Horseshoe Bat

The horseshoe bat is a genus that includes 100 or more species of insect-eating bats with large ears. They are classified under the genus Rhinolophus, the only genus included under the family Rhinolophidae. The horseshoe bats are close relatives of the leaf-nosed bats of the old world. The noses of the horseshoe bats have a large and complex nose leaf which comprises a fleshy appendage on the muzzle. Their common name is attributed to the fact that one section among the three ‘leaf’ sections bears resemblance to a horseshoe. 

In this article, we will study the genus Rhinolopus in detail also discussing the horseshoe bats adaptation, structure, habitat, etc. 

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Horseshoe Bat Habitat and Distribution

Horseshoe bats, typically, are found in temperate and tropical regions. They are distributed from Japan to Europe, from Africa to Asia. The greatest geographic distribution is seen in the case of the greater horseshoe bat. The habitat of horseshoe bats is dark and damp places such as caves,  tree hollows, buildings, and foliage where they are found in grooves and roosts. Their habitat can be both forested or unforested with species mostly occurring in tropical or sub-tropical areas. The species that are found in the temperate regions undergo hibernation during winter. 

Horseshoe Bat Structure and Morphology

Body Size and Structure: The structure of the horseshoe bat indicates that they are medium or small microbats. The head and body length of these bats range between 35 and 110 mm while their forearms range in length between 30 and 75 mm. The Rhinolopus hipposideros which is also known as the lesser horseshoe bat is one of the smaller species and weighs around 4 to 10 g. The greater horseshoe bat or R. ferrumequinum, on the other hand, weighs about 16.5 to 28 g. It is one of the larger species. The fur colour of the horseshoe bat also varies greatly among the species, from blackish to bright orange-red to reddish-brown. In a majority of the species, the fur is soft except for the R. beddomei and R. luctus, which are lesser woolly and woolly horseshoe bats, the fur is long and woolly.

Chest: The chest of horseshoe bats contains two mammary glands similar to other bats. The female abdomens additionally consist of two teat-like projections. These are known as false nipples or pubic nipples as they are not connected to the mammary glands. The armpits of the males of a few species of the horseshoe ba also have a false nipple under them.

Head: The nose-leafs are characteristic of the horseshoe bats and are composed of different parts, which is important for the identification of species. The nose-leaf in the front bears resemblance to that of a horseshoe which is flat and thin and positioned above the upper lip. The lancet that points up between the eyes of the bats is pointed, triangular and pocketed. At the centre of the nose, there is a ridge-like flat structure called the ‘sella’. This structure arises from the back of the nostrils and points out from the head perpendicularly. The ears of the horseshoe bat are leaf-shaped and large almost equal in length and breadth and lack tragi but bear conspicuous antitragi. They have very small eyes with the skull having a bony protrusion or rostral inflation on the snout. 

Teeth: The middle-lower premolars and the anterior upper premolars, in a horseshoe bat, are often missing. The bats are born with four erupted permanent canine teeth. These teeth enable the young ones to cling to their mothers. 

Horseshoe Bat Adaptation

• Diet: The horseshoe bats are primarily insectivorous although some species are seen consuming spiders. The horseshoe bats employ an adaptive strategy of flying low and slowly over the ground. They mostly hunt among bushes and trees. The second strategy employed by the horseshoe bats is called perch feeding with individuals roosting on feeding perches as they wait for the prey to fly past them and then the bats fly out to capture the prey. Horseshoe bats are mostly nocturnal hunting at night, however, R. lepidus, also known as Blyth's horseshoe bat is seen foraging on Tioman Island during daytime. This has been hypothesized to be an adaptation as the island lacks day-active bird predators.

• Flying: The horseshoe bats have rounded and especially small wingtips and large wings with respect to body mass combined with high camber. These adaptations provide the bats with increased agility allowing them to make tight, quick turns at low speeds. 

• Echolocation: The horseshoe bats are known to employ some very sophisticated echolocation techniques to help them navigate. Given their very small eyes, and limited field of vision owing to their large nose-leafs, they cannot rely on vision as their primary sense. Bats emit sound through their nostrils and use these sounds to echolocate. Horseshoe bats employ what is called single-frequency or constant-frequency echolocation. These bats have heavy-duty cycles, which implies that the individuals while calling, are producing sounds more than 30 percent of the time. This constant-frequency, high duty echolocation enables the bats to distinguish among prey on the basis of their size. 

• Hearing: The well-developed cochlea of the horseshoe bats attributes to their sophisticated sense of hearing. They are continuously able to receive and produce sounds given their ability to sense Doppler-shifted echoes. There exists an inverse relationship in horseshoe bats between their echolocation frequency and ear length. Species that have a higher frequency of echolocation tend to have ears with shorter lengths. The ears in horseshoe bats are also able to move in a flickering motion, independent of each other during echolocation with the simultaneous movement of the head up and down or sideways. 

Reproduction in Horseshoe Bats

There is not sufficient information available that allows us to understand the mating systems in bats. It appears that the greater horseshoe bat has a polygynous mating system where the male attracts multiple females while attempting to establish and defend their territories. Rhinolophus sedulus, on the other hand, is among the few species of bats believed to be monogamous. Temperate species, in particular, have their annual breeding season in the fall while other species are believed to mate during spring. 

Several species of horseshoe bat exhibit delayed fertilization by means of female sperm storage as is usually seen in the temperate species. In the species that hibernate, the timing of the sperm storage is coincident with hibernation. The Lander's horseshoe bat exhibits embryonic diapause, where fertilization directly occurs following copulation. There is no implantation of the zygote on the uterine wall for a prolonged time period. Delayed embryonic development is shown by the greater horseshoe bat. Here the embryo growth is delayed conditionally upon the female entering torpor. Thus, the gestation period varies from two to three months in this species of horseshoe bat. The usual gestation period is seven weeks. 

A single offspring, called a pup, is usually born. Sexual maturity is attained by the individuals by the time they are of two years of age. The lifespan of horseshoe bats generally does not exceed six-seven years, but some individuals may live longer. 

Behaviour of Horseshoe Bats

Horseshoe bats display sociality on various levels. While certain species are solitary where the individuals roost alone, other species are highly colonial where individuals aggregate in thousands. A majority of the species display moderately social behaviour. Segregation in some species is annual when maternity colonies are formed by the females while in other species the sexes remain together throughout the year. Individuals generally hunt solitarily. Owing to their poorly developed hind limbs, they are unable to climb adeptly or scuttle on flat surfaces. 

Conservation of energy is the main goal behind horseshoe bats entering torpor, during which their body temperature drops low up to 160 C and their metabolic rate is slowed down. Horseshoe bats across tropical, subtropical and temperate regions employ torpor. The duration of torpor is short. On the other hand, when there is consistent employment of torpor for days, weeks or even months, it is called hibernation. This is usually employed by the horseshoe bats native to the temperate regions, during the months of winter. 

Horseshoe Bats Predators and Parasites

A very few natural predators of the horseshoe bats have been observed, which include birds such as kites, eagles, hawks, owls and falcons. Some species are also preyed upon by snakes or hunted by cats while they roost inside the caves. 

The horseshoe bats, however, are known to have a number of internal as well as external parasites. Among the external parasites are mites belonging to the genus Eyndhovenia, ticks belonging to the genus Ixodes, "bat flies'' of the Nycteribiidae and Streblidae families and fleas belonging to the genus Rhinolophopsylla. The internal parasites of the horseshoe bats are cestodes (genus Potorolepsis), trematodes (genera Plagiorchis, Lecithodendrium, Prosthodendrium).

Significance of Horseshoe Bats

Reservoirs of Disease: Given as they are the source of coronaviruses, horseshoe bats are particularly studied in the areas of zoonosis and public health. The outbreak of SARS between  2002 and 2004 lead to the discovery of 45 SARS-related coronaviruses in different species of horseshoe bats. 30 SARS  related coronaviruses were discovered in R. sinicus, 9 from greater horseshoe bats, 2 from big-eared horseshoe bats, 2 from the least horseshoe bat and one each from R. blasii, the Blasius's horseshoe bat and R. affinis, the intermediate horseshoe bat. 

R. pusillus, or the least horseshoe bat was found to be seropositive after the SARS outbreak. On the other hand, the greater horseshoe bat was tested positive for the virus. R. sinicus, the Chinese rufous horseshoe bat, R. pearsoni, the Pearson's horseshoe bat and R. macrotis, the big-eared horseshoe bat were seropositive as well as tested positive for the virus. The viruses found in the bats showed a 88 to 92 percent similarity to the SARS-CoV. The intraspecies diversity displayed by the SARS-like coronaviruses is believed to have originated in Rhinolophus sinicus by the process of homologous recombination. It is likely that the direct ancestor of SARS-CoV was harboured by R. sinicus. Although the horseshoe bats are seen as the natural reservoirs of these viruses, humans are believed to have contracted the virus by means of masked palm civets, which were infected as these have been identified as being the virus’ intermediate hosts. 

More recently, a wet market in China’s Wuhan has been linked to the SARS-CoV-2 outbreak. The SARS-CoV-2 has been found to be highly similar (96% similarity) to the viruses isolated from R. affinis. The research to discover SARS-CoV-2’s evolutionary origin is indicative of bats being the natural reservoirs of the virus. The transmission of the virus to humans is, however, unclear as of yet.  

Apart from the SARS related coronaviruses, horseshoe bats are also associated with other viruses such as flaviviruses, orthoreoviruses, and hantaviruses. These bats have also tested positive for MRV or Mammalian orthoreovirus. Isolation of a  type 1 MRV from the lesser horseshoe bat and a type 2 MRV from the least horseshoe bat confirms the same. These specific MRVs detected in horseshoe bats have not been linked to infections in humans, although humans when exposed to other MRVs can become ill. 

The R. rouxii or rufous horseshoe bat has tested seropositive for a fever that is tick-borne, viral and hemorrhagic also known as the Kyasanur Forest disease, seen in southern India. The bite of infected ticks is known to transfer this disease to humans and is capable of causing a mortality rate between 2 and 10 percent. Longquan virus, which is a kind of hantavirus, has been isolated from the Chinese rufous horseshoe bat, intermediate horseshoe bat, and R. cornutus, the little Japanese horseshoe bat.

As Food 

Megabats are subjected to more intense hunting as opposed to microbats. In sub-Saharan Africa, horseshoe bats, in particular, are hunted for food. The species that are hunted in Africa include: 

• R. alcyone, the halcyon horseshoe bat

• R. guineensis, the Guinean horseshoe bat 

• R. hilli, the Hill's horseshoe bat

• R. hillorum, the Hills' horseshoe bat 

• R. maclaudi, Maclaud's horseshoe bat

• R. silvestris, the forest horseshoe bat

• R. ziama, Ziama horseshoe bat

Ruwenzori Horseshoe Bat

Several horseshoe bats are also consumed in Southeast Asia including the R. marshalli, the Marshall's horseshoe bat in Myanmar and R. rufus, the large rufous horseshoe bat in the Philippines. 

In Medicine

In the Northeastern part of India, the Ao Naga people have been reported to make use of the flesh of horseshoe bats in the treatment of asthma. Bat oil has also been known to be used traditionally in medicines for the removal of ‘earbugs’, which are millipedes that crawl through one’s ears and have been thought to gnaw at the brains. The oil is also used as a claimed treatment for treating partial paralysis and baldness. Anecdotal reports from Senegal report usage of horseshoe bats in potions recommended for treatment of mental illness while a pharmaceutical company in Vietnam reported using 50,000 kg of guano of horseshoe bats each year for medicinal purposes.