Wikipedia:Reference desk/Archives/Science/2012 June 27

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June 27

Why do dolphins kill sharks?

Why do dolphins kill sharks? — Preceding unsigned comment added by 68.83.98.40 (talk) 04:32, 27 June 2012 (UTC)

Because sharks kill dolphins. Looie496 (talk) 05:40, 27 June 2012 (UTC)

Not being a marine biologist, I'd say Looie's answer is pretty spot-on. My guess is that sharks and dolphins also compete for certain sources of food, but I may be wrong about that. Evanh2008 ^{(talk|contribs)} 05:55, 27 June 2012 (UTC)

OK, can someone point me to something saying that dolphins actually do kill sharks? Between a single dolphin and a single shark, I'd bet on the shark, though I could believe that dolphins might be able to use their intelligence to kill a shark with teamwork. --Trovatore (talk) 07:03, 27 June 2012 (UTC)

Here's something on the behaviour of bottlenose dolphins [1](bottom of page). Mikenorton (talk) 07:15, 27 June 2012 (UTC)

(e/c) I cannot speak as to reliability but per here: "The idea that dolphins will seek out sharks for undersea battle is largely incorrect. It is the unfortunate perception stemming from the 60's "Flipper" generation..." and continues from there.--Fuhghettaboutit (talk) 07:17, 27 June 2012 (UTC)

These sources [2] [3] seem to agree that dolphins rarely attack sharks. Nil Einne (talk) 07:19, 27 June 2012 (UTC) Edit: Also [4]

Interestingly, though, bottlenose dolphins are known to occasionally attack members of other species who seem to pose no obvious threat (porpoises, for example). Snow (talk) 09:29, 30 June 2012 (UTC)

What kills more dolphins: sharks or humans?165.212.189.187 (talk) 13:17, 27 June 2012 (UTC)

(Based on a more careful check of the mortality rate of dolphins from humans, I've reconsidered the answer hence the strike-out.) Considering the above sources suggest sharks don't really compete with dolphins that much, the likely answer is humans. This non-RS [5] makes the claim (edit: that humans kill more dolphins then sharks) but I don't vouch for it as it provides nothing to back up the claim, however it does mention tiger sharks, great white sharks and bull sharks as the likely shark predators of dolphins (one of the other sources mentioned tiger sharks as well). [6] claims that despite a frequent attacks by tiger sharks, the number of dolphins killed is small. (I don't know if the other source really contradicts this as it just said the person had found many dolphins in tiger sharks, the percentage may still be low.) Edit: In any case, it appears to me the mortality rate of dolphins from sharks is not that high although I don't have any specific numbers.

This will also depend on how you define 'kill'. I presume your excluding very indirect methods like overfishing. Are you including slightly less indirect methods like PCB contamination and other forms of pollution or habitat destruction? (I don't know if these are significant enough to be considered.) I presume you're including direct but inadvertent methods like deaths due to bycatch. On that point, I can't find any good non activist recent sources on overall mortality rates how what I've found suggests it's generally low. For example [7] suggests a rate of less then 5000 for those caught for the US market and [8] suggests a rate of 2600 in the eastern Pacific in 1996. You can add to that a few thousand killed by hunting, see e.g. Dolphin drive hunting. Based on this, the answer may very well be sharks in some cases although I have no idea of the mortality rate from sharks. (Definitely based on earlier rates of 100k+ humans seem hard to beat, see also [9].) Of course this does depend on the precise species of dolphin and where they live. I strongly suspect even ignoring habitat loss and degradation, many more Orcaella brevirostris are killed by humans then sharks. Nil Einne (talk) 18:48, 27 June 2012 (UTC) Edit2: Noticed we also have a Cetacean bycatch which seems to support the idea overall the numbers are low now although there may still be some at risk. Dolphin#Human threats also mentions the other threats.

While I wouldn't expect dolphins to kill large adult sharks, juvenile sharks (or adults of small species of sharks) would just be another fish they would eat. I believe the size threshold would be what the dolphins could swallow whole. StuRat (talk) 18:17, 27 June 2012 (UTC)

If humans kill more dolphins than sharks do, both directly and indirectly, then why don't dolphins kill us?165.212.189.187 (talk) 19:51, 27 June 2012 (UTC)

Because sharks have been killing dolphins for millions of years, which is enough time for dolphins to evolve an anti-shark bias, while humans have only been killing dolphins for a few thousand years. Also, if they did kill humans, it would be counter-productive, we'd just wipe them out. However, when dolphins are captured (with their families killed), held captive and forced to do tricks, like at Sea World, they do occasionally get even by killing people (although it's hard to tell if those deaths are on porpoise). Then there's the Simpson's episode where the dolphins take over the land and force all humans into the ocean. :-) StuRat (talk) 21:14, 27 June 2012 (UTC)

You do realize that didn't really happen, since it was a Halloween special? μηδείς (talk) 01:34, 28 June 2012 (UTC)

And the rest of the Simpson's, of course, is a factual documentary of their real lives. StuRat (talk) 01:46, 28 June 2012 (UTC)

Did you guys miss all the stuff up above, demonstrating that dolphins do not in fact kill sharks, at least in numbers large enough to be noticed? --Trovatore (talk) 21:21, 27 June 2012 (UTC)

And did you see Nil's 3rd link, which confirms, as I suggested, that dolphins will attack small sharks, which they don't see as dangerous ? StuRat (talk) 21:39, 27 June 2012 (UTC)

Are you referring to the link which shows that dolphins will attack certain sharks if trained by the US navy, but not large ones? If so I'm not sure that is particularly relevent. Unless I missed it, none of the links indicated there's any real evidence of dolpins regularly attacking sharks big or small. (It does demonstrate that dolphins have evolved an anti certain dangerous animals bias, but I'm not sure if Trovatore was disputing that. The bias appears to be in the form of avoiding dangerous animals rather then trying to kill them as is hardly surprising.) Nil Einne (talk) 19:34, 28 June 2012 (UTC)

Dolphins do kill humans [10] :-P (I primary mention this because a common question and one which confuses results when trying to find out dolphin mortality from humans vs sharks appears to be whether dolphins or sharks kill more humans. The answer appears to be sharks, since I only found one human death attributed to a dolphin in the wild. There are some near misses like a woman who got dragged down 100 feet and also some other results which confuse matters like people who died swimming with dolphins but where the death appears to have been natural causes. Not that sharks actually kill many people any way. BTW the other result which confuses search attempts on the dolphin mortality issues is of course dolphins occasionally saving humans.) More seriously beyond what has been said above, while dolphins are intelligent, as I mentioned above many of the mass killings in particular were from by catches so the connection to humans is rather indirect. Even if there was time to evolve, this would be to avoid nets or boats or otherwise escape from them rather then to kill humans. Note that while evolution may sometimes provide the tools to kill potential predators or other hazards, usually it also evolves towards using them sparringly. It doesn't tend to act towards killing these predators or hazards on sight, that's rarely a productive strategy. As StuRat has hinted at, it's particularly true with humans. While I'm not saying we would wipe them out, I think it's clear the idea of dolphins as cute, intelligent docile creatures you can swim with is one of the reasons people are so much more concerned with killing them then of the many, many 'killer'/'dangerous' sharks killed by humans each year. (Even if these concepts are not particularly accurate.) Nil Einne (talk) 19:55, 28 June 2012 (UTC)

Dolphins do mob sharks: Bottlenose_dolphin#Predators. μηδείς (talk) 01:33, 29 June 2012 (UTC)

Resolved

Which model of Citroën DS?

Citroën DS 21?

I saw and photographed this car in Paris earlier this month. It looks kind of neat IMO. But exactly what type of car is it? I suppose it is a Citroën DS, but this model has been manufactured in many variants. Being a car ignorant myself, could someone help me getting closer to which exact variant it is? --Slaunger (talk) 14:41, 27 June 2012 (UTC)

Looks like Citroen DS cabrio, as pictured at Citroën DS and [11]. Bazza (talk) 18:21, 27 June 2012 (UTC)

File:1974 Citroen DS23 Pallas.jpg Looks very similar to this one.--Canoe1967 (talk) 18:28, 27 June 2012 (UTC)

Thanks for the hints! I think it is some variant of the Citroen DS Cabrio. It looks very much like this DS Cabrio. But the Cabrio appears to be made with different front light systems, like this DS Cabrio. The Pallas referred to by Canoe has many similarities, but the elevated rear light system found in my photo is not on the Pallas, but seems to be on all the Cabrios found in the link provided by Bazza 7. --Slaunger (talk) 18:39, 27 June 2012 (UTC)

With help from elsewhere it has been confirmed that it is a Citroën DS 21 Cabriolet. --Slaunger (talk) 15:06, 28 June 2012 (UTC)

Resolved

BSP WASHER SIZES

IS THERE A STANDARD FOR BRITISH STANDARD PIPE THREAD WASHERS AND A TABLE OF SIZES ? — Preceding unsigned comment added by 195.74.125.225 (talk) 15:23, 27 June 2012 (UTC)

YOU MEAN THIS? OR MAYBE THIS? ←BASEBALL BUGS ^{WHAT'S UP, DOC?} CARROTS→ 19:47, 27 June 2012 (UTC)

Friction

Can anyone direct me to research regarding the ratio of static friction and kinetic friction, i.e. I am interested in $\mu _{s}:\mu _{k}$. — Fly by Night (talk) 20:04, 27 June 2012 (UTC)

I'd think the highest difference would be in a case where the surfaces actually break. For example, two sheets of sandpaper might have the sand grains ripped off when the object starts to move, greatly reducing the friction. StuRat (talk) 22:06, 27 June 2012 (UTC)

Fiddlesticks. DriveByWire (talk) 00:34, 28 June 2012 (UTC)

How about a flour avalanche in a slowly turning drum? Plasmic Physics (talk) 04:05, 28 June 2012 (UTC)

Just to reiterate my enquiry: "Can anyone direct me to research regarding…" — Fly by Night (talk) 00:01, 30 June 2012 (UTC)

I'm not aware of any, and it would really be non-sense to try and study that non-relationship. Kinetic friction is only approximated using a coefficient. In reality it should be a function of speed, and whether it is also a function of the static friction, and how the static friction contributes, is dependent on the material(s), temperature, etc. 112.215.36.174 (talk) 07:15, 30 June 2012 (UTC)

Actually, I just found one paper that claims a useful empirical relationship between the two for some mechanical engineering purpose [12]. 112.215.36.174 (talk) 07:24, 30 June 2012 (UTC)

To reiterate my answer, the bowing of a stringed instrument (violin, celllo etc.) depends on the different static and kinetic frictions of the bow against the string. Over the course of centuries empirical research into the sound produced has found the effects of typical bow materials e.g. coarse horsehair, rosin, beeswax, and lately synthetic hair. DriveByWire (talk) 12:49, 30 June 2012 (UTC)