The life‐history theory of evolution posits that organisms are maximized for reproductive success through the processes of natural selection. Though believed to impact nearly every aspect of an organism, the life‐history theory of evolution shines little attention on the cellular aspects of organismal physiology. Tropical bird species, compared with temperate birds, are typically placed on the ‘slow' pace‐of‐life spectrum, and demonstrate a ‘slow' pace of life across every level of organization. Previously, we demonstrated that phylogenetically paired tropical birds have significantly smaller muscle fiber diameters than their temperate counterparts. No other muscle‐related properties have been addressed and correlated to life‐histories in the literature. Myonuclear domain (MND), the cytoplasmic volume that each myonucleus serves, is an under‐appreciated skeletal muscle property that has been largely ignored in bird muscle literature. Here, we measured MND in muscle cross‐sections in phylogenetically paired species of tropical and temperate birds. We found that tropical bird species had significantly higher numbers of nuclei per mm of fiber as compared with temperate bird species. This may be due to the fact that tropical birds could have a higher population of slow‐ oxidative fiber and/or due to developing in warmer climates, as compared with temperate birds. In both tropical and temperate birds, we found significantly positive associations between muscle fiber diameter and MND and between muscle cross‐sectional areas and number of nuclei per mm of fiber in tropical birds and temperate birds, as previously noted in other bird species.
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