Is aquatic life correlated with an increased hematocrit in snakes?

Abstract

Background: 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. 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 findings: We 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/significance: Contrary 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.

Figure 1. Phylogeny used for analyses with corresponding Hct for each species (see Online Appendix S1, S2 & S3 for details).

Branch lengths represent the arbitrary method of Pagel, as used for statistical analyses. Black bars are for terrestrial species, light grey bars for semi-aquatic species, hatched bars for aquatic species, and white bars for marine species. Left-to-right order matches order of species in Online Appendix S1.

Figure 2. Hct (simple mean ± SE) in relation to family (a) and habitat (b).

Numbers in the bars indicate sample size (number of species).