Ecological and life-history correlates of erythrocyte size and shape in Lepidosauria

Abstract

Blood oxygen-carrying capacity is shaped both by the ambient oxygen availability as well as species-specific oxygen demand. Erythrocytes are a critical part of oxygen transport and both their size and shape can change in relation to species-specific life-history, behavioural or ecological conditions. Here, we test whether components of the environment (altitude), life history (reproductive mode, body temperature) and behaviour (diving, foraging mode) drive erythrocyte size variation in the Lepidosauria (lizards, snakes and rhynchocephalians). We collected data on erythrocyte size (area) and shape (L/W: elongation ratio) from Lepidosauria across the globe (N = 235 species). Our analyses show the importance of oxygen requirements as a driver of erythrocyte size. Smaller erythrocytes were associated with the need for faster delivery (active foragers, high-altitude species, warmer body temperatures), whereas species with greater oxygen demands (diving species, viviparous species) had larger erythrocytes. Erythrocyte size shows considerable cross-species variation, with a range of factors linked to the oxygen delivery requirements being major drivers of these differences. A key future aspect for study would include within-individual plasticity and how changing states, for example, pregnancy, perhaps alter the size and shape of erythrocytes in Lepidosaurs.

Keywords: blood oxygen-carrying capacity; erythrocyte; life history; oviparity; viviparity.

FIGURE 1

Mean erythrocyte area (µm²) and mean elongation ratio for individual species across the phylogenetic relationship of Lepidosauria. For illustration, the approximate location of representative taxa (top to bottom: Sphenodon punctatus; Gehyra mutilata; Eumeces fasciatus; Lacerta agilis; Varanus komodoensis; Stellagama stellio; Boa constrictor; Vipera latastei; Notoechis ater; Elaphe quadrivirgata) are included as tip labels were too small to be visible. Species that are underlined are not in the phylogeny but represent closely related species. All images are taken from Phylopics (http://phylopic.org) under Public Domain Dedication 1.0 license(http://creativecommons.org/publicdomain/zero/1.0/)

FIGURE 2

Relationships between body mass (log10, g) and erythrocyte area (μm²). Raw data points are shown, with regression line (+95CI) drawn through predicted values derived from the phylogenetic MCMC model

FIGURE 3

Boxplots showing the effect of (a) diving and (b) reproductive mode on erythrocyte area (µm²) and scatterplot showing the effect of (c) altitude and reproductive mode (oviparity = grey circles, viviparity = black circles) on erythrocyte area (µm²). Raw data points are shown and both boxplots and regression lines (+95CI) are drawn through predicted values derived from the phylogenetic MCMC models

FIGURE 4

The relationship between elongation ratio of erythrocytes and altitude (m). Raw data points are shown, with regression line (+95CI) drawn through predicted values derived from the phylogenetic MCMC model

FIGURE 5

The relationship between erythrocyte area (µm²) and (a) foraging mode (active or ambush foraging) and (b) mean recorded body temperature (degrees Celsius) across Lepidosauria