With the release of a new Godzilla film scientists have tracked the evolution of the monster since its first screen appearance. They've found the irradiated reptile is changing 30 times as fast as its living counterparts, but in the opposite direction.

Godzilla was inspired by a nuclear test carried out by the US in the Pacific in 1954 that exceeded its expected yield, leaving the crew of a Japanese fishing vessel with radiation sickness and introducing unsafe tuna to the Japanese market. A nation still reeling from the bombing of Hiroshima and Nagasaki by the US nine years before found a way to embody its fears and anger. The latest adaptation of the story 65 years later and on the other side of the Pacific reveals its cultural resonance. With 35 films, not including such fan efforts as Bambi meets Godzilla, it is the longest running franchise in film history, while many of its contemporary screen monsters have been largely forgotten.

Fans have noted that Godzilla has been growing in size from film to film. Dartmouth University's Professor Nathaniel Dominy and Dr Ryan Calsbeek put hard numbers to this in Science, concluding its size has doubled since 1954. Even compared to the phenomenal Jurrasic-era growth of ceratosaurids, Godzilla's closest parallels, this is phenomenally fast.

By using a formula that tracks changes in the body sizes of lizards against selection pressures, Dominy and Caslbeek calculate the strength of selection for size on Godzilla is 4.9. For those unfamiliar with the scale, they note 2,500 estimates of selection pressures for wild animals produced a median value of 0.16. Hyper-charged on radiation and facing a rapidly changing world, Godzilla's selective pressures are 30 times those of other reptiles, they conclude.

Faced with ever-growing sky-scrapers, Godzilla has had to keep growing in order to convincingly tear down cities. The paper also notes the high correlation (r²=0.74) between Godzilla's size, and the budget for the United States military – whose attack aircraft are often presented as the monster's primary threat. Both shrank in the 1990s between periods of rapid growth.

The authors also argue fears of climate catastrophe have raised public demand for bigger and more fearsome monsters to keep pace with the genuine threats the world is facing. As Dominy and Calsbeek note, even Godzilla's mortal enemy has described climate change as the “mother of all risks”.

Nevertheless, if filmmakers are increasing Godzilla's size in response to planetary heating, they have their science wrong. Animals adapt to warmer conditions by shrinking, an effect that has been observed in recent decades across multiple animal families.

Dominy and Caslbeek's work may seem frivolous, but it follows a tradition of applying evolutionary principles to fiction. The great science communicator Stephen Jay Gould wrote one of his famous essays on the evolution of Mickey Mouse's ears. Other prominent biologists such as Gerald Durrell and Joan Slonczewski turned their work to science fiction and fantasy novels.