Flight first in bats: when
and where?
Arising from Simmons N.B. et al., Nature, 451,818-821 (2008) and Speakman J., Nature 451,774-775 (2008).
Ivan Horáèek
Department of Zoology, Faculty of Science, Charles
University, 128 44 Praha, Czech Republic, horacek@natur.cuni.cz
The earliest bat, Onychonycteris finneyi, recently
described from the late early Eocene of Wyoming 1 demonstrated that
a powered flight evolved prior to echolocation in bats 2 . Here I argue
that the earliest bat shows at the same time quite derived states in some
characters and that the major mysteries of bat evolution remain unresolved: we
have no idea where, when, and from which clade of non-volant mammals did bats
arise and what promoted the transition. New record of Eocene bats from India
suggests possible southern origin prior to early Eocene. A shift of insect
flight activity to night and crepuscular hours, possibly caused by passeriform
birds, might presented an essential factor promoting night aerial insectivory
and early evolution of bats.
The outstanding plesiomorphies of Onychonycteris in cochlea and wing (short radius, ossified terminal
phalanges with small claws), are accompanied with derived characters common for
early and modern bats (shoulder girdle, keel on sternum), and even the unexpectedly
advanced characters (dentition). Onychonycteris
exceeds all other Eocene bats in degree of reduction of premolar row (P2,3)
and M3, enlargement of upper canines and caninization of I2/. In extant bats, such dental characters
suggest specialisation for a larger prey and are often accompanied with
enlargement of body size, that is
further combined with greater wing area and greater wing loading but relatively
short wings 3,4. Onychonycteris exhibits exactly all
these tendencies. It is one of the largest Eocene bats and its wings are
extremelly short (due to short arm and small wing tip area) but surprisingly
broad (due to conspisuously long finger V and hind limbs). With addition of spacious tail membrane the
total wing area can be estimated to about 185 cm2 and wing loading
(for a body weight 30 g) to 0.162 g/cm2. Such values can hardly be
looked upon as a primitive or semi-functional state. In combination with a
strange wing design (ossified terminal phalanges!) it suggests for Onychonycteris a flight mode and
foraging specialisation that has no parallel in any extant bat 4.
All the eight genera of the early to middle Eocene bats were
reported from Europe (Palaeochiropteryx,
Archaeonycteris, Hassianycteris, Cecilionycteris, Matthesia, Ageina,
“Icaronycteris?”, Tachypteron) and North America (Icaronycteris, Onychonycteris), and all record-based analyses predicted thus the Laurasian roots of
the order 5,6. A rich bat
assemblage (at least seven new genera including those related to extant clades)
reported recently from late early Eocene lignite in Vastan Mine in W India 7,8 shows a different
perspective, however. It consists of at least seven new genera, including those
which exhibit possible relations to extant clades. In any case, the Vastan record demonstrates
that bats were quite a diversified group distributed over a considerable part
of the World in the early Eocene, and even that most of the early evolution of bats,
including the modern groups, might take place in southern continents 9 and most probably prior to
early Eocene.
The southern origin (Paleocene or Cretaceous) is assumed
also for the largest group of arboreal and aerial insectivores – passeriform
birds 10. This fact is
worth of mentioning especially because, most probably, it was just a pressure
from diurnal predators what shifted insect activity to crepuscular and night
hours. In consequence, density of aerial prey in the Paleocene and Eocene night
sky was perhaps enormous and possibly just that constituted the most essential
factor forcing the night aerial insectivory, notwithstanding disadvantages of
its transitional stages.
It cannon be excluded that Onychonycteris represents a clade which specificites arose just in
response to the conditions promoting the early bloom of bats and disappearing
perhaps in its consequence.
1. Simmons, N.B., Seymour, K.L., Habersetzer, J. & Gunnell G.F. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451, 818-821 (2008).
2. Speakman, J. A first for bats. Nature 451, 774-775 (2008).
3. Findley, J.S., Studier, E.H. & Wilson, D.E. Morphological properties of bat wings. Journal of Mammalogy 55, 429-444 (1972).
4. Norberg, U.M. in European Bat Research (eds Hanák, V., Horáèek, I. & Gaisler, J.) 197-211 (Charles Univ. Press, Praha, 1989).
5. Teeling , E.C. et al. A molecular phylogeny for bats illuminates biogeography and the fossil record. Science 307, 580-584 (2005).
6. Simmons, N.S. An Eocene Big Bang for Bats. Science 307, 527-528 (2005).
7. Bajpai S. et al. Early Eocene land mammals from Vastan lignite mine,
District Surat (Gujarat), western India. J.Palaeont.Soc.India 50,101-113 (2005).
8. Smith T. et al. High bat (Chiroptera) diversity in the Early
Eocene of India. Naturwissenschaften 94, 1003-1009 (2007).
9. Eick, G.N., Jacobs, D.S. & Matthee, C.A., A nuclear DNA phylogenetic perspective on the evolution of echolocation and historical biogeography of extant bats (Chiroptera). Mol.Biol.Evol. 22, 1869-1896 (2005).
10. Beresford, P.; Barker, F.K.; Ryan, P.G. & Crowe, T.M. African endemics span the tree of songbirds (Passeri): molecular systematics of several evolutionary 'enigmas'. Proc. Roy. Soc. Lond. B 272(1565), 849–858. (2005).
Fligh first in bats: when
and where? A correspondence prior to submission
Arising from Simmons N.B. et al., Nature, 451,818-821
(2008) and Speakman J., Nature 451,774-775 (2008).
Ivan Horáèek
Department of Zoology, Faculty of Science, Charles University, 128
44 Praha, Czech Republic, horacek@natur.cuni.cz
From: Ivan Horacek [mailto:horacek@natur.cuni.cz]
Sent: Wednesday, April 02, 2008 7:13 AM
To: simmons@amnh.org;
Gunnell, Gregg; jhaberse@senckenberg.de;
kevins@rom.on.ca;
j.speakman@abdn.ac.uk
Cc: Ivan.Horacek@seznam.cz
Subject: Onychonycteris comments
Dear all,
I compiled an enthusiastic comment on your exciting Nature paper on
Onychonycteris (for
Nature Correspondence) immediately after its appearance (attached file 1). Then
the
editor recommended to rearrange it for Nature Brief Communications Arising
section (600
words). After some hesitating I did it, eventually (attached file 2) but
overlooked
that a correspondence with the authors should preceed the submission (see the
editor´s
letter at attached file 3). That is why I beg to intrude upon you with this
spam-like
letter.
My primary intention with the comment article was to supplement your exposition
of the
topic of bat origin with few further regards that could be, at least in my
eyes, worth
of discussion.
First on curious features of Onychonycteris. A combination of the outstanding
plesiomorphies with surprisingly derived dentition and modern shoulder girdle
does not
fit well to an intuitive picture of the most primitive bat. Still I did not
come to term
with Gregg´s proposal (discussed in time of his stay in Prague) that the
primitive
dentition in bats could indeed look like that in Onychonycteris. Still, I
cannot
remember any case in mammals which would for sure demonstrate a reversed
morphocline
(e.g. from a reduced M3 to an unreduced state) without essential rearrangement
of basic
design of the teeth in question. Let us hope that more details on that matter
will be
available after next step of preparation. For the moment, without respecting
the Gregg´s
proposal and basing on paradigmatic model on direction of the
phylogenetic morphocline
in chiropteran dental evolution, one feels obliged to interpret the dentition
of
Onychonycteris as a derived state suggesting a specialisation for a large
prey. Wing
design of Onychonycteris is indeed quite strange and the complete ossification
of
terminal phalanges in particular. Concerning it, the following should be taken
in
account: (i) flexibility of chiropatagial margins can essentially reduce air
resistance
at the wing upstroke movement, and, hence, the energy required for it
(notwithstanding
the disbalancing effects), (ii) for that reason it seems quite probable that
the
ossification of terminal phalanges would disappear quite rapidly if (iii)
something else
would not be in the play. Your idea on non-volant locomotion and use of wing
claws at
climbing along tree branches sounds quite reasonable, of course. Nevertheless,
I cannot
abstain from doubts about serious role of tiny claws of fingers III-V in it:
the
positive effects in that direction could perhaps hardly compensate disadvances
of a
rigid chiropatagial margin. Getting away from the neontologic perspective, the
ossified
terminal phalanges and other specificities of wing design in Onychonycteris
need not to
be ultimately looked upon as primitive transitional state to wing qualities of
modern
bats but, alternatively, they might evolved in effect of a specific directional
selection. Extremely long finger V (with the longest metacarpus of all),
conspicuously
long hind limb, spacious uropatagium and spacious propatagium as well (note
long humerus
and short radius) by which Onychonycteris differs from other bats suggest what
could be
in focus of it : airfoil of the total wing area. I estimated it (with aid of
your
metrical data in Supplementary information) to 185 cm2, relativelly quite a
high value,
indeed. In respect to all that, one cannot exclude that Onychonycteris
was a form
specialised in exploitation of a kind of active gliding flight and a foraging
mode that
it could provide. Unfortunately, we have no analogy from extant bats and
it seems quite
probable that the configuration of selection cues responsible for the above
mentioned
curiosities were unique for the earliest stage of chiropteran evolution
and it is
largerly beyond our chance to identify them with aid of per analogiam
technique.
I further argue that the aerial insectivory could evolve (and pass succesfully
over
enormous disadvantages necessarily associated with the transitional states)
only under
conditions of extensive ecological release and enormous capacity of the
respective food
resources. The hypothesis of a diural stage of chiropteran insectivory, proposed
by John
Speakman, does not sound too probable, in these respects, as competent diural
insectivores (a number of bird clades, including one essentialy specialized in
insectivory - Passeriformes) existed throughout the World apparently prior to
bat
origin. On contrary, it seems quite probable that effect of diural predators
shifted
flight activity of insects onto crepuscular and night hours and that the night
sky of
Paleocene and early Eocene time was densely filled with insects of all sizes
possesing ,
at the same time, no adpatation preventing aerial predation. To pick up just
the largest
of them (which could be located with sufficient accuracy by passive listening
only)
could might become, under such conditions, quite a promising strategy. It
cannot be
excluded that Onychonycteris does illustrate just that.
Last but not least I feel obliged to remind extremelly exciting records from
Vastan
Mine, India. Besides of published items, I had opportunity to examine a large
series of
SEM pictures of maxillary molars sent me by Sunil Bajpai. My statement in the
article “
... including those which exhibit possible relations to extant clades"
refers namely to
Cambaya complexus (which exhibits close relations to Megadermatidae in
structure of
dentition , shapes of premolars and molars, and last but not least its size)
and to
Microchiropteryx foliae (which in certain respects reminds Craseonycteris),
besides of
several upper molars (Bajpai, unpublished) which belong to Rhinopomatidae, most
probably. I would agree, of course, with possible objections that other Vastan
forms
fall in a grade corresponding to Messel bats, and that the information on
Vastan bats is
still too scarce to draw any more detailed conclusion.
Nevertheless, diversity of Vastan bats suggets that bats were quite frequent
during
early to middle Eocene in southern continents and even in so distant regions as
India
was in that time (comp. just a recently appearing survey by Ali and Aitchison,
2008). By
the way, the paleopositions of India suggested for Eocene in the
respective paper
explain relations to SE Asia, including early apperance of megadermatids in
Australia
and rather delayed apperance of them in the Western Palearctic (comp. Hulva et
al. 2007).
Ali, J.R. &Aitchison, J.C. Gondwana to Asia: Plate tectonics,
paleogeography and the
biological connectivity of the Indian subcontinent from the Middle Jurassic
through
latest Eocene (166-35 Ma), Earth-Sci. Rev. (2008),
doi:10.1016/j.earscirev.2008.01.007
Hulva P., Horácek I., Benda, P. Molecules, morphometrics and new fossils
provide an
integrated view of the evolutionary history of Rhinopomatidae (Mammalia,
Chiroptera).
BMC Evol.Biol. 7, 165 (2007), doi: 10.1186/1471-2148-7-165
So that is a brief explanation on my motivations which are behind 595 words of
the
enclosed article. I thank you very much in advance for your kind view of
them and for
any comments concerning the matter of common interest.
With all best greetings,
Sincerely yours
Ivan H
Ivan
Horacek, Prof. Dr.
Department of
Zoology
Charles
University
Vinicna 7
CZ 128 44 Praha
Czech Republic
tel. 00420
221951851
fax
00420 221951841
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Date: |
Wed, 2 Apr 2008 09:09:14
-0400 [04/02/2008 03:09:14 PM CEST] |
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From: |
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To: |
Ivan
Horacek <horacek@natur.cuni.cz>, simmons@amnh.org,
jhaberse@senckenberg.de,
kevins@rom.on.ca,
j.speakman@abdn.ac.uk |
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Cc: |
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Subject: |
RE: Onychonycteris comments |
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Hi All – |
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Date: |
Wed, 2 Apr 2008 10:36:10
-0400 [04/02/2008 04:36:10 PM CEST] |
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From: |
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To: |
Ivan
Horacek <horacek@natur.cuni.cz>, "Gunnell,
Gregg" <ggunnell@umich.edu>, jhaberse@senckenberg.de,
kevins@rom.on.ca,
j.speakman@abdn.ac.uk |
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Subject: |
RE: Onychonycteris comments |
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Dear Ivan -- Thanks for sharing your thoughts with us, it
is always interesting to get |
|
---------------------------------------
Dr. Nancy B. Simmons
Chairman, Division of Vertebrate Zoology
Curator-in-Charge, Department of Mammalogy
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024
Voice: (212) 769-5483
FAX: (212) 769-5239
e-mail: simmons@amnh.org
WWW page: http://research.amnh.org/mammalogy/personnel/simmons.php
Information on visiting or borrowing specimens from the
AMNH mammal collections:
http://research.amnh.org/mammalogy/index.php
For access to searchable databases of AMNH mammal
specimens:
http://research.amnh.org/mammalogy/databases.php
|
Date: |
Wed, 2 Apr
2008 22:28:31 +0100 [04/02/2008 11:28:31 PM CEST] |
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From: |
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To: |
Nancy
Simmons <simmons@amnh.org>, Ivan
Horacek <horacek@natur.cuni.cz>, "Gunnell,
Gregg" <ggunnell@umich.edu>, jhaberse@senckenberg.de,
kevins@rom.on.ca |
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Subject: |
RE:
Onychonycteris comments |
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Priority: |
Normal |
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Hi Ivan,
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From:
Ivan.Horacek@seznam.cz
To:
simmons@amnh.org
,
ggunnell@umich.edu
,
jhaberse@senckenberg.de
,
kevins@rom.on.ca
,
j.speakman@abdn.ac.uk
Pøedmìt:
horacek@natur.cuni.cz
Datum:
5.4. 2008,
04:59 - dnes v 04:59
Dear Nancy and Gregg,
I thank you very much for your immediate answers to my letter, kind view of the
comment on Onychonycteris and a number of stimulating opinions.
I understand well to Gregg´s objection on „should be true“ argumentation and
accept it. In no way I wish to conduct like an eyewitness of the early Eocene
evolutionary dynamics. Unfortunately the fossil record available on the
earliest history of bats is still so incomplete (both in respect to a pre-bat
ancestor and roots of the extant clades) that do not provide sufficiently
robust indisputable answer for the implicite questions on various details of
the story and, hence, an inquiry about an indirect information can help in
orientation. It cannot be harmful, perhaps, to keep all possible alternatives
on mind to be ready to test them immediately after further information will
appear – similarly as it was the case with John Speakman´s exposure of „flight
first“ vs. „sonar first“ hypotheses. That was why I tried to formulate
supplementary hypotheses, partly alternative to your interpretations, without
neglecting the arguments you provided.
A prediction that a complete dentition with large unicuspids and unreduced M3
represents a primitive state while those with reduced number of teeth, and/or
reductions in sizes of premolars and M3 are derived is supported by huge
empirical evidence from many mammalian clades and acts as a default
interpretation tool of mammalian comparative odontology. Of course, no rule is
universally valid and I can agree with you that universal validity of that
prediction is in no way granted. Also, I can agree that „reevolving“ of reduced
teeth may occur eventually and am relatively well informed on experimental
support for such a possibility (e.g. reappearance of P4, lost in evolution, in
ectodysplasin++Eda- mice demonstrated by Renata Peterková and Jukka Jernvall
groups, cf. Kangas et al. 2004: Nature 432,211-214). Nevertheless, I am
hesitating to believe that such reversals could become a common trend,
particularly in the case of the tribosphenic dentitions where the functionality
is ultimately controlled by perfect interlocking pattern among all crests and
cusps and any small misaarangement means lost of functionality of whole
dentition. The reversals and/or reappearances of lost elements could take place
where no such strict control does operate - e.g. in frugivores, nectarivores
(including pteropodids or phyllostomids as you mentioned). Then, however, the
reappeared teeth apparently do not copy the „primitive“ form corresponding to
ancestral dentition. The fact that many extant bats (such as Natalidae,
Myotini, Thyropteridae, Furipteridae, Myzopodidae etc.) show a dentition in
some respect more primitive than Onychonycteris should hence be interpretted
either as appearance of „reevolving“ event, perhaps parallel in ancestors of
these clades, or as a support for derived state in the latter form. I just
argued that in regard to the Occam´s razor technique, the latter alternative
looks at least formally more parsimonious. But readily agree that we urgently
need more information to decide the dilemma.
I thank you for reminding me on Dizzya and Australonycteris - I will supplement
the reference in the manuscript - five words are still available.
So, once again: thank you very much for all.
Best regards,
Ivan
From:
Ivan.Horacek@seznam.cz
To:
j.speakman@abdn.ac.uk
,
simmons@amnh.org
,
ggunnell@umich.edu
,
jhaberse@senckenberg.de
,
kevins@rom.on.ca
Kopie: horacek@natur.cuni.cz
Pøedmìt:
Onychonycteris:
answer to John Speakman
Datum:
5.4. 2008,
05:03 - dnes v 05:03
Dear John,
I thank you very much for your prompt response and kind view of my comments on bat origin. I enjoyed your substantial contributions on that topic and, correspondingly, I am greatly appreciating stimulating ideas and questions in your letter. Anhow, I cannot agree completely with all your arguments and, in the following rows, will try to explain why.
Although
the recent evidence puts the passerine origins way back in the cretaceous it is
worth remembering that they are a vanishingly small part of the fauna, until
their explosive radiation relatively recently.
The standard view of the early Cenozoic past is essentially influenced by that the most detailed record is, for the historical reasons, available from Europe. The picture seen from the European perspective is necessarily biased, however, by the fact that Europe of the Paleogene time (or at least prior to Grand Coupere) was but an archipelago at outskirts of the World, isolated from evolutionary effervescence on continents and dynamics of adpative radiations beyond the European splendid isolation of that time. This concern not only bats but other groups as well, birds including, and first of all the most progressive clade of birds - passerines. The Australian and African fossil record published in recent decade supported by phylogenetic studies (Boles 1995, 1997, Baker et al. 2004, Beresford et al. 2005) suggests spread of all basal clades of passerines throughout Gondwana as early as in Paleogene and early Eocene.
At the same time, there are several good reasons to believe that the actual role of passeriform birds (and small-sized insectivourous birds in general) in real communities is largerly underestimated: (i) the small arboricolous forms appear much rarely in fossil record than large terrestrial forms, simply for the taphonomical reasons, and it is particularly pertinent for the lake deposits from which major bulk of the European Eocene record was obtained. (ii) The small-sized fragments, often poorly fossilized, were typically not taken in account in earlier stages of paleornitology, among other also for severe conceptual problems with taxonomic relevance of the available characters (comp. e.g. uniformity of passeriforms in humeral features).
(iii) The absence of literary data on insectivorous birds in the early Cenozoic record until the recent decade (for the European record see e.g. Mlikovsky 2002) is indeed stricking and judging from this, your statesment is undoubtedly quite substantiated. Nevertheless, the recent data show even from there a different picture.
The
messel formation for example has yielded many birds but no insectivorous
passerines or insectivorous non-passerines for that matter(see Rydell and
Speakman (1995) Evolution of nocturnality in bats - potential competitors and
predators during their early history.Biological Journal of the Linnean Society 54
Iss 2: 183-191.). Hence the idea that there were large populations of
insectivorous birds flying around driving insects into nocturnal niches is not
supported by any evidence.
The story of Messel bird record illustrates the above view quite a well. Until 1998 no small sized insectivorous birds were reported in contrast to numerous water birds, raptors etc. Gunther Mayr, who first started reexamine the puzzling group of small-sized insectivorous birds, demonstrated in a series of recent studies an unexpected diversity of non-passerine insectivorous birds, including those highly specialized in aerial insectivory (Aegialornitidae- Hemiproctidae, Caprimulgiformes, Coliiformes, and a number of variouos clades of Coraciformes) - comp. the references below. It provides indeed a picture quite different from the traditional view and, at the same time, a clear support for the idea that the Eocene bird community was disposed to perform a serious predation pressure upon insect populations.
On the other hand there are several diurnal predatory birds among the messel assemblage so the idea that a diurnal bat might be driven into the nocturnal niche by predation risk in the day is a much better supported hypothesis. Moreover, I have several papers clearly showing that nocturnality at present is sustained by predation risk and is not driven by competition with insectivores (or frugivorous competitors) - see attached.
I agree, eventually, that nocturnality in extant bats is controlled by diural predators and appreciated your studies illustrating that point. Nevertheless, hardly can imagine how could it act as the factor promoting a flight capacity in bat ancestors. Predation upon volant animals at that stage will simply result is extermination of disposals for flying. As a factor forcing nocturnality it could act only if the food in night sky could be obtained in effectivelly lesser cost than that available for the non-volant relatives. In other words, the key factor promoting the transition to flight would be the density of night aerial insects.
The
idea that populations of insects are at maximal density in the evening seems to
be a view held only by bat biologists who routinely do not sample insect populations
during the day. In fact populations in the temperate zone peak in mid afternoon
at levels way higher than levels in the nocturnal niche. So not only is there
no evidence supporting a historical pressure driving insects nocturnal, there
is also no contemporary evidence that modern levels of insectivorous birds
which are probably orders of magnitude more abundant than in the Eocene achieve
this either.
I readily agree, of course, with your statesment on temporal distribution of aerial insect density in temperate zone. Unfortunately, I do not know exact data from tropes. My fragmentary experiences make me sure that density of aerial insects in tropes at crepuscular hours is enormous and I tend to believe that it is much higher than in any other part of a day (it is particularly impressive with mass appearance of social insects such as Isoptera). Of course, I agree that a number of other factors can be in the play in promoting increased insect activity in crepuscular time, e.g. danger of overheating on diurnal sky, increase of air humidity in crepuscular time etc.
The fact there are considerably more insect
resources in the day is consistent with the fact that modern passerine
insectivorous birds are able to raise multiple broods of 5-6 offspring over the
course of a summer while bats struggle energetically to raise singletons.
I aggree again but am not convinced from that argument. It is a bit unfair to compare birds and mammals just based on the total energy invested to raise offsprings: the eutherian mammals are essentially handicaped with constitutional constraints of their reproduction and the pathways of redistribution of energetic investments into particular components of reproductive process are apparently much different in these two groups.
Besides that: why not look upon the extreme K-strategy in bats (and its correlates in social life etc.) as an incipient quality achieved in their evolution? Or, at least as an essential advantage compared to r-strategy characterizing insectivorous birds? I am no fan of selfish-gene optics and somehow hesitate to rely upon explanatory power of an argument "the more the better".
If
a bat had evolved in the late cretaceous/early tertiary the most abundant
resources available to it would be diurnal – as they remain today.
I am not completely convinced that under extremelly hot tropical conditions of the Late Creatceous or Paleogene, the temporal pattern of insect activity was a similar to that in the nowday temperate zone. More likely I would expect an incresed concentration at crepuscular time similar to recent tropes.
Besides of the above mentioned arguments that insectivorous birds were diversified in the early Tertiary (including passeriforms in southern continents) and widespread enough to produce a serious selection pressure on insect aerial activity, the following agruments (indirect, of course) are available in support of my proposal: (i) The birds as predominantly optical vertebrates are extraordinary well, much better than mammals, disposed by their constitution to orientation and foraging in diurnal sky, especially for small-sized prey. (ii) In contrast, sensoric design of a mammal (vision at low light intesity, sensitivity to low intensity and high frequency acoustic stimuli, integrative role of olfactoric stimuli) presents a robust preadaptation for effective foraging in crepuscular and night time. Diurnal activity is rather exceptional in mammals, except for ungulates and primates, which both show specific constitutional adpatations for it, that invariantly absent in bats. (iii) It seem quite improbable that a bat ancestor, apparently not well disposed for diurnal activity could succeed competition with birds which flying capacity was refined with more than hunderd millions of years of evolution and equally improbable is that bird would remain any rich food resource available to them untouched. (iv) The transitional stages with ineffective flying capacity, biomechanical rearrangements limiting versatility of the non-volant locomotion, and enormously enlarged energetic demands accompanying it, could only be promoted by extremelly high foraging efficiency. The respective half-bat animal could achieve it only with extraordinary high density of aerial prey not available to any possible competitor. The latter precondition seems to be partricularly significant in evolution of bats: also K-strategy as ESS of bats could only evolve under conditions of nearly complete absence of predatory and competitive pressure.
If
you postulate a nocturnal origin then you have to explain what kept the bats in
that niche and prevented them expanding out to exploit the much richer insect
resources available diurnally before there were any competitor or predatory
birds around. The cretaceous assemblage of birds contains neither and is
dominate by shorebirds. In fact the diurnal insect niche appears effectively
vacant for many millions of years.
I hope the question was at least partly explained: in my view it is not too much interesting what kept the bats in nocturnal niche - they have had no need to escape of it except for the situations, more or less marginal in evolution of bats (such as the conditions of the Late Cenozoic temperate zone). In any case, all the hypotheses you formulated in that respect (taken from your web page):
First, bats may face direct competition and be out-competed by
aerial insectivorous birds. Second, bats that fly in daylight may be
susceptible to predation by diurnal predatory birds. Third, bats may be
susceptible to uptake of solar radiation across their wing membranes which
might lead daylight flying bats into fatal hyperthermia.
can be undoubtedly looked upon valid, eventually - but: rather as the factors controlling nocturnality in extant bats than those promoting a transition from a vagile arboreal creature to a flying animal.
Barker, F. K., Cibois, A, Schikler, P.A.,. Feinstein, J. &
Cracraft, J. (2004):
Phylogeny and diversification of the largest avian radiation. PNAS 101(30):
11040-11045.
Beresford,
P., Barker, F.K., Ryan, P.G. & Crowe, T.M. (2005): African endemics span the tree of songbirds
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In any case, once again I thank you very much for your kind interest and a help.
With best regards,
Ivan