How to build a puncture- and breakage-resistant eggshell? Mechanical and structural analyses of avian brood parasites and their hosts

LÓPEZ, ANALÍA V.; BOLMARO, RAÚL E.; ÁVALOS, MARTINA; GERSCHENSON, LÍA N.; REBOREDA, JUAN C.; FIORINI, VANINA D.; TARTALINI, VANINA; RISSO, PABLO; HAUBER, MARK E.

JOURNAL OF EXPERIMENTAL BIOLOGY

COMPANY OF BIOLOGISTS LTD

Lugar: Cambridge; Año: 2021 vol. 224

0022-0949

Evolved eggshell strength is greater in several lineages of obligate avian brood parasites (birds that lay their eggs in other species? nests) compared to their hosts. Greater strength is typically indirectly implied by eggshell thickness comparisons between parasites and hosts. Nevertheless, there is strong evidence that the eggshell structural organization differentially influences its mechanical properties. Using instrumental puncture tests and SEM/EBSD and XRD techniques, we studied the most relevant eggshell mechanical, textural, ultra- and microstructural features between several host species and their parasitic cowbirds (Molothrus spp.) that display different egg destructive behaviors reducing host reproductive fitness, and include the more frequently host-egg puncturer M. rufoaxillaris and M. bonariensis, and the host-egg remover M. ater. The results, analyzed using a phylogenetic comparative approach, showed interspecific patterns in the mechanical and structural features. Overall, eggshell of both species of the two egg-puncturer parasites (but not of M. ater) were stronger, stiffer, and required greater stress to produce its fracture than the respective hosts? eggs. These features were affected by eggshell micro- and ultrastructures, related to the increased of the intercrystalline boundary network acting in cooperation with the increased of the palisade layers´ thickness. Both of these structural traits generate more options and greater lengths of intercrystalline paths, increasing the energy consumed in crack or fissure propagation. The reported patterns of all these diverse eggshell features support a new set of interpretations, confirming several hypotheses regarding the impacts of both reproductive strategies (parasitic vs. parental) and parasitic egg destruction behaviors (more vs. less frequently puncturing).