Small organ size contributes to the slow pace of life in tropical birds

Abstract

Recently, we demonstrated that tropical birds have an 18% lower BMR, 34% lower PMR C and 39% lower PMR E compared with temperate species (Wiersma et al., 2007a;Wiersma et al., 2007b). Along with a reduced metabolism, tropical birds tend to have a smaller clutch size than their temperate counterparts (Cardillo, 2002;Kulesza, 1990) Accepted 25 January 2012 SUMMARY Attributes of an animalʼs life history, such as reproductive rate or longevity, typically fall along a ʻslow-fastʼ continuum. Animals at the fast end of this continuum, such as temperate birds, are thought to experience high rates of mortality and invest more resources in reproduction, whereas animals at the slow end, such as tropical birds, live longer, have fewer offspring and invest more resources in self-maintenance. We have previously shown that tropical birds, compared with temperate species, have a reduced basal (BMR) and peak metabolic rate (PMR), patterns consistent with a slow pace of life. Here, we elucidate a fundamental linkage between the smaller mass of central organs of tropical species and their reduced BMR, and between their smaller flight muscles and reduced PMR. Analyses of up to 408 species from the literature showed that the heart, flight muscles, liver, pancreas and kidneys were smaller in tropical species. Direct measurements on 49 species showed smaller heart, lungs, flight muscles, liver, kidneys, ovaries and testes in tropical species, as well as lower feather mass. In combination, our results indicate that the benign tropical environment imposes a relaxed selection pressure on high levels of sustained metabolic performance, permitting species to reduce the mass of organs that are energetically costly to maintain. Brain, gizzard and intestine were exceptions, even though energy turnover of brain and intestine are high. Feather mass was 37% lower in tropical species compared with similarsized temperate birds, supporting the idea that temperate birds require more insulation for thermoregulation. Supplementary material available online at

Perspectives on the membrane fatty acid unsaturation/pacemaker hypotheses of metabolism and aging

“…In one of these studies, the selection for increased metabolic rate led to an increase in relative internal organ size (Brzek et al, 2007). Given that metabolic rate and relative internal organ size are positively correlated (Holliday et al, 1967;Wiersma et al, 2012), it seems likely in this case that selection for increased metabolic rate led to an increase in organ size, rather than changes in fatty acyl composition. Hamsters fed diets high in different PUFA did not have differences in resting metabolic rate, though differences in tissue unsaturation index were not confirmed and dietary manipulations of PUFA concentration are notorious for not changing unsaturation index without extreme and often unhealthy dietary changes Pannorfi et al, 2012).…”

Linkages between the life-history evolution of tropical and temperate birds and the resistance of their cells to oxidative and non-oxidative chemical injury

“…Evidence of a linkage between slow pace of life and low rate of metabolism in tropical birds came to light when it was shown that they had a significantly lower wholeanimal basal metabolic rate (Wiersma et al, 2007a) and peak metabolic rate as measured by cold-exposure or by exercise (Wiersma et al, 2007b). Later it was discovered that a contributing factor to the reduced rate of metabolism in tropical birds was their smaller metabolically active organs, such as the heart, liver, kidneys and pectoral muscles, compared with similar-sized temperate species (Wiersma et al, 2012). These findings offer evidence of a connection between the life history of tropical birds and their physiology, at least at the organismal and organ levels.…”

Cellular Metabolic Rate Is Influenced by Life-History Traits in Tropical and Temperate Birds

“…We think that part of the increase in self-maintenance of tropical birds manifests itself in a slower pace of life, reflected in decreased BMR and PMR, smaller metabolically active organs, and lower cellular metabolic rate [28], [29], [87]: collectively all of these are aspects of the birds’ physiology which positively correlate with increased survival. Physiological and fitness costs of particular life-history choices are dependent on environmental conditions [10], and it is difficult to separate the influences of climate per se and life-history attributes.…”

“…Our results suggest that isolated fibroblasts and myoblasts had the same metabolic profile, which suggests that fibroblasts are a representative cell system for the organism. Additionally, cellular metabolic differences could also occur for other organs, and since tropical birds generally have smaller metabolically active organs [87], then both cellular metabolic rates and organ masses are likely to alter organismal metabolic rate differences.…”