Fuel for the pace of life: baseline blood glucose concentration coevolves with life history traits in songbirds

1. It has been proposed that life histories have coevolved with a suite of physiological and behavioural adaptations, termed pace-of-life syndromes (POLS). Here, we hypothesise that basal concentration of blood glucose (G0), a major source of energy circulating in vertebrate blood, may constitute a key component of POLS. 2. To test this hypothesis, we measured G0 in 30 passerine species and tested its covariation with body mass and other life history traits. Importantly, body mass is a major life history determinant and, when its effect is controlled for, there may be no single fast-slow life history continuum in birds comprising both fecundity and lifespan. Hence, we used individual life history traits, rather than principal component analysis, to characterise life history variation in our analysis. 3. In support of G0 life history coevolution, we found G0 to be negatively correlated with body mass and positively with reproductive investment in a single clutch across 30 passerine speci...