Table 1.

Overview of the different mechanisms, grouped into mechanistic and evolutionary explanations. See text for further details

Explanations	Further reading
Mechanistic (proximate) explanations
Animals grow faster but develop even faster in warm conditions
– Different thermal sensitivity of DNA replication versus protein synthesis: DNA replication (limited by enzyme kinetics) is more sensitive to temperature than protein synthesis (limited by diffusion)	Section IV.1
– At high temperatures or low oxygen, animals may preferentially allocate resources towards development and away from growth	Section IV.5
– Thermal sensitivity of growth may be reduced to prevent oxygen limitation, whereas thermal sensitivity of development may depend on genome size	Sections IV.1, IV.3 and IV.7; Fig. 4
Larger requirements for resources (food, oxygen) in warmer conditions prevent animals from growing larger
– Different thermal sensitivity of catabolism and anabolism: growth efficiency is lower in warmer conditions	Section IV.3
– Different thermal sensitivity of size‐dependent changes in catabolism and anabolism: decline in growth efficiency with size is amplified in warmer conditions, resulting in a lower growth efficiency in warmer conditions for large (but not small) individuals	Section IV.3
– Insufficient capacity to extract oxygen constrains animals from growing larger, even more so under warm conditions	Section IV.2
Animals consist of smaller cells in warm conditions
– A large genome (resulting in a larger cell size) with multiple copies ensures sufficient enzyme activity in cold conditions	Section IV.7
– Smaller cells have more membrane surface area relative to their volume supporting a greater capacity for oxygen transport in warm conditions	Section IV.7
– The ratio between oxygen supply and demand may function as a threshold for cell growth, thus regulating cell size and possibly the critical size observed in insects	Section IV.7
Evolutionary (ultimate) explanations
It becomes more advantageous to grow larger in cold conditions because of reduced mortality
– Senescence and mortality are greater in warmer environments, favouring early maturation (at a smaller size)	Sections V.1 and V.3
It becomes more advantageous to grow larger in cold conditions because of gains in fecundity
– Fecundity may increases more strongly with body size in cold conditions, favouring large size	Sections V.1 and V.3
It becomes more advantageous to grow larger in cold conditions because of resource limitations
– Selection for starvation resistance typical for larger animals is stronger in cool conditions	Sections V.2 and V.3
It becomes more advantageous to produce an additional generation rather than growing to a larger size in growing populations
– Faster maturity (at a smaller size) allows for completion of an additional generation in multivoltine species	Section VI.1; Fig. 5.
The ‘Ghost of Oxygen‐limitation Past’ has led to the evolution of thermal reaction norms for adult size that are anticipatory to temperature and oxygen conditions experienced by ancestors
– Past occurrences of oxygen limitation have selected for a canalized response with smaller sizes under warmer conditions as a compensatory response to safeguard sufficient oxygen provisioning	Sections IV.2, VI.3 and VI.4