In this week's issue of Nature is an article about the analysis of a
52.5 million-year old bat fossil. The authors conclude that the bat
was able to fly but unable to echolocate, thus suggesting that bats
evolved flight first. Reproduced below is the News and Views article
describing the importance and implications of this work.
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Nature 451, 774-775 (14 February 2008) | doi:10.1038/451774a;
Published online 13 February 2008
Evolutionary biology: A first for bats
by John Speakman
Abstract: Which came first as bats evolved — flight or echolocation?
Newly described fossils favour the flight-first hypothesis. But these
creatures may have been otherwise equipped for flying at night.
A long-standing debate about the processes that led to the evolution
of modern bats takes a new twist with the discovery of remarkable
fossil bats recovered from the Green River formation in Wyoming. The
fossils, which constitute a new genus and species, are described by
Simmons et al. on page 818 of this issue(1). Phylogenetic analysis
and comparison with other fossil bats recovered from the same
formation, and from the Messel formation in Germany, indicate that
this is the most ancient species of bat yet discovered.
The problem of understanding bat evolution dates back at least to
Charles Darwin, who in The Origin of Species enumerated a list of
difficulties he saw with the theory of evolution by natural
selection. The example often discussed is the origin of the eye. But
Darwin also mentioned the vexed issue of how bats had arisen from
terrestrial ancestors. The discovery of echolocation in bats about 50
years ago(2) added an additional feature to the conundrum of the
early evolution of bats. This currently boils down to one big
question: which came first, echolocation or flight(3,4)?
For a long time, 'echolocation first' held sway. Ancestral 'pre-bats'
were hypothesized to have been small terrestrial or arboreal
echolocators that detected passing insects using their echolocation
and snatched them from the air4. This favoured the extension of the
arms and digits to facilitate prey capture, perhaps with webbing
between the digits. Eventually, these animals started leaping out to
capture insects, using their echolocation to guide them to a landing
spot and their extended arms and digits as an aerofoil. From this
point they started hunting from perches (known as perch hunting) and
eventually developed fully powered flight (called aerial hawking;
Fig. 1).
Supporters of the echolocation-first hypothesis pointed to the
existence of terrestrial animals, such as certain shrews, that have
rudimentary echolocation systems; and to the fact that the most
primitive extant bats often use perch hunting, and lack a feature
known as the calcar, which is also absent in the most ancient fossil
bats. (The calcar is a cartilaginous spur projecting from the base of
the lower limb and running along the edge of the membrane between the
hind limbs and tail.) Moreover, the idea that bats might have evolved
the ability to fly before they could orient themselves in darkness
was seen as highly unlikely.
However, around the end of the 1980s, evidence accumulated, including
work from my own group, that favoured the 'flight-first' hypothesis.
One paper(5) showed that, for a bat hanging at rest, echolocation is
extremely energetically costly. This high cost probably explains why
no terrestrial mammals have evolved full-blown echolocation systems
such as those used by bats. However, a second paper(6) showed that
when a bat takes flight these costs disappear. This is because of a
remarkable coupling of the beating of the wings with the ventilation
of the lungs and production of the echolocation pulses(7). When a bat
hangs stationary and echolocates, it must contract its muscles
specifically to generate a forceful expiratory burst, and this is
where the large costs come from. When a bat is flying, it is already
contracting these muscles, so in effect echolocation when flying is
free (or at least substantially cheaper).
But what about the problem of bats flying in darkness before they
could orient themselves? A hypothesis I favour(8) is that the
earliest ancestors of bats may have been diurnal, and had visual
means of orientation — but were perhaps forced to become nocturnal by
the appearance of avian predators, shortly after the dinosaurs became
extinct around 65 million years ago. Some then evolved echolocation,
whereas others became nocturnal vision specialists.
Until the discovery of the specimens reported by Simmons et al.(1),
the fossil record has been rather unhelpful in resolving these
issues: the earliest-known bats, which have been recovered from
Eocene deposits around 50 million years old, are fully formed bats
very similar to extant ones(9, 10). It has been possible to show that
these bats were all already capable of echolocation by examining the
size of the cochleae in their ears; cochleae are massively enlarged
in echolocators. Previously described fossil bats were all already
capable of both flight and echolocation(11, 12).
The bat described by Simmons et al.1 is represented by two fossils
dating to about 52.5 million years ago; one is shown on the cover of
this issue. The bat's wing morphology is very similar to that of
extant species, except that it has claws on its digits. But in all
other respects this is clearly a bat capable of powered flight.
Unlike other primitive fossil bats, this species also has a calcar —
indicating that the absence of a calcar and perhaps therefore perch
hunting are not ancestral traits. But the real insight provided by
this fossil is the spectacular finding that it does not have enlarged
cochleae. By inference, therefore, it was not capable of
echolocation, providing the first direct evidence supporting the
flight-first hypothesis. Examination of the bat's limb proportions
suggests that it was probably arboreal, as has generally been assumed
by proponents of both echolocation-first and flight-first hypotheses.
A remaining question is whether this bat was nocturnal or diurnal.
Examination of the size of the eye sockets might help, as nocturnal
non-echolocating animals generally have enlarged eyes and so enlarged
eye sockets. Unfortunately, the two specimens described by Simmons et
al. cannot answer this question, because their upper skulls are
crushed and their eye sockets cannot be reconstructed. Perhaps that
was too much to hope for. As it is, these outstanding fossils
considerably advance our understanding of bat evolution.
References
1. Simmons, N. B., Seymour, K. L., Habersetzer, J. & Gunnell, G.
F. Nature 451, 818–821 (2008).
2. Griffin, D. R. Listening in the Dark: The Acoustic Orientation
of Bats and Men (Yale Univ. Press, New Haven, CT, 1958).
3. Teeling, E. C. et al. Nature 403, 188–192 (2000).
4. Jones, G. & Teeling, E. C. Trends Ecol. Evol. 21, 149–156 (2006).
5. Speakman, J. R., Anderson, M. E. & Racey, P. A. J. Comp.
Physiol. A 165, 679–685 (1989).
6. Speakman, J. R. & Racey, P. A. Nature 350, 421–423 (1991).
7. Suthers, R. A., Thomas, S. P. & Suthers, B. J. J. Exp. Biol.
56, 37–48 (1972).
8. Speakman, J. R. Mamm. Rev. 31, 111–130 (2001).
9. Jepsen, G. L. Science 154, 1333–1339 (1966).
10. Gunnell, G. F. & Simmons, N. B. J. Mamm. Evol. 12, 209–246
(2005).
11. Habersetzer, J. & Storch, G. Naturwissenschaften 79, 462–466
(1992).
12. Novacek, M. J. Nature 315, 140–141 (1985).
John Speakman is at the Institute of Biological and Environmental
Sciences, University of Aberdeen, Aberdeen AB39 2PN, UK.
* * * * * * * * * * * * * * * * * * * * * * * * * * * *
Diana R. Tomchick
Associate Professor
University of Texas Southwestern Medical Center
Department of Biochemistry
5323 Harry Hines Blvd.
Rm. ND10.214B
Dallas, TX 75390-8816, U.S.A.
Email: diana.tomch...@utsouthwestern.edu
214-645-6383 (phone)
214-645-6353 (fax)
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