Features -  October 27, 2008
Taking Wing: Uncovering the Evolutionary Origins of Bats
At last, fossil and genetic findings elucidate the evolution of bats--and 
settle a long-standing debate over the origins of flight and echolocation

By Nancy B. Simmons

Editor's Note: This story will be published in the December 2008 issue of 
Scientific American.

Survey the sky at twilight on a summer’s eve, and you just might glimpse one of 
evolution’s most spectacular success stories: bats. With representatives on 
every continent except Antarctica, they are extraordinarily diverse, accounting 
for one in every five species of mammal alive today. The key to bats’ rise to 
prominence is, of course, their ability to fly, which permits them to exploit 
resources that other mammals cannot reach. But their ascension was hardly a 
foregone conclusion: no other mammal has conquered the air. Indeed, exactly how 
these rulers of the night sky arose from terrestrial ancestors is a question 
that has captivated biologists for decades.

Answers have been slow in coming. This past February, however, my colleagues 
and I unveiled two fossils of a previously unknown species of bat that provides 
vital insights into this mysterious transformation. Hailing from Wyoming, the 
species—dubbed Onychonycteris finneyi—is the most primitive bat ever 
discovered. These fossils and others, together with the results of recent 
genetic analyses, have now led to a new understanding of the origin and 
evolution of bats.

Winged Wonder
To appreciate just how distinctive bats are, consider one of their trademark 
traits: wings. A few mammals, such as flying squirrels, can glide from tree to 
tree, thanks to a flap of skin that connects their front and hind limbs. And in 
fact, experts generally agree that bats probably evolved from an arboreal, 
gliding ancestor. But among mammals, bats alone are capable of powered flight, 
which is a much more complex affair than gliding. They owe this ability to the 
construction of their wings.

The bones of a bat’s wing consist of greatly elongated forearm and finger bones 
that support and spread the thin, elastic wing membranes. The membranes extend 
backward to encompass hind limbs that are quite a bit smaller than those of a 
terrestrial mammal of comparable body size. Many bats also have a tail membrane 
between their hind legs. A unique bone called the calcar projects from the 
bat’s heel to support the trailing edge of this membrane. By moving their 
fingers, arms, legs and calcars, bats can maneuver their wings in innumerable 
ways, making them superb fliers.

Most bats can also echolocate. By producing high-pitched sounds and then 
analyzing the returning echoes, these nocturnal animals can detect obstacles 
and prey much better than by using vision alone. (Contrary to the expression 
“blind as a bat,” all bats can see.) More than 85 percent of living bat species 
use echolocation to navigate. The rest belong to a single family—the Old World 
fruit bats, sometimes called flying foxes, which apparently lost the ability 
and instead rely strictly on sight and smell to find the fruit and flowers they 
feed on.

Echolocating bats have a distinctive suite of anatomical, neurological and 
behavioral characteristics that enable them to send and receive high-frequency 
sounds. Three bones in the skull have undergone modification. The first is the 
stylohyal, a long, slender bone that connects the base of the skull with an 
array of small bones—collectively termed the hyoid apparatus—that support the 
throat muscles and voice box. In most echolocating bats, the upper tip of the 
stylohyal is expanded into a kind of paddle that helps to anchor the hyoid 
apparatus to the skull.

The other two bones that bear the signature of echolocation occur in the ear. 
All mammals perceive sound by way of a chain of bones, known as ear ossicles, 
that transmit sound between the eardrum and the fluid-filled inner ear. The 
malleus is the first bone in this chain, and in echolocating bats it has a 
large, bulbous projection that helps to control its vibration. Once sounds pass 
through the ear ossicles, they travel to the inner ear, where they impinge on a 
coiled, fluid-filled structure known as the cochlea (Latin for “snail”) that 
contains special nerve cells responsible for sound perception. When compared 
with other mammals, echolocating bats have a cochlea that is enlarged relative 
to other skull structures, which makes them better able to detect 
high-frequency sounds and to discriminate among different frequencies of these 
sounds.

Which Came First?
The revelation more than 60 years ago that most of the world’s bats can “see 
with sound” made clear that echolocation contributed significantly to the great 
evolutionary success and diversity of bats. But which of the two key bat 
adaptations—flight and echolocation—came first, and how and why did they 
evolve? By the 1990s three competing theories had emerged.

The flight-first hypothesis holds that bat ancestors evolved powered flight as 
a way of improving mobility and reducing the amount of time and energy required 
for foraging. Under this scenario, echolocation evolved subsequently to make it 
easier for early bats to detect and track prey that they were already chasing 
in flight.

In contrast, the echolocation-first model proposes that gliding protobats 
hunted aerial prey from their perches in the trees using echolocation, which 
evolved to help them track their quarry at greater distances. Powered flight 
evolved later, to increase maneuverability and to simplify returning to the 
hunting perch.

The tandem-development hypothesis, for its part, suggests that flight and 
echolocation evolved simultaneously. This idea is based on experimental 
evidence showing that it is energetically very costly for bats to produce 
echolocation calls when they are stationary. During flight, however, the cost 
becomes nearly negligible, because contraction of the flight muscles helps to 
pump the lungs, producing the airflow that is required for intense, 
high-frequency vocalizations.

The only way to test these hypotheses about the origins of flight and 
echolocation is by mapping the distribution of relevant traits—wings and 
enlarged cochleas, for example—onto a family tree of bats to determine the 
point at which they evolved. Back in the 1990s, we simply did not have any 
fossils of bats that had some of these signature characteristics but not others.

Bat fossils are extremely rare. Ancient bats, like their modern counterparts, 
were small and fragile, and they tended to live in tropical habitats, where 
decay occurs very rapidly. Just about the only way a bat can become fossilized 
is if it dies in a place where it is swiftly covered with sediment that 
protects it from scavengers and microorganisms alike.

Until recently, the oldest and most primitive bat on record was the 
52.5-million-year-old Icaronycteris index, named for the boy of Greek legend 
who flew too close to the sun. Icaronycteris was discovered in the 1960s in 
lake deposits in Wyoming’s famed Green River Formation, whose fine-grained 
mudstone and limestone rocks have yielded beautifully preserved fish, plants, 
mammals, insects, crocodiles and birds.

For the next four decades Icaronycteris formed the basis for understanding the 
earliest stage of bat evolution. Ironically, however, perhaps the most 
remarkable thing about Icaronycteris is just how much this ancient beast 
resembles extant bats. The shape of its teeth indicate that it ate insects, as 
do most bats today. Its limb proportions are similarly modern, with long, 
slender fingers, elongated forearms and diminutive hind legs. The creature’s 
scapulas (shoulder blades), sternum (breastbone) and rib cage also attest to a 
fully developed ability to fly. And it possessed the requisite anatomy for 
echolocation.

In fact, if it were alive today, Icaronycteris would be hard to tell apart from 
other bats. Its most distinctive feature is a tiny claw on the index finger 
(hence the species name index). Most bats retain a claw only on the thumb. Over 
time, the tips of the other four fingers were reduced to thin, flexible rods or 
nubs completely enclosed in the wing membrane. Icaronycteris’s index claw seems 
to be a holdover from a terrestrial ancestor.

Filling the Gap
In retrospect, Icaronycteris was never much of a “missing link.” But another 
fossil bat from the Green River Formation would turn out to fit that 
description nicely. Enter Onychonycteris. The two known specimens, unearthed by 
private collectors in the past decade and later made available for scientific 
study, were discovered in the same rock layer that yielded Icaronycteris and 
are thus considered to be of comparable antiquity. Onychonycteris, however, has 
a combination of archaic and modern traits that make it exactly the sort of 
transitional creature evolutionary biologists have longed for.

I was fortunate enough to lead the team that described and named O. finneyi. We 
chose the genus name Onychonycteris (“clawed bat”) because the fossil displays 
claws on all five fingers, just as its terrestrial predecessors did. The 
presence of these claws is not the only feature of Onychonycteris that recalls 
nonflying mammals. Most bats have very long forearms and tiny hind limbs. 
Onychonycteris, however, has proportionately shorter forearms and 
proportionately longer hind limbs than those of other bats. Compared with other 
mammals, the limb proportions of Onychonycteris are intermediate between those 
of all previously known bats (including Icaronycteris) and those of arboreal 
mammals that rely heavily on their forelimbs for locomotion, such as sloths and 
gibbons. These animals hang from branches much of the time as they climb around 
in the trees. Perhaps bats evolved from arboreal ancestors that used a similar 
form of locomotion.

Despite these primitive limb features, other aspects of the anatomy of 
Onychonycteris indicate that it was capable of powered flight. Its long fingers 
would have supported wing membranes, and robust clavicles (collarbones) would 
have helped anchor the forelimbs to the body. Meanwhile a wide rib cage and a 
keeled sternum would have supported large flight muscles, and a faceted scapula 
would have secured other specialized, flight-related muscles.

Additional clues to how Onychonycteris traveled come from the proportions of 
its arm and finger bones, which reveal that the animal’s wings had a very low 
aspect ratio and relatively small tips. Among living bats, only mouse-tailed 
bats possess similarly short and broad wings. These animals have an unusual 
gliding-fluttering flight style involving brief glides between periods of 
flapping flight. Our best guess is that Onychonycteris flew the same way. It 
may be that gliding-fluttering flight was the transitional mode of locomotion 
between the gliding of prebat ancestors and the continuous flapping flight seen 
in most modern bats.

Beyond illuminating how early bats flew, Onychonycteris has brought some 
much-sought-after evidence to bear on the debate over when flight and 
echolocation emerged. Unlike the other known bats that date back to the Eocene, 
the epoch spanning the time from 55.8 million to 33.5 million years ago, 
Onychonycteris seems to have lacked all three of the bony correlates of 
echolocation. It has a small cochlea and a relatively small protrusion on the 
malleus, and its stylohyal lacks an expanded tip. Yet features of its limbs and 
thorax clearly indicate that it could fly. Onychonycteris therefore seems to 
represent a stage in early bat evolution after flight had been achieved but 
before echolocation evolved. Fossils have finally given us an answer: flight 
first, echolocation later.

Ancient Diversity
The emergence of flight and echolocation set the stage for a dazzling adaptive 
radiation of bats. Such rapid periods of diversification are known to occur 
after a breakthrough adaptation. Living bats are classified into 19 families; 
fossil bats comprise an additional seven families. Remarkably, time-calibrated 
studies of the DNA sequences of multiple genes indicate that all 26 of these 
groups were already distinct by the end of the Eocene. This “big bang” of 
diversification is unprecedented in mammalian history.

Flight and echolocation certainly were not the only factors contributing to 
this radiation, however. The origin of these major bat lineages apparently 
coincided with a rise in mean annual temperature, a significant increase in 
plant diversity and a peak in insect diversity. From fast-flying beetles to 
caddis flies, cockroaches and tiny, fluttering moths, an aerial predator would 
have had a veritable buffet of insects from which to choose. And as the only 
nocturnal flying predators other than owls and nightjars, bats would have had 
few competitors for the rich resources of the Eocene night.

Fossils from a site called Messel in Germany provide a glimpse of this early 
diversification. Although at 47 million years old these fossils are only 
slightly younger than the bats from the Green River Formation, they are 
considerably more variable. Seven bat species have been found at Messel since 
scientific excavations began there in the 1970s, including two species of 
Archaeonycteris, two species of Palaeochiropteryx, two species of 
Hassianycteris, and Tachypteron franzeni, the oldest known member of a family 
of bats known as the Emballonuridae (the sheath-tailed bats) that is still 
alive today.

It is not hard to see why bats thrived at Messel. During the Eocene, it would 
have had a balmy climate, and it was home to several lakes surrounded by lush, 
subtropical forest. Judging from the abundance of their fossilized remains, 
thousands of aerial, aquatic and terrestrial insects were available for the 
taking. That, in fact, is just what the Messel bats did. All seven species were 
insectivorous, but each one specialized in a particular subset of insects, as 
evidenced by their preserved stomach contents. Whereas Palaeochiropteryx seems 
to have fed on small moths and caddis flies, Hassianycteris apparently favored 
larger moths and beetles. Archaeonycteris, on the other hand, may have only 
eaten beetles. As for Tachypteron, no stomach contents are preserved. Yet we 
know it was insectivorous based on the shape of its teeth.

What did Onychonycteris and Icaronycteris subsist on? We lack the stomach 
contents to answer this question in detail. Insects are a good bet, though, 
based on the form of the bats’ teeth and the wealth of insect fossils in the 
rocks of the Green River Formation. Most bats today subsist on insects, too. 
Only later in the evolutionary history of the group would some bats begin 
eating meat, fish, fruit, nectar, pollen and even blood.

Bat Relationships
The fossils recovered thus far from Messel and the Green River Formation have 
proved critical in helping researchers chart the rise of bats. Still, we lack 
fossils that establish how bats are related to other mammals. Tree-dwelling, 
gliding placental mammals known as colugos are sufficiently similar to bats 
that they were long thought to be close relatives. Over the past 14 years, 
however, Mark S. Springer of the University of California, Riverside, and 
others have conducted studies of DNA from large numbers of mammalian species 
that have shown that bats are not close relatives of any of the groups of 
placental mammals that include gliders such as colugos and flying squirrels. 
(Such creatures nonetheless offer compelling models for what the limb structure 
of bat ancestors might have looked like.)

Rather these genetic analyses place bats firmly in an ancient lineage known as 
Laurasiatheria. Other modern members of this group include such diverse beasts 
as carnivores, hoofed mammals, whales, scaly anteaters, shrews, hedgehogs and 
moles. Primitive laurasiatheres, however, were probably mouse- or squirrel-size 
creatures that walked on all fours and ate insects. Laurasiatheres are thought 
to have evolved on the ancient supercontinent of Laurasia, which comprised what 
is now North America, Europe and Asia, probably in the late Cretaceous period, 
some 65 million to 70 million years ago. The exact position of bats within this 
group is uncertain, but clearly a considerable amount of evolutionary change 
separates Onychonycteris and other bats from their terrestrial forebears.

Some of this change from land dweller to flier may have occurred surprisingly 
quickly, if recent discoveries in the field of developmental genetics are any 
indication. Though short by bat standards, the fingers of Onychonycteris are 
greatly elongated as compared with those of other mammals. How could this 
elongation have evolved?

In 2006 Karen E. Sears, now at the University of Illinois, and her colleagues 
reported in the Proceedings of the National Academy of Sciences USA that the 
key may lie in the activity of genes that control the growth and elongation of 
digits in the hand during development. Their comparisons of growth patterns in 
bat and mouse embryos revealed significant differences in the proliferation and 
speed of maturation of cartilage cells in developing fingers. A class of 
proteins called bone morphogenetic proteins (BMPs) plays a pivotal role in 
controlling these processes and in determining the length to which fingers 
grow. As it turns out, one of these proteins, BMP2, is produced at 
significantly higher levels in bat fingers than in those of mice, and 
manipulation of the gene that makes this protein can alter digit length. It is 
therefore possible that a small change in the genes regulating BMPs underlies 
both the developmental and evolutionary elongation of bat
 wing digits. If so, that might explain the absence in the fossil record of 
creatures intermediate between short-fingered, nonflying mammals and 
long-fingered bats such as Onychonycteris and Icaronycteris: the evolutionary 
shift may have been very rapid, and few or no transitional forms may have 
existed.

Despite many new discoveries about the rise of bats, mysteries remain. Bat 
ancestors must have existed prior to the Eocene, but we have no fossil record 
of them. Likewise, the identity of the closest relatives of bats is still 
unknown. Investigators are also eager to learn when the bat lineage first 
became distinct from that of the other laurasiatheres and how much of early bat 
evolution and diversification took place in the northern continents versus the 
southern continents. We therefore need fossils that lie even closer to the 
beginning of bats than Onychonycteris does. With luck, paleontologists will 
find such specimens, and they will help solve these and other riddles about the 
origins of these fascinating animals.
Further Reading

    * Is Focusing on "Hot Spots" the Key to Preserving Biodiversity?
    * Why Dogs Don't Enjoy Music
    * Science of Snacks: Thinking Makes You Hungry
    * Un-Netting Trade in Endangered Species: eBay Vows Crackdown on Illegal 
Ivory Sales

 

    * "Voluntourism": See the World--And Help Conserve It
    * Medical Mystery: Only One Person Has Survived Rabies without Vaccine--But 
How?
    * Deadly by the Dozen: 12 Diseases Climate Change May Worsen
    * Bar Code of Life: DNA Tags Help Classify Animals



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