Topic: Bats: Insights into convergence
Bats show a fascinating array of convergences, from echolocation to flight to nectar feeding. Vampire bats can even detect infrared radiation, while others might be able to see into the ultraviolet end of the spectrum.
With more than 1,000 species, bats are the second largest order of mammals. One key factor in their success is their ability to fly, which allowed them to colonise numerous habitats and achieve an almost global distribution. Although the exact nature of their transition to an aerial existence is not yet known, the subsequent fossil record is richly informative. By the Eocene they were adept fliers and also employed echolocation. Equipped with this exceptional sensory capacity, bats could navigate in the dark and radiated into the virtually empty niche of nocturnal insect hunter. Echolocation represents a striking example of evolutionary convergence, not only within the bats, but also more widely with corresponding equivalents in several groups, notably the cetaceans and birds. In addition, bats are highly instructive with respect to many other cases of convergent evolution, from nectar feeding to infrared detection.
Bats show a number of fascinating convergences that are linked with echolocation. This sensory modality has arisen early in their evolutionary history, and they have developed two very different echolocation mechanisms. Fruit bats in the genus Rousettus click their tongue (reminiscent of some echolocating birds), whereas insectivorous species generate ultrasonic calls in their larynx. There is evidence suggesting at least two independent origins of laryngeal echolocation in the insectivorous bats (including the sequence of Prestin, a gene potentially involved in echolocation). Alternatively, it could have been present in the common ancestor of bats and secondarily been lost in the fruit bat lineage, but then re-evolved in different form in Rousettus.
Several acoustic features of the echolocation calls are also convergent. As call design is largely shaped by the environment, distantly related species with similar ecologies often employ very similar signal types. A particularly fascinating example is the so-called high-duty cycle echolocation, a highly sophisticated form of echolocation that has arisen independently in Old World horseshoe bats (Rhinolophidae) and Parnell’s moustached bat (Pteronotus parnellii) of the New World. These bats have converged upon a very similar auditory physiology, including an acoustic fovea that increases sensitivity of the cochlea to a very narrow frequency band. Calls can be emitted through the mouth and through the nose, and nasal emission has evolved from the more ancestral method of oral emission several times (perhaps in relation to large prey, as it allows for simultaneous chewing and calling). Analogous to electric fish, many bats have evolved a jamming avoidance response (JAR), where they modify the acoustic properties of their calls to avoid interference with the signals of conspecifics in close proximity.
Powered flight seems to have arisen at least four times in animals (in birds, bats, insects and the extinct pterosaurs), but probably only once within the bats. Compared with birds, there are profound differences in the anatomical structure of the wing. While the entire forelimb of birds has been modified into a feathered wing, with the bones of the digits fused together to form the carpometacarpus, bat wings consist of a skin membrane (called patagium) that covers the very long spread out digits. Interestingly, detailed studies of the airflow over the bat wing have revealed convergences with the sphinx moths.
Also hovering, a form of flight that requires huge amounts of energy, has evolved repeatedly. Among vertebrates, the masters are the hummingbirds, but their abilities are rivalled by those of insects, particularly (and obviously) hoverflies. Some nectar-feeding bats can hover as well, but only for a short while and close to sea level. The flight performance of bats is often considered inferior to that of birds and although bats cannot travel such long distances and lose out with respect to efficiency and stamina, they outperform birds in terms of manoeuvrability. As the patagium contains muscle and elastic fibres, bats can adapt the shape of their wings using their fingers, which allows them to perform highly acrobatic movements and catch insects in flight.
Recurrent ecomorphs are a regular feature of evolutionary convergence, and such occur amongst the bats. Molecular phylogenetic analyses have revealed repeated morphological convergences in the evolution of the speciose genus Myotis. Several groups of species with similar feeding strategies have evolved independently, and they share a remarkably similar morphology. Therefore, these ecomorphs had been assumed to be closely related and so grouped together until the molecular data showed the similarities were in fact convergent.
A particularly striking instance involves the fish-eating bat (Myotis vivesi), which is endemic to Mexico. It is one of only two truly piscivorous bat species, the other being Noctilio leporinus, a Neotropical bat, which has evolved piscivory relatively recently from an insectivorous ancestor. As a consequence of their unusual feeding strategy, these two species share a number of morphological characteristics, such as long hind legs and strong feet with enlarged, laterally compressed claws for gripping fish prey. But some other Myotis species in different parts of the world, such as the Eurasian Daubenton’s bat (M. daubentoni) and the Chinese Rickett’s big-footed bat (M. ricketti), also have similar hind legs for gaffing prey from the water surface, including (at least occasionally) fish. However, neither of them is closely related to M. vivesi, so these adaptations represent an example of ecomorphological convergence in multiple geographic regions.
Bats as a group are characterised by an extremely wide range of diets. Most species are insectivores, but a number of carnivorous species feed on animals such as frogs or fish, and the notorious vampire bats suck the blood of other vertebrates. In contrast, the Old World fruit bats eat fruit, pollen or nectar and play important ecological roles as plant pollinators and seed dispersers.
Nectar feeding has most likely evolved independently several times in fruit bats, accompanied by a number of anatomical changes. Nectivorous bats are characterised by an elongated, protrusible tongue with a brushy tip, reduced dentition and several other osteological skull features. Traditionally, Asian nectivores were united in the subfamily Macroglossinae, but molecular analyses have indicated polyphyly of this group, thus supporting convergent evolution of specialisations for nectivory. Also in birds has the use of tongue morphology as a character in phylogenetic reconstruction led to mistakes in classification, for instance when the distantly related sunbirds, honeyeaters, flowerpeckers and white-eyes were once included in the same higher-order group
Vision and opsins
A common misconception is that bats, all of which are active in low-light conditions, are virtually blind, but vision is in fact highly variable in this group. Although the eyes of the nocturnal, echolocating, insectivorous bats are generally degraded (albeit to different degrees in different species), there is evidence suggesting that dim-light vision is still of importance, particularly for seeing over longer distances. The crepuscular, non-echolocating fruit bats have larger, better-developed eyes and rely on vision to a greater extent.
Low-light vision is mediated by the extremely sensitive visual pigment rhodopsin, a protein found in rod cells. An analysis of the rhodopsin gene RH1 in a number of echolocating and non-echolocating bats has provided evidence for convergent evolution, with multiple parallel amino acid changes in the two groups. These changes could be related to ecological specialisations for different photic environments as bats became nocturnal. For example, fruit bats share an amino acid change with the black-bearded tomb bat Taphozous melanopogon that lives in a less dark environment compared to other echolocating bats and has relatively normal eyes. However, it remains to be shown that these substitutions actually alter the spectral sensitivity of rhodopsin, and at least some could alternatively reflect constraints upon the amino acid sequence.
Several types of cone opsin permit colour vision, and most mammals are dichromatic with long- to middlewave-sensitive L-cones and shortwave-sensitive S-cones. Some horseshoe bats as well as fruit bats in the genera Rousettus, Eidolon and Epomophorus, however, have lost their functional S-cones, rendering these species monochromatic and thus basically colour-blind. Monochromacy involving the loss of S-opsin has independently evolved in other groups of mammals, particularly the marine cetaceans and pinnipeds. There is evidence suggesting that the S-opsin in at least some bat species may be sensitive to UV light, which is relatively more abundant in low-light conditions. This would render bats the third mammalian group with UV vision, besides rodents and marsupials. Intriguingly, one species of fruit bat (Fischer’s pygmy fruit bat Haplonycteris fischeri) was shown to have a duplication of the L-opsin gene, the function of which remains unclear. This is the first case of opsin duplication outside the primates.
As their name suggests, the American vampire bats (Desmodontinae) are famous for their sanguinary habits - they cut incisions in vertebrate prey and lick up the blood. Interestingly, they seem to be able to detect infrared radiation, which makes sense, as they feed on ‘warm-blooded’ mammals and birds. This capacity has evolved independently in snakes and several groups of insects, notably some beetles and hemipterans (bed bugs). Vampire bats evidently perceive the thermal stimuli via three pits located on the nose. These exposed areas of skin are thermally insulated from the surrounding warm tissue of the head, which makes it easier to detect small changes in temperature. The arrangement of three pits could aid in providing directionality when locating a heat source. Additional evidence for an infrared capacity in vampire bats comes from the brain structure, with a specific nucleus that appears to have important similarities in terms of histology and location to the equivalent infrared nucleus found in snakes.
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Map of Life - "Bats: Insights into convergence"
March 3, 2021