title: Alternative mechanisms of increased eggshell hardness of avian brood parasites relative to host species.
abstract: Obligate brood parasitic birds lay their eggs in nests of other species and parasite eggs typically have evolved greater structural strength relative to host eggs. Increased mechanical strength of the parasite eggshell is an adaptation that can interfere with puncture ejection behaviours of discriminating hosts. We investigated whether hardness of eggshells is related to differences between physical and chemical traits from three different races of the parasitic common cuckoo Cuculus canorus, and their respective hosts. Using tools developed for materials science, we discovered a novel correlate of increased strength of parasite eggs: the common cuckoo's egg exhibits a greater microhardness, especially in the inner region of the shell matrix, relative to its host and sympatric non-host species. We then tested predictions of four potential mechanisms of shell strength: (i) increased relative thickness overall, (ii) greater proportion of the structurally harder shell layers, (iii) higher concentration of inorganic components in the shell matrix, and (iv) elevated deposition of a high density compound, MgCO(3), in the shell matrix. We confirmed support only for hypothesis (i). Eggshell characteristics did not differ between parasite eggs sampled from different host nests in distant geographical sites, suggesting an evolutionarily shared microstructural mechanism of stronger parasite eggshells across diverse host-races of brood parasitic cuckoos.
authors.affiliation: School of Biological Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Branislav
authors.initials: B
authors.lastname: Igic
authors.forename: Kim
authors.initials: K
authors.lastname: Braganza
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Hyland
authors.forename: Heather
authors.initials: H
authors.lastname: Silyn-Roberts
authors.forename: Phillip
authors.initials: P
authors.lastname: Cassey
authors.forename: Tomas
authors.initials: T
authors.lastname: Grim
authors.forename: Jarkko
authors.initials: J
authors.lastname: Rutila
authors.forename: Csaba
authors.initials: C
authors.lastname: Moskát
authors.forename: Mark E
authors.initials: ME
authors.lastname: Hauber
mesh_terms: D000220:Adaptation, Biological / Q000502:physiology*; D000818:Animals; D005075:Biological Evolution; D001717:Birds; D001837:Body Weights and Measures; D004528:Egg Shell / Q000737:chemistry* / Q000648:ultrastructure; D005843:Geography; D006244:Hardness; D006245:Hardness Tests; D008274:Magnesium / Q000032:analysis; D008855:Microscopy, Electron, Scanning; D009425:Nesting Behavior / Q000502:physiology*; D013045:Species Specificity
chemical_list: C005479:magnesium carbonate; D008274:Magnesium