How may people does a Kaiju need to eat every day?

Warning this may contain spoilers

 pacificrim_trailer_hd_screencaps_01The short answer is not as many as you think.

I spent Saturday watching Pacific Rim.  The movie has everything I want in a flick—big-ass sea monsters, big-ass robots, and big-ass robots fighting big-ass sea monsters.  Pacific Rim is undoubtedly the no-holds-bar-over-the-top-action-flick-who-gives-damn-about-plot-or-character-development-o-yeah-it-has-Ron-Motherf’n-Perlman kind of movie we all need.  My wife disagrees but I still love her.

The stars of Pacific Rim are the Kaiju (怪獣) a Japanese word that literally translates to “strange creature”.  Kaiju films are a staple of Japanese cinematography with Kaijuu, like Godzilla, Mothra, or Rodan, attacking each other or better yet a whole city.

Kaiju_CategoriesBeing the complete nerd I am, I waited patiently until I heard the magical words I needed to hear during the move. “That Kaiju weighed 2500 tons.” At 2500 tons and a puny category 3, this monster didn’t even top the scales as the largest.

Kaiju are creatures of a highly toxic nature and have been categorized on the “Serizawa Scale”. Each Kaiju is classified under five different categories. Categories 1 through 2 represent the weakest of the Kaiju, while Categories 3 through 5 are the strongest. The Serizawa Scale measures water displacement, toxicity and ambient radiation levels given off by their bodies when they pass through the breach.

ku-xlargeKnowing a Kaiju’s weight I can tell you a lot about Kaiju biology, like how many humans they need to eat per day to survive.

First we need to calculate what is the field energy expenditure, i.e. the number of joules per day to survive, for a Kaiju.  During the film the “scientist” states the Kaiju are related to the dinosaurs.  A previous paper suggests the equation for dinosaurs should be based on those for Komodo Dragons, i.e. active carnivorous lizards.  So

2500 tons = 2,267,961,850 grams

The equation is


FFE=8.05*10^6 kJ/day

Given the 2500 ton size of the Kaiju, you might be surprised this doesn’t come close the energy demand, 10.14*10^6 kJ/day, for a blue whale at a mere 160 tons.  The Kaiju estimate is just a single order of magnitude higher than that of elephants and rhinos, 5.36*10^5 and 7.10*10^6 kJ/day.

Pacific-Rim-kaiju-footprintSo what’s going on? The FFE is higher for carnivores than herbivore. More energy is required to chase and subdue prey.  That constant search for prey may also require muscle structure for endurance [pdf], increasing muscle mitochondria density, and requiring more energy.  But we accounted for that by using the total bad us flesh munching, bone crushing Komodo Dragon equation.

The real reason? Lizards don’t run as hot as mammals, i.e. they don’t regulate their internal temperature. Komodo Dragons, an active carnivorous lizard, actually do heat up bit, but still don’t suck energy like a mammal.  Keeping the body warm is energetically expensive to maintain, as exampled by heating bills in Boston to keep my Southern butt warm.

PACIFIC RIMSo real question is how many humans would a Kaiju need to eat daily to survive?  The human body contains, depending on athleticism, anywhere from 600,000 to 750,000 kilojoules of energy.  Per day the Kaiju would need to eat anywhere between 10.7 to 13.4 humans.  This would mean that it would take a Kaiju, at the quick side, 1,472 years to to eat through the population of Hong Kong, home of the Shatterdome base.

If we assume a Category 5 Kaiju weighs 5,000 tons, then it would only need 17.9 humans per day, taking it 1,102 years to eat its way through Hong Kong.  Perhaps we shouldn’t be so worried about them.


53 Replies to “How may people does a Kaiju need to eat every day?”

  1. Not having seen the movie, I think you might be somewhat underestimating the food requirements of Kaiju. Your calculations assume 100% kill-to-food ratio and 100% digestive efficiency. Also, your range for energy stored in the human body relates to healthy, normal adults, disregarding the fact that the first hunted are usually the small and the weak (e.g. children and elderly), whose body contains significantly less energy. There is also the hidden assumption that the Kaiju in question is expected to maintain constant weight, and is not in a growth phase.
    Of these issues, the digestive efficiency is potentially the most variable. While some digestive systems in the animal kingdom are above 90% efficient in converting food to energy, this need not be the case with the Kaiju (who did not have to go through enough evolutionary pressures to improve, being an apex predator in relatively friendly environments). Taking, for instance, a 10% efficiency (accounting for things like the unlikelihood of the ability to digest bones, nails and hair, or the energy required to deal with toxins and refuse), a mean 450,000kJ/person in the target population of the prey, and a 75% kill-to-food conversion gives you a figure of 238.5 people per day, without accounting for Kaiju growth. While it still would take decades to take out the population of Hong Kong even under these assumptions, this is by no means a trivial death-toll.

    Oh, and thanks for the fun read!

  2. A komodo dragon can eat up to 80% of its body mass in a single meal. That would mean that a 2,500 ton Kaiju could eat about 20,000-25,000 adults in a single meal and then not have to eat again for about 4 years.

  3. So I guess the mere 2040 ton crab-like kaiji Onibaba bothering to chase down a little kid wasn’t a total waste of time then, since I’d imagine it would have had an even slower metabolic rate than a reptile.

  4. It may indeed only need to eat a dozen or several people daily, but I’d be more worried about all the collateral damage: the people it steps on, people in the buildings it destroys, the effects on the health of the whole population due to the constant terror, not to mention the economic effects of buildings and city infrastructure being destroyed every time the bloody thing moves.

  5. Lizards are not the only analogue for kaiju. Perhaps they are more like theropods and we should use, say, ratites as the analogue. Also, we can get a feel for how much energy they consume by looking at their running speed, perhaps food consumption scales similarly.

  6. Okay, so they couldn’t eat us all to death all that quickly, but they sure made a mess of the infrastructure in a hurry. There were a lot more deaths from collapsing buildings, being stepped on, etc than were eaten. Even after the kaiju was killed, there would be a lot of work to do to rebuild.
    This article doesn’t make me feel much safer! ;)

  7. If I go with your refinements, that means a Kaiju would eat 20,000-25,000 people in a single rampage and then wouldn’t have to eat again for 2.5 months. I haven’t seen the movie yet, so I’m not sure of the number of Kaiju attacks per year, but I think the problem is only getting worse for humans.

  8. Good analysis. But if based on what I observed in the movie, have yet to see one eat humans, except for the pre matured one which ate “Hell Boy” so they may only rely on fish/aquatic/marine for food. On hindsight, since they come in via a bridge/portal. It is also likely that they have their own food source in their own realm.

  9. During the film the “scientist” states the Kaiju are related to the dinosaurs[…] Lizards don’t run as hot as mammals

    But birds do. And dinosaurs were not lizards. How would your calculation be changed if kaiju are at least somewhat “warm blooded”?

  10. I thought there was enough conversation in the movie that said they were fish related how does that change calculations? Wouldn’t the body temps and surface area create even greater joule usage? A great white or a giant squid might be a better example.

  11. All of this presumes that the Kaiju are natural beings undergoing a natural biological process.


    The movie makes it clear that the Kaiju are engineered bio weapons used for hostile planetary takeovers. They can effectively be calorically “tanked-up” for a ten plus year run before ever transiting the portal. They wouldn’t necessarily need to consume humans or any other biomass to survive long enough to accomplish their designed mission goals if a significant portion of their mass was in the form of stored calories. They could also effectively be omnivorous, and be able to process any biomass they can consume including trees and carrion. It would just depend on how versatile their designers wanted them to be in field operations.

    My take is that as directed engines of destruction, they are only going after humans because their mission profile directs that activity against the dominant species of the planet under a takeover attempt. Not particularly because they find humans the only acceptable food source available.

  12. Ha! I don’t have to run faster than a kaiju, I just have to run faster than 18 other people!

  13. Are the weights of the Kaiju off by a factor of 5 or more? The category 3 Kaiju is 295 feet tall and weighs 2500 tons. But they are roughly humanoid in build and even more bulky than a person. Since mass increases with the cube of height, and the category 3 Kaiju is roughly 50 times taller than a person, should it weigh at least 125,000 as much, which would make it 12,500 tons?

  14. When did a scientist say that the kaiju were related to dinosaurs? I remember one of them saying that the kaiju have a second brain, like the (mythical) second brain of some dinosaurs. But I thought that was just a comparison and didn’t imply an actual relationship.

  15. ah the old how many humans could a Kaiju chew question. Depends on the humans as well. If the Kaiju eat some really porky humans, they may be satisfied.

  16. As zmortis stated, it was made clear in the movie that they were destroying civilization in an attempt to wipe us out, not as collateral damage in trying to eat us. It doesn’t matter if they don’t need to eat at all they’ll still ravage our cities.

  17. they are not related to dinosaurs. why do ppl keep saying this? They just considered taking over the earth back when the dinosaurs were around.

  18. The kaiju are not related to dinosaurs. One was pregnant, so they don’t lay eggs, and are therefore likely to be warm-blooded if they’re similar to Earth animals at all. Which they’re not.

  19. Although starvation resistance and increased energy reserves may work it still would carry a Kaiju very long. Female blue whales can go for 3-6 months between major feedings so they can calf in warmer waters where food is scarce. Even that is pushing it. The ability to live off reserves usually requires a hibernation process.

  20. I referring to the part after the scientists goes through the mind meld and mentions that mysterious monster world which exists in another dimension and is the true origin of our dinosaurs. I assumed that then the second wave sent from the monster world would then be related, and an analog, to the dinosaurs. But see my other post where I make different assumption on what Kaiju are.

  21. It clearly states in the movie that mysterious monster world which exists in another dimension is the true origin of our dinosaurs. Thus Kaiju must at the minimum be related.

  22. It states that their last attempt to take over involved the dinosaurs, but not that they *were* the dinosaurs. The implication I’ve seen people conclude was that they were involved in the *extinction*, not the creation.

  23. They don’t seem to have any digestive enzymes or acids so I think they were genetically engineered to go without food.

  24. First of all I love the article, science is pretty amazing, I do have a few thoughts though. I am not sure that all of the kaiju can be classified as reptiles, since a number of them seem to have mammalian and crustacean attributes. The movie doesn’t really state that the kaiju are related to dinosaurs it simply says that the last time the interdimensional beings tried to take over the world was in the time of the dinosaurs, and the planet was too clean for them to colonize.

    There was a comment earlier on whether kaiju actually need to eat because they are engineered. I am fairly certain they do since Hannibal mentions that their fecal matter sells for a lot of money, gotta eat to poop.

    My thought is that humans would be a really inefficient food source, especially because kaiju spend much of their time in the ocean where they could easily bag a whale or large squid. Unless humans are in a big cluster (e.g. a bunker)it would be too much of an energy expenditure to try and catch one, and the payoff wouldn’t be worth it. Onibaba, the kaiju in Mako’s memory, couldn’t even catch a little girl very easily.

    Anyway, I love talking about Pacific Rim so I am really glad I ran across your article. I think it’s interesting, and fun to speculate about fictional monster biology. And I really dig evolutionary science.

  25. It’s important to note that the Kaiju, even if they resemble lizards or earthly animals, are not lizards at all considering that they come from another universe entirely. I would think they’d need to eat way more if their bodies are to the point of radioactivity and their brains have secondary brains.

  26. *****SPOILER ALERT*****
    If you’ve seen the ending, you would know that a Kaiju is seen being built or bio-engineered, by those “things”, and that the scientist, points out that they are like clones. So they could be fed before, not need food, or still eat fish. Remember the Kaijin were also seen NOT eating. Great movie though.

  27. Fun and interesting article! And I loved the Pacific Rim movie. However I think the Kaiju where not necessarily designed to actually eat humans, but rather to squish them. So basically the aliens were creating clones that weren’t supposed to feed properly and live long, just do as much damage as possible to humans and their settlements.
    All the same, your calculations are rather awesome.

  28. I am vietnamese i’m 12 year old, i like this movie.
    I don’t speak english much, here’re the question
    How many jeager do you think can kill kaiju.

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