Abstract
BackgroundPhysiological adaptations that allow air-breathing vertebrates to remain underwater for long periods mainly involve modifications of the respiratory system, essentially through increased oxygen reserves. Physiological constraints on dive duration tend to be less critical for ectotherms than for endotherms because the former have lower mass-specific metabolic rates. Moreover, comparative studies between marine and terrestrial ectotherms have yet to show overall distinct physiological differences specifically associated with oxygen reserves.Methodology/Principal FindingsWe used phylogenetically informed statistical models to test if habitat affects hematocrit (an indicator of blood oxygen stores) in snakes, a lineage that varies widely in habitat use. Our results indicate that both phylogenetic position (clade) and especially habitat are significant predictors of hematocrit. Our analysis also confirms the peculiar respiratory physiology of the marine Acrochordus granulatus.Conclusion/SignificanceContrary to previous findings, marine snakes have significantly–albeit slightly–elevated hematocrit, which should facilitate increased aerobic dive times. Longer dives could have consequences for foraging, mate searching, and predation risks. Alternatively, but not exclusively, increased Hct in marine species might also help to fuel other oxygen-demanding physiological adaptations, such as those involved in osmoregulation.
Highlights
Evolutionary habitat transitions often involve substantial modifications in morphology, physiology, and behaviour, if the new habitat imposes novel physical challenges
A significant effect of Clade on a non-hierarchical tree (OLS model) indicates that hematocrit varies among major branches of the tree, while the fact that the OLS model fits better than either Phylogenetic Generalized Least Squares (PGLS) or RegOU models indicates no statistically significant "phylogenetic signal" [20], [21], [27], [29] remains in the residuals after accounting for Clade and Habitat
Contrary to previous findings [9], [12], [13], [31], our study suggests that marine snakes display increased blood-oxygen stores
Summary
Evolutionary habitat transitions often involve substantial modifications in morphology, physiology, and behaviour, if the new habitat imposes novel physical challenges. Transition to an aquatic life leads to selection on such attributes as the ability to move through water, to remain underwater for long periods, and to dive to considerable depths [1]. Features facilitating such tasks are different from those required in most terrestrial organisms. Physiological adaptations that allow air-breathing vertebrates to remain underwater for long periods primarily involve modifications of the respiratory system, essentially through increased oxygen reserves (e.g. blood volume, hemoglobin concentration, myoglobin concentration [3]). Physiological adaptations that allow air-breathing vertebrates to remain underwater for long periods mainly involve modifications of the respiratory system, essentially through increased oxygen reserves. Comparative studies between marine and terrestrial ectotherms have yet to show overall distinct physiological differences associated with oxygen reserves
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