Fig. 1

Swimming is more energetically demanding than crawling for C. elegans. a A microcalorimeter measuring unit composed of a reference ampoule (without animals) and a test ampoule (with C. elegans) within a precisely temperature-regulated water bath. Heat flows are monitored by super-sensitive heat detectors. b Microcalorimetry measurements of standard metabolic rate (SMR, metabolic rate at rest) at 20 °C on a solid (nematode growth medium (NGM) agar) or in a liquid (M9 buffer) environment. We immobilized animals for SMR measurements by levamisole-induced paralysis for N2 animals (n = 4 independent trials) or by genetic paralysis of unc-54 mutants deficient in a major muscle myosin (n = 5 independent trials), and measured heat output normalized to the total amount of protein in each sample. c Microcalorimetry measurements of active metabolic rate (AMR) at 20 °C on a solid (NGM agar) or in a liquid (M9 buffer) environment by N2 animals (n = 7 independent trials). Heat output was normalized to the total amount of protein in each sample. Because unc-54 mutants cannot move, their AMR cannot be measured. d Calculating the energy cost of locomotion. The energy cost of crawling or swimming equals the difference between the AMR and the SMR on a solid or in a liquid environment, respectively. e Energy cost of both locomotion forms calculated based on the microcalorimetry measures presented in (b) and (c). Note that when using unc-54 SMR to calculate the energy cost of locomotion, we compared it to N2 AMR, and the genetic background of these two strains might differ slightly. Statistical significance was determined by paired two-tailed Student’s t test. **P < 0.01; ***P < 0.001; ****P < 0.0001. ns non-significant