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Giant Spiders are a traditional element of fantasy worlds. They prey upon most human's primal fears, and are easy to make as evil as possible.

Spider in Canada (Note: Not to scale)

But are they actually plausible? What kind of environment would be required to sustain a sizable population of giant spiders?

What can they eat, and how much of it would be required?

Assume:

  • Fantasy world, ala Middle Earth/Azeroth/etc...
  • These spiders are social creatures. Most depictions of them in fiction have them ganging together in swarms of over a dozen at least.
  • Sentient creatures are only going to be victims a couple of times before the spiders' territory becomes known and avoided.
  • Giant spiders, for some biological or environmental reason, exist.
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    $\begingroup$ Size, weight and species would be really helpful. $\endgroup$ – DonyorM Nov 10 '14 at 12:56
  • $\begingroup$ @ DonyorM It would, but I know little about spiders. Perhaps you can glean some information from: en.wikipedia.org/wiki/Shelob or lotr.wikia.com/wiki/Great_Spiders $\endgroup$ – Danny Reagan Nov 10 '14 at 12:58
  • $\begingroup$ Ok, assumed some values in my post. You might want to put some arbitrary weight numbers in the question so that people have a standard to answer by. $\endgroup$ – DonyorM Nov 10 '14 at 13:08
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    $\begingroup$ Is this question specifically about diet (as implied in the title)? There might be other reasons for them not to be plausible, but I think a diet-focused question will be more valuable than the more-general one. $\endgroup$ – Monica Cellio Nov 10 '14 at 17:35
  • $\begingroup$ @MonicaCellio Yes, this is diet focused. Whether they can exist or not is interesting, but out of scope. $\endgroup$ – Danny Reagan Nov 10 '14 at 17:36
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Summary: Yes it would be possible. If the spiders are 40 kg, one deer would feed about 47 spiders for one day. A large forest area could support many spiders.

Math note: assume that the word "calorie" actually means kilocal or Cal. I'm going by food calories and not chemistry calories.


A white-tailed deer weighs about 100 kg on average. Of that 100 kg, we will assume that about 80 kilos will be eaten by the spider. The "shell" of the deer won't be eaten by the spider, and it is most likely that the spider won't be able to suck the deer dry. 30 grams (1 ounce) of deer meat has about 45 calories in it. So one deer will approximately have: $80,000/30 * 45 = 120,000 Cal$

How food a spider needs per day varies widely depending on the lifestyle of the spider and the species, for this post I will use a tarantula as an example. One tarantula owner said that his 200 g tarantula ate roughly 5-50 Cal a day. For the rest of these calculations, I'm going to use 25 calories a day because that's medium range.

Let's assume that the giant spiders weigh 40 kg. That's about 200 times the size of a normal tarantula. So $200*25=5,000$ calories needed a day.

That means one deer can feed about 24 spiders for one day. Now spiders can last a long time on one meal, so the spiders wouldn't necessarily eat a deer every day. A spider that large might also need more energy for a better circulatory system or other bodily functions. But even so these spiders could easily survive on the food in a normal forest. A Pennsylvanian forest (with human hunters) has roughly 30 deer per sq. mile. A group of twelve spiders would need to have only a couple square-mile territory.

Thanks to Shempi and Shisha for noticing math and information errors in this post.


Now for some fun. MiraAstar asked how dwarves would figure into this. According to a Lord of the Rings wiki, average dwarf height is 4' 9'' (1.4 meters). Dwarf weight was not recorded, but I guessed that a dwarf would have a BMI of roughly 26, due to their tendency to be overweight. This would put the average dwarf at 130 pounds (59 kg). I'm also going to extrapolate the dwarf meat would be about as high energy as pork, which has 229 calories per 100 g. Figure that a dwarf is only 70% edible, and you have 41.3 kg of edible meat. $41,300 g *229 Cal/100 g = 94,577 Cal$. In other words, an average dwarf has about 94,577 calories in him. $94,577/5000 = 18$, so one dwarf could feed about 18 of our 40 kg giant spiders.


Original Post. The part assumed that spiders only drank the blood of their victims (which is incorrect). Left here because I spent time writing it so I don't want to delete it.

Let's do some math:

A liter of blood (in a human) has roughly 450 calories in it. An average white-tailed male deer weighs around 100 kg. A deer that size has about six liters of blood in it. So $6*450 = 2700$ calories. That's very rough numbers, but for this question it will do.

It's not easy to tell how much a spider will eat, it depends largely on the species and/or the behavior of the spider. This question will assume that the spiders act like a tarantula. I'll also assume that the spiders weigh about 20 kg. That's about 100 times the weight of an average spider. According to one spider owner, a 200 g tarantula eats about 25 calories a day. So $25*100=2500$, meaning that these massive spiders would eat about one deer a day.

In Pennslyvania, there are an average 30 deer per square mile. This makes the numbers of deer run pretty low if there are twelve spiders. But with other animals (a bear could probably feed a couple of spiders), and a range of hunting several square miles, then their would be enough feed.

Sometimes spiders eat more than that in a day, sometimes less. Most of the numbers in this post are average.

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    $\begingroup$ Spiders do not life from blood.... $\endgroup$ – Sempie Nov 10 '14 at 13:33
  • $\begingroup$ Did you read a little too much Could a humanesque creature derive all its nutrients from drinking blood?? $\endgroup$ – a CVn Nov 10 '14 at 13:52
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    $\begingroup$ Most liquefy internal tissues in the prey and suck that out, but I don't know of any that simply feed on blood. If you figure out the average caloric content of deer tissue (it's ~1200/kg for the meaty parts), you could probably estimate a percentage of what would remain as the "husk" from a 100kg deer. $\endgroup$ – Geobits Nov 10 '14 at 14:04
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    $\begingroup$ I'd really like to know how many dwarves this would translate into. $\endgroup$ – MiraAstar Nov 10 '14 at 15:24
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    $\begingroup$ @MiraAstar Added dwarves! $\endgroup$ – DonyorM Nov 11 '14 at 10:54
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As far as dietary requirements go, there's absolutely no problem with giant spiders. They'd have essentially the same dietary needs as any other large predator, and so any environment able to support, say, wolves or bears or tigers could also potentially support giant spiders of similar size.

(In fact, since spiders are ectotherms, they could potentially survive on a sparser and/or less regular food supply than mammalian predators. In that respect, reptilian predators like large snakes or crocodiles might provide a better comparison.)

What prevents spiders from growing a big as a bear in real life is not food supply, but difficulties with scaling up the arthropod body plan. The main issues appear to be the exoskeleton (and the ensuing need for moulting), as well as oxygen intake:

  • The bigger the animal, the thicker, even in proportion, its skeleton must be to support its weight. A thicker skeleton is heavier, and thus requires stronger (and thus thicker, and heavier) muscles to move it, which in turn need more skeletal support, and so on. At some point, the skeleton just can't support muscles that would be strong enough to lift it.

    This is a simple consequence of the square–cube law, and applies equally well to animals with internal skeletons as to those with external ones, but it appears that, at large sizes, an internal skeleton is more efficient at supporting the body, and can therefore support larger body sizes.

  • Another big issue with arthropod exoskeletons is that they can't grow with the body, but must be shed and replaced by moulting. A moulting spider is weak and vulnerable, and the bigger the spider (and thus the thicker the exoskeleton), the longer this "callow" phase lasts.

    For large tarantulas, like the Goliath birdeater, the new exoskeleton can take hours to dry and harden; for a hypothetical bear-sized spider, it would likely take days. During this time, the spider can't move and is essentially helpless. About the only way a giant spider could survive moulting would be to dig a burrow and hide in it, hoping that no opportunistic predator spots it before it's capable of defending itself again.

    (If the giant spiders were social, they could potentially guard and defend each other while moulting, but this presupposes a very advanced social structure and comes with costs: the spiders that are guarding their moulting pack-mates can't be out and hunting at the same time. Still, if you want to have marginally realistic giant spiders in your fantasy setting, this seems like the way to go.)

  • For many terrestrial arthropods, such as the insects, the actual size-limiting factor appears to be their (relatively) passive respiratory system, which relies on air diffusing directly into the tissues via a network of open-ended tubes called trachea. This works well at small sizes, but scales poorly as the body gets bigger. Some evidence for this hypothesis is provided by fossils of very large insects from the Paleozoic era, when oxygen levels in the atmosphere were higher than today.

    Many spiders, however, do have an active oxygen transport system with book lungs, so they might not be so susceptible to this particular problem. Still, your giant spiders are going to need efficient lungs to absorb oxygen, and an efficient heart to distribute it into their tissues. (The last part is a non-trivial one; the fully divided double circulatory system in birds and mammals is the result of considerable evolution.)

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  • $\begingroup$ well said, the upper size limit isn't so much food supply as physiology (and of course a bigger spider may well find it harder to find food, as it can't rely on the stealth that most actual spiders rely on. $\endgroup$ – jwenting Nov 12 '14 at 8:26
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First question is, How large could they get?

Therefore we must know, how they grow. Simplified: Insects and Spiders have growth-hormones, just like mammals. But their exoskeleton can't grow with them. They have to change it from time to time, as they get too large for their skeleton. So while growing, the growth-hormones do not increase and at some point the hormone per blood rate will be so low, that they do not grow anymore. This is the time when they change their skin. After they take off their old skin, the growth-hormone production increases and they start to grow again, this is a loop that lasts for their whole life, but decreases more and more over time.

www.iflscience.com/plants-and-animals/why-monstrously-large-insects-and-spiders-dont-exist

So this tells us that they will grow slower and slower as they get larger. A huge spider (let's say dog-size) must be very old, multiple 1000's of normal spider lifetimes I would guess.

But there is another (real(!)) problem: The circulatory system of Spiders works differently than what we know from mammals ! Image of a spider

They do not have a heart like we have, but some kind of tube that pumps the blood from one end of the body to the other. Their organs just "hang" in the blood-soup.

This kind of circulatory system is not able to supply a large body with blood due to pressure-problems. There are fossils of bigger insects and spiders, but scientists assume that the air pressure was significantly different than today.

So if you want to make big spiders in a realistic environment, you must change either the air pressure or the gravity (which will also lead to lower air pressure).

Second question, what can they eat and how much? Big Spiders like Tarantulas eat small mammals and birds, when they can get them. Also, many spiders practice cannibalism. So if they can't find easy prey, they will eat their brothers, sisters and parents instead. So a big population will probably not starve completely when prey is low, they will simply eat each other. When prey is plentiful, a single female can lay hundreds and thousands of eggs and push the population back to old size with ease.

Essentially, they eat every living thing that they can get down.

But they still must be aware of dangerous fights. If they do not have to take a risk, they avoid it. How often they must eat depends on their behavior. Hunting spiders have to eat much more often than web-building species. But both can last long periods with none or low food. The bigger problem is water supply. Because they don't get their liquid from prey, they have to drink. The real spiders in our world do this from waterdrops and puddles.

Huge Spiders won't be able to drink waterdrops, so they have to find lakes or something. Therefore I assume that they cannot live in dry caves.

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  • $\begingroup$ Do spiders have the same problems with breathing that insects would encounter if increased in size? $\endgroup$ – glenatron Nov 10 '14 at 18:18
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    $\begingroup$ One potential modification would be to allow the spiders some type of condensation gathering hairs, and have them thrive in humid environments. $\endgroup$ – Wayne Werner Nov 10 '14 at 19:56
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Spiders don't need to eat that much. In terms of food requirements, giant spiders would be fairly supportable by most environments.

As per Kleiber's Law:

For the vast majority of animals, an animal's metabolic rate scales to the ¾ power of the animal's mass. Symbolically: if q0 is the animal's metabolic rate, and M the animal's mass, then Kleiber's law states that q0 ~ M^¾. Thus a cat, having a mass 100 times that of a mouse, will have a metabolism roughly 32 times greater than that of a mouse.

The reasoning behind this is that smaller an animal, larger the fraction of their body mass consisting of structure rather than reserve. Structural mass involves maintenance costs, while reserve mass does not; ergo small animals respire faster and need more calories per mass than larger ones.

Assuming Kleiber's Law to hold true for these giant spiders, we can calculate their metabolic requirement by comparing it to a "real-life" spider of comparable activity levels, plus caloric requirement of silk production.

A Goliath Bid-Eater, the largest tarantula species by mass, weighs around 70-80 g on average (though it can grow to be 170 g). According to spider-care websites[2], a diet of 6-8 crickets per week is sufficient for "the larger tarantula species". That's only about 10 Calories a week!

By this calculation:

70 g spider => 10 Cal / week
∴ 70 kg spider => 10 * (1000^¾) Cal / week ~ 1780 Cal / week.

Let's take that number as "Caloric requirement at rest", since a captive Tarantula would have limited activity as compared to a 'wild spider' who would need to actively hunt for prey.

For spiders, a significant source of metabolic cost is also silk-creation. [1]. So the more web/silk a giant spider produces, the more calories it will need. Spider silk is light - enough spider silk to go around the world once would only weigh 500 g, but let's say these giant spiders spin proportionally thicker silk, so that the same length of silk weighs a 100 times more.

Caloric value of silk ~ 4500 Cal/g

So a 70 kg spider producing just 1 g of silk a day would need an additional caloric intake of 4500 Cal per day for silk production.

Since real spiders eat enough to tide them over for days at a stretch:

Assuming 70 Kg Spiders that produce 1 g Silk per day
And 100 Kg Deer that are 80% Consumable at 1.5 Cal/g with total 120K Cal
=> One (1) Deer would feed Four (4) Spiders for Seven (7) Days

ETA: However, even with a thread 10 times thicker (10 microns => 100 microns) and 100 times heavier than normal spider silk, 1 g of spider silk would be more than 166 m (1.31 g/cc /(0.005 cm *0.005 cm *pi)), so it is highly unlikely that even a giant spider would be producing 1 g of silk a day. [Thanks to @Sempie for bringing my attention to this].

So more realistically, unless silk production is significant enough (in terms of width/density/length) to weigh a lot, it would have insignificant calorie cost.

Additionally, Spiders often recycle their web silk. If these giant spiders are not expanding territory, they could be eating old silk to 'touch up' their webs, which could eliminate external (non-silk) caloric requirements even more.

Hence:

Assuming 70 Kg Spiders with negligible Silk production
And 100 Kg Deer that are 80% Consumable at 1.5 Cal/g with total 120K Cal
=> One (1) Deer would feed Thirty-Three (33) Spiders for Fourteen (14) Days

The paper linked above also gives an equation for activity cost of web building. Total Cost of Web Construction ~ (4.5 * Weight of Web in mg) + [Weight of Spider in grams * (2.79 * Weight of Web in mg)] calories
I'm assuming this activity cost gets taken care of by Kleiber's Law, but if it doesn't, then the dietary requirements would be:
=> (4.5 * 1,000) + [70,000 * (2.79 * 1,000)] = 4500 + [70,000 * 2790]
= a whopping and unsustainable 195,304,500 Calories! (An adult African elephant needs around 70,000 Cal per day for comparison)

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  • $\begingroup$ Interesting! I never realized that web spinning was such a draining activity for spiders. +1 for you $\endgroup$ – Danny Reagan Nov 11 '14 at 12:09
  • $\begingroup$ The Silk is an interisting point, but in the end hunting spiders eat more often than web-building spiders. The web of a normal spider holds for days or weeks. The web of such a giant spider(70kg) would be so strong, that nearly nothing can destroy it. So they probably wont have to fix it very often. $\endgroup$ – Sempie Nov 11 '14 at 12:10
  • $\begingroup$ @Sempie That's true! But I was also considering the typical fictional narrative in that the web of these spiders would be much thicker than normal web. So even if they wouldn't need to fix their webs very often, even a little bit of fixing could take it to 1 g of weight (spider silk density = 1.31 g/cc) $\endgroup$ – Shisa Nov 11 '14 at 12:15
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    $\begingroup$ @Sempie Nevermind, thanks for bringing my attention to the actual silk calculations :) You're right, it really wouldn't be required in general circumstances at all. Have corrected my answer to reflect this. $\endgroup$ – Shisa Nov 11 '14 at 12:58
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The problem is not so much dietary as the two thirds power law. As you scale up a creature, the mass increases as the cube of the size, while areas increase as the square. That means the stress on the cross section of the legs goes up linearly with he size, which is why large creatures have larger legs (proportionate to their body size) than small creatures. Similarly, spiders do not have a circulatory system to move oxygen deep into the body. They depend on diffusion, which works over fraction of an inch distances, but not over many inches.

A great essay is "On Being The Right Size", J. B. S. Haldane

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  • $\begingroup$ Except Haldane is reasoning (we live in the best of all possible worlds) from looking around at what currently exists; where we (and the climate) have wiped out all the large fauna. ie: T-REx couldn't exist, given his reasoning. So it happens to have valid points (maybe by accident?), but it's not so great on the limits of what's possible. $\endgroup$ – user3082 Nov 17 '14 at 16:46

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