The response to the Kaiju post has been amazing and of course everyone has suggested alterations to the calculations.

For background, biologists know that much of an animal’s biology, everything from limb length, heart volume, lung capacity, territorial range, and urine production, all scale with body size. We use an equation, based on data from lots of animals, to calculate all of these. The equation usually takes the form of Variable of interest = a (mass)^b. a the intercept usually varies among groups like birds, mammals, frogs, carnivores, nocturnal, carnivores, etc.. b the slope can as well but usually scales as a multiple of 0.25.

By knowing the mass of an animal, we can gain some insights even in the absence of direct observation, like how many people a day a Kaiju would consume.

Many people questioned basing my equations on Komodo Dragons, arguing that Kaiju were more like mammals, other reptiles, birds, fish, or crustaceans. A commenter also noted that the weights of each Kaiju are known.

So with actual weights and assuming that Kaiju are like different types of organisms in their physiology, the number of people per day needed to sustain a Kaiju is

Kaiju Name Category Kaiju Weight (tons) Komodo Dragon Mammal Bird Reptile Marine Fish Crustacean Onibaba II 2040 11.6 64.4 35.1 2.9 14.7 8.0 Knifehead III 2700 14.2 79.4 42.4 3.6 18.3 9.9 Leatherback IV 2025 11.5 64.1 34.9 2.9 14.6 7.9 Otachi IV 2690 14.2 79.2 42.3 3.6 18.2 9.9 Scunner IV 3230 16.2 90.7 47.9 4.1 21.1 11.4 Raiju IV 3475 17.1 95.8 50.4 4.3 22.3 12.1 Mutavore III 2630 13.9 77.8 41.7 3.5 17.9 9.7 Trespasser III 2700 14.2 79.4 42.4 3.6 18.3 9.9 Slattern V 6750 27.8 157.1 79.1 7.2 37.7 20.3

Of course I assumed that the tons were not metric tons but long tons. At least one commenter disagreed. If we use metric tons instead

Kaiju Name Category Kaiju Weight (tons) Komodo Dragon Mammal Bird Reptile Marine Fish Crustacean Onibaba II 2040 12.4 69.3 37.5 3.1 15.8 8.6 Knifehead III 2700 15.3 85.4 45.3 3.9 19.8 10.7 Leatherback IV 2025 12.3 68.9 37.3 3.1 15.7 8.6 Otachi IV 2690 15.2 85.1 45.2 3.9 19.7 10.7 Scunner IV 3230 17.4 97.6 51.2 4.4 22.8 12.3 Raiju IV 3475 18.4 103.0 53.8 4.7 24.1 13.1 Mutavore III 2630 15.0 83.7 44.5 3.8 19.4 10.5 Trespasser III 2700 15.3 85.4 45.3 3.9 19.8 10.7 Slattern V 6750 29.9 168.9 84.5 7.7 40.8 22.0

A commenter also suggested that digestive efficiency would not be 100%, i.e. a that all the energy of human would not be accessible to a Kaiju. Perhaps Kaiju are complete crap at digesting bone or their guts are just designed really poorly. Assuming they have an efficiency of 50% and long tons

Kaiju Name Category Kaiju Weight (tons) Komodo Dragon Mammal Bird Reptile Marine Fish Crustacean Onibaba II 2040 23.1 128.8 70.1 5.8 29.3 15.9 Knifehead III 2700 28.4 158.8 84.8 7.2 36.6 19.8 Leatherback IV 2025 23.0 128.1 69.8 5.8 29.2 15.8 Otachi IV 2690 28.3 158.3 84.6 7.2 36.5 19.8 Scunner IV 3230 32.4 181.5 95.8 8.2 42.2 22.8 Raiju IV 3475 34.2 191.6 100.7 8.7 44.7 24.2 Mutavore III 2630 27.9 155.7 83.3 7.1 35.8 19.4 Trespasser III 2700 28.4 158.8 84.8 7.2 36.6 19.8 Slattern V 6750 55.7 314.2 158.2 14.3 75.5 40.7

So if Kaiju are essentially mammals, complete rubbish at digestion, and assuming long tons, then the largest Kaiju, Slattern at 6,750 tons, would require 314.2 people per day. In 61.7 years it could eat its way through Hong Kong, assuming of course there were no births and no other non-Kaiju related deaths. Who has time for sex when they’re running from a 6750 ton alien?

But what about other aspects of Kaiju biology?

Slattern Weight (tons) 6,750.00 Weight (kg) 6,123,498.75 Pulse (beats/sec) 0.08 Mammal Pulse (beats/sec) 0.07 Bird Stride Frequency (seconds) 0.50 Mammal Skin Mass (tons) 379.03 Mammal Lung Mass (tons) 37.58 Bird Lung Mass (tons) 61.29 Mammal Lung Mass (tons) 202.50 Reptile Brain Mass (ton) 0.92 Mammal Brain Mass (ton) 0.07 Bird Brain Mass (ton) 0.67 Primate Brain Mass (ton) 0.03 Fish Heart Mass (ton) 30.43 Mammal Heart Mass (ton) 3.83 Bird Heart Mass (ton) 22.41 Fish Heart Mass (ton) 27.71 Turtle Heart Mass (ton) 1.98 Shark Litter Size 0.03 Mammal Litter Size 0.00 Bird Litter Size 169,008,565.50 Reptile Gestation Time (years) 1.55 Carnivorous Mammal Incubation Time (days) 4.61 Bird Lifespan (years) 264.53 Mammal Lifespan (years) 399.29 Bird Urine Production (gallons per day) 1,966.75 Mammal Urine Production (gallons per day) 1,489,082.95 Frog Density (no. per square kilometer) 0.00 Mammal 21,859.1 No. supported on Earth Density (no. per square kilometer) 0.32 Bird 162,342,390.0 No. supported on Earth Density (no. per square kilometer) 27.43 Aquatic Invertebrate 13,991,321,452.0 No. supported on Earth Territory (square kilometer) 2,761,876,776.05 Carnivorous Mammal 0.2 No. supported on Earth Territory (square kilometer) 90,246.21 Hunting primate 5651.2 No. supported on Earth Territory (square kilometer) 16,412,303,719.57 Carnivorous Bird 0.0 No. supported on Earth Territory (square kilometer) 339,183.74 Lizard 1503.6 No. supported on Earth

In the table above, I calculate different biological features of the largest Kaiju, Slattern, based on different animals (mammals, birds, frogs, fish, etc.). Slattern’s heart beat would be an order of magnitude slower than most humans, beating less than 5 times per minute. The heart would be anywhere from 2 tons upwards of 30.5 tons. Based on either birds or mammals, it wouldn’t even be able to even muster a single offspring per birthing event. On the other hand if Kaiju are reptilian, we are all severely screwed. Lifespan would be measured in 100’s of years.

Piss…how much would it produce? Every day a Kaiju, if mammalian, would produce about 40 barrels worth of urine. If Kaiju are like frogs, then Slattern would produce 92 Olympic swimming pools worth of urine per day (roughly 0.8% of flow rate of the River Thames).

With the scariness of that settling in, the density of Slatterns supported on Earth could be alarming. At the smallest this would still top out over 20,000. It strikes me that Kaiju are probably territorial. Why else would they always be so damn angry and smashing things all the time? If this is true then the numbers drop radically. If they are avian or mammalian, then Earth could support one, like beta fish in an aquarium. If on the other hand they are like hunting primates or lizards, we are looking at thousands.

McNab, B.K. (2009) Resources and energetics determined dinosaur maximal size. PNAS 106:12184-12188

Nagy, K.A., Girand, I.A., and Brown, T.K. (1999) Energetics of free-ranging mammals, reptiles, and birds. Annu. Rev. Nutr. 19:247-277

Peters, R.J. (1983) The ecological implications of body size. Cambridge University Press.