title: Differences in muscle fiber size and associated energetic costs in phylogenetically paired tropical and temperate birds.
abstract: Tropical and temperate birds provide a unique system to examine mechanistic consequences of life-history trade-offs at opposing ends of the pace-of-life spectrum; tropical birds tend to have a slow pace of life whereas temperate birds the opposite. Birds in the tropics have a lower whole-animal basal metabolic rate and peak metabolic rate, lower rates of reproduction, and longer survival than birds in temperate regions. Although skeletal muscle has a relatively low tissue-specific metabolism at rest, it makes up the largest fraction of body mass and therefore contributes more to basal metabolism than any other tissue. A principal property of muscle cells that influences their rate of metabolism is fiber size. The optimal fiber size hypothesis attempts to link whole-animal basal metabolic rate to the cost of maintaining muscle mass by stating that larger fibers may be metabolically cheaper to maintain since the surface area∶volume ratio (SA∶V) is reduced compared with smaller fibers and thus the amount of area to transport ions is also reduced. Because tropical birds have a reduced whole-organism metabolism, we hypothesized that they would have larger muscle fibers than temperate birds, given that larger muscle fibers have reduced energy demand from membrane Na(+)-K(+) pumps. Alternatively, smaller muscle fibers could result in a lower capacity for shivering and exercise. To test this idea, we examined muscle fiber size and Na(+)-K(+)-ATPase activity in 16 phylogenetically paired species of tropical and temperate birds. We found that 3 of the 16 paired comparisons indicated that tropical birds had significantly larger fibers, contrary to our hypothesis. Our data show that SA∶V is proportional to Na(+)-K(+)-ATPase activity in muscles of birds.
authors.affiliation: Department of Evolution, Ecology and Organismal Biology, Ohio State University, 318 West 12th Avenue, Columbus, Ohio 43210.
authors.forename: Ana Gabriela
authors.initials: AG
authors.lastname: Jimenez
authors.forename: Joseph B
authors.initials: JB
authors.lastname: Williams
mesh_terms: D000818:Animals; D001481:Basal Metabolism; D001717:Birds / Q000502:physiology*; D005843:Geography; D018613:Microscopy, Confocal; D018485:Muscle Fibers, Skeletal / Q000166:cytology*; D009820:Ohio; D010176:Panama; D010802:Phylogeny; D014329:Tropical Climate