Nest substrate and tool shape significantly affect the mechanics and energy requirements of avian eggshell puncture

Abstract

Some host species of avian obligate brood parasites reject parasitic eggs from their nest whereas others accept them, even though they recognize them as foreign. One hypothesis to explain this seemingly maladaptive behavior is that acceptors are unable to pierce and remove the parasitic eggshell. Previous studies reporting on the force and energy required to break brood parasites’ eggshells were typically static tests performed against hard substrate surfaces. Here, we considered host nest as a substrate to simulate this potentially critical aspect of the natural context for egg puncture while testing the energy required to break avian eggshells. Specifically, as a proof of concept, we punctured domestic chicken eggs under a series of conditions: varying tool shape (sharp vs. blunt), tool dynamics (static vs. dynamic), and the presence of natural bird nests (of three host species). The results show a complex set of statistically significant interactions between tool shapes, puncture dynamics, and nest substrates. Specifically, the energy required to break eggs was greater for the static tests than for the dynamic tests, but only when using a nest substrate and a blunt tool. In turn, in the static tests, the addition of a nest significantly increased energy requirements for both tool types, whereas during dynamic tests, the increase in energy associated with the nest presence was significant only when using the sharp tool. Characterizing the process of eggshell puncture in increasingly naturalistic contexts will help in understanding whether and how hosts of brood parasites evolve to reject foreign eggs.

Investigation of the rate-mediated form-function relationship in biological puncture

“…The positively correlated form-function relationship between the sharpness of puncture tools (measured by height-to-base aspect ratio, cusp angle, or tip radius) and puncture performance (characterized by the force or energy required for puncture) has been experimentally demonstrated by numerous studies on biological puncture 15 – 18 , 30 – 32 . Therefore, it is not surprising that Fig.…”

Modelling biological puncture: a mathematical framework for determining the energetics and scaling

“…Other performance metrics extract force/energy measurements directly from puncture tests [1][2][3][4][5]8,11,17,48,49,58]. These measures do not contain explicit information about either puncture tool shapes or material properties.…”

“…The visualization and verification presented here lay the foundation for implementing the puncture energy theory to help in characterizing biological puncture systems. However, it is important to recognize that biological puncture systems associated with brittle failure response, such as egg shell puncture [49] and chitinous beetle shell puncture [50], are not uncommon. Brittle crack extension and branching may markedly change the damage morphology [28,36,42] and, consequently, increase the damage size and the contribution to the released fracture energy relative to the stored strain energy—an avenue for future research.…”