Fracture toughness of horns and a reinterpretation of the horning behaviour of bovids

Abstract

The keratinous horns of bovids are used in intraspecific combat to gain access to females in oestrus. Horn sheath keratin is a composite material consisting of stiff protein fibres and a pliant protein matrix. Unlike antlers, horns are permanent structures which are likely to accumulate damage during fighting. Therefore, horn sheath keratin should be resistant to fracture (tough) and insensitive to surface defects (scratches and cracks) which may weaken horns by acting as stress concentrators. The effect of water on the toughness and notch‐sensitivity of horn sheath keratin was investigated in three‐point bending and tensile tests. Several measures of toughness were made on dry (0% water content), fresh (20%) and wet (40%) horn keratin, including total work of fracture, Gurney & Hunt work of fracture, critical strain energy release rate and critical stress intensity factor. The mean total work of fracture of fresh horn is about 40 kJ/m2 which is relatively much greater than most biological and synthetic materials. Most of the work of fracture is due to plastic yielding of the matrix (50–75%); the rest is due to crack‐tip specific fracture mechanisms such as fibre pull‐out and Cook Gordon crack‐stopping. Dehydration reduces the total work of fracture of horn keratin by preventing the yielding of the matrix. The strength of fresh and wet horn is insensitive to notches, but dry horn is very notch‐sensitive. Therefore, bovids must avoid dehydration of their horns due to the desiccating effect of the environment. The ‘horning’ behaviour of bovids may be a maintenance activity which ensures that the horn sheath is adequately hydrated to remain tough and notch‐insensitive.

Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

“…) at 8% water content [189], similar to other studies that horn sheaths of gemsbok, mouflon and waterbuck show maximum toughness when they are in the fresh state [141]. This explains why the bovid animals dip their horns into mud or plants to avoid the over dehydration and to ensure optimized mechanical properties.…”

“…The relationship of hydration and work of fracture can also explain the horning behavior of bovids [141]. The horn keratin has the highest work of fracture (41.63 ± 1.69 MJ m…”

“…On the other hand, the enhanced understanding of keratins has fueled the research area of biological keratinous materials with the aim to create bioinspired materials. Some keratinized materials with interesting properties, such as skin [137], quills [138,139], fingernails [140], horns [141,142], whelk egg capsules [116], and bird feathers [143,144], have been studied, with the hopes to obtain mechanisms and principles to design new functional materials, such as light-weight composites, and energy-absorbent materials [145]. This is a new and fascinating area, awaiting more and in-depth explorations.…”

“…They are remarkably tough, resilient and highly resistant to impact forces, since they are usually subject to extreme loading impacts during life and will not grow back once broken [185,186]. Horns possess very high energy absorption before breaking [141]: the work of fracture of fresh waterbuck horns was found to range from 10 to 80 kJ/m 2 along the length of the horn; taking [187,188]. This excellent mechanical performance results from the complex hierarchical structure and the crack-stopping mechanisms.…”

Regulation of hard α-keratin mechanics via control of intermediate filament hydration: matrix squeeze revisited

“…Lower matrix keratins may be able to absorb more energy (i.e. have a higher work of fracture), as the matrix component has been found to have lower fracture toughness than IFs [29], and higher hydration sensitivity may also improve work of fracture of keratins [30,31]. Since work of fracture may be more important than stiffness at high humidity for Table 1.…”