Metabolism – the chemical process that generates energy for cells to function – is so fundamental to existence that it has been dubbed ‘the fire of life’. However, the debate over how the rate of metabolism can influence an animal's survival has burned on for decades. The ‘pace of life’ theory predicts that species with high metabolic rates accrue more damage and succumb to death earlier than animals with slower metabolism. Whilst this makes sense in theory – a gently burning flame lasts longer than a firework – experimentally proving it has been a tricky business.
To shed light on how metabolic rate correlates with survival in wild tropical birds, a team led by Micah Scholer from the University of British Columbia, Canada, studied a diverse cohort of 37 species in the Peruvian Andes, from the humid foothills up to 3000 m altitude. The group caught birds in mist-nets and released them, tagged but unharmed, at different sites over several years to assess annual survival. They then combined this dataset with previously acquired measurements of basal metabolic rate of the same species from the same field sites – achieved by measuring oxygen consumption in resting animals – in a sophisticated mathematical model to dissect the interplay between metabolism, survival and habitat.
Survival varied greatly between different species; in some cases, annual survival exceeded 70%, whereas in the most vulnerable species, less than half this proportion survived each year. The team further revealed that, although metabolic rate was similar between birds inhabiting different altitudes, montane species exhibited lower survival than those from the foothills. Avian life at high altitude is something of a double-edged sword; the harsher, drier environment provides challenges, yet this also discourages predators. It appears here that the costs of living at altitude outweighed the benefits of lower predation, although the research team was unable to exclude the possibility that the high-altitude birds were emigrating from the study site instead of perishing.
The standout result was that, irrespective of altitude, the species with the lowest metabolic rate – or the slowest pace of life – indeed exhibited the highest rate of survival. Whether annual survival per se equates to evolutionary success is doubtful, because even the species with the lowest survival rate are, by definition, successful enough to prosper today. Apparently, they are able to reproduce effectively in their short but busy lifetime, which may be fuelled by their high metabolic rate. The fact that species with both short and long lifespans persist suggests that neither strategy is optimal.
By revealing a well-defined link between metabolism and survival, this study not only represents a great advance for environmental physiology but also helps pave a path for future work. For example, researchers could measure metabolic rates in animals’ natural environments, which is tricky but technically feasible, to allow us to describe how metabolism changes seasonally in the different habitats. Ultimately, this will help paint an ever-clearer picture of how and why survival and metabolism vary between different species.