Wikipedia:Reference desk/Archives/Science/2015 June 16

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June 16

An idea about the existence of a big planet far away

A lighter body doesn't orbit the center of the heavier body, but both bodies orbit their barycenter. Suppose now there's a hard-to-see planet the mass of Jupiter at 1,000 AU from the Sun, call it Planet X. That would put X's and Sun's center of mass at about 1 AU from Sun, or near Earth's orbit. X, orbiting 999 times farther away from the Solar System barycenter than Earth, would take 999^3/2 ~= 31,575 years to complete an orbit. The Sun would take one year to complete this orbit. Now, X, being 200 times as distant as Jupiter, will have a gravitational influence 40,000 times weaker than Jupiter on the other planets, so the rest of the Solar system (with respect to Sun) should revolve almost the same as if X weren't there, meaning the whole system would wobble slightly during Sun's voyage through the galaxy. This wobble would mess up our method of measuring the distance of celestial objects by stellar parallax, so not having encountered such a problem, we can conclude there are probably no Jupiter-mass planets 1,000 AU away from the Sun. Am I missing anything? 78.0.193.74 (talk) 01:23, 16 June 2015 (UTC)

If you're trying to demonstrate non-existence by showing the absence of an observable effect, it might help to start by studying the resolution limits we can achieve on these types of meaaurement. Our article on stellar parallax estimates state of the art resolution to around 10 micro-arc-seconds. Any perturbation larger than that value should be measurable. If you're careful, you can couch your statement in the language of statistics: "based on so many observations, with such and such confidence, and within statistically measured instrumental error, we will not expect to find such a planet...."

Are you missing anything? Sure! It's hard to even know what else to account for. Have you a mechanism or theory which would explain how a large planet forms at such a distance? Have you any numerical orbital analyses that show the orbit would be stable? Would you need these analyses to establish prerequisite confidence in your hypothesis? The only way to know is to read what other scientists have established as the "norm" for this type of investigation.

If you don't already have access through a school or library, get an individual subscription to Icarus, and read some back-issues.

If you're looking for a great book on applying numerical physics to planetary science, I recommend Planetary Sciences, which has a section on using stellar wobbles to place confidence bounds on extrasolar planets. If you can finagle the math a little bit, you could probably apply a similar technique to our own sun. Nimur (talk) 10:22, 16 June 2015 (UTC)

Thanks for the advice, Nimur. Sorry if I sounded too sure about my idea, it was just an amateur thought experiment, I'm not an astrophysics student or anything similar. Planetary Sciences sounds interesting; I have an interest in astronomy and astrophysics and I think I've already read pretty much anything Wikipedia has to offer about the Solar System. Unfortunately, the book is a bit outside my price range at the moment. 93.136.80.168 (talk) 17:35, 16 June 2015 (UTC)

Reduce it to a two body problem and ask yourself how the sun can take one year to orbit the barycenter when the distant planet X takes 31,575 years, as that would place the barycenter on the opposite side of the sun from planet X once a year. (It's not much of a barycenter if that can happen, is it?) Hint: Was your application of Kepler's Third Law a valid method for determining the sun's period about the system's barycenter? -- ToE 11:37, 16 June 2015 (UTC)

Yeah, that seems obvious now. It looks like Kepler's 3rd Law holds only for the less massive bodies of the system. I'm guessing the real solution would be the inner Solar System making approximately a small ellipse every 31,575 years to counter for X's motion. If I'm right, this would account for up to 10^-5 AU in extra motion perpendicular to the Earth-star vector, which would, I presume, be next to undetectable. So there's still hope for a tenth planet... :) 93.136.80.168 (talk) 17:35, 16 June 2015 (UTC)

When you refer to a tenth planet, that implies that you are counting Pluto/Charon as one planet and not any of the other dwarf planets. I know that many people dislike the downgrading of Pluto, but one of the reasons for the downgrading is that there is no consistent criterion for counting Pluto and not counting Eris, or, for that matter, Ceres. Robert McClenon (talk) 21:47, 16 June 2015 (UTC)

If there any fundamental reason not to count Eris and Ceres? From my admittedly non-expert perspective, it almost seems as if this whole hooha has been driven by people not wanting the solar system to have too many planets, and redefining the term to exclude the new ones (with Pluto losign out as colleteral damage). Iapetus (talk) 12:34, 17 June 2015 (UTC)

It wasn't really a re-definition, because there hadn't previously been an official (i.e. IAU) definition. That said, the reason you give was indeed one consideration (wrongly, in my own opinion). From accounts I followed at the time (being an Ex-Astronomy student), the crucial meeting at which the definition was voted in was deliberately arranged for the last day of the XXVIth General Assembly (see the previous link, Section 4), when many attendees had already left or were leaving, and was packed with partisan Dynamicists. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 13:33, 17 June 2015 (UTC)

Planet X article says that the existence of another Saturn has been ruled out to 10,000 AU, another Jupiter to 26,000 AU. I suspect, but certainly don't know, that an issue with such huge, slow orbits, I think, is that it is difficult to imagine that planets in that or a smaller size range could "clear" them in a meaningful way, simply because so small a variation in ellipticity would make things intersect them. Our article on Hill sphere makes a comment about the approximate nature of the concept. Can anything small enough to be undiscovered out there get counted as a non-"dwarf" planet? Wnt (talk) 19:57, 17 June 2015 (UTC)

Auto-darkening welding helmets and sunglassses

Here's some info on auto-darkening welding helmets: [1]. They don't work like the sunglasses, which darken due to a (slow) chemical reaction. Instead, they have a quick electronic reaction. I believe they sense when the light level increases and turn on LCDs to block some of the light. Some also use high-dynamic-range imaging to darken only the bright areas.

My question:

1) Do any sunglasses offer similar technology ?

2) Specifically, do any use high-dynamic-range imaging ? I'm thinking this isn't far from the capabilities of Google Glass. StuRat (talk) 13:29, 16 June 2015 (UTC)

Nuclear shield glass aside

This is really old technology. Read Photochromic visor protects solders' eyes from nuclear glare --Aspro (talk) 15:11, 16 June 2015 (UTC)

That says they had a chemical reaction based glass that could respond within a millionth of a second, and they predicted it would soon replace the slow, temperature dependent reaction used in self-darkening sunglasses. That was 1975. AFAIK it never happened. What went wrong ? StuRat (talk) 15:25, 16 June 2015 (UTC)

Speculation- It seems the glass that shields your eyes from the negative effects of nuclear blast/welding is always straight glass. Sunglasses have curved glass. Perhaps it is not reasonable/efficient to try and make curved glass with the special shielding?Agent of the nine (talk) 16:02, 16 June 2015 (UTC)

Continued speculation: If you look at the link aspro provided Here in the picture it looks like it's just glass no battery or circuitry? So maybe it can be manufactured both ways?Agent of the nine (talk) 16:32, 16 June 2015 (UTC)

So they claim, but the glass in the article doesn't seem to be in widespread use, so I am asking why. StuRat (talk) 16:39, 16 June 2015 (UTC)

Back to the original Q

More speculation... The welding glass is a huge LCD screen. It needs electricity to go black. In other words, it is the same technology used by digital watches. Is that good technology? For a watch. Is it good technology for something that will be shoved in a purse, tossed in a glove box, or repeatedly fall to the ground from 5-6 feet up in the air? Probably not. Further, where should you put the battery? Bulky glasses went out of style in the 70's. Only people who want to prove they are "Retro" wear them now. Overall, I don't see the demand to be high enough to justify producing glasses that cost a lot more, require a battery, and provide a heightened risk of damage from a drop or bump (just recalling smashing a digital watch display that I liked by simply bumping by wrist on a doorknob while reaching for a light switch). 209.149.113.240 (talk) 16:27, 16 June 2015 (UTC)

I would be willing to wear something bulky while driving. Or, maybe a car windshield would be a better application, then. It could even track the driver's eyes so it knows where to darken the windshield and where not to. Passengers would just have to wear regular sunglasses, which is OK, since they don't need to see details like the color of a traffic light with the Sun behind it. On the downside, if the windshield ever malfunctioned and went all black, you can't just take it off, like a pair of sunglasses, and would be in serious danger. Perhaps a flip-up screen could show the dynamically adjusted contrast view, with a normal windshield available as a backup, by flipping the screen back down. StuRat (talk) 16:38, 16 June 2015 (UTC)

With all of that work, wouldn't it be better if we had a little hover ball that went everywhere with us and always aligned itself perfectly between our eyes and the sun? When indoors, it could sit between what we look at and the sun to block glare. 209.149.113.240 (talk) 17:46, 16 June 2015 (UTC)

If that technology actually existed, perhaps. StuRat (talk) 19:28, 16 June 2015 (UTC)

Well it would be nice if car headlamps had polarizing filters set to 45 degrees and then drivers could also wear ones also at 45 degrees to cut out most of the light of oncoming cars. I think we'll have self driving cars before we have any really good way in general use for people to do it! Dmcq (talk) 21:53, 16 June 2015 (UTC)

Surveilluminescent wand (see pic)

How does it work ? StuRat (talk) 14:55, 16 June 2015 (UTC

Having done a bit of commercial photography myself, I think you will find it is better known as Light writing.--Aspro (talk) 15:29, 16 June 2015 (UTC)

But how does the wand know what the camera's (the one being photographed, not the one doing the photograph ) FOV is? ApLundell (talk) 15:52, 16 June 2015 (UTC)

I agree that it is a bit different than simple lightwriting. It could work with a special camera if the camera shone an infrared light through the lens that could be detected by the wand, but the materials I see seem to imply that it can work with any camera. I googled around a bit and couldn't find much. You could try asking him on twitter [2]. The only academic work that uses the word is this paper [3] - I'm not sure if I have access but I can try later if you'd like a copy. SemanticMantis (talk) 16:00, 16 June 2015 (UTC)

Never mind, that's not really a research paper, it's just basically a pamphlet talking about a workshop, lots of hype and buzzwords and neologisms but no science (and it would have cost you $15 to download it!). Here's a "science" paper describing some related [4] work, and also seems to mention some of the welding glass things you were asking about above. Still really light on the details and high on showmanship (a whole section titled "Veillance in the Vironment" :-/), but at least it's freely accessible. SemanticMantis (talk) 16:10, 16 June 2015 (UTC)

The ref cited at File:Surveilluminescent Lights in Motion handwash faucets 03.jpg says that you tap into the camera signal to determine when it senses something. It seems to describe a feedback loop of sorts: "emit some light of a certain color at a location, see if that leads to a camera response, change color of light to indicate response." The image we see is then a superposition of the light as it is placed in many different locations (via a second camera that observes the whole test). DMacks (talk) 16:16, 16 June 2015 (UTC)

OK, thanks, that makes sense. They should be able to extend the concept to a planar surface rather than a linear one, to create a visual cone image. I also notice the tops and bottoms of the lines don't quite align, which I guess is due to a delay in the feedback, during which time the stick was moved. StuRat (talk) 16:57, 16 June 2015 (UTC)

Why V formation and not a row?

Would it be more energy efficient to fly in a row? --Yppieyei (talk) 17:55, 16 June 2015 (UTC)

As the article says, the advantage is being in the air current behind the wingtip, which means being behind and somewhat off to the side. Perhaps you are thinking of drafting (aerodynamics)? I guess the speeds are not high enough and/or the gain in lift or overall ease of flying is at least as advantageous. Plus the view is less cloacal:) DMacks (talk) 18:16, 16 June 2015 (UTC)

No. If you are not convinced by the explanation and refs in the linked article, have a look at this recent research, where scientists were able to actually observe birds catching updrafts from birds ahead of them [5] [6]. Since the vortex sheds from the wing tip (Wingtip_vortices), a bird directly behind another would not get that small updraft. SemanticMantis (talk) 18:34, 16 June 2015 (UTC)

Why not a triangular grid ?

Wouldn't the following arrangement be more efficient still, as the birds inside the grid get a double wing draft ?

     ^
    ^ ^
   ^ ^ ^
  ^ ^ ^ ^

StuRat (talk) 19:12, 16 June 2015 (UTC)

I think that the V formation also has to do with the birds being able to communicate to turn, up, down, etc. there might be too much confusion with your V shape Sturat. what do you think?Agent of the nine (talk) 19:20, 16 June 2015 (UTC)

Yes, I thought of that possibility, but decided not to say it so as to not influence the answers (although schools of fish manage to do simultaneous turns with complex 3D formations). StuRat (talk) 19:24, 16 June 2015 (UTC)

There is no top-down communication in either case. The current understanding is that flock dynamics are governed by small sets of rules that only involve watching how neighbors move. This is pretty much a whole subfield of applied math and wildlife biology. See Swarm_behaviour and Flocking_(behavior) for starters - also note that many birds (e.g. starlings) do fly in dense 3D clouds [7], but aerodynamic efficiency is not their prime motivator. No one bird is making decisions, and no specific maneuver can be commanded - it's all just an emergent phenomenon. Different animals, different objectives. SemanticMantis (talk) 19:31, 16 June 2015 (UTC)

(EC) Sometimes they do fly sort of like that. See the pic here [8], which has 2-3 birds thick on each arm, but none in the center. Or just skim some photos on google images - or better yet go watch some geese. The near-perfect V gets the most attention, but there are several variations that drift in and out of pattern. Sometimes they will fly in multiple Vs linked together, like if each ^ in your picture were ~10 birds or so. Geese at least will take turns flying the lead position - the front is the most tiring, so nobody wants to do it the whole time. Since the V is just a line with an angle, it's easy to rotate each bird to the front (usually front bird goes to the end of one arm, and the two adjacent positions take turns filling front, but there's considerable variation). There's also maneuverability concerns - in V, you don't have to worry about bumping your neighbor, just follow the guy ahead of you. Also you have to figure out how to get the pattern started. The V usually starts as a clump, then elongates into a line, then crimps into a V. It's very fault tolerant. I'm not sure what rules you'd need to get a packed grid going. Basically, there are many other issues besides pure aerodynamic efficiency, so even if your packed triangle would be slightly more energy efficient, that doesn't mean it's better for the birds. Here's an even better write up of V formations in Canada Geese [9], and here's a nice science article on flocking dynamics in general [10]. SemanticMantis (talk) 19:31, 16 June 2015 (UTC)

I'm wondering if there's any disadvantage to a close packed pattern with regards to predators? A spread out like would certainly be harder to attack than a more tightly packed "grid". Do geese have any airborne predators? Vespine (talk) 23:01, 16 June 2015 (UTC)

Our article lists some avian predators, but doesn't mention whether they're hunted on the wing. My first guess would be that they're not: Canada geese are large birds and very powerful fliers; attacking an entire group of them while on the wing would be a high-risk/low yield hunting maneuver. Matt Deres (talk) 12:31, 17 June 2015 (UTC)

To visualize the flow pattern in the air after the passage of an airfoil, such as a bird in flight, see horse shoe vortex. Dolphin (t) 12:47, 17 June 2015 (UTC)

There are many migratory avian species, with greatly varied behaviours for this activity, so one can't rule out the occasional raptor taking advantage of an obvious meal, particularly in cases of altitudinal migration, but longer-range migratory behaviour (particularly with regard to geese) can often take the birds to fairly high altitudes, relative to their species and avians in general -- that's part of evolutionary pressure that lead to streamlined flocking, as flight at the heights some species achieve can be markedly more taxing on stamina, and geese often typify the more extreme examples of altitude and endurance in migration. Raptors overwhelmingly prefer to attack in a dive from above, and tend to take their time hovering in lofty positions before an opportunity presents itself, often relying on thermal currents to achieve these positions. So the situation is all less than ideal (or outright untenable) for the raptors, most of whom rely more heavily on terrestrial and aquatic species to begin with. And then too, one might assume that a tight-packed formation is more favourable to a hunter, but this is not necessarily the case; in fact, many organisms swarm not just for social, migratory or mating behaviours, but also specifically because it frustrates the senses and predatory approaches of the hunter, through "target confusion" and other factors. So, while I don't think anyone can rule out every form of raptor/goose predation in this context, it's unlikely this factor has exerted any significant selective pressure upon the formations that have evolved to be employed by geese in general, which will be much more heavily influenced by the obvious environmental factors and, as SM already pointed out, communicatory/organizational constraints. Snow ^{let's rap} 22:14, 18 June 2015 (UTC)

I know I once read that the lead Canada Goose is often a widow, but it seems much more likely that the lead keeps changing (as mentioned above). Is there *any* truth to the widow theory?176.46.102.175 (talk) 02:06, 19 June 2015 (UTC)

I'd like to know how anyone could tell... AndyTheGrump (talk) 02:08, 19 June 2015 (UTC)

That's a new one to me, but no, I can't imagine there's any truth to it. For one, as noted, there is a fairly well-established "rules" by which the geese trade places and share in the burden of flying lead. For another, as Andy points out, I don't know how we'd have ever caught on to the fact that this widow tendency was a thing, as even a dedicated researcher who could recognize individuals and know when one had lost their breeding partner would still not be able to observe the exact flying formation they employed for their migration for more than the smallest portion of their trip (unless they were taking the lead role onto themselves). And then also, I don't know how the goose would recognize herself as a widow, unless the implication is that some physiological change takes place after a year non-productive breeding that then causes her to take this role onto herself. But I've never heard of any evidence for such and a cursory search on the matter didn't even turn up the topic for me, even as a myth. Snow ^{let's rap} 06:21, 19 June 2015 (UTC)

Two-headed animals

In the case of an animal born with two heads (or is it two animals born with one body? for the sake of argument, I'm going with the former), and assuming that both heads are 'viable' and of equal size/strength/completeness/health - who typically controls the body and how is this 'decided'? Is one head usually dominant over the other?

I've seen videos of two-headed reptiles that seem to manage okay, and I think (a long time ago) a two-headed cat. I think it's more common in reptiles than mammals though. --Kurt Shaped Box (talk) 23:05, 16 June 2015 (UTC)

One possibility is that each head controls one half of the body. I believe that's the case in the dicephalic parapagus conjoined twin women. StuRat (talk) 23:21, 16 June 2015 (UTC)

Yeah, I was wondering if they learned to coordinate like that. Or maybe it's one of those things that's literally impossible to know/understand unless you have shared a single body with another member of your species yourself... --Kurt Shaped Box (talk) 23:24, 16 June 2015 (UTC)

As I understand, the way a centipede or millipede works is that each set of legs just respond when the prior set moves. I picture something similar with conjoined twins, that each learns to move her leg right after the other one does. Maybe one of the two does dominate as far as initiating and stopping walking, however. StuRat (talk) 00:01, 17 June 2015 (UTC)

Conjoined organisms and semi-autonomous body segments in a developmentally normal and healthy organism are very distinct situations. The manner of innervation, the degree to which motor control is mapped in (and slaved to) the brain (or brains), the degree to which these movements are formed through autonomic or volitional pathways and the origin point of the stimuli between central and peripheral nervous systems all drastically vary in these different scenarios, so the parallels you suggest really make no sense. The reason conjoined twins control different halves of the "same body" is that they originate as two organisms that end up fused. In utero, they still develop a sensory/motor map which allows them to have proprioception and motor control for those limbs which are "theirs". In the case of two-headed animals, some are conjoined is a similar fashion to the twins referenced, but others develop in a manner where the extra head (or other superfluous body parts, as is sometimes the case) is thought to be just "along for the ride" -- though as Kurt noted himself, to some degree we can't really know to what extent this extra head complicates sensation and body-identity mapping in the creature or how either of the minds tied to each brain perceive the situation. There may very well be competing signals to the same areas of the body in some cases. But the case of body segmentation is radically different from any of these scenarios; a centipede or millipede moves its segments in relation to one another in the same way you or I coordinate the movement of the individual fingers of our hands. That is, it's a volitional movement but we don't consciously coordinate each individual joint. Rather we impel the basic movement, but the coordination is handled by various different elements of the central and peripheral nervous system of which we are only superficially cognizant. we can of course exert overt control over a specific joint if we consciously recognize the need, and segemented creatures can generally do likewise (a centipede that was coordinating its segments for simple forward locomotion can instead torque its segments in different directions in order to bring its forsections around in a hurry in order to meet a threat, for example). But the volitional component derives from just one brain and its hard to imagine this complex system could be coordinated efficiently with competing signals from two brains, if they each had innervation to the same areas.

So, just to be clear in relation to Kurt's inquiry, if the conjoinment is one of those infinitesimally small fraction of circumstances in which that leads to a viable, physiologically stable organism, then each brain has control over different parts of the body that have motor innervation and if there is sufficient ability to communicate coordination (and its hard to imagine that trial and error working for other species besides humans) then they could learn to become mobile. In cases of none-human mammals that have survived and functioned into adulthood (a still rarer situation than even a viable birth), then one head almost certainly had dominate control over the limbs and coordinated them in a roughly normal fashion. Our article on this topic, by the way, is polycephaly. Snow ^{let's rap} 05:59, 19 June 2015 (UTC)

I don't agree that it's hard to imagine such trial-and-error communication in non-human species. I don't expect communication to be in the form "I just move the left leg, now you move the right". I expect it to be more automatic than that, where each can feel when the other has moved their leg and respond by moving their own leg. Even fairly primitive species would fairly quickly learn that they get nowhere when each tries to act independently. StuRat (talk) 14:45, 19 June 2015 (UTC)

Well, that may be what you expect, but in reality that is not what happens (which is probably why you should not speculate here, but then it doesn't seem you're ever going to take that lesson to heart, no matter how many times other editors point out to you that this is not what we are here for and that responses here are meant to be sourced or at least based on sourced materials and not an unending stream of your best guesses in areas you clearly aren't well-versed in). Even in the rare occasions where conjoined organisms do not have physiological defects that guarantee death from systemic causes, they still live short, miserable lives because they cannot coordinate movement and thus cannot readily secure nourishment or avoid threats. The only exceptions to this are snakes, which have a very different form of bilateral locomotion (and even then, the only known survivors were kept as pets and attractions and cared for). Why you expect two non-human animals to have the self-awareness to even begin to understand their unique circumstances and be able to compensate for this situation, I don't know, but it's truely a silly idea. First off, these creatures aren't just dealing with coordination -- their conjoined bodies also don't have the normal biomechanics by which their species' bodies operate and leverage movement. Even if they did each have control of one-half of a near-perfect body, the process of coordinating motor control between different muscle groups is massively more complex than you seem to appreciate, and two brains sending two different complete streams of signals to two different halves of a body as part of their instinctual responses to their environment is just a non-starter.

Even Brittany and Abigail have to get by with stilted, inefficient movement, and they have the game-changing advantages of human language, human-level mechanical intellect, and an upbringing in a community of mature human beings dedicated to their survival. And then too, most all other animals operate at a threshold much closer to instinctual response to their environment, so it's not even just a matter coordinate the deeply complex aspects of quadripedal motor function -- they also would have to be able to communicate complex plans for where they are going, for what purpose, and how (between all the myriad options and complications) they are going to tackle the layout of the environment around them. Most animals just don't do a whole lot of planning in this regard to begin with and even those that don't have the intellect to plan them in concert with another creature in this particular way. Indeed, no known animal other than a human and perhaps some primate species has the cognitive array necessary to tackle any of the above. So while you apparently expect the two-headed creatures to have the presence of mind to understand that "I can't get anywhere without working with this guy" and to then proceed with successful trial-and-error methodology, the reality is that such creatures with a higher profile to their shared center of gravity (kittens for example) stumble around and fall over without getting anywhere, and creatures lower to the ground that can't easily topple (lizards) just move around in circles or in random directions, each relying on its own instinctual expectations of its body mapping and responses of its surroundings, until some lucky predator gets a snack with a little extra protein. Snow ^{let's rap} 20:48, 19 June 2015 (UTC)

Re: speculation, that's all I see in your reply. Re: "human-level mechanical intellect", if you mean designing suspension bridges, then clearly humans have the advantage, but that's hardly relevant. If you mean "ability to learn to move their bodies to accomplish the desired tasks", then I don't see how humans are much better at that than, say, an octopus, which can learn to open a bottle. As for language, I don't think Brittany and Abigail say "I'm going to move our right foot now, then you move our left", it just becomes automatic. Would a conjoined twin animal, given the choice to move it's legs to keep up with the dominant twin's legs, or be dragged, opt to be dragged ? No. It would quickly learn to do whatever happens to lessen the pain, which is to go along. I tend to agree that this will make for an inefficient gate and that they wouldn't tend to survive in nature, but that's not really the issue.

We can also look at how people adapt to drive bicycles or cars. We don't consciously think, "I am turning the steering wheel right, which will result in the two front wheel of the car turning, which will in turn turn the car to the right". We just know we want the car to turn right and the rest is "on autopilot". In cases where animals are fitted with artificial limbs or wheels, they similarly seem able to adapt. For example, if they can no longer fit through the dog door with their prosthetics, they learn not to try that again. The same thing would happen in learning to adapt to a conjoined twin. And they would have the advantage of being young when first learning to deal with their conjoined twin, when their brain is more malleable. StuRat (talk) 22:47, 20 June 2015 (UTC)

You might enjoy this nice little (freely accessible) article from PNAS, 1926, "INHIBITION OF REGENERATION IN TWO-HEADED OR TWO-TAILED PLANARIANS" [11] - it is discussing "making" two-headed planarians by cutting them in the appropriate ways, but it is a nice simple animal that might be a good way to start thinking about two-headed biology. SemanticMantis (talk) 14:02, 17 June 2015 (UTC)

You might enjoy this nice little (freely accessible) article from PNAS, 1926, "INHIBITION OF REGENERATION IN TWO-HEADED OR TWO-TAILED PLANARIANS" [12] - it is discussing "making" two-headed planarians by cutting them in the appropriate ways, but it is a nice simple animal that might be a good way to start thinking about two-headed biology. SemanticMantis (talk) 14:02, 17 June 2015 (UTC)