Incoprea Theory
Volume V · 2026

Theory 20/Paleontology & Mass Extinction /Palaeobiology/3 min

The Lilliput Effect Is Metabolic

After a catastrophe, animals get smaller. The reason may be breathing, not eating.

The claim

The shrinking of body size after mass extinctions is a metabolic response to low oxygen rather than a response to scarce food, so the effect should track oxygen proxies rather than productivity proxies and should appear first in groups with high oxygen demand.

01The evidence

  • The Lilliput effect has been documented in the aftermath of the end-Permian extinction, a setting in which oxygen and productivity both changed, which is why the two explanations have been hard to separate.Palaeogeography, Palaeoclimatology, Palaeoecology
  • The effect has also been documented in scolecodonts from the Late Devonian biodiversity crisis, which extends its taxonomic range and supports a general mechanism rather than a group-specific one.Paleobiology

02The tension

Body-size reduction is usually attributed to resource scarcity: less food means smaller adults. That explanation is intuitive and hard to test, because food supply and oxygen tend to change together in extinction intervals and neither is directly measurable.

03The theory

Smaller body size reduces the distance over which oxygen must diffuse and lowers absolute metabolic demand, which is advantageous when oxygen is low. The same logic does not apply to food scarcity in the same way, because a smaller body also reduces absolute food requirement.

That makes size reduction a response to oxygen specifically, and it predicts that the effect should appear in groups with high demand first, and in settings where oxygen proxies indicate hypoxia even if productivity proxies do not.

The prediction is that size change should correlate with oxygen proxies better than with productivity proxies across extinction intervals, and that high-metabolism groups should shrink more.

The consequence is that body size becomes a proxy for oxygen in the fossil record, which is useful because oxygen is otherwise hard to reconstruct.

04What this is not

Resource-scarcity explanations

Scarcity accounts make size a function of food. This makes it a function of oxygen, which predicts a correlation with oxygen proxies and a rank order by metabolic demand.

05Predictions

  • P1Size reduction should correlate with oxygen proxies better than with productivity proxies.
  • P2High-metabolism groups should shrink more.
  • P3The effect should appear in hypoxic settings without productivity collapse.

06What would refute it

Size change tracking productivity at controlled oxygen would leave scarcity as the driver.

07What it would change

It would make body size a palaeo-oxygen proxy, and change extinction narratives.

08Evidence base

  1. The Lilliput effect in the aftermath of the end-Permian extinctionhttps://www.sciencedirect.com/science/article/abs/pii/S0031018207001137
  2. The Lilliput effect in scolecodonts from the Late Devonian crisis — Paleobiologyhttps://www.cambridge.org/core/journals/paleobiology/article/another-worm-bites-the-dust-the-lilliput-effect-in-scolecodonts-from-the-late-devonian-biodiversity-crisis/E5EEFB470B1CF52D7FF0ED258FF69F24