Standard metabolic rate (SMR), defined as the minimal energy expenditure required for self-maintenance, is a key physiological trait. Few studies have estimated its relationship with fitness, most notably in insects. This is presumably due to the difficulty of measuring SMR in a large number of very small individuals. Using high-throughput flow-through respirometry and a Drosophila melanogaster laboratory population adapted to a life-cycle that facilitates fitness measures, we quantified SMR, body mass, and fitness in 515 female and 522 male adults. We used a novel multivariate approach to estimate linear and non-linear selection differentials and gradients from the variance-covariance matrix of fitness, SMR, and body mass, allowing traits specific covariates to be accommodated within a single model. In males, linear selection differentials for mass and SMR were positive and individually significant. Selection gradients were also positive but, despite substantial sample sizes, were non-significant due to increased uncertainty given strong SMR-mass collinearity. In females, only nonlinear selection was detected and it appeared to act primarily on body size, although the individual gradients were again non-significant. Selection did not differ significantly between sexes although differences in the fitness surfaces suggest sex-specific selection as an important topic for further study.

Methods are described in Videlier, M., V. Careau, A.J. Wilson, and H.D. Rundle. Quantifying selection on standard metabolic rate and body mass in Drosophila melanogaster. Evolution: in press.

Each row corresponds to an individual fly (total: 1060 individuals, 532 females and 528 males). Columns are as follows:

BLOCK: Experimental block, a categorical variable with six levels (1 to 6) corresponding to the 6 temporal blocks over which the experiment was performed.
Sspop: Day of adult emergence, a categorical variable with three levels (1 to 3) corresponding to flies that ecolosed 8, 9 or 10 days after eggs were laid.
B_S: A combination of block (six levels) and day of emergence (three levels).
ID: A categorical variable corresponding to a unique individual identifier.
SEX: A categorical variable with 2 levels (0 for female, 1 for male).
MASS: A continuous variable representing the wet body mass (g) of the individual.
TEMP: A continuous variable consisting of the average temperature (Celcius) during the 20 sec measurement of standard metabolic rate (i.e. SMR).
WV: A continuous variable representing the average water vapor  (ppt) during the 20 sec SMR measurement.
LUX: A continuous variable representing the average light intensity (Lux) during the 20 sec SMR measurement.
FRC: A continuous variable representing the average flow rate (ml/min) during the 20 sec SMR measurement.
ACT20: A continuous variable representing the average locomotor activity during the 20 sec SMR measurement.
ACT20p: A continuous variable representing the average locomotor activity 20 sec before SMR measurement.
SMR: A continuous variable representing the standard metabolic rate (ml CO2/min) of the individual.
MW_WTnb: Total number of wild-type offspring produced by the focal male; a continuous variable.
NbFemTot: A continuous variable representing the number of mutant (bw) females which were used for male fitness measurement.
FW_W: Total number of wild-type offspring produced by the focal female; a continuous variable.