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We are working on creating a room that is a self-sustaining ecosystem (like ecosystem aquariums but with creatures) that is an obstacle for PCs to pass (as part of an entire “Island of Dr. Moreau” filled with these ecosystems.)

Our current thought is a mad wizard who has set up a room with hundreds of stirges fed by a regenerating troll in a box that only allows as many stirges to feed on him at once as it can regenerate. Ideally, the troll could occasionally grab and eat some of the stirges as well.

The question we have is what guidance is there anywhere across the game or previous adventures or interviews with designers to calculate any or all of the following:

  1. The amount a creature needs to eat each day to survive.

(This would give the number of stirges the troll could support and the number of stirges the troll needs to eat per day.)

  1. Whether there is any known gestational period for stirges

(This would allow us to answer the question whether or not a troll could actually survive on something feeding on its regenerating blood - and allow us to calculate the equilibrium state for number of stirges supported.)


We use DnD to teach kids math, biology and physics. If we can make scenarios that teach various principles while also fitting with existing game guidance - it is a double win.


Of note, the underlying assumption in this scenario, of course, is that a troll’s regeneration has a magical aspect to it given how fast trolls regenerate and given that there is nothing describing how much they have to eat based on what they regenerated. (Proportional eating would be required if regeneration was a purely physical phenomena.)

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    \$\begingroup\$ What is the goal of this question? Why do you need or even think there would be "guidance" that level of finicky detail? How is this not just something the DM can hand-wave and gloss over? Are the players really going to question/argue over this sort of thing? What problem is actually being solved by this? \$\endgroup\$ – Purple Monkey Nov 18 '18 at 3:42
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    \$\begingroup\$ @PurpleMonkey - Back in the days of AD&D, Dragon Magazine ran a series of "Ecology of [insert monster here]" articles, so there is precedent for this sort of information being available from official sources. \$\endgroup\$ – Dave Sherohman Nov 18 '18 at 10:44
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    \$\begingroup\$ @PurpleMonkey a cogent problem is a recent lack of good - uh - cheese. This question fulfills that desperate need. :-) \$\endgroup\$ – KorvinStarmast Nov 20 '18 at 17:32
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Yes

...but no self-respecting troll would want such a wretched life.

A troll can sustain almost limitless stirges

A stirge (MM 284) takes on average two attempts to hit a troll (MM 291), dealing an average of 5 damage, and draining an average of 5 hit points worth of blood until until it has drained 10 hit points. Since the troll regenerates 10 per round, he can sustain two feeding at a time indefinitely.

In theory, the troll can regenerate enough each day to feed 9,600 stirges, given that there are six seconds in a round, 86,400 seconds in a day, the troll regenerates 10 per round, and each stirge deals 15 damage to feed. This is a hypothetical maximum and doesn't account for the practical logistics of moving this many sitrges around.

In practice, the stirge's instinct is to swarm its prey in large numbers. In this case, twenty Tiny creatures can surround a single Large creature, not even considering three dimensions. On average half will hit in the first round, dealing 50 damage, of which 10 will regenerate. In the subsequent round, the attached stirges will drain the troll of enough blood to reduce it to 0 hit points.

However, a stirge only detaches when the creature is dead, and the troll is not dead at 0 hit points. Thereafter the troll remains in a constant loop of regenerating 10 and being sapped for 10 either by suckers or new joiners.

According to The Ecology of the Stirge by Ed Greenwood (Dragon Magazine #83), the stirge eats once every 72 hours (3 days), and will starve after another day if it cannot feed. Therefore, a troll can support a theoretical maximum of 28,800 stirges, or 38,400 if they all wait until the last possible moment to feed. Again, this is only a theoretical maximum, and would be impractical in reality.

However, we must also consider the possible mutations which will occur when a troll is subjected to such an ordeal over the long term (MM 291):

Their regenerative capabilities make trolls especially susceptible to mutation. Although uncommon, such transformations can result from what the troll has done or what has been done to it.

The troll may mutate a harder hide or poisonous blood to stop the stirges, or it may start to produce even more blood to feed its swarm. This would, of course, by up to the DM to speculate on.

Do stirges reproduce fast enough to sustain a troll?

The troll is described in earlier editions of D&D (particularly the D&D 3.5 Monster Manual) as weighing 500 pounds, around the same size as a male lion, similarly a predatory carnivore. A lion eats up to 66 pounds of meat in one session and requires around 15 pounds per day.

However, trolls are "born with horrific appetites", and are not described as having the lion's feline habit of resting for up to 20 hours per day. While there's no fixed definition of how much a troll eats, Dragon #301's Ecology of the Troll says:

Much of their activities focus on acquiring food, whether exploring their habitats for sources of meat, preparing ambushes, or actually tracking prey.

We can infer that the troll eats considerably more than 15 pounds per day, but the exact amount is unknown. It is much more active than the lion and would reasonably require more energy, but the exact amount is not specified.

There's no reference that says a troll who regenerates becomes hungrier. The third edition Monster Manual says its regeneration is not magical or supernatural in nature, so it could be a biological process which requires energy, but even in that edition it's not explicitly stated as such.

An average a stirge is one foot long (Dragon #83). Supposing its weight is similar to that of a winged animal of similar size and shape, the grey-headed flying fox](https://en.wikipedia.org/wiki/Grey-headed_flying_fox), which has similar wingspan and length, it may weigh around 2 pounds.

Considering that our troll probably eats at least twice as much as a lion, at a conservative estimate, he would need to eat at least 15 stirges per day, and it would not be at all unrealistic for the troll to eat 30 or 50 per day.

Unlike the troll, the stirges have no fast regeneration (although Dragon #83 suggests that they can regenerate wounds over a period of days, or missing body parts over a period of months). We're reliant on their breeding rate to keep the troll fed.

As per Dragon #83:

They reproduce by live birth, in litters of one to three young, with a gestation period of six months.

They become adult size after another seven months.

In optimal circumstances, a swarm of 100 stirges would produce on average 200 young each year (assuming 50% female producing an average of 2 offspring every 6 months). This isn't accounting for stirges which die of natural causes or are eaten by a troll.

Unfortunately, we don't have figures for the stirge's lifespan. However, based on its description of infrequent reproduction, long pregnancy, long period of parental care, and relatively low number of offspring, the stirge surprisingly sounds like a K-selected species, who tend to have relatively long lifespans.

It's possible, then, that the stirge has a natural lifespan as long as ten or even twenty years, especially with a limitless supply of food. Barring disease, accidental injury and the like, we may be looking at a growth rate of over 150%.

Assuming the troll needs to eat somewhere between 20 and 50 stirges per day, a colony of stirges would need to produce anywhere from 7,300 to 18,250 offspring per year to satisfy the troll's hunger.

This is within the possibility of the troll's ability to feed the stirges, but with three caveats:

  1. The troll would need to spend a substantial amount of time lying around in a weakened state being bitten by stirges.
  2. The troll would need to have an absolutely massive colony of stirges. They would fill an entire cavern and the sound of their wings would be deafening. Stirges (according to the 3.5 Monster Manual) tend to live only in groups of 12 or so, and breeding the necessary 3,000 or more over the course of five or six years (without eating any) would be time-consuming and require patience which is not typically attributed to the troll.
  3. The troll would have to go against his instincts of hunting large prey, and perhaps any religious teachings of Vaprak the Destroyer. Other trolls who learn of the stirge cavern would not only find his lifestyle immoral, they would also take great interest in eating their way through a massive store of delicious stirges.
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    \$\begingroup\$ What a great answer. I suspected there was guidance out there - but then - I am old and have been playing the game across 40 years and remember all those random articles in Dragon. I am opening a bounty on this simply to award this answer for indulging curiosity! \$\endgroup\$ – Praxiteles Nov 19 '18 at 11:18
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    \$\begingroup\$ Entered into the cheese hall of fame on meta. Well played, QW. \$\endgroup\$ – KorvinStarmast Nov 20 '18 at 17:32
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This goes beyond RAW but does not violate it, and is feasible

It's a stretch to find supporting literature

The 5e RAW is silent about these kinds of mechanics; there are not specific answers to your questions 1, 2 and 3 in published works in 5e (though perhaps you could find something in pre-5e materials). At any rate, there is precedent in the background literature that inspired the creation of D&D for this kind of “realism” in a magic ecosystem. Specifically, there is strong representation of a law of “conservation of organic material” even when magic is active.

For example, in Fritz Lieber’s universe, when a character magically enlarges or reduces their size, they either suck out organic material from nearby creatures in order to enlarge (thus rendering the bystanders shrunken mummies), or ooze out a pool of their own organic matter (in order to reduce in size). This was in the material Gygax stated was his inspiration to invent D&D, and while that fact is by no means a sufficient argument, it’s supportive of the reasonableness of your proposition, because it says that when magic creates bodily flesh, it has to get the raw material from some place.

Thus I think your proposal goes beyond RAW but violates nothing in it, and I’d like to think it would make Gygax smile.

If you’re going this direction, then you could note that the troll has to “go to the bathroom” periodically, which provides for dung beetles (a specific swarm of insects?) and the stirges could snap up some of the beetles now and then – thus cycling even more of the organic material in the room.

To arrive at an "equilibrium" you'll need to make some assumptions

To arrive at an "equilibrium" you could make a bunch of assumptions and determine some ratio of hit points. In 5e, hit points seem roughly proportional to the mass of the creature, and dietary needs would seem to be, as well. So, the DM could determine (estimate) what percentage of its own HP does a troll need to consume daily if it is surviving and powering occasional regeneration? This is pure conjecture and might be a number like 10% or 20%.

Next, assuming stirges breed annually (like many creatures) and their brood is double their number (like many birds), the attrition they can sustain in hit points needs to match what the troll needs to survive, i.e. their population needs to be at least double the hit points that the troll needs to consume in a whole year. It's a lot of stirges.

Again it's beyond RAW and I'm not sure all the detail is really needed, but it's a reasonable thing to place into your game.

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    \$\begingroup\$ On the example of enlarging and reducing, I would caution that in 5e there does exist the spell enlarge/reduce which enlarges or reduces a creature, yet it does not conserve mass. While conservation of mass may appear in some fantasy literature, it does not appear (and is frequently contradicted) in the rules of D&D itself. \$\endgroup\$ – BBeast Nov 18 '18 at 6:52
  • \$\begingroup\$ @BBeast Good cautionary note; of course those spells in 5e don't conserve mass in the precise way depicted in Lieber's literature, but I do not think that is grounds for concluding that in 5e "it does not conserve mass" period. It just doesn't say much about how the spell works. Those spells could be drawing inorganic material in and out of the earth or another plane, for all we know. The Enlarge spell, as written, doesn't address how the enlargement happens or whence the material comes, and it certainly does not assert that it is created out of nothing. \$\endgroup\$ – Valley Lad Nov 18 '18 at 15:53
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    \$\begingroup\$ Wait if HP equals consumption, does that mean that a 20th level Barbarian needs to eat more each day than a mammoth? A giant? This reasoning seems to fall apart a bit when you start taking class levels. \$\endgroup\$ – Theik Nov 19 '18 at 12:57
  • \$\begingroup\$ @Theik you're absolutely right... and for that matter, there are lots of things in the game that are very different for players taking class levels, in contrast to all the creatures in the Monster Manual. I was making a generalization over "monsters" or non-player creatures. \$\endgroup\$ – Valley Lad Nov 19 '18 at 16:12

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