What is it about?
The rates of various biological processes, collectively known as the ‘pace of life’, vary considerably among organisms and environments, for reasons that are not completely understood. Since all biological processes are fueled by metabolism or the ‘fire of life’, one widely accepted explanation for variation in their rates is that they are driven by variation in the rate of metabolism, which is in turn dictated by body size, temperature and resource availability. However, this ‘metabolic pacemaker’ view is largely based on correlative evidence (i.e. covariation between the rates of metabolism and that of other biological processes), that can be explained in multiple cause-and-effect ways. Although such correlations (pace-of-life syndromes) may in some cases be driven by the rate of metabolism, reverse causal relationships may also occur: i.e. the rate of a biological process may drive the rate of metabolism. This causal bidirectionality is consistent with growing evidence that the biochemical reactions constituting metabolism are both ‘pushed’ by resource supply and ‘pulled’ by metabolic demand. A third possibility is that the rates of metabolism and other biological processes may be independently controlled or co-adjusted in an integrative way by various regulatory factors. These multiple possibilities are supported by evidence showing that the rate of metabolism may be unrelated to, dissociated from, co-adjusted with, or driven by the rates of other biological processes. As a result, I argue in my review that the metabolic pacemaker view is inadequate because it emphasizes energetic control based simply on resource supply, while ignoring informational control of resource uptake and use based on the effects of various genetic, cellular and neuroendocrine regulatory systems. Understanding the temporal dynamics of life requires a holistic perspective involving multidirectional causal pathways and the effects of both resource supply and demand, as orchestrated by adaptively regulated and evolved responses to various intrinsic (biological) and extrinsic (ecological) factors.
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Why is it important?
The tempo of life is critical for the evolutionary fitness of organisms and the proper functioning and coordination of their vital processes. An understanding of the factors driving the rate of living processes is thus of fundamental concern in biology. Contrary to common belief, my review shows that metabolic rate is not a universal driver of the pace of life. Instead, the pace of life appears to be driven by multiple regulatory factors that synchronize or dissociate the rates of various biological processes. This understanding may have multiple theoretical and practical applications.
Perspectives
My review was spurred primarily by my own research on the effect of growth rate and locomotor activity on the body-mass scaling of metabolic rate. For example, my students and I have shown that predator-induced changes in growth rate appear to drive metabolic rate and its scaling with body mass in the freshwater amphipod Gammarus minus. In springs with fish predators, the slopes of the scaling relationships for rates of growth, food intake and metabolism of amphipod populations all show similar parallel decreases compared to those observed in populations inhabiting springs without fish (see Glazier et al. 2011, Ecol. Monogr. 81: 599-618; Glazier et al. 2020, Biology 9: 40). This ‘parallel allometry’ is best explained by predator-induced size-selective mortality and inhibition of amphipod locomotor activity primarily driving the rates of growth and food intake, which in turn result in changes in the rate of supporting metabolism. Similarly, several intra- and interspecific analyses show that changes in locomotor activity cause significant changes in the body-mass scaling of metabolism in a variety of animals (reviewed in Glazier 2008, Proc. R. Soc. Biol. Sci. 275: 1405-1410; Glazier 2009, J. Comp. Physiol. B 179: 821-828; Glazier 2010, Biol. Rev. 85: 111-138; Glazier 2014, Systems 2:425-450). These effects are best explained by the energetic demand of muscular activity dictating the rate of metabolism, rather than the reverse as specified by the metabolic pacemaker view. In short, these studies indicate that the rate of metabolism may not necessarily drive the rates of other biological processes, such as growth and locomotor activity, but rather may be driven by these processes.
Professor Douglas S Glazier
Juniata College
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This page is a summary of: Is metabolic rate a universal ‘pacemaker’ for biological processes?, Biological Reviews, May 2014, Wiley,
DOI: 10.1111/brv.12115.
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