A rare mineral, vaterite, acts as a shock absorber in the eggshell of a communally nesting bird

Abstract

Birds' eggshells are primarily composed of calcite, an abundant polymorph of calcium carbonate (CaCO 3 ). However, the eggshells of some species are coated with spherules of vaterite, a rare and thermodynamically unstable polymorph of CaCO 3 , the function of which remains unknown. We experimentally tested the mechanical and physiological effects of the vaterite coating on eggshells of the Greater Ani Crotophaga major, a tropical cuckoo. Vaterite removal did not affect vapour conductance rates across the eggshell, indicating that the vaterite coating does not influence gas exchange during embryonic development. However, nanoindentation revealed that the hardness and elasticity of vaterite is similar to that of calcite, and white light interferometry showed that the vaterite layer increased the total thickness of the shell cuticle by up to 10%. Furthermore, calculations of contact mechanics found that when two eggs come into contact, the depth of the surface deformation caused by the contact is far less than the thickness of the vaterite coating. These results suggest that the layer of vaterite spherules may act as a shock absorber for the underlying calcite shell, protecting it from mechanical damage caused by collision with other eggs in the nest and reducing the risk of eggshell fracture during incubation.

Synthesis Methods and Favorable Conditions for Spherical Vaterite Precipitation: A Review

“…Vaterite can be precipitated in some mineral springs when specific glacial conditions take place [3]. Also, vaterite crystals have been identified in materials produced by living organisms, e.g., otolith organs of fishes [4][5][6], spicules of the ascidian Herdmania momus [6,7], freshwater pearls, crustacean tissues, or bird eggs [6,8] as well as the chalky crust on the surface of leaves of the alpine plant, Saxifraga scardica [9].…”

Structure and Composition of the Eggshell of a Passerine Bird,Setophaga ruticilla(Linnaeus, 1758)

“…Tullett et al (1976) have found a compositional difference in the outer layer of sea-bird eggshells: the organic cuticle usually described in "terrestrial" birds is replaced by a vateritic layer. Vaterite was also detected at the surface of the eggs of Crotophaga major (Portugal et al, 2018) and in nonparasitic cuckoos (Board & Perrott, 1979). However, the main part of these eggshells was calcite.…”

Properties, Genetics and Innate Immune Function of the Cuticle in Egg-Laying Species

“…Cuticles might also provide mechanical protection against damage produced by the incubating parent or by the hard substrate inside the nest. For example, vaterite nanospheres on bird eggshells and thick layers of aragonite on turtle shells can act as very effective shock absorbers (145) and reduce shell erosion in acidic nesting environment (146). Although we have a general knowledge of the function of eggshell cuticles, mechanistically, we know very little about how diversity in cuticle morphology and chemistry affects function, particularly in reptiles, or how environmental pressures shape the relationship between form and function.…”