Standard metabolic rate

SMR is the lowest metabolic rate of an ectotherm while in rest, measured at a specified ambient temperature¹¹. SMR was measured in 156 females and 151 males on the day after adult emergence (Sample sizes females/males per category: north-woodland-cool: 19/23; north-woodland-warm: 11/14; north-agri-cool: 16/16; north-agri-warm: 20/13; south-woodland-cool: 25/20; south-woodland-warm: 24/22; south-agri-cool: 24/26; south-agri-warm: 17/17). Before the experiment, butterflies were kept in individual cages (30 cm³), had free access to moist cotton wool and were weighted to the nearest 0.1 mg (Ohaus Balance). Next, the individual was placed in a 25-cl transparent plastic metabolic chamber connected to a stop-flow respirometer with a CO₂ analyzer (Sable Systems Co) and with an internal thermoprobe (Physitemp probe type MT-29/1B connected to a Physitemp BAT-12 thermometer). Raw data from the system and the thermoprobe were processed by a UI-2 analogue/digital converter (Sable Systems) and stored using Sable Systems Expedata software. Ambient temperature (±SD) was 30.6 ± 0.7 °C (range: 28.7–33.8 °C). Note that the test temperature was chosen to reflect the likely body temperature of the butterflies when warmed by the sun and able to fly, which is considerably above the ambient temperature of the environment^12,50. The individual was left in the chamber to acclimatize (25 min). The chamber was covered with a non-transparent cloth; as butterflies are heliotherms and sunlight triggers their active flight behaviour, they typically remain inactive under damped conditions, as was confirmed in our experiment. CO₂-free dry air was pumped through the respirometer system. Next, the chamber was closed for 10 min and the CO₂ emitted by the resting butterfly accumulated. The chamber was then flushed again (flow rate: 1981 ± 37 ml/min; Sable Systems subsample TR-SS3 pump) and the previously accumulated CO₂ measured for 2 min (i.e., until reaching an almost null CO₂-level). After the measurement of SMR, we measured simultaneously flight performance and FMR.