Bat - Classification and Evolution

Classification and Evolution

Bats are mammals. They are often mistakenly called "flying rodents" or "flying rats". In many languages, the word for "bat" is cognate with the word for "mouse": for example, chauve-souris ("bald-mouse") in French, murciélago ("blind mouse") in Spanish, летучая мышь ("flying mouse") in Russian, and nahkhiir ("leather mouse") in Estonian, vlermuis (winged mouse) in Afrikaans, from the Dutch word vleermuis. An older English name for bats is flittermice, which matches their name in other Germanic languages (for example German Fledermaus and Swedish fladdermus). However, they are not directly related to rodents, and much less to birds, and do not in fact have any closely related orders (their uniqueness can be demonstrated by the fact their closest living genetic relatives are thought to be carnivorans, certain hoofed animals, such as alpacas and hippopotamuses, and sea mammals, such as dolphins.)

The two traditionally recognized suborders of bats are:

• Megachiroptera (megabats)
• Microchiroptera (microbats/echolocating bats)

Not all megabats are larger than microbats. The major distinctions between the two suborders are:

• Microbats use echolocation: megabats do not with the exception of Rousettus and relatives.
• Microbats lack the claw at the second toe of the forelimb.
• The ears of microbats do not close to form a ring: the edges are separated from each other at the base of the ear.
• Microbats lack underfur: they are either naked or have guard hairs.

Megabats eat fruit, nectar or pollen, while most microbats eat insects; others may feed on the blood of animals, small mammals, fish, frogs, fruit, pollen or nectar. Megabats have well-developed visual cortices and show good visual acuity, while microbats rely on echolocation for navigation and finding prey.

The phylogenetic relationships of the different groups of bats have been the subject of much debate. The traditional subdivision between Megachiroptera and Microchiroptera reflects the view that these groups of bats have evolved independently of each other for a long time, from a common ancestor already capable of flight. This hypothesis recognized differences between microbats and megabats and acknowledged that flight has only evolved once in mammals. Most molecular biological evidence supports the view that bats form a single or monophyletic group.

Researchers have proposed alternate views of chiropteran phylogeny and classification, but more research is needed.

In the 1980s, a hypothesis based on morphological evidence was offered that stated the Megachiroptera evolved flight separately from the Microchiroptera. The so-called flying primates theory proposed when adaptations to flight are removed, the Megachiroptera are allied to primates by anatomical features not shared with Microchiroptera. One example is the brains of megabats show a number of advanced characteristics that link them to primates. Although recent genetic studies support the monophyly of bats, debate continues as to the meaning of available genetic and morphological evidence.

Genetic evidence indicates megabats originated during the early Eocene and should be placed within the four major lines of microbats.

Consequently, two new suborders based on molecular data have been proposed. The new suborder Yinpterochiroptera includes the Pteropodidae or megabat family, as well as the Rhinolophidae, Hipposideridae, Craseonycteridae, Megadermatidae, and Rhinopomatidae families The new suborder Yangochiroptera includes all the remaining families of bats (all of which use laryngeal echolocation). These two new suborders are strongly supported by statistical tests. Teeling (2005) found 100% bootstrap support in all maximum likelihood analyses for the division of Chiroptera into these two modified suborders. This conclusion is further supported by a 15-base-pair deletion in BRCA1 and a seven-base-pair deletion in PLCB4 present in all Yangochiroptera and absent in all Yinpterochiroptera. The chiropteran phylogeny based on molecular evidence is controversial because microbat paraphyly implies one of two seemingly unlikely hypotheses occurred. The first suggests laryngeal echolocation evolved twice in Chiroptera, once in Yangochiroptera and once in the rhinolophoids. The second proposes laryngeal echolocation had a single origin in Chiroptera, was subsequently lost in the family Pteropodidae (all megabats), and later evolved as a system of tongue-clicking in the genus Rousettus.

Analyses of the sequence of the "vocalization" gene, FoxP2 was inconclusive as to whether laryngeal echolocation was secondarily lost in the pteropodids or independently gained in the echolocating lineages. However, analyses of the "hearing" gene, Prestin seemed to favor the independent gain in echolocating species rather than a secondary loss in the pteropodids.

In addition to Yinpterochiroptera and Yangochiroptera, the names Pteropodiformes and Vespertilioniformes have also been proposed for these suborders. Under this new proposed nomenclature, the suborder Pteropodiformes includes all extant bat families more closely related to the genus Pteropus than the genus Vespertilio, while the suborder Vespertilioniformes includes all extant bat families more closely related to the genus Vespertilio than to the genus Pteropus.

Little fossil evidence is available to help map the evolution of bats, since their small, delicate skeletons do not fossilize very well. However, a Late Cretaceous tooth from South America resembles that of an early microchiropteran bat. Most of the oldest known, definitely identified bat fossils were already very similar to modern microbats. These fossils, Icaronycteris, Archaeonycteris, Palaeochiropteryx and Hassianycteris, are from the early Eocene period, 52.5 million years ago. Archaeopteropus, formerly classified as the earliest known megachiropteran, is now classified as a microchiropteran.

Bats were formerly grouped in the superorder Archonta along with the treeshrews (Scandentia), colugos (Dermoptera), and the primates, because of the apparent similarities between Megachiroptera and such mammals. Genetic studies have now placed bats in the superorder Laurasiatheria along with carnivorans, pangolins, odd-toed ungulates, even-toed ungulates, and cetaceans.

The traditional classification of bats is:

• Order Chiroptera
  • Suborder Megachiroptera (megabats)
    • Pteropodidae
  • Suborder Microchiroptera (microbats)
    • Superfamily Emballonuroidea
      • Emballonuridae (Sac-winged or sheath-tailed bats)
    • Superfamily Molossoidea
      • Molossidae (Free-tailed bats)
    • Superfamily Nataloidea
      • Furipteridae (Smoky bats)
      • Myzopodidae (Sucker-footed bats)
      • Natalidae (Funnel-eared bats)
      • Thyropteridae (Disk-winged bats)
    • Superfamily Noctilionoidea
      • Mormoopidae (Ghost-faced or moustached bats)
      • Mystacinidae (New Zealand short-tailed bats)
      • Noctilionidae (Bulldog bats or fisherman bats)
      • Phyllostomidae (Leaf-nosed bats)
    • Superfamily Rhinolophoidea
      • Megadermatidae (False vampires)
      • Nycteridae (Hollow-faced or slit-faced bats)
      • Rhinolophidae (Horseshoe bats)
      • Hipposideridae (Old World leaf-nosed bats)
    • Superfamily Rhinopomatoidea
      • Craseonycteridae (Bumblebee bat or Kitti's hog-nosed bat)
      • Rhinopomatidae (Mouse-tailed bats)
    • Superfamily Vespertilionoidea
      • Vespertilionidae (Vesper bats or evening bats)
      • Antrozoidae (Pallid Bat and Van Gelder's bat)

Megabats primarily eat fruit or nectar. In New Guinea, they are likely to have evolved for some time in the absence of microbats. This has resulted in some smaller megabats of the genus Nyctimene becoming (partly) insectivorous to fill the vacant microbat ecological niche. Furthermore, there is some evidence that the fruit bat genus Pteralopex from the Solomon Islands, and its close relative Mirimiri from Fiji, have evolved to fill some niches that were open because there are no nonvolant or non-flying mammals in those islands.