Body size, energy metabolism and lifespan

Abstract

Bigger animals live longer. The scaling exponent for the relationship between lifespan and body mass is between 0.15 and 0.3. Bigger animals also expend more energy, and the scaling exponent for the relationship of resting metabolic rate (RMR) to body mass lies somewhere between 0.66 and 0.8. Mass-specific RMR therefore scales with a corresponding exponent between -0.2 and -0.33. Because the exponents for mass-specific RMR are close to the exponents for lifespan, but have opposite signs, their product (the mass-specific expenditure of energy per lifespan) is independent of body mass (exponent between -0.08 and 0.08). This means that across species a gram of tissue on average expends about the same amount of energy before it dies regardless of whether that tissue is located in a shrew, a cow, an elephant or a whale. This fact led to the notion that ageing and lifespan are processes regulated by energy metabolism rates and that elevating metabolism will be associated with premature mortalitythe rate of living theory.The free-radical theory of ageing provides a potential mechanism that links metabolism to ageing phenomena, since oxygen free radicals are formed as a by-product of oxidative phosphorylation. Despite this potential synergy in these theoretical approaches, the free-radical theory has grown in stature while the rate of living theory has fallen into disrepute. This is primarily because comparisons made across classes (for example, between birds and mammals) do not conform to the expectations, and even within classes there is substantial interspecific variability in the mass-specific expenditure of energy per lifespan. Using interspecific data to test the rate of living hypothesis is, however, confused by several major problems. For example, appeals that the resultant lifetime expenditure of energy per gram of tissue is 'too variable' depend on the biological significance rather than the statistical significance of the variation observed. Moreover, maximum lifespan is not a good marker of ageing and RMR is not a good measure of total energy metabolism. Analysis of residual lifespan against residual RMR reveals no significant relationship. However, this is still based on RMR.A novel comparison using daily energy expenditure (DEE), rather than BMR, suggests that lifetime expenditure of energy per gram of tissue is NOT independent of body mass, and that tissue in smaller animals expends more energy before expiring than tissue in larger animals. Some of the residual variation in this relationship in mammals is explained by ambient temperature. In addition there is a significant negative relationship between residual lifespan and residual daily energy expenditure in mammals. A potentially much better model to explore the links of body size, metabolism and ageing is to examine the intraspecific links. These studies have generated some data that support the original rate of living theory and other data that conflict. In particular several studies have shown that manipulating animals to expend more or less energy ...

Energetics and longevity in birds

“…In our analysis, however, DEE and RMR were not significantly related to MLSP in birds after the effects of body mass and phylogeny were accounted for. These data are consistent with a previous analysis focussing primarily on mammals (Speakman 2005b), and highlight the critical importance of accounting adequately for size and phylogeny effects in comparative analyses (Promislow 1991;Speakman 2005a). One of the most important insights provided by Rubner was the observation that the product of lifespan and energy metabolism (based on RMR) was independent of body size.…”

“…Techniques such as the doubly labelled method are now available to quantify total daily energy expenditure (DEE) (Nagy 1983;Speakman 1998), and when these are used in comparative analyses rather than RMR, the supposed constancy of lifetime mass-specific energy expenditure in relation to body size disappears. In other words, when DEE is substituted for RMR data, our analyses have shown that smaller mammals actually burn more energy per gram of tissue during their lives than larger animals (Speakman 2005a).…”

Environmental Predictors of Seabird Wrecks in a Tropical Coastal Area

“…Cabot’s terns have smaller body size and shorter life spans than the other investigated species [56, 98]. Body size is positively correlated with longevity, since small birds need to feed more frequently than large birds to fulfil their high metabolic rates [99, 100]. Furthermore, thermoregulation capacity is correlated with body size, making small-bodied species more vulnerable to low temperatures and the negative effect of oiling in the heat insulating properties of feathers [6, 101].…”

Magellanic penguin telomeres do not shorten with age with increased reproductive effort, investment, and basal corticosterone

“…Across species, longevity is positively correlated with body mass (Haussmann et al., 2003; Lindstedt & Calder, 1976; Speakman, 2005). Generally, bird species live significantly longer than mammals of similar body size; however, understanding of the mechanisms that affect the increased longevity of birds is limited (Holmes and Austad, 1995a,b; Holmes, Fluckiger, & Austad, 2001).…”