Why adult mayflies of Cloeon dipterum (Ephemeroptera: Baetidae) become smaller as temperature warms

We reared Cloeon dipterum from egg hatch to adult at 10 constant temperatures (12.1-33.5 degrees C) to test 3 hypotheses (thermal equilibrium hypothesis, temperature size rule [TSR], and O-2- and capacity-limited thermal tolerance [OCLTT]) that account for variation in life-history traits across thermal gradients. Male and female adult size declined similar to 67 and 78% and larval development time declined similar to 88% with warming; chronic survivorship (thermal limit for population growth) was highest from 16.2 to 23.9 degrees C (mean 5 85%) and declined to 0 at 33.5 degrees C; thresholds for 0 growth and development were 10.0 and 10.7 degrees C, respectively; peak rate of population increase (r) occurred at 27.8 degrees C; rates of growth and development were maximal at 30 degrees C; fecundity was greatest at 12.1 degrees C; and between 14.3 and 30 degrees C, growth and development rates increased linearly and the number of degree days (> 10.7 degrees C) to complete development was nearly constant (mean = 271). Acute survivorship during short-term thermal ramping was 0 at 40 degrees C. Warming temperature caused development rate to increase proportionately faster than growth rate; male and female adult size to decrease as per TSR, with adult females similar to 5x larger at 12.1 than 31.7 degrees C; adult size to decrease proportionately more for females than males; and fecundity to decrease proportionately more than adult female size. TSR was related to differences in the responses of growth and development rates at temperatures above thresholds rather than to thresholds for growth or development per se. Respirometry suggested that OCLTT is more applicable to acute than chronic thermal limits. Cloeon dipterum appears to have a thermal 'acclimation zone' between 14.3 and 30 degrees C where development and growth rates change linearly and degree-day requirements to complete metamorphosis are constant. The optimum temperature is similar to 27.8 degrees C where r is maximum. We propose 5 hypotheses to explain these patterns.