| Popis: |
Obligate avian brood parasites lay their eggs in nests of others avian species, the hosts; the latter provide all parental care to the parasitic eggs, chicks and fledglings. Brood parasites depend entirely on the hosts for reproduction, which in turn suffer fitness losses as a result of parasitism. Parasitic cowbirds (Molothrus) reduce host fitness through grasp−ejection (e.g., M. ater) or puncture (e.g., M. rufoaxillaris and M. bonariensis) of the host’s egg(s). In addition, cowbirds typically lay their eggs from an elevated position, dropping them into the host nest and damaging some eggs already in the nest. Therefore, adaptations in parasites and defense in hosts are expected to differentially shape the evolution and the resulting properties of host and parasite eggs. Here I tested this hypothesis by comparing the mechanical and structural features of eggs and shells. M. ater’s eggs were not stronger and stiffer than their large or frequent hosts’ eggs. However, such traits are still advantageous against attempts of some of their smaller hosts to pierce−eject parasitic eggs when grasp−ejection is bill−gape size−wise impossible. In turn, the two egg–puncturer cowbird species’ eggs were more puncture resistant and stiffer than host eggs, and displayed eggshell crystallographic structure that differed from those of other species. These traits would thus confer also a lower probability of being punctured by other female parasites during multiple parasitism events. One property shared by cowbirds that stood out over hosts was mechanical ’toughness’ (better ability of the shell to absorb energy elastically and to resist the propagation of small cracks). This trait provides protection to the parasite egg against damage during laying from an elevated position, while causing damage to host eggs, and this trait provides protection when other cowbird eggs are laid in nests with multiple parasitism. By studying the shifts in mechanical and ultrastructural features of the incubated egg in egg−puncturer parasites and frequent hosts, where no differences in shell strength, stiffness, and toughness at hatching were detected between these two parasites and hosts. These findings indicate that parasitic embryos do not need to engage greater effort during hatching relative to host embryos. Finally, grasp−ejection and egg−puncture parasitic behaviors appear to induce evolutionary defenses in their frequent hosts by differentially affecting the egg shape and size (larger, rounded and asymmetrical eggs), and the shell strength and stiffness, to reduce the chance of cowbird mediated host−egg removal and puncture, in the respective host species. |
