Bovid Horns Research Articles

Geo (Im) pulse Caprovis savinii (Bovidae, Mammalia) rediscovered: horn core finds of an Early Pleistocene antelope from the North Sea floor

AbstractFor more than 120 years since the name Caprovis savinii was founded on the basis of a bovid horn core from the ‘Forest Bed’ at Overstrand (Norfolk, UK), only one other find has been assigned to this species. Recently, two horn cores and one horn core fragment of the species have been discovered at two sand and gravel sorting centres in the Netherlands. The bone-bearing sediments are exposed on the North Sea floor, just off the coast of Great Yarmouth (Norfolk, UK). By comparing fossil and extant taxa, a classification within the bovid tribe Antilopini is proposed. Based on the accompanying fauna from the specific dredging area and the litho- and chronostratigraphy at Overstrand, C. savinii is placed within the Early Pleistocene.

Differences in the thermal conductance of tropical and temperate bovid horns

Bovid horns consist of a highly vascularized bone core covered by a keratin sheath which seems to offer limited resistance to heat flow. Based on dynamic cooling curves measured for inverted horns filled with warm water, we developed estimates of the thermal conductance of keratin and the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air boundaries to allow us to quantify heat flux through the horn sheath. Coupled with measurements of the internal and external horn dimensions, we constructed a simplified conceptual model of sheaths from 68 horns of 14 bovid species to test the prediction that the horns of temperate bovid species offer greater resistance to heat flux than the horns of tropical bovids. The specific heat capacity of the keratin sheath was 1.53 ± 0.07 (SD) J g−1°C−1. The coefficient of conductive heat transfer for keratin was 6.30 ¥ 10−3 ± 0.30 ¥ 10−3 (SD) W cm−1°C−1. We estimated the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air interfaces to be 8.79 ¥ 10−3 ± 5.20 ¥ 10−3 W cm−2°C−1 and 2.49 ¥ 10−3 ± 1.98 ¥ 10−3 W cm−2°C−1, respectively. A reduction in the size of the bone core and overlying vascular bed and an increase in the thickness of the keratin sheath in temperate bovids acted to reduce the surface area through which heat was lost to the environment. Because the surface-specific thermal conductances of temperate sheaths were lower than those of tropical sheaths, we estimate that a temperate bovid having horns of the same length and external surface as a tropical bovid would experience only 75.7% of the heat loss when facing a thermal gradient of 20°C. We argue that differences in horn morphology between temperate and tropical Bovidae appear to have evolved as adaptations to restrict heat loss in the former while facilitating heat loss in the latter group.

Horn Growth in Mountain Goats (Oreamnos americanus)

Horns of bovids are important social organs, their growth is often indicative of population characteristics and habitat quality, but little is known of the factors affecting their growth in individuals. We studied horn growth of 135 (51 males, 84 females) marked mountain goats (Oreamnos americanus) in Alberta, Canada, for 9 years. In both sexes, horn growth was quadratic during the first 5 years of life and not significant after 5 years of age. Goats completed 93% of horn growth by 3 years of age. Horns of males grew more than those of females during the first 1.5 years of life. Horns of females grew more than those of males during the third year. Although males maintained longer horns than females because of their longer first increment, adult males had shorter horns than females for a given body size. Males had thicker horns than females at all ages, absolutely or relative to body size. Horn growth early in life was correlated negatively with later growth. Annual growth increments of horns of females aged 4–5 years were affected negatively by lactation. Horn growth was not affected by total rainfall during the vegetation-growing season. Total length and basal circumference of horns were correlated positively with mass, chest girth, and hind-foot length in both males and females. While sexual dimorphism in body mass of mountain goats increases until at least 5 years of age, most dimorphism in horn growth is achieved by 2 years of age. Therefore, body mass of mountain goats may be a more important factor than horn size for male mating success.

Morphology of Horns and Fighting Behavior in the Family Bovidae

Horns of bovids are remarkably diverse. This diversity may reflect functional differences associated with the use of horns as intraspecific weapons. I use measurements from museum specimens and behavioral data from the literature to examine the relationship between morphology of horns of males and fighting behavior in 21 species of bovids, representing 11 of the 12 bovid tribes. A high correlation between morphology of horns and fighting behavior was found. In particular, a short horn reach and undeveloped catching arch is associated with stabbing behavior; a long horn reach, with wrestling and fencing behavior; a well-developed catching arch, with wrestling behavior; and robust, recurved horns, with ramming behavior. A phylogeny of bovid tribes suggests that these features of morphology of horns and fighting behavior are rapidly evolving and frequently convergent.

The cost of horniness: Heat loss may counter sexual selection for large horns in temperate bovids

:While sexual selection may drive the evolution of large horns in male Bovidae, heat loss from the horn’s vascularized bony core in cold winters imposes an energetic cost on large-horned males. We compared horn morphology between tropical and temperate bovids to test for evolutionary adaptations to heat loss. In temperate species, the surface area of the vascularized bony core is reduced while thickness of the outer keratin sheath is increased, limiting heat flow from horns. Within the genus Ovis, temperate species and subspecies have thicker keratin sheaths than desert subspecies. We argue that thermal stress affecting the morphology of horns is a selective force countering sexual selection in temperate bovids.

Strain patterns in the horncores, cranial bones and sutures of goats (Capra hircus) during impact loading

The design of bovid horns and skulls for the function of head‐to‐head fighting has been well studied using mechanical and kinematic analyses, but actual strain measurements from the horncores, cranium and sutures during impact loading have never been made. During in vitro impact loading of one or both horns on the heads of female domestic goats, principal strains were measured from the surfaces of the horncore bases, frontal and parietal bones, and interfrontal and frontoparietal sutures. Impact produced a bending moment at the bases of the horncores, with compressive strains on the posterior surfaces more than doubling the anterior tensile strains. These ratios of compression to tension exceed our predictions (and those in the literature) based on curved beam theory. Principal strains in the cranial bones dropped by as much as 50% crossing the sutures, which acted like springs or hinges to allow cranial bone movement. Sutures also experienced very high strain magnitudes, being more than 10 times greater than those of the surrounding cranial bones. Sutural strains during impact loading of the horns also greatly exceeded the strain levels recorded during mastication in other studies. The decrease in strain between adjacent cranial bones across a suture and the large strains at the sutures indicate that the sutures function as shock absorbers during impact.

Bovid Horns: An Important Site for Heat Loss during Winter?

Bovid horns play a prominent role in social and sexual interactions. In cold climates, however, heat loss through the horn surface may represent a major energy cost. We measured surface temperatures of horns (Th) for two Barbary sheep ( Ammotragus lervia ; one male and one female) at the Granby Zoo, Quebec, using temperature-sensitive radiotrans-mitters. Over ambient temperatures (Ta) of 5 to — 19°C, Th never dropped below 3°C. At Ta ≈ −10°C, the difference between Th and Ta was 17°C for the male and 21°C for the female. Using empirical models to predict heat loss through horns and resting metabolic rates, we estimate that, at Ta of ≈−10°C, heat loss through the horn surface is 20% of resting metabolic rate for females and 29% for males which have larger horns than females. We argue that the metabolic costs of possessing large horns in cold climates may impose constraints on morphology and sexual selection.

Spiral waves in excitable media: a model for diverse aspects of organismal development

Conical skeletal elements of various morphologies are a feature common to numerous Lower and Middle Cambrian organisms of problematic affinity, not to mention their occurrence in multiple extant metazoan phyla. In some cases these display helicospiral morphology, sometimes with dextral and sinistral morphs that may have been paired structures in a bilaterally symmetrical body plan. Such paired elements, including spines of coeloscleritophorans, appear to show a spicular substructure with some suggestion of spatial organization comparable to that of phyllotactic systems in vascular plants. These recurring structural themes raise old questions regarding the significance and mode of development of spiral forms and patterns in many organisms, both unicellular and multicellular.Most recent work on phyllotaxis has interpreted patterns of spiral organization as emergent geometrical properties of systems controlled by local reaction-diffusion interactions around the developing plant axis, without recourse to any explicitly spiral process. The term “generative spiral,” for the pattern linking consecutively formed elements, has thus taken on a purely descriptive, rather than mechanistic, meaning. Likewise, helicospiral shells and comparable structures are generally assumed to reflect no truly spiral growth mechanism, but rather, the regular scaling and displacement of a fundamentally concentric, cone-within-cone, accretionary system. Explanations for the common appearance of such forms include the structural strength offered by adherent whorls (though many forms are evolute), and the simplicity of isometric growth (though departures from logarithmic spirals are common).An alternative perspective was suggested initially by discovery of a higher order pattern in the phyllotactic systems of some conifers, wherein the handedness of the generative spiral reverses as each parent axis gives rise to daughters, through the full range of levels in the branching hierarchy. Although this may be explicable in conventional terms, it may also be interpreted as involving a spiral wave of activation passing rhythmically around the plant axis, at or within some critical distance of the apical meristem. Models for such phenomena are provided by several physical systems involving excitable media, the most well known of which is the Belousov-Zhabotinsky reaction, and by some biological systems, such as neural networks or aggregations of slime mold. At nearly the opposite end of the organismic realm, vertebrate teeth, most of which are subtly but fundamentally helicospiral in form, show features that also suggest rotating waves of activity. These include the common presence of an axial canal in the dentin underlying tooth cusps, which could represent the phase singularity of a rotating wave in the layer of odontoblasts responsible for dentin formation, and asymmetries in autoradiographic patterns associated with developing teeth. In such systems it may be generally true that the period of other physiological cycles that modulate secretory activity is not a simple multiple of the period of rotation of the spiral wave. This would result in a regular change in phase relationship, generating helicospiral form. Additional applications of this principle may illumine features as diverse as bovid horns and the handedness of supernumerary appendages in grafting experiments on insects and amphibians. Spiral waves in excitable media also have properties of symmetry and capacity for generating serial structure that make them intriguing candidates for involvement in even earlier stages of metazoan development.

The significance of horns and other male secondary sexual characters in female bovids

A new model for the evolution of horns in female bovids postulates that females mimic male secondary sexual characters to buffer their male offspring against the aggression of dominant males, thereby prolonging the presence of their sons in the natal herd and home range. The survival and reproductive success of male offspring and the mother's own genetic fitness are, thus, all enhanced. This form of heterosexual mimicry is viewed as simply another form of maternal investment, strongly selected for because the enforced dispersal of male offspring during adolescence is a major source of male mortality and of the skewed adult sex ratios typical of mammals with polygynous mating systems. Horns, and other secondary characters that provoke despotic competition as male sex symbols, become ambiguous and no longer release aggression when females possess similar ornaments. In the case of horns — permanent structures essential for males to achieve dominance and reproductive success — their acquisition by females not only postpones the natal dispersal of male progeny but also enables them to begin developing horns much earlier than the progeny of hornless females can afford to do. The competitive advantage of early horn development could be so decisive that, once the alleles for female horns became established, this genotype would spread rapidly throughout a population. How long male-mimicry functions to buffer males against despotic competition depends on the developmental stage tracked in females. Under conditions that favor seasonal or permanent bisexual grouping, the process could have led to the minimal sexual dimorphism seen in many migratory and nomadic bovids. The importance of despotic competition as an antagonist to peer competition, capable of countering selection for development of male secondary characters, calls for a redefinition of sexual competition. When the dual nature of sexual competition is taken into account, Darwin's theory of sexual selection becomes applicable to various polygynous mammals that fail to develop the expected degree of sexual dimorphism.

Landmark-based morphometrics of spiral accretionary growth

Many organisms continue to grow their skeletons throughout ontogeny. In the shells of molluscs, protists, and brachiopods and in bovid horns, accretionary spiral growth provides a detailed and continuous growth history. Although a shell may be described as a single static form, the overall morphology is a summation of the ongoing accretionary process. For this reason, an explicitly ontogenetic characterization of form provides insight into the final form achieved. Analysis of landmark transformations offers direct access to major components of morphological variation, both among adult individuals and through an individual's ontogeny. Parameters of preconceived, abstract geometric models can also be used to characterize morphological variation, but there is no guarantee that these parameters will coincide with the major features of shape variation.In order to locate landmarks at equivalent ontogenetic stages, features that indicate ontogenetic stage of coiled forms must be identified (e.g., growth increments, age, size, whorls). The gastropod Epitonium (Nitidiscala) tinctum exhibits prominent varices that provide landmark locations throughout ontogeny. Recent specimens of this species were obtained from three localities in Baja, Mexico. The morphological variation among individuals, treated as whole shells and within individual ontogenies, was analyzed using shape coordinates of landmark configurations. Deformation of shape is expressed in the uniform and nonuniform shape subspaces. The empirical components of shape variation found are similar to those generated by two parameters of an equiangular spiral: θ, the angle between consecutive varices, and W, the whorl expansion rate. The distribution of individuals is examined within morphospaces constructed from these shape features.Three scales of analysis are necessary to characterize adequately the shape variation within and among specimens. The smallest scale is equivalent to increment-by-increment changes in θ and W. The middle scale comprises variation equivalent to whorls resulting from systematic changes in θ and W during an individual's ontogeny. Finally, there is the overall ontogenetic trajectory. Mean shape must be a function of initial shape and ontogenetic trajectory in shape. Mean forms that are found to have similar shapes at the same arbitrary growth increment may achieve that shape in different ways.

An analysis of the forces of fighting of the blackbuck (Antilope cervicapra) and the bighorn sheep (Ovis canadensis) and the mechanical design of the horn of bovids

The costs and benefits of fighting in bovids are high in terms of injury and reproductive success, respectively. The breakage of a horn would curtail reproductive success permanently. Therefore, the horns of bovids should include sufficient material so that they are strong enough to be unlikely to break in fighting but without being too heavy to carry around. The forces developed during fighting were measured in a computerized analysis of film of blackbuck and bighorn sheep. All possible modes of failure were investigated using a mechanical analysis to see how the horns are most likely to fail.The maximum possible force developed during fighting is 3400 N for the bighorn sheep and 456 N for the blackbuck. Bending stress, shear stress, deflection, strain energy and critical crack lengths were calculated for the horns of these two bovids. Horns are most likely to fail in bending as indicated by safety factors. Most of the force is taken in compression due to the curvature of horns. Shear stresses are insignificant and deflections are negligible during the most forceful encounters.The safety factor in bending of the horn of the bighorn sheep is greater (10) than that of the horn of the blackbuck (3.4) because the forces are probably more variable and unpredictable in the fighting of the former.All of the energy of fighting is absorbed by the body musculature because horns store less than 1% of the energy produced in fighting as strain energy when they bend. Cracks and scratches mustbe more than 60% of the transverse basal dimensions of horns in order for there to be catastrophic failure at the maximum stresses developed during fighting.The horns of the blackbuck and the bighorn sheep appear to be minimum weight structures given the variability of the forces acting on them and are unlikely to break in fighting given the forces calculated from the analyses of films.

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

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.

A quantitative method for bovid horn classification

A quantitative method for the classification of bovid horns and horn-cores is proposed. Selected horn characteristics are expressed as ratios of intercepts along three mutually perpendicular reference axes of which one is anteroposteriorly orientated and tangent to the frontal bone midway between the horn bases. Horns can be compared interspecifically and intraspecifically without regard to their actual size. All bovid horns may be classified, by immediate inspection, into one of four classes, from least to most complex, under the proposed system. The notation identifies the class and provides means of comparing individuals within it. The actual horn length may be included as an additional parameter, to permit conversions. Other parameters (e.g., horn thickness) can be added when needed without interfering with the formula.