect, regardless of which explanation is best, or whether they're both true to an extent. In that sense, the study doesn't take us any further. But by confirming that a species not previously looked at in this way at least doesn't contradict what theory says should happen, we at least know we're not completely on the wrong track. Knowing how the animals move about may also be useful to the dolphin watching industry, and for fisheries management.
There's a lot we still don't know about cetaceans and their habits. But sometimes, just by going out with a camera, we can learn a bit more.
[Picture from Wikimedia Commons. Cladogram adapted from Harlin-Cognato & Honeycutt, 2006]