Göran Arnqvist, Biljana Stojković, Johanna L. Rönn and Elina Immonen
Animals tend to be either fast reproducers or long-lived, because it is hard to be both. This kind of negative relationship between reproduction and longevity arises due to limiting amounts of resources available to individuals. Species optimise the use of available resources differently, which results in different life history strategies. Also individuals within species differ in such strategies: males, for example, often show a fast “life-style” compared to females. This may come at the expense of self-maintenance, which can lead to sex differences in longevity and in the so-called pace of life. Metabolism converts available resources into growth and self-maintenance, which should make metabolic rate one of the key determinants of pace of life. How these two evolve together within species is not well understood. We addressed this question by using the seed beetle Acanthoscelides obtectus, where populations have been selected to either reproduce early or late in life for over 160 generations. This selection has resulted in evolutionary changes in many life history traits, such as juvenile development, body size, fecundity and lifespan. We measured metabolic rate in these selection lines and observed that, as we predicted, metabolic rate was indeed positively associated with the pace-of-life: individuals selected for slow life history with long lifespan had slower metabolic rate, which also declined more slowly as the beetles aged. Compared to females, males had up to 50% faster metabolic rate which also declined more rapidly with age. Our findings support the idea that males benefit from a faster pace of life and show that metabolic processes are at the heart of evolved differences in life-style.
Image provided by authors.