Does the rate of metabolism drive the pace of life?

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.

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