Meeting Abstract

53.5  Monday, Jan. 5  Small organ size contributes to the slow pace of life in tropical birds WIERSMA, P; RO, J; WILLIAMS, JB*; Ohio State Univ.; Ohio State Univ.; Ohio State Univ. wiersma.6@osu.edu

Previously we have shown that lowland tropical birds have a reduced basal metabolic rate (BMR) and peak metabolic rate (PMR), induced by cold exposure or exercise, compared with temperate species. Here, we test whether reduced mass of central organs contributes to a reduction in BMR in tropical birds, and whether smaller flight muscles might contribute to reduced peak metabolic rates. In addition, we searched for correlations of metabolic rate and organ masses within tropical birds. For 14 species of tropical birds, BMR was positively correlated with mass of pectoral muscle, heart, and lungs. PMR as elicited by cold correlated positively with mass of skin, intestines, liver and kidneys (n=14). When we compared organ masses of tropical birds with those of temperate birds, using body mass as covariate, we found 13 to 34% lower mass of heart (n=424 species), liver (n=65), kidneys (n=60) and flight muscles (n=304) in tropical species. Mass of lungs (n=47), spleen (n=34), and gizzard plus intestine combined (n=35), showed similar trends. No such trend was visible in size of the brain (n=154). In a separate analysis, we paired tropical and temperate species by genus and compared heart mass. This analysis confirmed lower heart masses in tropical species. If organ masses are reduced in tropical birds, after correcting for body mass, other structures must be larger. To explore this idea, we analyzed the skeletal mass of 60 museum specimens from both tropical and temperate locations. We found that mass of the skeleton was 17% higher in tropical species. In combination, our results indicate that the benign tropical environment has relaxed selection on high levels of sustained metabolic performance, permitting species to reduce the size of organs that are costly to maintain, which results in a lower BMR in tropical species.