Abstract

Complex and at times extreme environments have pushed many bird species to develop unique eggshell surface properties to protect the embryo from external threats. Because microbes are usually transmitted into eggs by moisture, some species have evolved hydrophobic shell surfaces that resist water absorption, while also regulating heat loss and the exchange of gases. Here, we investigate the relationship between the wettability of eggshells from 441 bird species and their life-history traits. We measured the initial contact angle between sessile water droplets and the shell surface, and how far the droplet spread. Using phylogenetic comparative methods, we show that body mass, annual temperature and eggshell maculation primarily explained variance in water contact angle across eggshells. Species nesting in warm climates were more likely to exhibit highly hydrophobic eggshells than those nesting in cold climates, potentially to reduce microbial colonization. In non-passerines, immaculate eggs were found to have more hydrophobic surfaces than maculate eggshells. Droplets spread more quickly on eggshells incubated in open nests compared to domed nests, likely to decrease heat transfer from the egg. Here, we identify clear adaptations of eggshell wettability across a diverse range of nesting environments, driven by the need to retain heat and prevent microbial adhesion.

Highlights

  • Avian eggshells are fine tuned to the needs of the embryo and fulfil multiple adaptive functions, including crypsis, mechanical and microbial protection, gas and water exchange and providing calcium for bone growth [1,2]

  • (i) we examined whether eggshell wettability is influenced by shared evolutionary history between species

  • (ii) we explore whether eggshell wettability is associated with life-history strategies of species, after accounting for phylogeny

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Summary

Introduction

Avian eggshells are fine tuned to the needs of the embryo and fulfil multiple adaptive functions, including crypsis, mechanical and microbial protection, gas and water exchange and providing calcium for bone growth [1,2]. Bird species differ greatly in their ability to retain or repel water droplets on their eggshell surfaces [3,4]. Differences in eggshell wettability reflect distinct approaches to assist embryonic development in different nesting environments, and under different parental incubation strategies. Hydrophobic eggshells minimize water and debris coverage on the surface, and are expected to be more prevalent in those species which experience a high bacterial load in their nests, either due to nest location or parental behaviour [6].

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