Wikipedia:Reference desk/Archives/Science/2014 December 29

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December 29

Is there a correlation between G6PD deficiency to allergy to penicillin?

I mean to ask if people who have G6PD have allergy to penicillin 149.78.225.73 (talk) 05:14, 29 December 2014 (UTC)

A pubmed search: [1] gives only 10 hits, and none that deal with type I, IgE mediated penicillin allergy. Judging by the titles and abstracts, the most relevant papers are about hemolytic adverse drug reactions. If a patient with G6PD deficiency were to have a penicillin-induced hemolytic drug reaction, it is possible that the reaction would be more severe than in a patient without the deficiency. Penicillin-induced hemolytic drug reactions are very rare (less than 1/10 000 Source in Norwegian, sorry). This information is not intended as medical advice. If you are a G6PD deficiency patient, and worried about taking penicillin, consult a doctor. --NorwegianBlue ^talk 14:48, 29 December 2014 (UTC)

Why is jet fuel measured in pounds and not in gallons?

I wasn't sure which Reference Desk to use, so I settled on this Science Desk. I was reading about that Indonesian plane that went missing a day or two ago. I read, in the news, the following statement: "Looking at the flight's paperwork, the plane had more than 18,000 pounds of jet fuel at takeoff, enough to fly about 3 1/2 hours ...". It struck me as quite odd that jet fuel (a liquid) would be measured in "pounds". Is this an error in the news report or is "pounds" the correct measurement for jet fuel? If the latter, why would that be the case? Thanks. Joseph A. Spadaro (talk) 05:29, 29 December 2014 (UTC)

It's usually in pounds or other units of weight, just as fuel for ships is measured in tons. I've always assumed that in both cases it is because weight is a critical factor in performance (e.g., keeping an airplane aloft or a ship afloat) though I don't really know. Short Brigade Harvester Boris (talk) 05:40, 29 December 2014 (UTC)

Yes, I imagine that weight is a critical factor for planes and boats. But, wouldn't the same concerns be addressed by measures of liquid volume, just as easily as by pounds and tons? I assume it's a simple conversion. So, let's say that we have this hypothetical situation: this airplane can only be flown with a maximum of XXX pounds of fuel. Can't they just convert pounds to a typical liquid measure and say the same thing, with the same result? Namely: this airplane can only be flown with a maximum of YYY gallons of fuel. Am I missing something? Also, if it's so "easy" and "convenient" and "practical" to simply use pounds, why do we bother with other units of liquid measure, as a general rule, anyway? Thanks. Joseph A. Spadaro (talk) 05:53, 29 December 2014 (UTC)

Could it be because the volume of fuel changes with altitude? -- Calidum 06:00, 29 December 2014 (UTC)

Hmmm. I have no idea. But, if the volume changes, wouldn't the weight also proportionally change? Or no? Joseph A. Spadaro (talk) 06:06, 29 December 2014 (UTC)

I suspect that the preference for weight measurement rather than volume is that the volume of a given mass of fuel varies with temperature - and aircraft may be refuelling at locations with large variations in temperature. AndyTheGrump (talk) 06:04, 29 December 2014 (UTC)

Yes, volume will vary by pressure and temperature. Weight should only vary by gravitational field changes, although acceleration of the plane may appear to make it vary. Mass should never vary at all. StuRat (talk) 06:10, 29 December 2014 (UTC)

Well, the total weight of the plane matters, not just to determine if the plane can take off, but in determining how much fuel it takes per minute to keep the plane aloft. So, you'd need to constantly convert fuel to pounds and then add it to the weight of the planes, passengers, crew, luggage, and cargo. It's a lot easier just to start with pounds.

As to why not always use pounds, I suppose you could, but it's generally easier to measure liquid by volume, as all you need is a graduated cylinder, not a precise balance scale. StuRat (talk) 06:05, 29 December 2014 (UTC)

OK, thanks. So ... the volume and the weight do not vary proportionally? They vary at different rates, according to the temperature (and other factors)? Joseph A. Spadaro (talk)

That suggests an interesting question. As the plane increases altitude, everything on board the plane, as well as the plane itself, loses weight. So, since I can't recall from 9th grade science class, does the weight vary proportionally for everything? I would think it would, but maybe someone can confirm that. As a simple example, a 10 pound little plane or drone is carrying a 1 pound payload. Let's also assume it can go ridiculously high. By some altitude or another, the plane's weight should have dropped to 9 pounds, for example, but its payload should also have dropped, to .9 pounds - so the plane is lighter, but is also carrying proportionally less weight. Right? ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:29, 29 December 2014 (UTC)

Hmmm. That is interesting. But, I was actually suggesting something altogether different. I assume that there is some "conversion factor" (i.e., a formula) that translates pounds into gallons and vice versa. So, according to this (How Do I Convert Pounds To Gallons?) ... Pounds = (Gallons * 8.345). But, wouldn't that conversion formula only work in a certain temperature? And then, if we move to a different temperature, we have to use a completely different conversion formula? Joseph A. Spadaro (talk) 06:40, 29 December 2014 (UTC)

Pounds at a particular altitude, such as see level. A quick rule of thumb is, "a pint's a pound the world around". That works in the kitchen, anyway. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:52, 29 December 2014 (UTC)

Um, nope. The U.S. pint is approximately 473 mL, while the Imperial pint is approximately 568 mL - a difference of 20%. Even in the kitchen, that is rather a large error. And come to think of it, the fact that there are two different non-metric measures of volume may be another reason to prefer weight for jet fuel. A 20% error there could be rather embarrassing. AndyTheGrump (talk) 06:57, 29 December 2014 (UTC)

Well, we have the metric system to thank for this incident. -- Calidum 07:01, 29 December 2014 (UTC)

Not really - we have a chain of events starting with malfunctioning equipment, and ending with the incorrect use of a figure for pounds per litre instead of kilogrammes per litre to thank for it. If the metric system had been used consistently from the start, and the aircraft hadn't been flown in a state where fuel usage couldn't be properly monitored, the accident wouldn't have happened. AndyTheGrump (talk) 07:11, 29 December 2014 (UTC)

In principle the weight of things decreases with increasing altitude; gravitational attraction varies inversely with (the square of) one's distance from the center of the Earth. However, the radius of the Earth is about 4000 miles, and most air traffic tops out at less than 10 miles: a trifling 0.25% further out, reducing weights by about 0.5%. (Even the "edge of space" – the Kármán line – is drawn at 62 miles (100 km), so the distance from the center of the Earth is only increased by about 1.5% even at that boundary.) TenOfAllTrades(talk) 07:54, 29 December 2014 (UTC)

Yes, density changes with temperature. Like most substances, jet fuels (and avgas) are denser at lower temperatures. The volume of a pound of fuel changes by more than 5% when going from -40°C to +20°C: page 22. (That's the difference on the ground between Alaska and the California coast in the winter, say, or the difference between San Francisco on the ground and San Francisco at about 40,000 feet: cruising altitude.) As with most liquids, the effect of pressure – from 1 atmosphere at sea level to about 0.2 atmospheres at cruising altitude – on density technically exists and is measurable with suitable tools, but is practically negligible for this purpose. TenOfAllTrades(talk) 07:54, 29 December 2014 (UTC)

Thanks. But, I'm still confused on this point. There is some "conversion factor" (i.e., a formula) that translates pounds into gallons and vice versa. So, according to this (How Do I Convert Pounds To Gallons?) ... Pounds = (Gallons * 8.345). But, wouldn't that conversion formula only work in a certain temperature? And then, if we move to a different temperature, we have to use a completely different conversion formula? I thought (per above discussion) that changes in weight and volume are not proportional and are dependent upon the temperature? So, if that's the case, wouldn't we need (and use) different conversion formulas for different temperatures? Joseph A. Spadaro (talk) 07:27, 29 December 2014 (UTC)

To more clearly and directly answer that question: The formula is approximately correct for determining the mass in Avoirdupois pounds (of about 453.5g) of one US liquid gallon (of approximately 3.785 litres) of water at 4 °C (and normalish pressures). It's not suitable for most jet fuels (probably not for any), and its accuracy for water goes down as temperature goes up, although it remains good enough for most household purposes at naturally occurring temperatures. --Stephan Schulz (talk) 15:01, 29 December 2014 (UTC)

OK, thanks. So, let's ignore the formula for water and discuss the formula for another liquid (namely, jet fuel). I assume there is such a formula, and that it happens to be somewhat different than the "water formula" I cited above. Nonetheless, would not the formula always change, dependent upon what the temperature is in a given spot? Joseph A. Spadaro (talk) 15:54, 29 December 2014 (UTC)

Yes, there is such a formula - in fact, it's essentially the same formula with a different constant. If you check jet fuel, you will find that there are different kinds with different physical properties, but standard jet fuel has a density of about 0.81kg/l or 1.79lb/l or 6.76lb/US gallon. That formula strictly only applies at 25 °C. If the fuel is significantly warmer or colder, it needs to be adjusted. Since jet fuel is a mixture of different carbohydrates, I suspect the pragmatic solution is to just have a sufficiently narrowly specified table, or an empirical formula. The effect is not huge for realistic temperature ranges, but easily measurable - I guess on the order of a few percent, if you go from -30 °C to 20 °C. --Stephan Schulz (talk) 17:13, 29 December 2014 (UTC)

Thanks. Got it. Joseph A. Spadaro (talk) 16:59, 31 December 2014 (UTC)

Going back to the original question, jetfuel is commonly measured by mass (pounds or kilograms) since it lets you a) know how heavy your aircraft is and where the CG is (weight and balance is rather critical to know for obvious reasons), and b) how much energy you have available. Volume - and therefore energy density - can change rather dramatic between a hot tarmac and 50.000' in the air, but the total mass do not. Luckily the dielectric permittivity of jet fuel changes more or less proportionally with the temperature (jet aircraft commonly uses capacitive probes as fuel gauges). WegianWarrior (talk) 07:43, 29 December 2014 (UTC)

The pilots of most aircraft work in litres or gallons. The capacity of the tanks in their aircraft is specified in litres or gallons; they purchase their fuel from the fuel supplier by specifying the required number of litres or gallons; they memorise their specific fuel consumption in litres per hour or gallons per hour. However, to calculate the total weight of their aircraft at take-off, and at landing, they must multiply the fuel volume by the density of the fuel. Pilots memorise fuel densities such as 0.75 kg/L or 6 lb/US gall or 7.3 lb/Imp gall.

For very large aircraft that fly very long distances and carry very large amounts of fuel, the aircraft manufacturer specifies fuel capacities as a mass (or weight). They specify specific fuel consumptions and specific air ranges as kg/hour or kg/km or pounds/nautical mile etc. Also, operators purchase their fuel by specifying the required mass (or weight), and they do their flight planning using mass (or weight). There are several advantages in this approach. Firstly, when the pilot calculates the total weight of the aircraft there is no need to multiply by the fuel density. Secondly, to reach its destination safely the aircraft requires a certain amount of energy - this is closely related to the mass (or weight) of the fuel and less so to the volume of the fuel because the density of fuel varies with temperature. It is the responsibility of the fuel supplier to know the prevailing density of his fuel and to work out how many litres or gallons will be required to supply the number of kilograms or pounds specified by the pilot. For a small aircraft, the variation in density of the fuel with temperature has an insignificant effect on the amount of fuel required to reach the destination. But for a very large aircraft, especially one travelling large distances between usable airports, the variation in density of the fuel with temperature is not insignificant. Dolphin (t) 08:00, 29 December 2014 (UTC)

The "conversion factor (i.e. a formula)" is just the density of the fuel. To convert pounds to gallons, multiply by the density in pounds per gallon. To convert gallons to pounds, divide by it. But, as stated, the density is not a constant but depends on temperature. The mass is more important than the volume for the reasons Dolphin states.

There was an incident in 1983 that is relevant to this. The electronic fuel gauges on a Boeing 767 had failed, and due to some miscommunications, the pilots thought it was acceptable to fly in that situation as long as the fuel quantity was checked by a "drip test" and entered into the flight computer. The drip test was done after fueling and provided the volume in the tanks, in liters, this being Canada. The fuel supplier was responsible for telling the pilots the density of the fuel and provided the number 1.77, meaning 1.77 pounds per liter. Unfortunately, whereas the rest of the Air Canada fleet was calibrated for fuel quantity in pounds, the 767 used kilograms. 1.77 was a familiar number and the pilots did not think about whether it was pounds per liter or kilograms per liter. Disaster was avoided only by good luck and skilled piloting when the plane ran out of fuel in mid-flight.

(There are several slightly different versions of this story in different sources, so there may be minor errors of detail here. I don't have the official report of the flight, and it doesn't seem that anyone has scanned it onto the Internet.) --65.94.50.4 (talk) 08:30, 29 December 2014 (UTC)

Roughly speaking jet fuel has a mass of 0.8 kg per litre, or 6.7 lb/US gallon.It'll vary a little with temperature and scarcely at all with pressure. Any of the apparently erudite opinions expressed above that differ significantly from this are wrong.Greglocock (talk) 10:12, 29 December 2014 (UTC)

Clearly a 5% change in volume due to temperature between a standard day at sea level and "normal cruise altitude" is not going to confuse the pilot or the flight computers, and all Technical Orders and Manuals issued for the F-5 and F-16 are wrong. Got it.

There are reasons - as outlined higher up - why military aircraft measure fuel by mass and not volume, and I'm assuming the same reasons apply to airlines which also experience large changes in ambient temperature. WegianWarrior (talk) 10:54, 29 December 2014 (UTC)

The relevant temperature variations are those occurring on the ground and affecting the fuel before it is pumped into the aircraft. Temperature variations at cruise altitude are not relevant because, by then, the fuel has been loaded. The fuel gauges on the flight decks of large aircraft are calibrated in pounds (or kilograms), not gallons or litres.

If the pilot of a large airliner orders 15,000 pounds of fuel the refueler must divide that number by the density of the fuel in the underground tanks in order to determine the number of gallons (or litres) of fuel to be pumped into the aircraft. The density of the fuel will vary according to the temperature of the fuel in the tank, but will also vary slightly depending on the chemical composition of the fuel. Dolphin (t) 11:39, 29 December 2014 (UTC)

Which part of 'roughly speaking' isn't in your dictionary WegianWarrior? Greglocock (talk) 23:44, 29 December 2014 (UTC)

Note that the long-range versions of the Boeing 747 have a fuel capacity of about 240,000 L. A 5% change in volume (as when fuelling at a warm-weather versus a cold-weather airport; think winter service between Honolulu and Chicago or Minneapolis) is a difference of 12,000 liters, which is about 10 tons of fuel in round numbers—and also about what the jet burns in a hour of flying time: [2]. It's also probably pretty close to the total amount of reserve fuel the aircraft is required to carry: [3]. So while it is true that density only varies 'a little' with temperature, it's definitely enough to matter. TenOfAllTrades(talk) 17:19, 29 December 2014 (UTC)

Hey guys! Look what I found! A reliable and encyclopedic reference: the Weight and Balance Handbook, part of the series of free aviation training textbooks made available at no cost by the United States' Federal Aviation Administration. There's an entire chapter on fuel weight. Large aircraft (above 12,500 pounds, including most commercial jets) tend to measure fuel loads in pounds or kilograms. Small piston planes tend to measure their fuel loads in gallons, and the pilot must manually crunch numbers to ensure the aircraft is safely loaded.

I sincerely hope no aviators take heed of the rampant and uninformed speculation that has proliferated earlier in this discussion. Nimur (talk) 16:47, 29 December 2014 (UTC)

The amount of chemical energy in a given mass of fuel is a constant - irrespective of pressure, gravity, temperature or whatever. Measuring the weight of some amount of fuel is a reasonably close (and practical) way to estimate the mass of the fuel because the force of gravity doesn't change all that much from one airport to another. This means that the weight of the fuel is a reasonably good measure of the energy content. The energy content determines how long and how far the plane will fly - and that's what really matters. Hence they dispense fuel by weight, not volume.

In the case of your car, you buy fuel by volume - and this turns out to be a really, REALLY stupid idea. If you buy a gallon of gasoline when it's cool, you'll be able to drive further than if you bought a gallon of the exact same formulation from the exact same gas station when it's hot. Since what you care about is how far you can drive - and not how much volume of gasoline you have - it makes zero sense to sell the stuff by the gallon. There has been much controversy about that here in Texas where the weather gets hot and the gasoline manufacturers are (in effect) charging more for their product than they should. It's been argued that gasoline pumps could easily be equipped with a thermometer to measure the fuel temperature as it's dispensed and have the price of a gallon of gasoline be dynamically adjusted accordingly.

I've seen several sources recommending that you develop the habit of buying fuel on your morning commute (when the weather tends to be cooler) rather than on the way home. That's because you get more energy for your money if you buy it when it's cooler. I'm kinda skeptical that it makes a significant difference because the fuel is held in huge underground tanks and the temperature probably doesn't track the ambient air temperature very closely. Better advice is to not refuel until you absolutely need to...the additional energy required to haul 100 to 200lbs of gasoline around is significant and you'll use less fuel if your car (including fuel) weighs less.

In the case of an airplane, the amount of energy content in the fuel is just as important - but the weight of the fuel also alters the weight of the plane - and that's rather important for the pilot to know, so knowing the number of gallons he has on board isn't very interesting...it's the number of pounds that determines both how long he can keep flying and how he has to configure the aircraft during landings and takeoffs.

SteveBaker (talk) 16:57, 29 December 2014 (UTC)

I'll note that in some places – including Canada – gasoline pumps can account for density changes with temperature, and charge for a corrected volume of fuel: [4]. TenOfAllTrades(talk) 17:19, 29 December 2014 (UTC)

Virtually all commercial airlines will use weight. They have a maximum takeoff weight and a maximum landing weight. Jet A I thought was about 7 lbs per gallon while water is 8 lbs per gallon and gasoline is 6 lbs per gallon. whether fuel burn is measured in gallons or pounds per hour, the only meaningful quantity is how many pounds are on the plane at landing as landing weights can be much less than takeoff weights. --DHeyward (talk) 05:41, 31 December 2014 (UTC)

Thanks, all. Joseph A. Spadaro (talk) 17:04, 31 December 2014 (UTC)

Lichen and moss identification

cladonia chlorophaea?Oregon, USA

cladonia chlorophaea?Oregon, USA

Can anyone identify these species? The lichen looks like the picture of cladonia chlorophaea in Plants of the Pacific Northwest by Pojar and MacKinnon p.501, but the entry mentions that there are a number of similar species and I was hoping to narrow it down. Given the tip on each sporophyte on the moss, I'm guessing twisted ulota (p.476) but the pictures of the mosses in the book are at higher magnification than I can get with my camera, so I'm very unsure of this one.--Wikimedes (talk) 05:34, 29 December 2014 (UTC)

I get the distinct impression it's crawling over the wall, like in The Blob (and I suppose it is, on a time lapse movie). StuRat (talk) 06:08, 29 December 2014 (UTC)

Hemorroids!

The hemorrhoids article is unclear. It seems to start off suggesting that a hemorrhoid is a normal physiological structure that can sometimes develop abnormalities. Then it seems to suggest that a hemorrhoid is an abnormality. My attempt to reconcile this contradiction in the context of my limited prior knowledge is that in medical terms a hemorrhoid is a normal physiological structure which is referred to as an abnormality by the layman (leading to the ambiguity in the article). What's the situation actually? --78.148.105.13 (talk) 08:21, 29 December 2014 (UTC)

Haemorrhoids are swollen and inflamed veins around the anus. These veins occur as part of the normal anatomy of the anus but the acute or chronic inflammation or swelling of the veins which leads to discomfort, pain itching or bleeding is an abnormal pathology. The swelling and/or inflammation is usually caused by excessive pressure exerted on that part of the venous system which causes the veins to swell and ooze blood. Perianal itching occurs most commonly from inadequate personal hygeine which leave small amounts of faeces within the folds of the anus. Causes of increased pressure may be constipation leading to the need to strain, raised inter-abdominal pressure during later pregnancy or increased weight. [5]. Richard Avery (talk) 08:35, 29 December 2014 (UTC)

So would you agree that the first two sentences of the hemorrhoids article are misleading/inaccurate? If what you have said is correct, then hemorrhoids are inherently pathological which contradicts that part of the article, right? 78.148.105.13 (talk) 10:26, 29 December 2014 (UTC)

I agree that the opening paragraph needs rewriting and the phrase "pathological haemorrhoids" needs reviewing (I'm going to do it now) as it gives the impression that there are "non-pathological haemorrhoids". Richard Avery (talk) 10:56, 29 December 2014 (UTC)

Sidetrack: Where to discuss article improvement.

The place to discuss an inconsistency in the article is the talk page, Talk:Hemorrhoids. Robert McClenon (talk) 22:04, 29 December 2014 (UTC) Poppycock. I could discuss it Costa Coffee if I want. It's not out of place here. I'd have a long wait for an answer on the damn article talk page and you know it. I might as well write my query with my finger in the sand as put it on that talk page. 78.148.105.13 (talk) 22:32, 29 December 2014 (UTC) Issue discussed at WT:RDS#Hemorroids. -- ToE 02:28, 30 December 2014 (UTC)

Doc James has responded at Hemorroids#Edit with a link stating that the term "hemorroid" is used for the anatomical structure both in their normal condition and when inflamed -- ToE 12:09, 31 December 2014 (UTC)

Moment capacity of beam

Am I correct in thinking that the moment capacity of a beam has no effect on the bending moment distribution on it? 82.132.213.121 (talk) 15:21, 29 December 2014 (UTC)

I dont understand your Question. Moment capacity is an abilty and moment distribution sounds like an actual force. You have to add some link or detail to make sense. --Kharon (talk) 09:12, 30 December 2014 (UTC)

The crossing of a small female toy dog and a large male dog

In nature, speciation occurs when, say, two populations of the parent species cannot mate with each other in some way. In domesticated animals or animals in captivity, this can be a bit tricky, as humans can intervene in the mating process between a female dog and a male dog (or a female lion and a male tiger) or assist in the birthing process. This may suggest speciation occurs in animals in the wild, but there have been documented cases in which a wolf species successfully blends with the coyote species to form the coywolf species in the wild. Now what? Is this new hybrid species the opposite of speciation? In general, some hybrids are viable and fertile offspring of the parent generation, while other hybrids can't even be viable, let alone reach adulthood and mate naturally. In domesticated animals, I am wondering if it's possible to mate a large male dog (Newfoundland) and a small female dog (Toy poodle). I don't think viable puppies should be the size of the mother, because that would certainly suffocate the life out of her. In that case, are Newfoundlands and Toy Poodles different species, or are they considered the same species merely because they share the same ancestral lineage to the wolf? 71.79.234.132 (talk) 16:30, 29 December 2014 (UTC)

Domestic dogs are all the same sub species Canis lupus familiaris- See our article Dog breed. Two quotes from the article: "Dog breeds are groups of closely related and visibly similar domestic dogs, which are all of the subspecies Canis lupus familiaris, having characteristic traits that are selected and maintained by humans, bred from a known foundation stock" and "Dog breeds are not scientifically defined biological classifications, but rather are groupings defined by clubs of hobbyists called breed clubs". The only obstacles to breeding any dog breed with another are the practical ones of differing size etc. You could, however, artificially inseminate a Newfoundland bitch with the sperm of a toy poodle dog and produce offspring. The results of crosses between different breeds are called Mongrels. Richerman (talk) 17:16, 29 December 2014 (UTC)

Why do domesticated animals get an exception? 71.79.234.132 (talk) 17:23, 29 December 2014 (UTC)

Domestic animals do not get an exception. Richerman wasn't saying that. Both you and Richerman were referring to the practical issue of size. What is extremely unusual about Canis lupus familiaris is the extreme range of sizes in what is still, in the DNA, a single subspecies. The size problem is unlikely to be observed in non-domesticated species that do not have breeds with a weird range of sizes. As noted below, see species problem, as this is a special species problem. The breeds, which are the same subspecies in the DNA, can conceive, but the size difference is likely to be problematic with a small female and large male. I don't know what the experience of breeders is, although breeders may not care, because they try to maintain purebred species. I don't know what the experience of veterinarians is with size-discrepant mongrels. Robert McClenon (talk) 22:28, 29 December 2014 (UTC)

See ring species. Also note that humans are in the same situation as dog breeds, as a very tall male human and a very short female probably couldn't have normal childbirth. However, the fact that a midget male and giant female can breed makes it not quite a ring species, in both cases. StuRat (talk) 17:59, 29 December 2014 (UTC)

I mostly disagree as to Homo sapiens. The size range in H. sapiens, which is not the result of breeding, is no greater than other species, so that the problem of large fathers and small mothers is probably present in other mammalian species. H. sapiens does have a special problem, however, which is that full-term infants are at the limit of carrying capacity of their mothers, and some births are problematic due to cephalopelvic disproportion. That is because human babies have (and must have) large heads in order to start to learn human knowledge immediately after birth. Some other species have other special problems, such as giant pandas are too small. Robert McClenon (talk) 22:28, 29 December 2014 (UTC)

It sounds like you agree with me. Specifically, if human mothers are already at the limit in normal circumstances, a large baby and small mother can easily push them past that limit. StuRat (talk) 01:06, 30 December 2014 (UTC)

See also species problem. Richerman (talk) 18:16, 29 December 2014 (UTC)

There's an old joke: "I crossed a Great Dane with a Chihuahua." "What'd you get?" "A dirty look from the Chihuahua!" However, googling the subject yields examples of success with such cross-breeding. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:17, 30 December 2014 (UTC)

I would have said "Well, the chihauhau was definitely cross with me !" StuRat (talk) 03:28, 30 December 2014 (UTC)

• This is a matter of physiology, and has very little to do with species problem, and absolutely nothing to do with ring species like similar sized gulls or herps per se. μηδείς (talk) 02:05, 1 January 2015 (UTC)

Question about contraception--Can RISUG/Vasalgel theoretically fail?

I have heard about this contraceptive for a while by now, including how it has apparently been 100% effective in clinical trials so far; thus, my question here is this--can this contraceptive theoretically fail? I myself am guessing that the answer to this question is Yes, but I am wondering as to what all of you think about this.

Also, for reference: Reversible inhibition of sperm under guidance

Futurist110 (talk) 18:41, 29 December 2014 (UTC)

You've been here for long enough to know you shouldn't be asking what we think of anything. However in terms of the general question of failure rates, the article doesn't give any reasons to expect things will necessarily be better than Vasectomies which do have a non zero failure rate. Nil Einne (talk) 19:30, 29 December 2014 (UTC)

Maybe I should have phrased this better, but what I meant was if anyone here would be able to utilize his or her knowledge on this to try answering this question of mine. Futurist110 (talk) 06:23, 30 December 2014 (UTC)

Reading a bit more [6], it sounds like the early failure rate could be significant lower. However it's less clear for the late failure rate, and still no particularly reason to think it will be definitely zero (even with a medical procedure there's still the human component of the doctor as of now). The source also mentions a non surgical reversal method and while it sounds like something unlikely to occur accidentally (but people do have a lot of different sexual preferences), it may not always be necessary to go to such a level for reversal. P.S. To be clear, when I say "it" above, I'm actually thinking of the overall failure rate rather than just the late failure rate. My point is that the early failure rate may be significantly lower than a vasectomy, for reasons given in the source, the info given is insufficient to even suggest the late failure rate will be significantly lower nor to suggest that the overall failure rate will be zero. Nil Einne (talk) 19:36, 29 December 2014 (UTC)

Thank you for this info; also, I've got a question--can a male's body theoretically reject and/or expel this injection (before 10 or 15 or whatever number of years are up)? Futurist110 (talk) 06:23, 30 December 2014 (UTC)

No particularly reason to think it will never happen. Nil Einne (talk) 13:48, 30 December 2014 (UTC)

Reversible_inhibition_of_sperm_under_guidance#Advantages says that 1 of 250 test subjects did have an unplanned pregnancy as a result of improper placement. So, including this in the mix, we get a 0.4% failure rate. However, I suspect that the medical personnel administering this method in a test environment would be better trained than average, were this to go into wider use. So, the human error rate might go way up. StuRat (talk) 19:48, 29 December 2014 (UTC)

Can't one make sure that the doctor performed this procedure correctly afterwards when one goes/returns to the doctor's office for a check-up, though? Futurist110 (talk) 06:23, 30 December 2014 (UTC)

Well if you read our article, it does discuss Smart RISUG, and also link to the source (from 2009) discussing this concept of adding iron oxide and copper particles to make visualation with x-ray and magnetic imaging easier. I would suspect it would probably show up under x-ray even without the additions, but x-raying the male gonadal area may not be ideal if you're looking for reversible contraception and I guess the additions probably increase the contrast or allow a lower dosage. Don't know about MRI without the additions (well I presume they mean MRI), I'm actually a bit surprised it's okay to use this after the additions of iron oxide and coppper but I guess the quantity is low enough that it's safe. In any case, the source does seem to suggest Smart RISUG is important to enable easier testing and does mention contrast. But Smart RISUG seems even less tested than the others, actually I can't find any further followup since 2009. Of course an MRI isn't exactly cheap anyway. Nil Einne (talk) 13:48, 30 December 2014 (UTC)

The iron oxide is funny. If you tell somebody your balls are full of rust, they might assume you haven't had sex for a very long time. StuRat (talk) 15:47, 30 December 2014 (UTC)

Looking at the article - I assume (and hope) they apply the radiation to the polymer _before_ it's been injected? 300 rads to your intimate bits in 10 seconds will be very effective in preventing reproduction, and, indeed, respiration. I think some clarification of the article is in order. Tevildo (talk) 20:45, 29 December 2014 (UTC)