Calcar
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The calcar, also known as the
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. ...
, is the name given to a
spur A spur is a metal tool designed to be worn in pairs on the heels of riding boots for the purpose of directing a horse or other animal to move forward or laterally while riding. It is usually used to refine the riding aids (commands) and to back ...
of
cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
arising from inner side of ankle and running along part of outer
interfemoral membrane The patagium (plural: patagia) is a membranous body part that assists an animal in obtaining lift when gliding or flight. The structure is found in extant and extinct groups of flying and gliding animals including bats, birds, some dromaeosaurs ...
in
bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most ...
s, as well as to a similar spur on the legs of some
arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
. The calcar serves to help spread the interfemoral membrane, which is part of the wing membrane between the tail and the hind legs. Calcar (femorale) also refers to the dense, vertically oriented bone present in the posteromedial region of the femoral shaft inferior to the
lesser trochanter The lesser trochanter is a conical posteromedial bony projection of the femoral shaft. it serves as the principal insertion site of the iliopsoas muscle. Structure The lesser trochanter is a conical posteromedial projection of the shaft of the fe ...
.


Usage history

It is unclear who first coined the word "calcar" to apply to bat anatomy; records of its usage date to
Joel Asaph Allen Joel Asaph Allen (July 19, 1838 – August 29, 1921) was an American zoology, zoologist, mammalogy, mammalogist, and ornithology, ornithologist. He became the first president of the American Ornithologists' Union, the first curator of birds and ma ...
in 1893. The word calcar is derived from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
"
calx Calx is a substance formed from an ore or mineral that has been heated. Calx, especially of a metal, is now known as an oxide. According to the obsolete phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in t ...
," meaning "heel". Other terms or phrases that refer to the same feature include "supplementary calcaneal bones", "styliform bones", ''"les éperons"'' (French), ''"Fusswurzelstachels"'' (German), "spurs", and "stylets".


Prevalence

Not all bats have a calcar, as not all bats have a well-developed
uropatagium The patagium (plural: patagia) is a membranous body part that assists an animal in obtaining lift when gliding or flight. The structure is found in extant and extinct groups of flying and gliding animals including bats, birds, some dromaeosau ...
. Of the bats that have a developed uropatagium, most, but not all, have a calcar. The
Kitti's hog-nosed bat Kitti's hog-nosed bat (''Craseonycteris thonglongyai''), also known as the bumblebee bat, is a near-threatened species of bat and the only extant member of the family Craseonycteridae. It occurs in western Thailand and southeast Myanmar, where i ...
is the only species of bat that has an extensive uropatagium while lacking a calcar.


Structure

The calcar varies widely among bats. It can be as small as , or longer than . In some species of bat, the calcar is very long and bladelike. Examples of this include species in the genera '' Noctilio'' and ''
Diclidurus ''Diclidurus'' is a genus of bats whose common name is the ghost bats (not to be confused with the Australia ''Macroderma gigas''). ''Diclidurus'' all inhabit tropical South America, and ''D. albus'' is also found in Mexico and Central America.E ...
''. In other species, the calcar is very small or absent, such as the
Kitti's hog-nosed bat Kitti's hog-nosed bat (''Craseonycteris thonglongyai''), also known as the bumblebee bat, is a near-threatened species of bat and the only extant member of the family Craseonycteridae. It occurs in western Thailand and southeast Myanmar, where i ...
or species in the genera ''
Rhinopoma Mouse-tailed bats are a group of insectivorous microbats of the family Rhinopomatidae with only three to six species, all contained in the single genus ''Rhinopoma''. They are found in the Old World, from North Africa to Thailand and Sumatra, i ...
'', ''
Diaemus The white-winged vampire bat (''Diaemus youngi''), a species of vampire bat, is the only member of the genus ''Diaemus''. They are found from Mexico to northern Argentina and are present on the islands of Trinidad and Margarita. Etymology and ...
'', ''
Mystacina ''Mystacina'' is the sole surviving genus of the family Mystacinidae. The New Zealand lesser short-tailed bat is the only member of this group confirmed to survive today, since the closely related New Zealand greater short-tailed bat The New Z ...
'', '' Syconycteris'', '' Harpyionycteris'', and ''
Notopterus The bronze featherback (''Notopterus notopterus''; as, কান্ধুলি ''kandhuli'', bn, ফলি, bn, কাংলা,, th, ปลาสลาด, ปลาฉลาด, ปลาตอง, Vietnamese: ''Cá thát lát'', my, င ...
''. In the
hairy-legged vampire bat The hairy-legged vampire bat (''Diphylla ecaudata'') is one of three extant species of vampire bats. It mainly feeds on the blood of wild birds, but can also feed both on domestic birds and humans. This vampire bat lives mainly in tropical and ...
, the calcar has a unique, finger-like form that extends approximately beyond the edge of the uropatagium. Some insectivorous species of bats have an elaborate, pronounced calcar. This form of calcar is referred to as a "keeled calcar", and it is hypothesized that it may be useful in converting the uropatagium into a basket for catching insects. For species in some genera (for example, ''
Vampyrum The spectral bat (''Vampyrum spectrum''), also called the great false vampire bat or Linnaeus's false vampire bat, is a large, carnivorous leaf-nosed bat found in Mexico, Central America, and South America. It is the only member of the genus '' ...
'' and '' Phyllostomus''), the calcar is entirely made of cartilage. In other genera (e.g. ''
Saccopteryx ''Saccopteryx'' is a genus of sac-winged bats from Central and South America. The species within this genus are: * Antioquian sac-winged bat ''Saccopteryx antioquensis'' *Greater sac-winged bat The greater sac-winged bat (''Saccopteryx bili ...
'', ''
Pteronotus ''Pteronotus'' is a genus of bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bat ...
'', '' Molossus''), the calcar is calcified. Intermediate between these two forms is when one end of the calcar is calcified, while the other is cartilaginous (e.g. '' Noctilio'' and ''
Trachops The fringe-lipped bat (''Trachops cirrhosus'') is a leaf-nosed bat from southern Mexico to Bolivia and southern Brazil. It has three subspecies and no known fossils. It is the only species within its genus. Morphology The fringe-lipped bat has ...
''). All
megabat Megabats constitute the family Pteropodidae of the order Chiroptera (bats). They are also called fruit bats, Old World fruit bats, or—especially the genera ''Acerodon'' and ''Pteropus''—flying foxes. They are the only member of the su ...
s, however, lack any calcification in their calcar. In
microbat Microbats constitute the suborder Microchiroptera within the order Chiroptera (bats). Bats have long been differentiated into Megachiroptera (megabats) and Microchiroptera, based on their size, the use of echolocation by the Microchiroptera a ...
s, movement of the calcar is controlled by several muscles, including the
gastrocnemius muscle The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It runs from its two heads just above the knee to the heel, a three joint muscle (knee, ankle and subtala ...
, m. calcaneocutaneous, and m. depressor ossis styliformis. M. depressor ossis styliformis abducts the calcar towards the foot, which spreads the uropatagium. M. calcaneocutaneous works in opposition to m. depressor ossis styliformis, helping to stabilize the calcar. The gastrocnemius muscle aids in flexion of the foot, working in conjunction with m. depressor ossis styliformis to spread the uropatagium. Unlike microbats,
megabat Megabats constitute the family Pteropodidae of the order Chiroptera (bats). They are also called fruit bats, Old World fruit bats, or—especially the genera ''Acerodon'' and ''Pteropus''—flying foxes. They are the only member of the su ...
s lack the m. calcaneocutaneous muscle for calcar control; megabats do share the other two muscles for calcar control, however.


Function

The calcar can assist the uropatagium in forming a basket or pouch to help catch and hold insects captured in flight. The calcar helps spread the uropatagium during flight, managing its
camber Camber may refer to a variety of curvatures and angles: * Camber angle, the angle made by the wheels of a vehicle * Camber beam, an upward curvature of a joist to compensate for load deflection due in buildings * Camber thrust in bike technology * ...
. For species of bats that forage via
trawling Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different spec ...
, such as the
greater bulldog bat The greater bulldog bat or fisherman bat (''Noctilio leporinus'') is a species of fishing bat native to Latin America (Spanish: ''murciélago pescador''; Portuguese: ''morcego-pescador''). The bat uses echolocation to detect water ripples made ...
and the fish-eating myotis, the calcar is used to prevent the uropatagium from dragging along the surface of the water. In the hairy-legged vampire bat, the calcar is not used for flight, but rather as a sixth digit to aid in tree-climbing.


Evolutionary history

The oldest known ancestor to present day bats, '' Icaronycteris index'', apparently did not have a calcar or spur as evidenced by fossil remains.Ontogeny, Functional Ecology, and Evolution of Bats () The oldest known bat with a calcar is ''
Onychonycteris ''Onychonycteris'' is the more primitive of the two oldest known monospecific genera of bat, having lived in the area that is current day Wyoming during the Eocene period, 52.5 million years ago. Taxonomy Two specimens of ''Onychonycteris'' we ...
'', which lived in the early
Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
. ''Onychonycteris'' had a long calcar, indicative of a broad tail membrane. In describing ''Onychonycteris'' in 2008, Simmons hypothesized that its calcar was more relevant to the mechanism of flight than capturing prey. Some authors have treated the calcar as a
synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
, or a unique trait shared by all bats, derived from a common ancestor. However, major differences in calcar structure between the two major clades of bat (formerly Megachiroptera and Microchiroptera, now
Yinpterochiroptera The Yinpterochiroptera (or Pteropodiformes) is a suborder of the Chiroptera, which includes taxa formerly known as megabats and five of the microbat families: Rhinopomatidae, Rhinolophidae, Hipposideridae, Craseonycteridae, and Megadermatid ...
and
Yangochiroptera Yangochiroptera, or Vespertilioniformes, is a suborder of Chiroptera that includes most of the microbat families, except the Rhinopomatidae, Rhinolophidae, Hipposideridae, and Megadermatidae. These other families, plus the megabats, are seen ...
) have led other authors to challenge this designation. In a paper published in 1998, Schutt and Simmons advocated for different names for this structure for the two suborders, with the Yangochiroptera (microbats) retaining "calcar" and the Yinpterochiroptera (megabats) using "uropatagial spur." The calcar-like structure is not a synapomorphy, they argued, but rather a similar structure that evolved independently in each suborder. In 2000, Adams and Thibault published a book on bat
ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
that supported this assertion, stating that various evidence " upportsthe hypothesis of independent origins for the microchiropteran calcar and the megachiropteran ‘uropatagial spur.’ Hence, the calcar, as currently understood, does not meet the assumptions of biological homology." Instead, the calcar of the microbats and the uropatagial spur of the megabats are an example of
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
.


References

{{reflist Mammal anatomy