Evolution of Head Ornaments in Ungulates

From UBC Wiki

Even-toed ungulates are mammals with an even number of toes, they also usually (Artiodactyla) have head ornaments. This order consists of animals like the deer, moose, antelopes, oxen, cows, and giraffes. Odd-toed ungulates are mammals with an odd number of toes (Perissodactyla) such as rhinoceroses, these mammals also have head ornaments. Head ornaments are usually refereed to as horns or antlers and are classified as a projection of the skin on the head. This projection is either coated in keratin, protein or velvet nerves, while the core is usually composed of bone [1]. Horns and antlers have distinct shapes and sizes varying within each species, they can be curved, spiraled or branched. In many species only the males have head ornaments. The terms horn and antler are often used synonymously, but in fact they are completely different structures.

A are the bones of a Tapir (Tapirus indicus), an Artiodactyla. B are the bones of a Rhinoceros (Rhinoceros sumatrensis), Perrisodactyla. C are the bones of a Horse (Equus caballus), Perrisodactyla

Antlers are easily recognized in the Artiodactyla family and are present only in males (with the exception of caribou and reindeer). They have the ability to grow to astonishingly large sizes and length and their shape varies among species. Antlers grow from pedicels; these are boney structures that develop at the sides of frontal bone [1]. An antler’s growth cycle is regulated by hormones, and growing season of new antlers typically begins in early spring and they grow for the rest of the time they remain connected to the animal. Antlers are shed towards the end of the winter season as fewer hormones are produced from the pituitary and mating season is complete.

In early antler development, the appendages have high water and protein content and as they continue to grow they are covered with a thin layer of skin and a soft hair referred to as velvet [2]; velvet contains blood vessels and nerves. As antlers continue to develop, spongy bone which is present in the outer edge of the appendages is replaced with compact bone. Antlers provide a variety of purposes; males use their antlers throughout the breeding season for social interactions whilst competing for female attention and to defend territory.

Horns are usually curved, or spiraled. Unlike antlers which begin to grow at 1-2 years old, horns begin growing soon after birth and continue to grow throughout the organism’s life [1]. Pronghorns are the exception to this, their outer layer is shed annually but they retain the inner bone core. Horns contrast antlers in a variety of ways, one of these being the presence of horns in females and another is that horns are never branched.

The growth of horns differs in that from antlers, the horns core or outer layer are never shed and the horn never stops growing. Horns begin as a small growth under the skin above the skull in the connective tissue. They are not connected to the skull and fuse secondarily to the skull bones[3]. Similarly to antlers, horns are used by males in competition for females throughout the breeding season. In species where both males and females have horns, often sexual dimorphism occurs and males will have thicker horns used for fighting while females have straight thin horns used more for defense [2].

History of Ungulates

Ungulate Phylogenetic Tree

Horns and antlers date back to prehistoric times; although their exact origin is unclear due to their diverse nature. Evidence in the Ceratopsians species, the most common member being the Triceratops exhibits horns which existed millions of year ago. It is believed they were used as a defense mechanisms and for courtship. Evidence of horns being used as a defense mechanism exists in fossils that have been found. Tyrannosaurus rex fossils were discovered which showed injuries from some sort of horned animal in its abdomen region. Dinosaurs not only had horns, they had what is now known to be frill, this is the area of the neck which is surrounded by bone covered in skin. Because of the wide assortment of hornlike organs, head ornaments are believed to have evolved independently from a series of mammalian families, it is thought through as many as four times in different lineages [4]

In primitive large mammals it is believed head ornaments developed from a series of head blows. Over time as fights continued with head hitting being the dominant tactic of defense, it is thought protrusions began to develop; first as a small projection, later as larger full scale defense weapons[4].

In the traditional account of evolution, Perrisodactyls and Artiodactyls first appeared in the Eocene era 50 million years ago, however there is evidence that small horned animals existed a few million years before that in the Paleocene era [5]. Small ungulates with omnivorous diets (modern day pig) were abundant throughout the Paleocene era, as their numbers began to decline in the middle of Eocene era there was an increase in population of ungulates and larger specialized forms began to develop in the Oligocene era [6]. Fruit eating herbivore ungulates although abundant throughout the Paleocene era underwent an extreme diversification in the late Eocene and doubled their genetic diversity. However, shortly after their populations began to dwindle and by the time the Oligocene era (around 27 million years ago) rolled around, many species were extinct. Plant/cellulose eating herbivore ungulates also appeared in the Paleocene era, they were moderate in diversity and fairly small in size. In the late Eocene era they also underwent an extreme diversification and their bodies began to triple in size, unlike the fruit eating herbivore, they were able to maintain their taxonomic diversity and are now the predominate ungulate to date [5]. The diversification is believed to have been caused by a profound cooling period during the late period of the Eocene [4]., which the plant herbivores were most successful in surviving.

Bone vs Keratin

Antlers are primarily made up of bone except for their epithelial velvet covering.[7] The bone of the antlers need this velvet for oxygen and nutrients.[8] Bone tissue is dense connective tissue.[9] Bone can be made one of two ways, either by intramembranous ossification or endochondral ossification.[9] Intramembranous ossification is the direct development of bone whereas endochondral ossification is the development of bone from cartilage.[7] The majority of antler bone formation is done by a modified version of endochondral ossification,[10] somtimes direct formation can occur.[7] In endochondral ossification, cartilage is developed and grown first. After this is primary and secondary ossification.[9] Ossification is the formation of new bone by cells called osteoblasts.[11] In antler bone development, once the caritilegde is minerilized to become bone, the velvet is lost and the bone tissue dies.[8] This is when the antler is full sized and shedding can occur.[8] Hormones are needed for skeletogenesis,[7] especially testosterone in antler bone development.[7] Antler bone growth is done at the tip of the antler.[10]

Horns are made primarily of keratin and are produced by epithelial cells known as keratinocytes.[7] Keratin has a high sulfur content[7] and this is from the amino acid cysteine. Mammals are only able to produce α-keratin, while birds and reptiles can produce both α- and β-keratins.[12] Keratin has cross links of individual molecules by disulfide bonds.[7] This is what gives keratin is flexibility[7] and how flexible the tissue can be is dependent on the number of disulfide bridges, as they are also called.[12] In horn development, the epidermis collects and builds up layers of keratin fibers.[7] makes the growth of horns basal, meaning that the old layers are pushed forward as the new layers grow, making the horn unable to change shape.[7] The horn is a dead tissue because of all these layers of keratin fibers[7] Most horns have a bony centre in the middle of all the keratin,[7] though horns arise as a separate bony ossicle from the frontal bone.[7] Some horns, such as rhino horns, do not have a bony centre and these are referred to as keratin-fibre horns.[7]

Giraffe head ornaments are neither classified as horns nor antlers but are ossicones, simply referred to as knobs.[7] These are permanent bony outgrowths that are covered with vascularized skin.[7]They begin as nodules of bone under the skin though they are separated from the skull like keratin horns, but these knobs later fuse to the skull.[7]

Perrisodactyla

Overview

Woolly Rhino

Perissodactyla comes from the Greek words perissos and daktulos, meaning odd numbers and toe respectively.[13] Rhinoceroses have been around since the Paleocene epoch (about 65 million years ago) and from about 35 – 20 mya (late Oligocene to early Miocene) they used to be the largest terrestrial mammals on all the northern continents.[13] From the Oligocene to the Miocene they ranged over all ecosystems. They were an abundant family with many different sizes and morphological adaptations and were even able to survive the ice age through species like the Woolly Rhino. The Paraceratherium, a hornless species of Perissodactyla, is believed to be the largest terrestrial mammal that ever existed.[13] Horns, however, only evolved into a defining characteristic about 48 million years ago when the Rhinocerotidae family appeared. These were the ancestors to the five species of rhino we have today. Horn and antler evolution has occurred, independently, several times and in closely related groups like the rhinos and ruminants.[14] The Rhinocerotidae family, even, independently evolved horns at least twice throughout their evolution.


Habitat

Rhino

The family Rhinocerotidae had members that inhabited Asia during the Oligocene epoch (about 37 mya) and Europe during the Miocene epoch (about 23 mya). [13] They only arrived in Africa, from Asia, 23 million years ago. From the late Miocene to the early Pliocene epoch (about 10 – 5 mya) they were on a decline. Many, obviously, became extinct as there are no more, wild Rhinocerotidae, in Europe (about 12,000 years ago).[13] The five species found today (white, black, Sumatran, Javan, and greater one horned) are all from different lineages. Today they are found in Southeast Asia (India, Nepal, and Indonesia) and in Eastern and Southern Africa. African rhinos have more open terrain, grasslands with small shrubs and few trees, whereas Asian rhinos inhabit more forested areas.[15] The consistent threat of poaching and loss of habitat continue to push Rhino’s to extinction.[15]


Diet

All Perissodactyla where placed in one of two different groups when it came to their diet. They were either grazers, eating mostly grasses, or browsers, eating whatever vegetation they could find with the majority of Rinocerotidae evolving into the later. Ceratotherium simum or white rhinos only graze on grasses where as the all of the other rhinos eat a variety of vegetation including flowers, leaves and fruits. [16]


Head Ornamentation

All rhino horns are made up of a compact Keratin (hair like material). This is the part that poachers want when they kill rhinos for their horns. Around 5 million years ago (Pliocene epoch) rhinos diverged into two types the two horned rhino species and one horned species.[16] They can have frontal or nasal horns depending on which bone in the skull the horn is over.[13]

Rhino Sizes
  • White Rhinos: These rhinos have two horns on their snout. The horn on the front of the rhino’s snout is larger then the one behind it and it averages 90 cm in length. Some can even reach up to 150 cm in length.[17] Horns occur in both males and females of this species.
  • Black Rhinos: These rhinos have two horns like White rhinos but a third horn (smaller than the rest) can sometimes develop. The front horn is generally larger and averages about 50 cm in length. Horns occur in both males and females of this species.[17]
  • Greater one horned Rhinos: These rhinos, as the name indicates, only have one horn and it is usually 20 to 100 cm in length.[17] Horns occur in both males and females of this species.
  • Javan Rhinos: These rhinos also only have one horn. In males it can grow up to 26 cm in length, where as female horns are more like knobs but may not be present at all.[17]
  • Sumatran Rhinos: These rhinos have two horns like the two African types with their front horn averaging between 25 to 79 cm and the other horn averages less than 10 cm in length.[17]

Artiodactyla

Overview

Artiodactyls (even-toed ungulates) first appeared around 54 million years ago, (mya) in the early Eocene epoch, when the Perissodactyls dominated.[18] These mammals were much like the chevrotains of today, small ruminants with no cranial ornaments. Occupying small niches, they began developing complex digestive systems to survive on low-grade plant foods. Artiodactyls were a distinct order by the end of the Eocene, with 3 suborders, Suina, Tylopoda, and Ruminantia.[18] The changing environment during the Oligocene (33.9-23mya) and decline of the Perissodactlys allowed the Artiodactyls to become the prominent ungulates and begin diversifying. Evidence suggests that horns first began to develop in ruminants of the Artiodactyla order in the North American Miocene, around 23 mya.

Habitat

Since the spread of Artiodactyls over 33 mya from Asia they have become exceptionally diverse and globally distributed.[19] Artiodactyls are native to all continents except Antarctica and Australia, having been introduced by humans to the latter. Being globally distributed means that Artiodactyls have adapted to nearly all types of habitats provided there is sufficient forage and predator defence.[19]

Although artiodactyls occur in all biomes from deserts to tropical forests to tundra, preferred habitat types fall into four major categories. Open grasslands have sufficient forage and allow for early detection of predators.[19] This is the preferred habitat of Pronghorns, although they are also found in semi-desert regions and grassy brush lands. Pronghorns range from southern Alberta and Saskatchewan all the way down to Northern Mexico.[20] The second preferred habitat of Artiodactyls is grasslands or meadows near steep cliffs. Rocky Mountain Goats and similar Artiodactyls prefer this type of habitat in alpine and subalpine areas as most predators have difficulty on the rocks. Rocky Mountain goats range from Southeast Alaska to Idaho, staying on the Rocky, Cascade, and Coastal Mountain ranges.[21] Forests are the third habitat preferred by many species in the Cervidae family, including Moose. Moose are distributed across a large range, from Coastal B.C. through the rest of Canada and northern European countries and into Siberia. Moose prefer to stay near forest cover and close to a fresh water source such as lakes, rivers, or wetlands.[22] Many species of Artiodactyla, such as moose, prefer the fourth category. The ecotone between open areas and forests is highly preferred because the open areas provide abundant forage and adjacent forests provide dense cover from potential predators.[19]

Many Artiodactyls will migrate for the changing of seasons, Pronghorns are one of the few mammals in North America to migrate long distances, over 300 miles.[23] Rocky Mountain Goats also migrate from high elevation in the summer to lowlands during the winter months.

Diet

Mountain Goat

Nearly all species of Artiodactyls are herbivorous, with exceptions in the Suidae and Tayassuidae families.[19] Certain members of the Tayassuidae and Suidae families are omnivorous, meaning they eat both plant and animal foods. Herbivorous Artiodactyls have a very diverse diet, however, all species have developed larger stomachs and longer intestines needed to digest plant material. They have also evolved multi-chambered stomachs and many species of Artiodactyla will regurgitate their food to further improve digestion of plant foods.[19] Salt licks are often utilized as well in order to meet mineral requirements. Diet and diet patterns vary greatly within and among species of Artiodactyla. Moose, for example, graze on higher grasses and shrubs as well as aquatic vegetation in the summer.[22] In the winter moose will eat almost any forage available including twigs and moss. Rocky Mountain Goats, conversely, will graze almost constantly on grass, woody plants and moss.[21] They will also travel in the spring to find salt licks. Pronghorns have a poor diet of grass, flowers, and brush meaning they must regurgitate for proper digestion.

Head Ornamentation

Head ornamentation does not occur in all species of Artiodactyla. Pigs, peccaries, hippopotamuses, camels, and chevrotains have no horns or antlers.[18] Antlers are prominent in the cervidae family; all true male members of the deer family possess antlers. Antlers only occur on female caribou and reindeer. Horns are prevalent in the Bovidae family, occurring in males of all species, and in females of larger species. Larger females likely possess horns as a means of defense, smaller female species would choose to run from predators, therefore selecting for an absence of horns.[2] There is some sexual dimorphism in antlers and horns; males having the larger head ornamentation.[2] Horns and antlers are extremely diverse in size and shape, however, all males use them primarily to attract females, from direct male-male fighting, intimidation, and for displays for females.

Moose have the largest antlers of all mammals worldwide, measuring up to 2 meters across, from tip-to-tip.[22] Moose antlers are very wide and flat, with many prongs. Moose have adapted other uses for there antlers as well. The antlers are used to clear away snow to obtain the shrubs and moss underneath. The huge antlers also act as hearing aids. Rocky Mountain Goats, conversely, have small horns. The pure black horns are about 200 to 300 mm long, rather thin, and present on both sexes.[21] These horns have a slight backwards curve, are extremely sharp, and are used primarily for defense. Males will occasionally fight over mates, but rarely, as they stab each other rather than butting heads.

Pronghorns are the exception to the Artiodactyla, they do not have true horns or antlers. Their horns are similar to true horns except they branch and are shed annually.[2] Pronghorns are the only species with branching horns, and the only living species in their family. Both sexes have horns; only males have branching horns, the longer fork curves back and the shorter points forward. Male horns grow to between 25 and 30 cm, female horns are only small bumps.[20] The horns are used for territorial contests in spring and then the keratin sheath is shed after mating in early fall. The bony core remains attached to the skull, while the keratin is regrown every winter.[20]

Male Pronghorn

Horn Development

True horns are the head ornaments of all species in the Bovidae family, both Rhinos and Pronghorns have head ornaments that are called horns, but are not true horns. True horns consist of a bony core covered in a sheath of keratin. These horns are never branched, and neither the core nor keratin sheath is ever shed. In most species horns never stop growing but growth will slow with aging. The bony cores of horns begin as a small growth under the skin, developing over the frontal bone but not yet attached to the skull.[2] Keratin growth will occur after birth, and form a sheath over top of the bony core.

True Horn Growth

Bone

The bony core of horns begins as a separate bony ossicle, the os cornu, with it’s own centre of ossification. The os cornu begins development in utero, as a separate bone from the skull. The bony growth develops above the nasal bone and over the frontal bone within the dermal connective tissue.[7] The bony cores will fuse secondarily to the skull bone.[2] There is little reliable evidence on the histogenesis of the os cornu, or on how it attaches to the skull. It is also unknown if ossification is direct or endochondrial.[7] There has been discussion of finding cartilage in goat and sheep horns, suggesting endochondrial ossification.

Keratin

Keratin is produced by specialized epithelial cells known as keratinocytes. Before horn development begins, these epithelial cells produce hair follicles. Hair follicle production ceases shortly after birth. The keratinoctyes then begin producing layers of keratin fibres that accumulate and elongate as the bony core grows inside.[7] The underlying layers of proliferative keratinocytes produce layers upon layers of keratin fibres, causing the horn to continue growth throughout the individuals life, even though keratin is not living tissue. It is not clear how hair follicle production cease and horn development is initiated.[7]

Minerals and Hormones Involved

The bone layer of horns are composed primarily of calcium and phosphate.[9] The hormones and factors of bone growth in horns are unknown as the exact mechanism of bone growth in horns is unknown. Castration does cause the loss of horns, indicating horns reliance on testosterone for maintenance.

Keratin is composed of many proteins, the largest percentage being glycine and cysteine. Keratin is also composed primarily of two elements, Sulphur and Nitrogen.[24] Keratinocyte proliferation, the production of keratin, is influenced by a number of factors including transcription factor p63, Vitamin A, Epidermal Growth Factor (EGF) and Transforming growth factor alpha.[25]

Rhino Horn development

Horns are usually made up of a bony core with a hardened layer of keratin, the same material that makes up hair or nails, on the outside. Rhino’s however don’t have a bony core that extends from their skull. Their horns are made mostly of keratin with calcium deposits in the core to make it harder and stronger. They also have deposits of melanin to protect the calcium core from breakdown by UV light. The outer keratin layers are softer so they can be weakened by UV light and therefore can be worn down to a sharp point when rubbed against vegetation, the ground or by horn – horn contact in fights. One could compare rhino horns to pencils with their hard lead core and softer wood covering that can be worked to a sharp point. These deposits appear in rhino horns as yearly growth surges but the effects of temperature, diet and stress on the growth are still unknown.[26]

Rhino horns differ from true horns because these horns have no core or sheath. They are made up of multitude of epidermal cells and bundles of dermal papillae, extensions of the dermis.[2] Cells from each papilla form a horny fiber similar to thick hair. These fibers, which are held together by the mass of epidermal cells, are not true hairs. True hair grows from follicles that extend into the dermis, whereas rhino horns grow from dermal papillae, which extend up into the horn.[2] The rhino horn is situated over the nasal bones. In species that have two horns, the second horn lies over the frontal bones. Rhino horns commonly curve posteriorly.[2]

Pronghorn Keratin Shedding

Pronghorn horn development occurs in much the same way as true horns. The ‘horns’ on pronghorns consist of a pair of permanent, unbranched bony projections on the skull that are covered in a layer of epidermal cells that form keratin.[7] This epidermal layer is shed annually and then replaced, leaving only the permanent bony core for months at a time. The keratin growth becomes subdivided into prongs, hence the name Pronghorn. The Pronghorn horn hardening and shedding are synchronic with annual testosterone levels, much like antlers.[7] The exact mechanism that has evolved in pronghorns for keratin shedding is not fully understood.

Antler Development

Antlers are head ornaments present on the front of an animal’s head. They consist of pure bone and are shed yearly. Most Artidodactyls show the presence of head ornaments, such as moose, elf, caribou and deer. Antlers are derived from the frontal bone, they grow by an accumulation of cartilage which is transformed to bone. Antlers normally begin their growth cycle in spring; they are covered in a coating of nerves which also provide nutrients and pheromones unique to each individual, this coating is known as velvet [1]. By the end of fall, velvet dies and antler bone becomes denser and mineralizes. Antler growth cycles are synonymous to mating seasons, so by the end of fall/start of winter when antlers are there largest, females are looking for mates; after mating season, antlers are cast.

Antler pedicles begin to develop in the fetus from osteogenic centers on the frontal bone [3]. The pedicles can be viewed at 6 months of age via ultrasound. Removal of the pedicles results in the absence of the antlers, similarly any damage or abnormality in the pedicles later in life can results in irregular antler formation.

Testosterone stimulation is required for pedicles to initiate and begin growing [1]. It is believed testosterone is required to maintain the changes in ossification during late pedicle growth. The pedicle grows by intramembranous ossification which occurs when new bone is laid down by bone forming cells [3]. In the first year of growth, antlers differentiate from the pedicle to form spikes. Once these are shed, according the antler growth cycle, a new pair of antlers will develop yearly from the pedicles.

Rocky Mountain bull elk in Nebraska tall grass prairie

Antler Growth and Calcification

After shedding of the old antlers, the skin of the pedicle grows over and heals the wound left by the discarded antler. Mesodermal cells in the pedicle proliferate to make antler tissue. Fibroblast cells with deposit collagen at the point where antlers will growth, forming a mass on the pedicle. Growth continues and cells differentiate at the base into chondroblast and chondrocytes[3], both these cells are associated with the formation of cartilage. Antlers will continue to develop via the chondroblast and chondrocytes dividing at the outer edge and fibrocartiledge divisions at the inner edge. Once cell division is complete, the cartilage will ossificate via endochondral ossification and become bone [3]. Antler cartilage is heavily vascularized , throughout the antler there are elaborate capillary networks. Growth in antlered species is usually very rapid and composition of hard antler is similar to bone containing approximately 25% calcium, 19% phosphorous, organic matter making up 39% and water content is 8%. [3]

Mineral Requirements

Mineral requirements for antler growth exceed those for skeletal growth, for example elk antlers grow at approximately 100gm/day, while skeletal growth occurs at about 34 gm/day [3]. Mineral for antler calcification is partly fulfilled by bone resorption. Diet provides a great majority of the calcium and phosphorous needed for antler growth [27]. Other minerals involved in antler growth include collagen; a structural protein that binds joints together and serves as a main component of growing cartilage. Another is chondroitin; a carbohydrate that attracts fluid into proteoglycan molecules and protects cartilage from destructive enzymes. Lipids are also extremely important; they are responsible for building and improving cell energy. Glucosamine sulfate builds cartilage (to become bone). Hormones along with minerals are involved in antler growth and can be classified into two general categories, the first being those which control the timing of the antler cycle and those controlling the physical growth of antlers. Androgens, such as testosterone are the primary source of antler growth [27]. They are produced concurrently with the reproductive cycle. Pedicle initiation is induced by high testosterone levels but the transformation from pedicle to antler is induced by a change in testosterone levels. A main hormone involved in antler growth is growth hormone (GH) which releases insulin-like growth factor (IGF-1) to regulate antler extension [27]. Another hormone produced by the pituitary gland is lutenizing hormone which helps initiate antler hardening and velvet shedding during the fall season [28].

Velvet Shedding

Red Deer Velvet

Antlers are made of true bone that is fed by blood which is carried in the outer velvet covering [29]. Velvet could be thought of as a similar layer of sensitive skin in humans, it’s filled with blood vessels that feed the antler with necessary minerals and vitamins to aid bone growth. Velvet is hot to the touch, with brushy hair and a waxy coating [29]. Velvet has many components and hormones such as growth factors, IGF, proteins, minerals, chondroitin, glucosamine and many amino acids. These ingredients are naturally formed each year with the new growth of the antler velvet. The exact mechanism of how velvet is shed is not fully understood but scientists have concluded velvet shedding mimics vascular changes within the animal and as testosterone levels begin to increase [3] this triggers the shedding cycle. Antler growth lasts about 2 to 4 months, after that time – velvet is no longer needed so a ring forms at the base of the antlers and cuts off the blood supply to the velvet [30]. As a result, velvet will begin to wither up and fall off; often animals are seen rubbing their antlers against trees to assist the velvet in its shedding process.

Antler Casting

Casting is the process of shedding antlers; antlers are shed according to their physiological cycle; each species will have their own casting cycle (usually occurs after mating season). Antlers grow from pedicles; pedicles however are never cast, they may lose bone mass and shrink over time, but they are always present in an organism which grows antlers (i.e. female deer do not have pedicles). There is a line which indicates separation between pedicle and antler; this is called the abscission line. It is indicated by a narrow transverse band of blood vessels, osteoclastic activity between the dead bone of the antler and the living bone of the pedicle allow for differentiation between the two. An experiment was performed on deer who were administered high levels of testosterone or estrogen and the results concluded in the presence of high androgen levels casting of antlers did not occur, whereas if the deer were castrated, casting resulted within 2-3 weeks.

Sexual Selection and Reproductive Advantages

Horns and antlers are secondary sexual characteristics,[31] which means that they are characteristics not directly involved with reproduction. The size of the head ornament is very important when finding a mate. They can be used one of two ways, either to attract a mate or to fight off the other competitors.[31] It is not unusual for these secondary sexual characteristics to do both though. Sexual selection has caused evolution to favour animals with larger head ornaments because they are the ones with the most reproductive success,[32] as size and shape are genes that will be passed down. The size of the head ornaments can also show the mate that the male has survived many years or has superior genetics.[32] Larger antlers and horns can also determine dominance ranks.[31] There are different trade-offs associated with head ornament size. The larger ones do have a better chance of finding a mate and have a greater chance at reproductive success, but they are also the ones that are targeted by hunters. The ungulates with smaller head ornaments may have a lesser chance at finding a mate, but they are also not as targeted as males with larger antlers or horns.

Heritability

The size of head ornament can be heritable in ungulates and this is part of the evolution process since evolution will favour those with larger head ornaments. Different studies have shown that there are different traits of antlers, not just size, that are heritable. [33] There are different genes for shape, colour, and other traits. A study in white tailed deer showed that nutrition and age are also very important developmental factors, not just heritability [33]

Female Head Ornaments

Why do some ungulate species, whose males have horn like organs, have horned females and some have hornless females? [14] It is well know that males have horns or antlers in order to compete with other males for mating rights and to show females that they stronger and possess more advantageous traits than other males. All Recent families of horn or antler possessing males of ungulates contain genera both with and without horned females.[34] The only exception for this is Rhinocerotidae because all females of this group have horns.[34] What is less known are the reasons for the unequal distribution of horned and hornless females.[34] One hypothesis for why females do have “horns” is that of the Female Competition hypothesis, which says that female horn like organs evolved because of “intrasexual competition for resources”.[35] As well, a study, led by Ted Stankowich a professor and the University of Massachusetts Amherst, found that water buffalo females have horns because they aren’t able to hide well and they need to protect their feeding territory.[36] Some even suggest that the presence or absence of horn like organs in females is altered during radical speciation events.[34] Although these are all good hypotheses to explain why some females have cranial appendages and some do not, there is no general consensus regarding functional explanations.[35]

Different Shapes of Head Ornaments in Ungulates

  • Serengeti Buffalo

    Serengeti Buffalo

  • Big Horn Sheep

    Big Horn Sheep

  • Wapiti

    Wapiti

  • Male Moose

    Male Moose

  • Male Fallow Deer

    Male Fallow Deer

  • Male Caribou

    Male Caribou

  • East African Oryx

    East African Oryx

  • Sable Bull

    Sable Bull

  • Markhor

    Markhor

  • Capra Ibex

    Capra Ibex

  • White Rhino

    White Rhino

  • Giraffe ossicones

    Giraffe ossicones

  • Wildebeest

    Wildebeest

Human Applications

Trophies

One of the most common human uses for a horn or an antler is for trophy hunting. These trophies can have great value depending on their size and shape and large males are usually targeted.[37] The body mass of the animal being hunted is a major determinant for the trophy price. Rare species can also be targeted for a bigger trophy price[38] and this could be cause extinction.[37] Trophy hunting has many effects on a population and a study done in Alberta with bighorn sheep showed that it can have a more rapid effect that expected.[7] There was a rapid decline in mean ram weight, population size, and mean horn length.[7] Other animal body parts that are used as trophies are tusks, teeth, and skulls.[37]

Antler Chandelier
  • Furniture; In the modern century it's become increasingly popular to decorate homes with custom designed pieces of antler furniture. From chairs, to tables and even chandeliers. What once may have been considered tacky has now become pieces of art work.

Instruments

Musical instruments have been made out of ungulate head ornaments, though these instruments are generally now made of brass.[39] A shofar is made of ram horn and is used for Jewish ceremonies.[40] Goat horns also make musical instruments but contain the mouthpiece at the wide end of the horn, which is different from most instruments.[41]These are called Gemshorns, from the German language, and date back to the fifteenth century.[41]

Medicine

Both velvet from antlers and horns (specifically rhino horn) have been used for centuries in traditional Chinese medicine. In the case of antlers, the antler is cut off near the base after it's began growing and it about two thirds of its full capacity, this is to avoid calcification. The antler is then dried and crushed to a powder, it is said to be effective as an anti-inflammatory, anticancer, immune stimulant, and pro-growth agent, though these claims have not been verified through scientific study [42]. Velvet capsules are sold worldwide, claiming to improve everything from infertility to cancer. Deer velvet is rich in substances including the female sex hormones estrone and estradiol which aid in cells grow and function [43].

Rhino horn being sold

In Traditional Chinese Medicine, the horn, which is shaved or ground into a powder and dissolved in boiling water, is used to treat fever, rheumatism, gout, and other disorders. According to the 16th century Chinese pharmacist Li Shi Chen, the horn could also cure snakebites, hallucinations, typhoid, headaches, carbuncles, vomiting, food poisoning, and “devil possession.” [44]. Unlike the antler once the rhino's horn is removed it can never regrow and leaves the animal defenseless. Horns for medicinal purposes are most commonly used from the rhino, however Serengeti buffalo have very similar proteins and hormones and Chinese medicine has documented the use of both. These sought after remedies have driven many animals to near extinction, such as the Javan rhinoceros of which its estimated there are only 60 in the wild due to poaching for the horn, fetching as much as US$65,000 per kilo on the black market [45]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 http://www.bookrags.com/research/antlers-and-horns-ansc-01/
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 http://animaldiversity.ummz.umich.edu/collections/mammal_anatomy/horns_and_antlers/
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 http://www.usask.ca/wcvm/herdmed/specialstock/antlers/physio.html#Antler%20Casting
  4. ↑ 4.0 4.1 4.2 Prothero, Donald R., and Robert M. Schoch. Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals. Baltimore: Johns Hopkins UP, 2002. Print.
  5. ↑ 5.0 5.1 McCarthy, Eugene M. "Ungulates." Ungulates. Web. <http://www.macroevolution.net/ungulates.html>.
  6. ↑ http://www.rhinoresourcecenter.com/pdf_files/131/1311411400.pdf
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19 7.20 7.21 7.22 7.23 7.24 7.25 Hall, Brian K.. (2005) Bones and Cartilage: Developmental Skeletal Biology: Academic Press.
  8. ↑ 8.0 8.1 8.2 http://en.wikipedia.org/wiki/Antlers
  9. ↑ 9.0 9.1 9.2 9.3 http://en.wikipedia.org/wiki/Bone
  10. ↑ 10.0 10.1 http://herkules.oulu.fi/isbn9514266064/html/c394.htmlthough
  11. ↑ http://en.wikipedia.org/wiki/Ossification
  12. ↑ 12.0 12.1 http://en.wikipedia.org/wiki/Keratin
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 http://www.rhinoresourcecenter.com/pdf_files/117/1175860259.pdf
  14. ↑ 14.0 14.1 Geist, Valerius. “The Evolution of Horn-Like Organs.” Behaviour 27.3 (1966): 175-214. Web. 10 Sept. 2012. http://www.jstor.org/stable/4533157 Cite error: Invalid <ref> tag; name "”art" defined multiple times with different content
  15. ↑ 15.0 15.1 http://wwf.panda.org/what_we_do/endangered_species/rhinoceros/asian_rhinos/
  16. ↑ 16.0 16.1 http://www.mammalsrus.com/eutheria/perissodactyla/rhinocerotidae/rhinocerotidae.html\
  17. ↑ 17.0 17.1 17.2 17.3 17.4 http://en.wikipedia.org/wiki/Rhinoceros#Evolution
  18. ↑ 18.0 18.1 18.2 http://www.britannica.com/EBchecked/topic/37203/artiodactyl/51691/Evolution-and-paleontology
  19. ↑ 19.0 19.1 19.2 19.3 19.4 19.5 http://www.animaldiversity.ummz.umich.edu/accounts/Artiodactyla/
  20. ↑ 20.0 20.1 20.2 http://animaldiversity.ummz.umich.edu/accounts/Antilocapra_americana/
  21. ↑ 21.0 21.1 21.2 http://animaldiversity.ummz.umich.edu/site/accounts/information/Oreamnos_americanus.html
  22. ↑ 22.0 22.1 22.2 http://animaldiversity.ummz.umich.edu/accounts/Alces_alces/
  23. ↑ http://www.nwf.org/Wildlife/Wildlife-Library/Mammals/Pronghorn.aspx
  24. ↑ Block, Richard J.. (1939) The Composition of Keratins: Journal of Biological Chemistry
  25. ↑ http://en.wikipedia.org/wiki/Keratinocyte
  26. ↑ http://www.sciencedaily.com/releases/2006/11/061106144951.htm
  27. ↑ 27.0 27.1 27.2 http://www.harding.edu/plummer/animphys/studentwebsites/tnlee/Hormones.htm
  28. ↑ http://www.soyouwanna.com/causes-deer-shed-antlers-2448.html#ixzz2BnF52Kcw
  29. ↑ 29.0 29.1 http://www.suwanneeriverranch.com/WTinfoIantlers.htm
  30. ↑ http://www.whitetailantler.com/whitetail%20deer%20shed%20antlers.html
  31. ↑ 31.0 31.1 31.2 Bergeron, P.. (2008) Heterogeneity in male horn growth and in a highly sexually dimorphic ungulate. Oikos, 117, 77-82.
  32. ↑ 32.0 32.1 http://wdfw.wa.gov/living/antlers.pdf
  33. ↑ 33.0 33.1 http://www.tpwd.state.tx.us/huntwild/wild/game_management/deer/genetics/
  34. ↑ 34.0 34.1 34.2 34.3 http://www.jstor.org/stable/pdfplus/4535366.pdf?acceptTC=true
  35. ↑ 35.0 35.1 http://web.ebscohost.com/ehost/pdfviewer/pdfviewer?sid=6a351c70-8598-444a-a75d-b1320ad0b772%40sessionmgr11&vid=2&hid=7
  36. ↑ http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1985.tb00377.x/pdf
  37. ↑ 37.0 37.1 37.2 Palazy, L.. (2011) Rarity, Trophy Hunting and Ungulates. Animal Conservation.
  38. ↑ Festa-Bianchet, M.(2003). Exploitative wild life management as a selective pressure for life-history evolution of large mammals. Animal Behavior and Wildlife Conservation.
  39. ↑ http://en.wikipedia.org/wiki/Horn
  40. ↑ http://en.wikipedia.org/wiki/Shofar
  41. ↑ 41.0 41.1 Norman, John. (2010). The Goat-Horn. The Organists Review.
  42. ↑ http://en.wikipedia.org/wiki/Velvet_antler
  43. ↑ http://www.webmd.com/vitamins-supplements/ingredientmono-808-DEER%20VELVET.aspx?activeIngredientId=808&activeIngredientName=DEER%20VELVET
  44. ↑ http://www.pbs.org/wnet/nature/episodes/rhinoceros/rhino-horn-use-fact-vs-fiction/1178/
  45. ↑ http://articles.businessinsider.com/2012-05-02/lifestyle/31536010_1_rhino-horn-face-extinction-main-market
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