Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: Jenny Adcock on 26/08/2009 13:30:03
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Jenny Adcock asked the Naked Scientists:
As always I'm loving the Naked Scientists podcast (http://www.thenakedscientists.com/HTML/podcasts/) (almost an addiction!)
I have a question for the favourite naked marine biolologist!
I had the good fortune to live in Oman for a few years where we used to go out on our boat looking for the dolphins and were often lucky to be able to observe them.
There were mostly common dolphins and spinners. We would sometimes scan the horizon for the traditional small Omani fishing boats which would be fishing for tuna and other fish amongst the pods of these beautiful mammals.
Although normally we saw groups of between twenty and fifty, at times, really NO exaggeration, we would see maybe five hundred, diving, spinning and jumping in graceful arcs!
My question is, do the dolphins live and travel in small or extended family groups, and then congregate in great numbers to co operate in catching their food and then separate again to swim and live in their smaller groups? Is this possible for them because of they have an intelligent method of communication?
ps Snorkelling is brilliant there too.
What do you think?
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Heh [;)] well, if the larger groups didn't split up again to form smaller groups then we'd end up with just one super-sized group consisting of all the dolphins in the world.
I don't think the reason for very large groupings is definitively known but having a number of smaller family groups temporarily coming together is a good way to preserve genetic diversity. Dolphins are also clearly social animals too and this probably plays a significant part in the formation of these large groups. Iirc, in one of the 'Blue Planet' sequences that featured a grouping of several hundred individuals, feeding was done in smaller groups and not by the entire large group all at the same time; a small group would split off to hunt and feed, and then return to the large group.
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Dolphins do normally live and hunt in groups, but at times they do amass into vast groups on feeding frenzies.
An example of this is the annual migration of Sardines off the African coast. These shoals can be 15 miles long. Superpods of 5000 Common Dolphins arrive at the best hunting grounds and await the arrival of the sardines.
The Dolphins heard the fish into great balls, then attack. Sharks and Gannets make the most of the Dolphins' efforts and join in the feeding frenzy.
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Dolphins don't actually indulge in mass feeding frenzies. As I said, smaller groups split off from the main group to feed, as and when they're hungry, and then return to the main group. At any one time, the majority of the super-group won't be feeding.
The sardines, and many other small fish too, instinctively form large 'balls' in the same way and for the same reason that many small birds form flocks; it's a defensive move intended to make it difficult for a predator to identify a specific target to attack. What the dolphins do is not to herd the sardines up into a large ball but split the large ball down into smaller and smaller groupings, making it easier to track and attack individual targets.
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Perhaps 'feeding frenzy' is a bit OTT, 'bumper banquet' might be a better description. As Lee said, the superpod does not all feed at once and can be spread over considerable distances.
Look at this http://www.environmentalgraffiti.com/featured/great-sardine-run-largest-gathering-predators-earth/13115 (http://www.environmentalgraffiti.com/featured/great-sardine-run-largest-gathering-predators-earth/13115)
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...or how about a Food Festival [:)], as it's over a period of several days? They've certainly picked that location because of the food, and it's probable that the amount of available food in a region limits the size of the groups that form there.