Wikipedia:Reference desk/Archives/Science/2017 June 28

= June 28 =

Characteristics of a predator

 * If a lone human gets eaten up by a crocodile, then is the crocodile the predator?
 * If a human kills a pig and eats its flesh, then is the human a predator?
 * If a human exchanges metal coins for a dead pig's flesh from a dead pig that is killed by another human or a slaughter machine, then is the human a predator? Or does being a predator automatically mean that the human must kill the animal and eat it?
 * If a very tiny population of creatures eat a human's flesh, then is the whole population of tiny things considered a "predator" or are they "many predators"? 50.4.236.254 (talk) 03:13, 28 June 2017 (UTC)
 * Small organisms that feed off of another living organism without killing that other organism are generally considered parasites rather than predators. Generally, a predator is an organism that obligately *kills* other animals to feed on them. Where this gets murky, of course, is in the case of parasitic animals that inevitably kill whatever they're parasitizing; these are called parasitoids (parasitoid wasps, for example). NorthBySouthBaranof (talk) 03:17, 28 June 2017 (UTC)
 * Agreed on the last point. (I retract that per Gem fr below; we both made unwarranted assumptions about the tiny things) For the others: technically, I think that exchanging metal coins for part of a dead pig makes you a scavenger, sure as if you had growled at the hyenas to drive them off.  Of course in terms of digestive adaptations you may not look quite like a scavenger, but to some degree you might... human ancestors were notorious for running to where the buzzards were going and cracking bones for the marrow.  Killing the pig is surely predation.  And crocodiles... well, if a crocodile isn't a predator, what is? :)  The term omnivore applies (humans aren't what you'd call carnivores or herbivores) but that doesn't rule out acting as the predator (just ask the grizzlies, they'll be happy to explain) Wnt (talk) 03:24, 28 June 2017 (UTC)
 * It should be noted that most predators are scavengers as well, which is to say that carnivores don't pass up a free meal even if they didn't kill it. This article notes that it's easier to thinking of carnivores as existing on a continuum of scavenger-predator and that there's few of any scavengers that never kill, nor predators that don't scavenge. -- Jayron 32 05:15, 28 June 2017 (UTC)
 * Also as is nearly always the case, definitions are fussy at the boundaries. In particular, I don't think it's clear whether something that eats prey hunted in a community setting is a scavenger. Is a lion cub a scavenger when it eats food caught by others in the pride in a hunt it didn't participate in? What about a male lion in a pride? (As our article mentions Lion group hunts in a pride generally involve the lionesses although male pride lions probably hunt more than we're previously appreciated.) When a lion (or lion pride) steals the prey of a hyena sure. For that matter if it steals the prey of a solo lion (or very unlikely, of a lion pride). Nil Einne (talk) 05:44, 28 June 2017 (UTC)
 * Also as is nearly always the case, definitions are fussy fuzzy at the boundaries. In particular, I don't think it's clear whether eating something hunted by your community as part of the community counts as scavenging or makes you a scavenger in that context. Is a lion cub a scavenger when it eats food caught by others in the pride in a hunt it didn't participate in? What about a male lion in a pride? (As our article mentions Lion group hunts in a pride generally involve the lionesses although male pride lions probably hunt more than we're previously appreciated.) When a lion (or lion pride) steals the prey of a hyena sure, it's often called a scavenger. For that matter if it steals the prey of a solo lion (or very unlikely, of a lion pride). You can look outside lions at e.g. animals that hunt for and bring home food for their mate. Once you start talking humans things get particularly fussy fuzzy (edit: and not just in terms of predator/scavenger) but the human context being referred to tends to be much more like these fussy fuzzy non humans animal examples than in a lions stealing from a hyena or another lion. (I wasn't totally happy with my earlier wording but it had been a while and since there were no replies decided it easier just to strike out the earlier response and repost it.) Nil Einne (talk) 11:02, 28 June 2017 (UTC)
 * I am surprised that predation wasn't mentioned in answers.
 * In the pig example, hunting is predation, it may be argued that animal husbandry is not
 * In the money example, the predator / scavenger / grazer categories just don't apply. Would you ask if a man that give money for a car is a carmaker, a mechanics, etc. ? The man is a consumer, ant that's it.
 * for you "many tiny" question, our predation article use the example of many ants killing and eating a much bigger prey, obviously each ant is considered a predator as well as the whole pack. See also meat ant, that takes for granted that they are indeed predators.
 * Bottom line: RTFM before asking questions
 * Gem fr (talk) 12:18, 28 June 2017 (UTC)

Side thought

 * I am not trying to prevent replies to the OP, but this got me thinking: Is this actually a science question? One could argue that it is a language question, since it is about the definition of predator. Or am i just being facetious, and actually defining such open ended terms is part of science? --Lgriot (talk) 11:58, 29 June 2017 (UTC)


 * It's both. It's linguistics of science, nomenclature, semantics, ontology, classification, etc. We take this stuff very seriously. IUCN, etc. Not to mention glossary of biology that captures "easy" definitions. Lots of research posits working definitions as par for the course. If you read research on e.g. predation, the authoring scientists often include the definition they will be using.
 * TLDR: making of definitions and discussing their limits is well within the scope of science, broadly construed. I personally think the question is best suited for this desk. SemanticMantis (talk) 14:20, 29 June 2017 (UTC)

Can cross spiders (Araneus diadematus) change colour to match their surroundings?
Most cross spiders I've seen are brownish (or redish brown or greyish brown) in colour. However, I've seen a lot of charcoal-grey ones living on a bridge that had been painted that colour, and once saw a very bright yellow one on a hedge that contained leaves of that colour. Can the spiders change colour to match their surroundings? And if not, what accounts for the colour difference? Or are these difference species that I've mistaken for A. diadematusIapetus (talk) 13:35, 28 June 2017 (UTC)


 * each spider do not change colour to match its environment, but since Araneus diadematus can have a range of colours "extremely light yellow to very dark grey", the whole local population surely will follow Peppered moth evolution scheme, so that colour that give some adaptive advantage will prevail.
 * Gem fr (talk) 14:08, 28 June 2017 (UTC)


 * Yep, color of insects and spiders is hugely variable. To the extent that serious ID almost never uses color as a key characteristic. Here is a nice scholarly overview of color in spiders, this  research is focused on color change during development of one (Araneae) species. SemanticMantis (talk) 14:45, 28 June 2017 (UTC)

Can hydrogen conduct electricity?
Science news had an article about Jupiter that said there is a theory that its magnetic field is caused by circulating electric currents in one of the planet's outer layers of molecular hydrogen. Does hydrogen conduct electricity? It does as a plasma but how about when it isn't ionized? Does it need a certain pressure? RJFJR (talk) 14:20, 28 June 2017 (UTC)
 * See metallic hydrogen. As is defining for a metal, this involves delocalized electrons (free to move) as a "cloud" within the metal, thus conduction.  Yes, it takes a great deal of pressure. Andy Dingley (talk) 14:25, 28 June 2017 (UTC)
 * Is that the only time hydrogen would conduct? (No one has proved metallic hydrogen exists and it was proposed it might exist at the core of Jupiter but the layer described in the article is an outer layer so probably not sufficient pressure for metallic.) RJFJR (talk) 14:31, 28 June 2017 (UTC)
 * It is estimated that metallic hydrogen extend ~80% of the radius of Jupiter. That's fairly "outer" to me.  Dragons flight (talk) 00:16, 29 June 2017 (UTC)
 * Here's a paper on the topic of metallization in Jupiter's atmosphere. Mikenorton (talk) 14:32, 28 June 2017 (UTC)
 * Thank you. RJFJR (talk) 14:42, 28 June 2017 (UTC)


 * If hydrogen is heated to a plasma (which some may say is not really hydrogen anymore because it is a different state of matter) that will conduct. As will any other element in a plasmic state. Aspro (talk) 21:57, 28 June 2017 (UTC)
 * But Jupiter's atmosphere isn't thought to be a plasma. Andy Dingley (talk) 22:20, 28 June 2017 (UTC)


 * Ionic hydrogen H+ conducts in solution. μηδείς (talk) 00:09, 29 June 2017 (UTC) (surprised that's a redlink. Maybe it should redirect to acid?)
 * I think you may be looking for the Grotthuss mechanism? Double sharp (talk) 07:36, 29 June 2017 (UTC)
 * Hydron (chemistry) ; i made the redirect Gem fr (talk) 12:38, 29 June 2017 (UTC)