There are numerous stories both in fantasy and sci-fi about some sort of biological swarm that consumes all it touches, stealing their genetics and then producing improved drones using the new DNA.

Lego Genetics not-with-standing, the main problem I see with this is growth speed. Ideally, this devouring swarm would be able to infest a new locale and produce an insurmountable tide of chitinous death as quickly as possible, but even an elephant (which is about 200lb at birth) takes almost two years to gestate. I'm not going to get my rapidly growing horde with those rookie numbers, even if the resultant spawn instantly matured.

So my question is, how fast could an intelligently designed species produce the most biomass in the form of new, mature offspring to cause my unstoppable flood of tyra-bug-lings?

If some other method than mammalian pregnancy would work better, feel free to use it.

  • $\begingroup$ Consider for example locusts. Locusts are swarming grasshoppers; they can produce a new generation in about 8 weeks, and their numbers can increase dramatically in each successive generation, because each female lays dozens of eggs at a time. But then, locusts don't "gestate". $\endgroup$
    – AlexP
    Jun 14, 2020 at 9:37
  • $\begingroup$ What, exactly, are the constraints for this "Tide of chitinous death"? Is it designed for a particular planet? Does the planet have a technological species resident? What sort of biosphere does the planet have? $\endgroup$
    – Monty Wild
    Jun 14, 2020 at 12:28
  • $\begingroup$ Star Trek's Jem'Hadar, arguably the ultimate intelligently-designed horde, matured to adulthood in 3 days... :-) Granted, this doesn't technically answer your question about gestation periods, but if they mature in 3 days their gestation is probably measured in single hours. Who needs the truth when you can have a good fantasy? $\endgroup$
    – JBH
    Jun 14, 2020 at 15:41
  • $\begingroup$ I've been picturing Tyranids, Zerg, and Xenomorphs mainly, although if you have numbers for those locusts so I can math out how many 200lb baby elephants they can make that'd work too. Wrt constraints, It's fairly open. I don't expect resident technological species to affect growth speed. The planet is a standard earth-like one. $\endgroup$
    – Kyyshak
    Jun 14, 2020 at 16:11
  • $\begingroup$ Question: How do the bugs know which part of the genes they take are the "good" parts? $\endgroup$
    – NomadMaker
    Jun 14, 2020 at 22:19

5 Answers 5


The limitations upon the duration of gestation and maturation of placental mammals are not based solely upon the rate of cell division, they are based upon the rate at which the mother can supply nutrients to the fetus for gestation, and the rate at which the newborn can consume nutrients or its mother supply them, and additionally the amount of time that it takes the creature to learn to be a fully functioning member of its society.

Since a fetus grows at a constant rate, this requires a constant rate of nutrient and energy input, and if the demand for either is not met, it can lead to health problems for both mother and offspring.

Likewise, meeting a particular post-birth growth rate requires meeting the demand for energy and nutrients, or suffering from health issues. Additionally, many species have reduced growth rates due to the time required to learn the social aspects of being a member of its species.

For a bioengineered species, assuming no limitations on the capabilities of the designers, the social limitations on growth need not apply, since the necessary information can be built in at design-time.

Additionally, for a species designed to be a bioweapon, there is no need for it to gestate its own young if its young can gestate within the species that it is intended to exterminate. In fact, the act of implantation and gestation could be a primary mechanism by which the target species is exterminated, as well as acting as a self-limiting control upon its population.

However, if this bioweapon species is designed to target a sentient, technological species such as humans, relying upon humans as a reproductive host may not be completely reliable, given humans' capability to rapidly recognise the threat and act to minimise it. By necessity, a bioweapon designed to grow as rapidly as possible would have a somewhat different biochemistry, and the fact that such a difference exists means that it could be targeted by selective poisons that affect only the bioweapon and not the human host.

So... if we assume no limits on the availability of energy and nutrients, how rapidly could cells divide? Human cells divide at most once every 24 hours, though some yeasts can divide every 52 minutes, maintaining a constant maximal cell mass, while some fly embryonic cells have been timed at 8 minutes between divisions, though these would be trading mass for quantity, and would not be growing.

Most of the time in a steady size cell division cycle is taken up by growth, though a significant time is spent replicating DNA. in order to minimise the time spent replicating DNA, the DNA may be pared down to the barest minimum necessary, and split up into many seperate chromosomes, allowing the DNA to be replicated in parallel. It is not beyond the bounds of possibility for the bioweapon's cells to divide each 30 minutes. However, from an initial single cell, which may be very large, we could have a number of rapid divisions without growth, in order to achieve a larger number of smaller cells as quickly as possible, thus multiplying the growth rate of the organism in as small a time as possible.

If we take an initial cell of about 128ng, and rapidly divide it down to the average cell mass of the organism, that most likely being 1ng, that would be 7 divisions. These 7 divisions could conceivably be achieved in 5 minutes each, for a total of 35 minutes to reach the 128-cell stage. From there, with cell growth included, each cell division could in theory be achieved in as little as 30 minutes, though in practice, it would take somewhat longer due to cell death: not all cells survive (or are intended to survive), so it is reasonable to say that we could have a doubling in mass each 40 minutes, conditions permitting.

That just leaves the final mass of the bioweapon to consider. A critter weighing in at 300kg would be frightening, but is not necessarily all that practical: it takes longer and more nutrients to grow, but is a bigger target that is more easily killed or spotted and avoided. A bioengineered creature weighing around 7.5kg could quite easily be ten times stronger than a 75kg human, given that its musculature could also be bioengineered. Given an exoskeleton and a robust arrangement of internal organs, such a creature, perhaps the size of a small dog or monkey, could be expected to easily overpower and kill most humans all by itself... and such a small body size reduces the time necessary to grow it.

So, to go from 128ng to 7.5kg would take around 36 doublings, at 40 minutes per doubling, for a total time of 24 hours. Add the 35 minutes for initial cell division to 128 cells, and you have your dog-sized horror in just over one day.

The problem is going to be providing it with energy, nutrients and dealing with the waste heat from all this furious cell division. Supplying nutrients isn't an insurmountable problem: growing within a host could easily provide the necessary nutrients, and it's not as if the host has to survive. However, dealing with waste heat could be a problem. A live human host is actually ideal, since humans have a practically unmatched capacity for shedding excess body heat amongst all large land-dwelling species. However, given the rapid growth of a 7.5kg parasite, the death of the host is quite likely, and the corpse would become insulation rather than a cooling mechanism.

So... it may be necessary to reduce the growth rate of this bioweapon just so that its growth does not cause it to overheat. This is an actual problem in large commercial fermentation tanks: they must be actively cooled so that the rate of cell growth and division does not cause overheating and cell death. However, a reproductive cycle on the order of 25 hours to perhaps a week at most is going to give it rather short generations. Let's say that it has a growth time of 5 days.

By taking an r-strategy over a K-strategy, with short generations and a large number of offspring, such a bioweapon might be designed to implant its embryos into a large number of hosts. If each individual implants ten hosts and each host produces two offspring, and we start with one individual bioweapon, in 5 days, we have 20, in 10 days there could be 400, in 15 days 8,000, 160,000 in 20 days, 3,200,000 in 25 days, 64,000,000 in 30 days, 1.28 billion in 35 days, and 25 billion in 40 days... but if we're using humans as hosts, humanity would be extinct before then.


From a mammalian point of view, consider Australia's stripe-faced dunnart with an 11-day gestation period and the North American opossum, which gestates for approximately 12-14 days and delivers a litter of 16-20 babies.

On the insect side of the question, aphids reproduce asexually and are born pregnant. That's as fast as you can get. However, the problem with this is that aphid eggs, laid in the Fall, sit around until spring to hatch.

However, you need your critters to get to adulthood, not simply get born quickly. That honor is held by the African killifish, which can grow to adulthood in just two weeks.

What can we conclude from this? Small is obviously better when it comes to quick gestation and maturation — but I think a conception-to-adult cycle of 10-14 days for an intelligently designed species is believable.

Which suddenly makes my Jem'Hadar comment, which I posted because I think it's funny what Star Trek sometimes does with science, not so unbelievable....


You didn't define how big the lifeforms building your swarm have to be, so I would break it down to insects. As @alexP mentioned in the comments locusts produce a new generation every 8 to 10 weeks with exponential growth. Braking it down they could double their numbers within less than three days so if you take flesh-eating (or better 'nearly-everything-eating') bugs and start with two, you break 100k bugs in roundabout one and a half month, one million in two month.


Intelligent design means we can ignore requirements of evolution and play mix and match.

So I'd go with a genetically engineered fungus, spread via spores. It would grow unnoticed in forests, either feeding from decaying organic matter or as a plant parasite, or maybe it would grow in sewers as waste water contains plenty of nutrients.

Once it has spread over a large area and matured, it would grow fruits in the soil. That's where the genetic engineering bit comes into play: these fruits would simply be the chrysalis of your critters. Feel free to use any pseudoscientific explanation, for example an animal cell endosymbiont living inside a fungus, or a modified slime mold, etc.

Then, just like cicadas, triggered by temperature changes or maybe a signal from their creators, they would all crawl out of the dirt at the same time and devour anything that moves. They don't even need to reproduce.

I believe this makes a better bioweapon than xenomorphs because the initial phase is more stealthy and it can take its time to produce a large number of drones. There is no nest or queen to act as a single point of failure. Also it is more controllable, as the critters don't reproduce, and even the fungus can be delivered by spores but made unable to reproduce too. Until it decides to evolve the ability on its own, of course.


Manufactured drone soldiers: I'm assuming you want a big alien thingy, intelligent and fully functional. I'm also assuming very advanced biology & biochemistry and genetics. It kind of depends on what you're willing to call gestation. I think this process could result in a ready drone within perhaps 24 hours. It looks more like manufacturing, though.

  • An extracellular matrix can be essentially 3D printed and coated with differentiation markers, then flooded with a stem cell slurry that takes up residence in the matrix. Each variant on a drone can be printed differently and the same stem cells used to make whatever drone is desired. If you have an extracellular matrix manufactured, then fill it full of replicated alien cells in alien body form, with all the alien DNA already replicated, most of the work is done.
  • Now extract the DNA from a (human?) and identify what DNA is "superior" and replicate it with PCR or the alien equivalent. Load this DNA into a virus or equivalent plasmid delivery system and "infect" your premade drone blank with the DNA. I'm assuming they can build a system with 100% penetration of the added DNA in very short timeframes.This allows you to use the same slurry for drones with differing DNA requirements. Alternatively infect the stem cell slurry with the DNA before the manufacturing step if it is universally desirable.
  • Preferably before, but if needed after this, you add memory biochemically so your soldier is born with the skills and experience to start fighting/consuming immediately.
  • $\begingroup$ How does the intelligence identify the "superior" DNA? Maybe they read the comments in the code? $\endgroup$
    – NomadMaker
    Jun 14, 2020 at 22:20
  • $\begingroup$ @NomadMaker Only generations of selective pressure & experimentation would identify the best genes, but if this hive mind is assimilating DNA and improving the species on the spot, it would need to be be intelligently analyzing the sequences and deciding what to use and what not. I'm just the messenger, I didn't write the question. $\endgroup$
    – DWKraus
    Jun 14, 2020 at 22:46
  • $\begingroup$ This sort of thing has been used in a lot of sf fiction, and it wasn't until this question came up that I realized the hole in this gimmick. $\endgroup$
    – NomadMaker
    Jun 14, 2020 at 23:32

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