Even in the animal world, there are high rollers and those with cheaper lifestyles. The links between the lifestyle of an animal and the associated costs has long fascinated both ecologists and physiologists. One environment that is associated with a slower lifestyle is the tropics. In a recent study published in the Proceedings of the National Academy of Sciences,Popko Wiersma, Mark Chappell and Joseph Williams looked at the energy expenditure in tropical birds during expensive activities such as exercise and keeping warm. They hypothesized that rainforest birds have a reduced capacity to stay warm when exposed to cold, but also a lower endurance in flight. In other words, is it cheaper to live in the tropics?
The team headed down to the Smithsonian Tropical Research Institute in Panama, where they captured species of birds living in the rainforest. One test they performed consisted of placing the birds in a chamber where the rate of oxygen consumption was measured as the temperature in the chamber was lowered. This measurement allowed them to assess the peak metabolic rate of 19 species during exposure to the cold, as well as finding the lowest temperature at which the birds could maintain this peak metabolic rate. The researchers also measured peak metabolic rate during exercise; this was not a simple task but their flight wheel successfully motivated 45 species to take to the wing.
Ultimately the team compiled an impressive data set of physiological readings, across 14 species, of measurements of individual's cold temperature limit, basal metabolic rate, peak metabolic rate during exercise and peak metabolic rate in the cold. Having analysed the data, taking into account phylogenetic relatedness, the team confirmed their first hypothesis when they found that birds living in the tropics have a lower ability to stay warm;their peak metabolic rate when exposed to the cold is lower than the peak metabolic rate of temperate species when conditions get chilly. They also showed that the lowest ambient temperature at which the tropical birds could maintain their own body temperature is more than 8°C higher than for temperate species. Tropical birds do not do as well in the cold, maybe not so surprisingly, but they also appear to have a lower peak metabolic rate during exercise. The authors suspect that this is because tropical birds commute less or rely less on endurance; their comings and goings can be sustained on lower metabolic rates, so it is cheaper to live in the tropics.
Having gathered so much metabolic data on individual birds, the group were also able to tackle the issue of links between different species' energy expenditures in various situations. The team looked at whether a species that idles at a high basal metabolic rate does well in the cold or has a greater exercise capacity, and whether doing well in the cold is related to exercise ability. It turns out that idling rate and exercise ability are related, but not the ability to tolerate cold temperature. It begs the question, what is the mechanism that links metabolic rate at rest and during exercise? – a well-known old animal physiology riddle.
In this study, Wiersma, Chappell and Williams have highlighted the ecological causes and consequences of physiological diversity, as well as providing the raw material for further investigation of the mechanisms that connect metabolic rates of animals in different situations.
