The Mechanics of Antler Bone: A Weapon for Courtship

Laurianne Daoust, Annabelle Huynh-Rondeau, Félix Lavoie, Erica Song

Abstract

An organism’s evolutionary fitness is determined by its ability to pass on its genes to its offspring. Males of some species make use of their courting and fighting abilities to gain access to mates, thus passing on genetic material. When it comes to Cervidae, or the family of ruminant mammals, antlers allow males to gain high social ranking and access to mates through physical combat. Therefore, antlers have evolved towards a mechanically resilient and specific architecture to resist fights on one hand, and impress females on the other. In this essay, the architecture of deer antlers and how its multiple hierarchical levels combine to form a tough and stiff structure which can deflect crack propagation will be discussed. Also, an overview of the toughening mechanisms involved in male-on-male confrontations will be provided. Moreover, the comparison of the mechanical characteristics of antlers to those of a femur bone and a tympanic bulla will illustrate how the fighting capacities of antlers are reflected through their unique properties. The differences between the mechanics of wet and dry antlers will also be addressed to better understand how antlers are affected by their environment.

Introduction

In nature, there are many principles, phenomena and systems from which humankind could benefit. For instance, the development of air transportation was inspired by the flight of birds, while the creation of Velcro comes from the observation of burdocks. Evolution resulted in the acquisition of unique physiological traits that fit the lifestyle and the environment of specific species, such as antlers in the Cervidae family. Antlers are present in male Cervidae and appear as bone spikes anchored in the individual’s skull. One of the most important characteristics of antlers is their regenerative nature, meaning that they grow and fall off throughout the antler growth cycle. 

Their functions involve social ranking and physical combat (Goss, 1983). Most commonly, after observing their opponent’s antlers to evaluate their strength, males decide to go separate ways, thereby avoiding any physical contact. Other times, male Cervidae choose to engage in a battle, which is when their antlers become highly useful. Indeed, the strength of antlers allows Cervidae to better perform in one-to-one combat, which establishes a social ranking. They also symbolize the individual’s status and are suspected to follow the handicap theory (Goss, 1983). This hypothesis suggests that the most beautiful ornaments, namely the most imposing antlers, have negative consequences on their carrier’s health. Thus, only the fittest individuals can afford the cost of bearing extravagant sexual traits. In the case of Cervidae, the quality of their antlers is evaluated by their size, length, and the presence of fractures. As a result, those with the largest, longest, and most well-conditioned antlers are recognized by females for having the best survival abilities (Asher, 2014). 

Overall, these spiky structures have important functions and characteristics which can be better understood by analyzing the physics behind their strength.  The quality of these bones relies on their mechanical properties, the most important ones being the elastic modulus, the bending strength, the work of fracture, and the impact energy absorption. While the elastic modulus relates to the stiffness of the material, the bending strength is a measure of the minimal force needed per volume unit to cause a fracture. The work of fracture, on the other hand, is a measure of the work required to break the sample. Finally, the impact energy absorption is the amount of energy needed to crack a specimen by collision (Currey et al., 2002). 

This paper will answer the following question: How are the mechanical properties, shape, and bone structure of antlers related to their function? This question will be answered by addressing the architecture of the antler bone and the influence of the antler’s shape during a male-on-male confrontation. The mechanical comparison with common types of bone tissues and between dry antlers and wet bones will also be examined.

The toughening mechanisms of antlers

The architecture of antler bone

Biological materials exhibit their mechanical properties through complex systemic organisation into various levels (Salthe, 2013). These hierarchical structures evolved naturally from basic components as a result of the strategical self-organization during growth and development (Meyers et al, 2008).

Similarly, antler bone is organized into a complex hierarchical architecture resulting in its unique mechanical properties. As previously mentioned, the quality of a red deer’s antlers is key to the courtship process, as males battle for access to females. Thus, to be successful, the antlers require both a “high resistance to fracture” and a “strongly anisotropic behavior”, as stated by Kulin et al. (2010).

The question which arises is how antlers can be both stiff and strong, as manufacturing materials with these properties has challenged engineers for years. The answer is found in the antlers’ complex hierarchical structure.

Hierarchical structure of deer antlers

De Falco et al. (2017) examined the toughening mechanisms of antler bones at the nanoscale. As illustrated in Figure 1, the scientists identified that deer antlers encompass 8 levels of hierarchical organization, each layer of which collaborates with others to generate a highly resistant material. The core (VII) consists of cancellous bone and is surrounded by compact bone (VI), which runs longitudinally along the antlers’ main beam (Chen et al., 2008; Chen et al., 2009). The compact bone consists of osteons (V), which are built from concentric rings of laminated units (IV) arranged in a plywood structure and surrounding a central blood vessel (Chen et al., 2008). Each lamella has collagen fibrils (III) which are composed of tropocollagen (I), a helically arranged molecule with hydroxyapatite crystals interspersed between or along them (II) (Chen et al., 2008). 

Fig. 1. Hierarchical structure of the antler bone (De Falco et al., 2017). 

One of the main components which contributes to the high resistance of the antlers is the breakage of the sacrificial bonds (i.e., bonds that break before the main structural link does) in the non-collagenous proteins of the interfibrillar matrix (III). When the antler bone is under mechanical stress, the sacrificial bonds break and reform, protecting the collagen fibrils from breakage by dissipating the energy throughout the bone (De Falco et al., 2017). Thus, antlers are capable of withstanding much greater forces, allowing deer to fight for dominance and courtship.

Moreover, De Falco’s work shows that the staggered arrangement of the collagen fibrils plays a key role in energy dissipation and in enhancing the structural elastic properties of antler bone. As shown in Figure 2 (a), staggered collagen fibrils surrounded by breakable bonds behave elastically, while other fibrils reach higher stress values (De Falco et al., 2017). In fact, since staggered fibrils are capable of transmitting loading, breakable bonds allow the structure to deform up to 1.89 % under a mechanical stress of 60  MPa, while in a condition of non-breakable bonds, the tissue strain reaches 1.67 % at the same stress level (De Falco et al., 2017).

Figure 2 illustrates the different modes of binding between fibrils (left side) and the resulting relationships between the mechanical stress and tissue strain, as well as between the fibril strain and tissue strain (right side). Antlers correspond to configuration (a), as their fibrils have staggered structures and breakable bonds (De Falco et al., 2017). It can be observed that the presence of breakable bonds makes the structure more deformable, resulting in a higher tissue strain.

Fig. 2. The relationship between the mechanical stress and percentage tissue strain, as well as between the percentage fibril strain and percentage tissue strain. (a) Shows a staggered configuration and breakable bonds, (b) a staggered configuration and unbreakable bonds, and (c) a non-staggered (aligned) configuration and no bonds (De Falco et al., 2017).

The influence of antler bone’s hierarchical structure on crack propagation

There are two types of osteons in bone: primary and secondary. Primary osteons are generally smaller, while secondary osteons have a more rounded, uniform shape (Kulin et al., 2010). J.D. Currey and others (2002) have reported that primary osteons are stronger than secondary osteons, which usually arise in response to mechanical stress. Therefore, the skeletal structure, as it is constantly supporting new loads, is primarily composed of secondary osteons. However, antlers have a much shorter life span, are mainly utilized for display (with occasional combat), and do not undergo mechanical loading during their growth process (Chen et al., 2008; Kulin et al., 2010). Thus, according to J.G. Skedros et al., antlers are composed almost entirely of primary osteons.

Chen et al. (2008) have demonstrated that, intriguingly, the hierarchical structure and osteon composition of antlers influence crack propagation. In fact, in antler bone, cracks tend to propagate along the primary osteon growth direction, which leads to large crack deflections and eventual crack arrest in the transversal orientation (Fig. 3) (Kulin et al., 2010). Therefore, the antler bone’s ability to stop dynamic fracture propagation is believed to be one of the toughening mechanisms which make antlers highly resilient. This high-energy absorbing material may serve as a biomimetic inspiration for future material development (Kulin et al., 2010).

Fig. 3. Time-lapse images of a dynamic 4-point bending experiment on a dry antler sample with the notch cut transverse to osteonal growth direction (Kulin et al., 2010). It can be noticed that crack propagation was diverted and stopped by the primary osteons, preventing complete sample failure.

The benefits and disadvantages relative to different antler shapes based on their resistance to several loads sustained in fighting

Among cervid species, disputes occur more frequently than violent fights, and often between two disproportionate males or two males with relatively short antlers (Geist, 1974). These quarrels are harmless to both animals, as the less dominant male is aware that he has little chance of beating his opponent and thus withdraws from the fight (Brown, 1967). As for males with larger antlers, the altercations may turn into intense fights, although rather rare since these battles are energy-intensive and risky (Bergerud, 1973). 

Unrolling of the clashes between two males in Miocene muntiacinae

Miocene muntiacinae—an extinct deer species which inhabited the forests of South Asia—was among the first to possess antlers in addition to retaining their canine defenses (Barrette, 1977). Apart from the presence of tusks, which distinguishes this deer from other species, its way of bickering is like other cervids. Figure 8 illustrates the movements of a muntjac’s head during a fight. The two candidates first approach each other with their heads angled at 45° from the ground, coming into contact by touching the tip of their noses (Darling 1937; McCullough 1969; Struhsaker 1967). The deer then continue rotating their head until their antlers face their opponent, and a second contact occurs. From that moment on, the fighters push against one another until one of them withdraws (Darling 1937; McCullough 1969; Struhsaker 1967). When one of the competitors has withdrawn, he lifts his head and usually starts to clean himself. Sometimes a mutual cleansing occurs, marking the friendly side of the interaction (Darling 1937; McCullough 1969; Struhsaker 1967). 

Fig. 4. A schematic representation of a fight and friendly dispute between two muntjac deers. Images A and B are similar to the stages of a friendly dispute, while the other images illustrate defensive strokes and parades using the antlers during a fight (Barrette, 1977).

 
More violent confrontations are preceded by a signal of dominance, frightening the less dominant males who withdraw from the battle. The two fighters usually begin at 2 meters from one another, then raise their heads to an angle of 90° while gritting their molar teeth, producing a strident noise audible over 50 meters which marks the beginning of the confrontation (Bergerud, 1973; Geist, 1974; Brown, 1967). The battle involves tusk blows, especially when one of the competitors is losing balance due to exhaustion, so only a few can be exchanged per match. In these situations, the antlers must counter the blow and distribute its force evenly, but sometimes, the tusks manage to reach the opponent, inflicting serious injuries (Barrette, 1977).

Evolution of antler shape in cervids

In Miocene Muntiacinae, the antler shape is relatively simple. They are composed of two short unique branches with only two spikes, which increases the risk of injury to the eyes and face during frontal contact (Barrette, 1977). However, there are more advantages to having bigger antlers due to the social hierarchy they establish. This idea reduces fighting by improving communication during the dominance dance. For example, in species such as Odocoileus, Cervus, Dama, Alces and Rangifer which have a complex crown (shown in Figure 4), contact occurs distally and less frequently, thus reducing the risk of injury (Geist, 1971). There is reason to believe that antlers will continue to evolve into very wide, palmate structures with small spikes (Barrette, 1977). Fights could thus be won only by force and endurance without inflicting injuries. However, larger antlers require more weight to move and force to deploy; therefore, it is to be wondered if this is the best antler shape for battling—considering their resistance to stress and shock (Klinkhamer et al., 2019). A study was carried out to verify whether Megaloceros giganteus, an extinct deer species which carried the biggest antlers to date, could have supported the forces involved during a confrontation (i.e. the forces of the pushing phase and the twisting phase). Compared to existing deer species, M. giganteus’ antlers were oriented horizontally instead of vertically, reducing neck forces (Gould, 1974; Geist, 1986). Furthermore, the position of its proximal points protected its eyes as they faced downwards, but they were ineffective for anything related to the stability and strength of the animal when the antlers intertwined. 

Fig. 5. The different skull models of the four species studied. (a) Megaloceros giganteus, (b) Cervus elaphus (red deer), (c) Dama dama (fallow deer) and (d) Alces (elk) (Klinkhamer et al., 2019).

Comparison of the resistance of Megaloceros giganteus antlers with those of three existing deer when fighting

In a study conducted by Klinkhamer et al, the mechanical properties of M. giganteus antlers were tested using finite element analysis through comparison with three other deer species: C. elaphus, D. dama, and A. alces (Wroe, 2008; Wroe et al., 2018). These three specimens were selected based on their multiple similarities with M. giganteus, particularly with respect to DNA sequence, size, and morphology of their antlers. C. elaphus‘ antlers are distinguished from other taxa by their many branches and lack of palms (Hughes, 2006; Lister et al., 2005; Mennecart, 2017). To determine the modulus of the force to which individuals were subjected, the following equation was used: 

 where m represents the animal’s mass, and a is the rate of impact based on deceleration time (Klinkhamer et al., 2019). In the experiment, the value of a had been established at 30 m/s². A stress response factor was also considered, corresponding to the rate between the rigidity of a structure and its maximum stress rate (Klinkhamer et al., 2019). A body is at its safest state when this factor approaches 1. If it is less than 1, the risk of breakage is higher (Kitchener et al., 1991). The scientists carried out two tests, the pushing load test and the twisting load test, to evaluate the response to stress and establish the stress response factor of the four models. The forces applied were unidirectional and parallel to the neck for the pushing load test, whereas they created a moment of force at the neck in the case of the twisting load test (Klinkhamer et al., 2019). As a result, the point of impact of the different forces was on the distal parts of the antlers, as this is usually where they collide during combat. The collected data was compiled as diagrams illustrating the stress of von Mises experienced by the different regions of the skull of each of the taxa with a maximum limit of 180 MPa (Currey, 1979). Figure 6 presents the stress analysis of the four species’ antlers according to the von Mises stress scale. 

Fig. 6. Results of the push force test (left) and the rotation force test (right) on Megaloceros giganteus, Cervus elaphus, D. dama and Alces alces. The left column shows the average load that the bone can support, while the right-hand column shows the maximum load that it can withstand. Source: Adapted from (Klinkhamer et al., 2019).

According to Figure 6, the M. giganteus model (safety factor: 0.7) experienced the greatest stress for the push force test, with an average of 1480 MPa and a maximum stress of 2169 MPa, unlike other models which showed much lower results: 37 MPa and 55 MPa (C. elaphus), 281 MPa and 190 MPa (D. dama), and 154 MPa and 224 MPa (A. alces) (Klinkhamer et al, 2019). Similarly, M. giganteus showed the greatest stress response for the torsion force test with an average of 1723 MPa and a maximum of 2541 MPa (Klinkhamer et al., 2019). Unlike M. giganteus, C. elaphus (safety factor: 1.2) showed the smallest response among the 4 species with an average value of 41 MPa and a maximum value of 63 MPa. As for D. dama (safety factor: 1) and A. Alces (safety factor: 1.7), both showed lower responses than the first test with an average of 118 MPa and 64 MPa and a maximum of 174 MPa and 96 MPa (Klinkhamer et al., 2019). Thus, for both tests, M. giganteus showed the greatest stress response, which reveals that this species was not very well suited for one-on-one combat. Another test specific to M. giganteus and C. elaphus was performed by shifting the position of the applied forces; that is, instead of the forces being applied more distally for M. giganteus and more proximally for C. elaphus, they were applied more proximally  and distally, respectively. The results of this test are presented in Figure 7.

Fig. 7. Test results for when the push and torsion forces were administered differently in the M. giganteus and C. elaphus models, either proximally or distally (Klinkhamer et al., 2019).

As a result, the stress experienced by both models was greatly reduced. Indeed, when subjected to push forces, the average mechanical stress of the M. giganteus model decreased from 1480 MPa to 351 MPa and from 2169 MPa to 512 MPa for the maximum stress. On the other hand, when subjected to a torsion force, this value dropped by about 11.7 times (Klinkhamer et al., 2019). 

Therefore, in the alternative test where forces were applied proximally, M. giganteus showed lower stress response. That is, deer with webbed antlers tend to contact the opponent at distal points, as the fins made them stiffer and stronger (Clutton-Brock et al, 1982). The higher stress recorded (when the forces were positioned distally) can be explained by the massive size of M. giganteus antlers, resulting in a high distance between the pivot and the point of impact of the force. This increases the force moment on the antler and the risk of breakage (Kitchener et al, 1994). Moreover, the C. elaphus model exhibits the same behavior as M. giganteus (i.e., the stress recorded was higher when the forces were distal than proximal), reinforcing the idea that M. giganteus would be more likely to make contact at the proximal level.  As for the other species with webbed antlers, they showed lower stress in the twisting load than in the phase of pushing forces in contrast to C. elaphus which showed higher stress in the twisting load. As it has been suggested that M. giganteus establishes contact at the proximal level, this further supports previous studies showing that deer with webbed antlers tended to use rotatory motion to counter attacks (Clutton-Brock et al., 1982).  For the resistance factors of the specimens, they varied between 0.7 and 1.6. Generally, antler bone has smaller stress response factors than horns, which is a consequence of their complex structure and their capacity to regenerate (Kitchener et al, 1988). For M. giganteus which has the smallest factor, it would be too energetically demanding to increase this value, as its antlers would be too heavy to bear (Lincoln, 1992). 

Comparison of the mechanical properties of antlers to those of a femur and a tympanic bulla

One of antlers’ main functions is to act as a weapon and protection during battles. Thus, antlers must have the mechanical properties that make the bone resist considerable impacts during combat. The mechanical attributes of antlers can be better understood through their comparison with bones of differing functions, such as a bovine femur, a cow femur, and the tympanic bulla of a fin whale. Furthermore, these comparisons will illustrate how the mechanical properties of each of these bones are closely related to their respective functions.

Zoologist John Currey of the University of York studied the mechanical properties of a red deer antler, a fin whale tympanic bulla, and a cow femur aiming to relate their function to three specific properties: work of fracture, bending strength, and elastic modulus. The three bones have highly differing functions. While the tympanic bulla has a sound isolating purpose, the cow femur has a supporting and leverage function. Antlers, on the other hand, are used in combat and must be resistant to high impacts coming from their opponent. These distinctive functions are reflected in the measure of the three mechanical properties mentioned above (Currey, 1979).

The work of fracture is defined as “a measure of the amount of work necessary to drive a crack through a material and gives a good idea of the resistance of a material to impact loading” (Currey, 1979, p.313). The performance of each bone tissue was evaluated by dividing the work done on the bone by two times the area of the broken specimen . The bending strength and the elastic modulus were tested using an Instron testing machine which recorded the results in the following table.

Table 1 Measure of Work of Fracture, Bending Strength and Elastic Modulus of an Antler, a Femur, and a Tympanic Bulla (Currey, 1979).

The work of fracture of antlers, a femur bone, and a tympanic bulla

Table 1 suggests that antlers have a high level of work of fracture, and that the work of fracture is inversely proportional to the level of mineralisation of the bone. Indeed, the results show that the red deer antler requires the most energy to create a small breakage in the specimen. The high work of fracture of antlers is explained by the importance of avoiding fractures when cervids fight. Even when subjected to high impacts, the antlers did not break, but rather bent and then recoiled (Currey, 1979). This reflects how resistant to external forces antlers are. Compared to the femur and the bulla, antlers require three times the amount of energy a cow femur does to create a fracture in the bone tissue. The bulla, on the other hand, can be broken using very little energy, which is explained by the fact that the tympanic bulla is never subjected to collision, but rather to sound vibrations and is adapted to the lossless conduction of sound. The femur, on the other hand, has the work of fracture value between the bulla and the antlers. Its leverage function does require resistance to fracture to avoid breakage when lifting loads. However, it is rarely subjected to impacts as high as those of antlers. Furthermore, the muscle and skin that covers the femur bones increases the amount of energy needed to break the specimen (Currey, 1979).

The bending strength of antlers, a femur bone, and a tympanic bulla

As shown in Figure 4, the femur had the highest bending ability, followed by the antler. The difference in the bending strength of these two bone tissues is justified by the fact that the microscopic components of the antlers have a lower degree of orientation than those of the femur (Currey, 1979). In fact, data of other tests suggests that the femur is more anisotropic in the bending strength than the antler, meaning that the magnitude of certain properties will differ in the femur according to the direction from which it is measured (Britannica, 2021). Moreover, the low bending strength of the bulla can be explained by its extremely high mineral content. Mineralization significantly facilitates  crack propagation.

The anisotropic property of antlers

The anisotropic property of the antler, even though it is not as present as in the femur, is worth discussing. A study carried out by Blob and Labarbera in 2001 found that the mechanical properties of antlers do not vary along their length (Fang et al., 2018). Consequently, the bending strength, the work of fracture, and the modulus of elasticity of the antler will not differ depending on the section selected. However, another study led by Kitchener suggests that the volume of fraction of the cancellous bone varies in the cross-sectional area of the antler, as shown in Figure 8.

Fig. 8. Cross-Sectional Area of the Antler of a China Sika Deer (Fang et al., 2018).

The volume of fraction affects the toughness and the elastic modulus of the antler, which means that the results shown in Table 2 could differ depending on the orientation of the antler sample tested. (Fang et al., 2018). Consequently, experiments testing the elastic modulus, the ultimate bending strength, the ultimate strain, and the work of fracture of antlers along their longitudinal, radial and transversal orientations were performed to test the anisotropic property of antlers. The results obtained are shown in the following table.

Table 2 Mechanical Properties of Antlers Along their Transversal, Radial and Longitudinal Axes (Fang et al., 2018).

The transversal specimens yielded the highest values in terms of elastic modulus, bending strength, ultimate strain, and work of fracture for each antler orientation. This can be explained by the differences in microstructural characteristics of the three orientations. More precisely, these orientations differ in their crack-extending routes, which are the directions in which a crack will elongate after being subjected to a significant impact. The specimens of the three orientations also differ in their osteons’ orientation and the strength of the interaction between the osteons and the stroma of the bone (Fang et al., 2018). Indeed, the orientation of the osteons comparatively to the orientation of the specimen affects the direction in which the cracks extend. Figure 9 illustrates how the orientation of the specimen affects the crack-propagation process. 

Fig. 9. Crack Propagation in Transversal, Longitudinal and Radial Samples of Antler Bone (Fang et al., 2018).

Red sections in b, d and f represent the osteons. These illustrations show that cracks tend to extend between the osteons. In the transversal sample, the direction of propagation of the cracks is perpendicular to the osteons. In the longitudinal specimen, cracks do not need to go through the osteons to propagate because the direction of propagation is parallel to the direction of the osteons. Finally, in the radial sample, the cross-section of osteons has a circular shape, which creates multiple crack-extending routes between the osteons.  The transversal specimen is the only one for which cracks must cross the osteons to propagate, impeding crack extension (Fang et al., 2018). 

Overall, the energy required to separate a unit area of surface will differ due to the different crack-extending routes of the longitudinal, radial, and transversal specimens (Fang et al., 2018). The anisotropic property of antlers, although not as present as femur bone in bending, partly explains why Table 1 records a high bending strength of the antler.

The elastic modulus of antlers, a femur bone, and a tympanic bulla

Going back to the comparison between the bulla, the femur, and the antler, it is shown that the antler has the lowest elastic modulus, while the bulla has the highest level of elasticity. The elasticity is closely linked to the level of mineralization of the bone tissue. A high mineralization leads to higher elasticity. The elasticity of the tympanic bulla makes it stiff, which is essential to prevent the rotation of the bone around undesired axes. Indeed, the bulla is attached to other auditory ossicles in a way that restricts its rotation to only one axis. Rotation of the bulla around the other axes would have undesirable effects on the whale’s hearing. The difference in mineralization between the femur and the antler bone is relatively small compared to the difference in elasticity shown in the table. The table therefore shows that small variations in mineralization can have a large impact on the elasticity of the bone (Currey, 1979).

Differences in the mechanical properties of dry and wet antler

While most internal bones keep moist because of internal body fluids, deer antlers are effectively dry (Landete-Castillejos et al., 2012). Velvet, a smooth vascularized tissue covering the surface of young antlers, protects growing antlers (Currey et al., 2009). However, by the middle of summer, the antlers have fully mineralized, velvet growth has ceased, and the deers get ready to spar with their antlers (Currey et al., 2009). Dry antlers appear to be stiff (i.e., how well a material withstands distortion) but not tough (i.e., the ability of a material to absorb energy before breakdown) compared to wet bones; hence, antlers are predicted to be brittle and shatter readily when dried (Currey et al., 2009).

Currey et al. (2009) compared antler mechanical qualities to wet femur depending on antler hydration condition to determine how wet and dry antlers behave differently. Figure 5 shows the load-deformation curves of the three-point quasi-static bending test for wet femur, and wet and dry antler. Young’s modulus is proportional to the slope of the curve, bending strength is proportional to the greatest load achieved, and work to maximum load is parallel to the area under the curve (Currey et al., 2009). Elastic materials cannot withstand great strength and will bend elastically till they shatter. As a result, stiffer materials with a high Young’s modulus show a steeper slope in the graph because they can withstand higher strength. The scientists identified that wet antler has a very low Young’s modulus as expected, and is extremely deformable in bending (Currey et al., 2009). Dry antler, on the other hand, has a substantially higher Young’s modulus than wet antler, along with a higher bending strength than wet femur. In terms of work to fracture, it is crucial to note that the wet antler curve stays almost flat up to 8 mm as shown in Figure 5, implying that wet antler has a higher work to fracture compared to bone and dry antler (Currey et al., 2009).

Fig. 10. Load–deformation curves for wet femur, and wet and dry antler (Currey et al., 2009).

Another study led by the engineering department of the University of California San Diego compared the mechanical properties of antlers with those of a bovine femur. The results obtained are very similar to the ones shown in Table 2. However, researchers also recorded the data on the mechanical properties of dry and rehydrated antler. Rehydration of the sample was done in Hank’s balanced salt solution (Chen et al., 2009).  This liquid is made of glucose and inorganic salts, such as sodium chloride and magnesium sulfate. This solution keeps the bone tissue viable by maintaining physiological pH and osmotic pressure (Sciencell, (n.d.)). The dry, rehydrated, longitudinal and transversal samples of antler were tested for their elastic modulus, bending strength, compressive strength, and tensile strength. Table 3 displays the data obtained in the experiment.

Table 3 Mechanical Properties of a Dry Elk Antler and a Rehydrated Elk Antler (Chen et al., 2009).

Overall, the table suggests that the dry elk antler has different mechanical properties than rehydrated antler. The differences are more significant in the longitudinal sample, which indicates that the presence of the liquid between the osteons changes the properties of the antler bone tissue.

The influence of moisture on Young’s modulus

In terms of Young’s modulus, the question that emerges is how hydration conditions might influence antler stiffness, since wet antler showed much lower values than dry antler in the experiment.

In the experiment to examine the effects of moisture on the Microcrystalline Cellulose (MCC), Sahputra et al. illustrated the relationship between Young’s modulus and moisture content. Figure 6 shows that increasing displacements indicate a rise in molecular mobility with moisture content, which results in softening or plasticization of the MCC (Sahputra et al., 2019). It displays the longest displacements of carbon atoms in polymer chains across the measured time for each moisture content, with the highest moisture level exhibiting the greatest molecular mobility. (Sahputra et al., 2019). In the same way that increasing water content and molecular mobility softens cellulose in this experiment, increasing water content and molecular mobility may soften antler in the hydration states studies.

Fig. 11. Molecular mobility for each moisture content as expressed by the maximum displacement of carbon atoms in the polymer chains over a period of 100 ps (Sahputra et al., 2019).

The influence of moisture on antler bone’s mechanical characteristics in combat, particularly during the rut

As pictured in Figure 7, Cervidae mainly utilize their antlers in dominance battles for access to females during the rut season. Fights generally consist of an intense clashing of the antlers, which may result in serious damage such as antler and body deformation—or even death (Clutton-Brock et al., 1979). If antlers are damaged or lose their initial potency, deer have a significantly lower chance of reproductive success during the rut season (Clutton-Brock et al., 1979). It is thus critical to have favourable antler material qualities, such as reduced deformation and resistance to shattering during a battle.

Fig. 12. Two male deer clashing antlers in rutting season (Discover Wildlife, n.d.).

Throughout an experiment testing the mechanical properties of antlers led by Currey et al. (2009), the scientists realized that antlers need to have a balance between Young’s modulus and effort to fracture to succeed during fights. They studied the data set obtained and determined the ideal antler material required for combat. Dry antler has a 22% lower Young’s modulus than wet femur, as shown in Table 4; however, wet antler has a 67% lower Young’s modulus than wet femur (Currey et al., 2009). Moreover, wet antler has 56% less bending strength than wet femur, whereas dried antler bone has a significantly higher bending strength than wet femur (Currey et al., 2009). Young’s modulus of elasticity is only 22% lower in dry antler specimens than in wet bone specimens, yet the static bending strength of wet bone is 25% lower than that of dry antler (Currey et al., 2009).

As illustrated in Table 4, wet antler showed a considerably lower elastic modulus and bending strength than wet bone, but a greater effort to fracture (Currey et al., 2009). On the other hand, dry antler had a slightly lower Young’s modulus than wet bone, but a far higher bending strength and a substantially higher work to fracture (Currey et al., 2009). The energy absorption of dry antler in quasi-static bending was 2.4 times bigger than that of wet bone, although wet antler absorbs more work in bending before fracture than dry antler (Currey et al., 2009). In terms of impact, the difference is much more pronounced; dry antler absorbs 6.6 times more energy than wet bone (Currey et al., 2009).

Table 4 ​​ Mean results from three-point bending tests for Young’s modulus of elasticity E, bending strength BS, work under the load-deformation curve W, and impact energy U (Currey et al., 2009).

It appears that the antlers may benefit from being a little wetter to absorb more energy before fracture. However, a thoroughly wet antler would not be the most effective fighting material since it has extremely low Young’s modulus allowing the antler to be warped by bending in the antler pushing combat. For instance, especially for juvenile deer, it is not desirable to engage in mock fights because the developing velvet is especially prone to stress and wet velvet is not firm enough to prevent any injury from pushing match. Many deer antler abnormalities are caused by traumas during the velvet stage such as antler development in unusual directions or forms. (Goss, 1983). As previously stated, it may lead to less successful mating during the rut season since having an asymmetrical antler is a major flaw in the eyes of potential mates.

In conclusion, dry antler material appears to be the most favorable in fighting with minimal distortion. Dry antler is mechanically well adapted to its role, which is to absorb the shocks of a fight with a moderately high work to fracture and impact energy absorption yet possessing a sufficiently high elastic modulus and bending strength to protect the antler from bending too much. 

Conclusion

Antlers are characterized by their incredible toughness and stiffness which make them a powerful weapon for fighting. Toughness and stiffness are properties which can be combined in the antler bone thanks to its complex hierarchical structure. Antlers are composed of 8 hierarchical levels which interact to make the tissue highly resistant to fracture. The presence of sacrificial bonds is also partly responsible for the high work of fracture of these spiky structures. The high work of fracture also prevents the breakage of the bone after considerable impact. Their anisotropy allows them to have a quite high bending strength. Antlers also differ in their mechanical properties depending on the hydration conditions. Wet antlers are more resistant to impact, while dry antlers have a higher Young’s modulus. Moreover, the shape of antlers evolved from simple structure to a more complex, spiky shape to allow for safer one-to-one battles. Overall, antlers constitute a product of evolution which tends to fit the environment and the life conditions of Cervidae. 

References

Asher, C. (2014, October). Handicaps, Honesty and Visibility, Why Are Ornaments Always 

Exaggerated. ULC. https://blogs.ucl.ac.uk/ 

Barrette, C. (1977). Fighting Behavior of Muntjac and the Evolution of Antlers. SSE, 31(1), 169-

176. https://doi.org/10.2307/2407555

Bergerud, A. T. (1973). Movement and rutting behavior of caribou (Rangifer tarandus) at Mount 

Albert, Québec. Canadian Field-Naturalist, 87, 357-369. https://collections.banq.qc.ca/ark:/52327/1935968

Britannica, T. Editors of Encyclopaedia (2021, March 19). Anisotropy. Encyclopedia Britannica. 

Brown, L. (1967). The Deer and the Tiger, by G. B. Schaller. Cambridge University Press, 9(3), 

233-234. https://doi.org/10.1017/S0030605300006426 

Chen, P.-Y., Stokes, A. G., & McKittrick, J. (2009). Comparison of the structure and 

mechanical properties of bovine femur bone and antler of the North American elk (Cervus elaphus canadensis). Acta Biomaterialia, 5(2), 693-706. https://doi.org/10.1016/j.actbio.2008.09.011

Chen, P. Y.,  A. Y.-M. Lin, A. G. Stokes, Y. Seki, S. G. Bodde, J. McKittrick, & M. A. Meyers. 

(2008). Structural Biological Materials: Overview of Current Research. JOM, 60(6), 23-32. https://doi.org/10.1007/s11837-008-0067-2 

Clutton-Brock, T. H., Albon, S. D., Gibson, R. M., & Guinness, F. E. (1979). The logical stag: 

adaptive aspects of fighting in red deer (Cervus elaphus L.). Anim. Behav. 27, 211-215. https://doi.org/10.1016/0003-3472(79)90141-6  

Clutton-Brock, T. H. (1982). The functions of antlers. Behav. Brill, 79(2-4), 108-124. 

Currey, J. D. (1979). Mechanical properties of bone tissues with greatly differing functions. 

Journal of biomechanics, 12(4), 313-319. https://doi.org/10.1016/0021-9290(79)90073-3 

Currey, J. D. (2002). Bones, Structure and Mechanics. Princeton University Press.

Currey, J. D., Landete-Castillejos, T., Estevez, J., Ceacero, F., Olguin, A., Garcia, A., & Gallego, 

L. (2009). The mechanical properties of red deer antler bone when used in fighting. Journal of Experimental Biology, 212(24), 3985–3993. https://doi.org/10.1242/jeb.032292  

Darling, F. F. (1937) A Herd of Red Deer. Nature, 143, 453. https://doi.org/10.1038/143453a0

De Falco, P., Barbieri, E., Pugno, N., & Gupta, H. S. (2017). Staggered Fibrils and Damageable 

Interfaces Lead Concurrently and Independently to Hysteretic Energy Absorption and Inhomogeneous Strain Fields in Cyclically Loaded Antler Bone. ACS Biomater Sci Eng, 3(11), 2779-2787. https://doi.org/10.1021/acsbiomaterials.6b00637 

Discover Wildlife. (n.d.). Understanding the deer rut, plus the best places to see rutting red deer. 

BBC Wildlife Magazine. Retrieved October 3, 2022, from https://www.discoverwildlife.com/animal-facts/mammals/understand-the-british-deer-rut 

Fang, Z., Chen, B., Lin, S., Ye, W., Xiao, H., & Chen, X. (2018). Investigation of inner 

mechanism of anisotropic mechanical property of antler bone. Journal of Mechanical Behavior of Biomedical Materials, 88(1751-6161), 1-10. https://doi.org/10.1016/j.jmbbm.2018.07.043

Geist, V. (1971). On the relation of social evolution and dispersal in ungulates during the 

Pleistocene, with emphasis on the old-world deer and the genus Bison. Quat Res, 1(3), 283-315. https://doi.org/10.1016/0033-5894(71)90067-6 

Geist, V. (1974). On fighting strategies in animal combat. Nature, 250, 354. 

Geist, V. (1986). The paradox of the great Irish stags. Nat. Hist. 95(3), 54-65.

Goss, R. J. (1983). Deer antlers: regeneration, function and evolution. New York: Academic 

Press.

Gould, S.J. (1974). The origin and function of ‘bizarre’ structures: antler size and skull size in 

the ‘Irish elk,’ Megaloceros giganteus. Evolution, 28, 191-220. https://doi.org/10.1111/j.1558-5646.1974.tb00740.x 

Hughes, S., Haydenc, T. J., Douady, C. J., Tougard, C., Germonpré, M., Stuart, A., Lbova, L., 

Cardenc, R. F., Hänni, C., & Say, L. (2006). Molecular phylogeny of the extinct giant deer, Megaloceros giganteus. Mol. Phylogenet. Evol., 40(1), 285-291. https://doi.org/10.1016/j.ympev.2006.02.004

Kitchener, A., Bacon, G., & Vincent, J. (1994). Orientation in antler bone and the expected stress 

distribution, studied by neutron diffraction. Biomimetics, 2(4), 297-307.

Klinkhamer, A. J., Woodley, N., Neenan, J. M., Parr, W. C. H., Clausen, P., Sánchez-Villagra, 

M. R., Sansalone, G., Lister, A.M. & Wroe, S. (2019). Head to head: the case for fighting behaviour in Megaloceros giganteus using finite-element analysis. Proc. R. Soc. B., 286(1912). https://doi.org/10.1098/rspb.2019.1873

Kulin, R. M., Chen, P. Y., Jiang, F. McKittrick, J., & Vecchio K. S. (2010). Dynamic fracture 

resilience of elk antler: Biomimetic inspiration for improved crashworthiness. JOM, 62(1), 41–46. https://doi.org/10.1007/s11837-010-0009-7 

Landete-Castillejos, T., Estevez J. A., Ceacero, F., Garcia, A. J., & Gallego, L. (2012). A review 

of factors affecting antler composition and mechanics. Frontiers in bioscience (Elite edition), 4(7), 2328-2339. https://doi.org/10.2741/e545

Lincoln, G. A. (1992). Biology of antlers. J. Zool. 226(3), 517-528. 

Lister, A., Edwards, C. J., Nock, D., Bunce, M., Van, P. I., Bradley, D., Thomas, M. G., & 

Barnes, I. (2005). The phylogenetic position of the ‘giant deer’ Megaloceros giganteus. Nature, 438, 850-853. https://doi.org/10.1038/nature04134

McCullough, D. R. (1969). The Tule Elk. Its history, behavior, and ecology. Univ. California 

Publ., 88(3), 1-191. https://doi.org/10.7589/0090-3558-21.4.463 

Mennecart, B. DeMiguel, D., Bibi, F., Rossner, G.E., Métais, G., Neenan, J.M., Wang, S., 

Schulz, G., Muller, B., & Costeur, L. (2017). Bony labyrinth morphology clarifies the origin and evolution of deer. Sci. Rep. 7, 131. https://doi.org/10.1038/s41598-017-12848-9

Meyers, M. A., Chen, P.-Y., Lin, A. Y.-M. & Seki, Y. (2008). Biological materials: structure 

and mechanical properties. Progress in Materials Science, 53(1), 1-206. https://doi.org/10.1016/j.pmatsci.2007.05.002 

Sahputra, I. H., Alexiadis, A., & Adams, M. J. (2019). Effects of moisture on the mechanical properties of microcrystalline cellulose and the mobility of the water molecules as studied by the hybrid molecular mechanics-molecular dynamics simulation method. Journal of Polymer Science Part B: Polymer Physics, 57(8), 454–464. https://doi.org/10.1002/polb.24801  

Salthe, S. N. (2013). Biological Hierarchies. In: A.L.C Runehov. & L. Oviedo (Eds.), 

Encyclopedia of Sciences and Religions (pp. 229–231). Springer, Dordrecht. 

ScienCell Research Laboratories. (n.d.). Hank’s Balanced salt Solution. 

Skedros J. G., Durand P., & Bloebaum R. D. (1995). Hypermineralized peripheral lamellae in 

primary osteons of deer antler: Potential functional analogues of cement lines in mammalian secondary bone. Journal for Bone and Mineral Research, 10(Suppl. 1), S441. 

Struhsaker, T. T. (1967). Behavior of elk (Cervus canadensis) during the rut. Z Tierpsychol. 1, 

80-114. https://doi.org/10.1111/j.1439-0310.1967.tb01229 

Whitehead, G. K. (1993). The Whitehead Encyclopedia Of Deer. Swan Hill Press & Airlife 

Publishing.

Wroe, S. (2008). Cranial mechanics compared in extinct marsupial and extant African lions 

using a finite-element approach. J. Zool. 274(4), 332-339. https://doi.org/10.1111/j.1469-7998.2007.00389.x