It sounds like the beginning of a joke: Why do dolphins carry sponges? To…um…to…well technically, it’s to scoop up fish, but that’s not funny…to get to the other side? No? Dang.
This paper from PLoS ONE has recently been covered by the illustrious Ed Young (he of the recent book publication!), but it’s PLoS ONE’s birthday, and I really think this phenomenon is cool, and so I’m going to blog it, too. So there, Ed. *ppppbbbbtttttttth* 🙂
So why DO dolphins carry sponges?
Mann, et al. “Why do dolphins carry sponges?” PLoS ONE, 2008.
Dangit, I’m STILL trying to think of a punchline…
Anyway, this is the first time I’ve ever heard of a dolphin in the wild using tools, but I can’t say that I’m surprised. Some people would say that dolphins are the second smartest species on the earth:
For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much–the wheel, New York, wars and so on–while all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man–for precisely the same reasons.
Douglas Adams, The Hitchhiker’s Guide to the Galaxy.
Regardless of where you would place dolphins in the intelligence scheme of things, tool use is a very rare phenomenon. So far it’s only been found in 30 birds, 10 primates, and 0.01{9f43b4361d9a125bc126dd2a2d1949be02545ec69880430bc4fed2272fd72da3} of non-primate mammals. If you’re not a bird or a mammal, you clearly don’t use tools. And most dolphins have never been documented as using tools. But this paper found one population off the coast of Australia doing something that they like to call “sponging”, which I find to sound vaguely dirty. Only 11{9f43b4361d9a125bc126dd2a2d1949be02545ec69880430bc4fed2272fd72da3} of the females (far less of the males) do it.
Basically, it involves taking a marine sponge (one of the relatively cone shaped ones), and fitting it over your nose/beak/snout/”rostrum” (I’m going to call it a beak, that’s the Sci technical term). It pretty much covers the whole beak.
Then, the dolphins use their sponged beaks to poke at the seafloor in deeper channels of water. Apparently, they are after spothead grubfish, which hide themselves in the sand, and only move when disturbed. When they move, the dolphins can detect them, and the chase is on! The dolphin usually wins.
This tool use is particularly interesting for several reasons.
1) Relatively few dolphins do it, and those that do pass it on from mother to daughter (or rarely from mother to son). The fact that males don’t do it isn’t too surprising, female dolphins tend to follow their mother’s pattern more closely than males, and males also need to devote time to building alliances, they have a wider range, and of course they must devote time to the ladies.
2) As far as time use, sponging isn’t any “cheaper” than other types of hunting, the dolphins spend about the same amount of time hunting with a sponge as other dolphins do hunting without. And dolphins who sponged were just as reproductively fit, producing the same number of offspring. So why pick up a sponge in the first place?
3) For the very social dolphin, sponging involves a lot of time spent by themselves or with their calves, and away from the social group. Not only that, dolphins who sponged had fewer associates than those who did not. This seems like a negative for the social dolphin, are these dolphins less social?
There are a couple of options as to why this sponging takes place, and I also have some guesses (though they’re just guesses). Why pick up a sponge? The authors surmise that hunting in the deeper water channels is better adapted for sponging (the grubfish may not like more shallow water), and that dolphins who don’t sponge probably wouldn’t have as much success in those areas. This means that those areas have less foraging competition, which is good for the individual dolphin having a lucky hunting day.
And why these channels and these fish? They’re not very big. I’m wondering what the nutritional value of a grubfish is, maybe they are better for the dolphins, or maybe they’re just better tasting. Perhaps those willing to pick up sponges are the ones who are willing to put in the effort for a gourmet meal. You could probably test this with choice in captive dolphins, have them choose between different types of fish. I’m sure a dolphin could work an operant.
As for the social aspects of sponging, I have a hypothesis! I think the sponging dolphins are the geeks of the dolphin world! They’re high tech (check out those sponges) and slightly more solitary. Maybe the other dolphins tease them for carrying around their sponges, and the poor geek dolphins feel misunderstood. The dolphins did tend to carry the sponges less in public, perhaps they were trying to hide their high tech foraging? I’m kidding, of course. But it would be cool if dolphins had geeks.
And hey, WHY did the dolphin carry a sponge? I still don’t really know. I welcome input on punchlines…
Janet Mann, Brooke L. Sargeant, Jana J. Watson-Capps, Quincy A. Gibson, Michael R. Heithaus, Richard C. Connor, Eric Patterson (2008). Why Do Dolphins Carry Sponges? PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003868
