Why Did the Dolphin Carry a Sponge?

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?
ResearchBlogging.org 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% 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% 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.
Marine_sponge_header.jpg The sponge
journal_pone_0003868_g001.png The dolphin, sponge in situ.
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

12 Responses

  1. Do they use the sponge in order to protect their snouts while poking around on the bottom, like using gloves?

  2. I personally hypothesize that the sponge allows the dolphin to poke their nose into the sand without getting sand all up their snouts, which must be uncomfortable.

  3. There are not a small number of non-motile sea creatures that have pretty toxic venoms. Cone snail toxin is not something that a dolphin would want to get injected with. I suspect that the use of the sponge is to reduce the likelihood of getting stuck with venom by disturbing something like a cone snail.
    I also suspect that it is the lower status dolphins that are relegated to the poorer foraging territories, where they have to dive deeper and work harder to get sufficient food. I suspect that (just like in humans), the technology innovators tend to be the social outcasts, those on the ASD spectrum, rather than those that are NT.

  4. Dolphins carry sponges in case they get wet?

  5. Here’s what the first author had to say about the reason for using the sponge and it’s basically what Scicurious said. It’s a case of (a) greater surface area and (b) protection from “getting dinged”. 😉
    Daedalus2u – the authors actually reject the “lower status” idea you mention. In their view, the fact that the behaviour has a neutral effect on fitness outcomes suggests that it’s not a case of “making the best of a bad situation”.

  6. I TOTALLY linked you, Ed. It’s right up there!! I think they should test the sand up the snout hypothesis, personally.

  7. Hee hee. I wasn’t jumping for attention, honest! I just thought Mann’s explanation was cool. I personally think they should test the dolphins’ preferences for a variety of soft foraging tools. Mainly because I want to see a dolphin with a tea cosy on its snout.

  8. Interested in animal intelligence? Check out the episode of Scientific American Frontiers titled “Animal Einsteins.”
    You can watch the full episode online, it’s like the 3rd one down.

  9. I know the authors conclude the behavior didn’t have either positive or negative effects, but their data seems (to me) to indicate otherwise:
    “There were no ultimate costs to sponging in terms of calving success (our best proxy for female fitness). The average calving rate for sponge-carrying females, non-sponge-carrier females and all females was 0.156 (+/- 0.018), 0.132 (+/- 0.008) and 0.135 (+/- 0.007) surviving calves per year, respectively.”
    Their data is unable to exclude a substantial positive effect on calving rate. Why they then default to conclude there is no effect is not something that I understand. 0.156 is a 20% increase over 0.132. A 20% increase in offspring survival rate is a gigantic effect. In 10 generations the fraction of the population with the trait will increase ~10x.
    Dolphins have extremely good echo location. A sponge will be substantially transparent to sound because it has essentially the same density as water. Getting “dinged” by hitting a rock that is hidden by sand is not a credible (to me) adverse effect for a dolphin because their echo location sense would be able to easily detect rocks of a size that would cause “dings”.

  10. I like the geek explanation, and the sponges are the equivalent of “pocket protectors” of the dolphin social world. The ones that use them are convinced that they are eminently practical, but they look kind of goofy.
    If I were a boy dolphin I would totally want a dolphin girl with a sponge, at least until we were old enough to be responsible parent dolphins.

  11. Camouflage, most likely; accidentally initially, then repeated by originator and copied by infants during weaning. Note the camouflage pattern of the spothead grubfish, which resides at 10m depth, plenty of light. http://www.divingthegoldcoast.com.au/index.asp?PageID=animal&NextType=latest&CritterID=4357

  12. daedalus2u,
    The difference in effect is within the error bars. It would have been irresponsible of them to claim that there was an effect here, though they may have concluded that more research was needed to determine whether or not there was a difference in calving rate.

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