The Origin and Evolution of Bats part 3

Earlier here and here we looked at the origin and evolution of bats. See Part 4 here. It solves many of the problems attending the origin of bats.

Now let’s add a few more pieces to the puzzle to make the various transitions more complete, more logical and more gradual.

The bat flight stroke
(Fig. 1) is similar to that of birds during takeoff, when the airspeed is minimal. So, there is less reliance here on Bernoulli’s principle and more Newton’s third law as the bat wing scoops up and pushes down a mass of air with each downstroke. The incredible ability of bats to ‘turn on a dime’ is due to their ability to greatly modify their wings in flight, push a volume of air in any direction, and to play the mass of their own wings against the remaining 80% to succeed at maneuvers birds would never try. This includes the ability of bats to invert themselves at low airspeed in order to cling to ceilings and perform an upside-down, two-point landing.

Figure 1. Bat flight stroke in anterior view. This is a different stroke than birds or pterosaurs used, depending more on elbow and finger  flexure and extension.

Figure 1. Bat flight stroke in anterior view. Note how the wing tips gather toward the torso during part of the flight stroke.  Click to view YouTube video. Note the elbows don’t move a great deal here, but the wrist rises above the back, unlike other mammals.

In bats the wing stroke 
cycles from extension and maximum wing area (on the downstroke) to flexion with minimum wing area (on the upstroke). On some bats the wing tips ventrally touch one another during the flight stroke, but on this Myotis specimen (Fig. 1) the wing tips graze the torso. This sort of flight stroke appears to have evolved from a prey gathering stroke when the much smaller forelimbs were simply enlarged webbed hands (Fig. 5) gathering prey like a catcher’s mitt.

Bats were not always so incredibly gifted in flight.
Obviously there were millions of transitional generations that evolved from quadrupedal arboreal forms in the Late Cretaceous before bats became such supreme aerialists in the Early Eocene.

The question is:
what sort of behavior led to this sort of flight stroke on non-flying bat ancestors with small unwebbed hands? In other words, what can simple hands and small pre-wings do that presages and evolves into the flight stroke we see in bats with large wings? Fossils do not yet tell us all the details, but enough is known to create hypothetical transitional forms (Fig. 5).

Inverted bipeds
It should be obvious that pre-bats cannot begin to flap their wings unless their forelimbs are freed from typical support duties. Virtually all living bats are inverted bipeds (the rare exceptions are vampire bats and grounded bats). The legs and feet of bats have gone through such radical changes that ordinary quadrupedal locomotion is impossible for them now.

An evolutionary starting point: the pen-tailed ‘tree shrew’
Ptilocercus
(Fig. 2; actually a pygmy civet) is a small, extant, arboreal, quadruped at home both on tree trunks and narrow branches. It seems unlikely that the transition to bipedalism could have happened upside-down on a tree trunk because the hands would still have been used for support. Rather, the transition had to happen while inverted on a horizontal branch or vine (Fig. 2) with only the hind limbs hanging on and the forelimbs free to flap.

Figure 3. The pen-tailed tree shrew, Ptilocercus, the closest living non-flying relative of bats. Note the pose, perpendicular to the narrow branch. It arrived there by leaping from one branch to another, rather than walking along the length of the branch. When pen tails 

Figure 2. The pen-tailed tree shrew, Ptilocercus, the closest living non-flying relative of bats. Note the pose, perpendicular to the narrow branch. It arrived there by leaping from one branch to another, rather than walking along the length of the branch.

Figure 3. Bat and human scapula compared. Red arrows point to acromion process, reduced and moved away from the shoulder joint in bats to enable greater freedom of motion.

Figure 3. Bat and human scapula compared. Red arrows point to acromion process, reduced and moved away from the shoulder joint in bats to enable greater freedom of motion.

Ptilocercus
is the closet tested living sister to bats and dermopterans, like Cynocephalus. Notably, the pen-tail has no trace of any extradermal flight/gliding membranes anywhere on its body. Thus the gliding membrane of Cyncoephalus and the flying membrane of Pteropus are not homologous, but developed independently.

Inverted Flapping
Earlier I found a video of a full-fledged bat flapping on a horizontal branch, not releasing its feet until attaining a horizontal attitude. That’s the best data I’ve found so far for flapping while inverted before flying. (And I’m not forgetting that this bat had a fully evolved set of wings.) By comparison, smaller hands, like those in Ptilocercus, would have been nearly useless for pushing air around.

When an animal is hanging by its feet, no matter how much it ‘flaps’ its little hands and arms, it’s not going to go anywhere. It’s not going to find food and it’s not going to attract mates. I don’t see any evolutionary advantages to this sort of behavior.

Unlike quadrupedal mammals,
bats can raise their wrists and hands over their backs as part of their flight stroke. Brachiating primates (anthropoids including humans) can only lift their hands over their heads, but not over their backs. Bat shoulder flexibility is due both to their elongated clavicles, that extend above/behind the neck, and to the cranial shifting of the acromion process on the scapula (Fig. 3). That removes the scapular block that restricted dorsal humerus abduction. Only bats have this key trait.

Another question: Which key trait came first? The long clavicle? The dorsally open glenoid? The large webbed hand? At present, we just don’t know. We don’t have the fossils.

Which traits were lost in bats? 
As in birds and pterosaurs, the manus in bats is unable to supinate or pronate because the ulna becomes little more than a splint and the radius no longer axially rotates around it. Thus the bat manus is unable to operate in a normal fashion (palmar side in contact with the substrate, pointing in the direction of quadrupedal locomotion, flexing and extending in PIL sets). Rather in bats, birds and pterosaurs the palms face each other when adducted (like clapping), except at maximum extension (abduction, lateral forelimbs) when the palms face ventrally.

Figure 3. Fruit bat (Pteropus) skeleton with hypothetical muscles added).

Figure 4. Fruit bat (Pteropus) skeleton with hypothetical muscles added).

Ontogeny recapitulates phylogeny
We don’t have to rely solely on extant adults and fossils. We can also glean data from extant juvenile and embryo bats, which appear to replay the evolutionary journey of bats as they grow up. Their wings are much smaller than in adult bats. We also have behavioral clues to work with.

Newborns
of Ptilocercus are nest-bound. One, two, or three nestlings receive extremely sparse maternal care as the mother visits her young every other day for no more than ten minutes at a time. The parents themselves are solitary feeders. By contrast, single newborn bats are carried by their mothers everywhere they go and adult bats are communal.

Bat juveniles
do not fly shortly after birth, but do so only as subadults when their wing fingers reach adult length. By this evidence we can draw an analogy that bat ancestors did not achieve competent flight until they had adult-length fingers. The evidence also shows that less than competent flight was good enough to survive and flourish in that Paleocene environment devoid of predators and likely filled with prey and nearby landing sites. Picture a tangle of branches and vines with pre-bats leaping and flapping between them.

The Paleocene insect fossil record
is currently underrepresented, but giant ants and beetles are known. Some of these would have been on Paleocene bat menus.

If you’re going to evolve wings like a bat,
you have to stop using them as hands. Whenever a digit extends or membranes develop, they are going to get damaged and become increasingly awkward with increasing size, unless folded, as pterosaurs and birds do. In adult and juvenile bats the carpus rotates the hand posteriorly for complete wing folding. However, the embryo bat manus (Fig. 5) does not appear to rotate and fold in the same fashion. Even so, in the bat embryo there is little indication of inter-metacarpal muscle. Instead that area looks indistinct from the inter-digital membrane. Note the unguals of manual digits 2-5 in all bats, including embryos, are vestiges and not in use.

Figure 4. Ptilocercus, Icaronycteris and a hypothetical transitional taxon based on the ontogenetically immature wing of the embryo Myotis. If you're going to evolve wings it looks like you have to stop using them as hands early on. Note in the bat embryo there is little indication of inter-metacarpal muscle. That area looks identical to the web.

Figure 5. Ptilocercus, Icaronycteris and two hypothetical transitional taxa based on the ontogenetically immature wing of the embryo Myotis. If you’re going to evolve wings it looks like you have to stop using them as hands early on. Note in the bat embryo there is little indication of inter-metacarpal muscle. That area looks identical to the web.

Lessons from gliding squirrels, dermopterans and sugar gliders
These living mammal gliders are not related to each other and not related to bats. None develop large hands or a flapping behavior. None of these gliders produce thrust while gliding and therein lies the difference with large-handed bats. Furthermore none of these gliders develop a larger clavicle and scapula, which bats use to anchor large flapping muscles.

Why would a sister to Ptilocercus start to flap?
Elephants flap enlarged ears to cool off. Baby birds flap tiny wings to encourage their parents to feed them. Before they could fly, pre-pterosaurs and pre-birds likely flapped to threaten rivals and seduce mating partners. Bats don’t appear to use their wings except to fly, catch insects and perhaps to wrap themselves for insulation from the cold and rain.

Prey Acquisition > Locomotion 
Given the large gap between Ptilocercus and Eocene bats (Fig. 5), along with the present evidence from embryo/juvenile bats, we can only imagine transitional stages starting with an enlarged hand (HTTaxon 1) and continuing to enlarge that hand while developing extradermal membranes (HTTaxon 2). Here’s how the current data can be used to rebuild the missing portions in the evolutionary stages of bat evolution (Fig. 5).

Figure 5. Ptilocercus in vivo, holding prey with its small hands while eating it.

Figure 6. Ptilocercus in vivo, holding prey with its small hands while eating it.

  1. The Ptilocercus state Ptilocercus is an omnivore that pounces on insects that have landed on trees. Ptilocercus grabs its prey with its hands and kills with its mouth (Fig. 6). That’s pretty much what bats do, too.
  2. Hypothetical Transitional Taxon 1 stage – The five fingers are longer here. That increases the ability to grab prey, like a catcher’s mitt (Fig. 7) does with baseballs. The metacarpals were more loosely joined at their carpometacarpal joints. Conversely the fingers were bound by webbed membranes creating more flexible mitts. Perhaps lateral extradermal membranes, like those found in gliding squirrels, dermopterans and opossums, also had their genesis at this stage. Flight, such as it was, would have been restricted to close vines or branches. Frantic flapping would have supplemented each glide in a rudimentary fashion.
  3. Hypothetical Transitional Taxon 2 stage – Only fingers 3-5 are longer here and they form support structures for more extensive wing trailing membranes. At this stage the hand could no longer be used for traditional quadrupedal locomotion, so bats probably became inverted bipeds at this stage. When forced to, bats rested and walked on the medialventral rim of their folded hands with only their thumbs able to grab surfaces, as they do today. Flight duration continued to expand.
  4. Icaronycteris state – The lumbar vertebrae were longer. The tail was shorter. The clavicle, chest and scapula were enlarged at this stage. The ulna was a splint. The hand was full-sized and capable of both folding completely and flying competently. Flight duration had no practical limits.

Behavioral stages
The following hypothetical behavioral stages gradually grade from quadrupedal pygmy civets to aerial bats. As you’ll see, bat wings developed principally for prey gathering, whether that prey was on tree branches or in mid-air.

  1. Arboreal prey grabbing with small hands, then eating at leisure (Ptilocercus).
  2. Arboreal prey gathering with larger hands, making large sweeping motions toward the body and mouth (the genesis of the flight stroke, Fig. 1) during capture and transfer to the mouth (HTT1).
  3. Branch-to-branch or vine-to-vine leaping/flapping with even larger hands now transforming into flying organs. Loss of the tibial malleolus increases pedal rotation. (HTT2).
  4. Tree-to-tree leaping/flapping with even larger hands that could be folded during inverted hanging. At this stage the whole body starts to act like a catcher’s mitt as flying insects are added to the menu. After capture the tail or wing curls in to transfer the prey to a descending skull to bite and kill while on the wing (Icaronycteris).
  5. Open air flapping/flying with wings large enough that juveniles/babies could be carried by mothers. Shorter tail and expanded uropatagium adds to the surface area of the ‘catchers mitt’ as these posterior elements are increasingly used for prey acquisition (Myotis) and the wings less so.
  6. Radiation/Evolution into various species, some frugivorous, others with echolocation.

Figure 7. Everything evolves, even catcher's mitts. By analogy, bat's wings evolved to become better catcher's mitts.

Figure 7. Everything evolves, even catcher’s mitts. By analogy, bat’s wings evolved to become better catcher’s mitts. Click to enlarge.

All we have to do now
is find fossils of a Paleocene pygmy civet with large fingers, like those of an embryo bat.

References
Jepsen GL, MacPhee RDE 1966. Early Eocene bat from Wyoming. Science 154 (3754): 1333–1339. doi:10.1126/science.154.3754.1333. PMID 17770307.
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309.
DOI – 10.1111/j.1096-3642.1926.tb02241.x
Simmon NB, Seymour KL, Habersetzer J, Gunnell GF 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451 (7180): 818–21. doi:10.1038/nature06549. PMID 18270539.

wiki/Icaronycteris
wiki/Onychonycteris
wiki/Ptilocercus

The Origin and Evolution of Bats

Parts 2 and 3 of this subject were posted here and here.
See Part 4 here. It solves many of the problems attending the origin of bats.

Scientists have long wondered about the origin and evolution of bats. Bats seem to have appeared ready to fly at their first appearance in the fossil record. Even so, it is possible to determine their ancestors with cladistic analysis and a sufficient number of taxa.

The most primitive known bats include Onychonycteris and Icaronycteris. Modern bats, like Myotis, are either small insectivores (with some nectar-, blood- and fish-eating thrown in) or large fruit-eaters, like Pteropus.

Current Views
Gunnell and Simmons (2005) reported, “The phylogenetic and geographic origins of bats (Chiroptera) remain unknown.” Wiki reports, “Little fossil evidence is available to help map the evolution of bats, since their small, delicate skeletons do not fossilize very well. Bats were formerly grouped in the superorder Archonta along with the treeshrewscolugos, 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 carnivoranspangolinsodd-toed ungulateseven-toed ungulates, and cetaceans.”

That’s a big list. Way too general. Most workers nest bats between Insectivores and Carnivores. Again, way too general. Let’s get specific, shall we?

Figure 1. Hypothetical bat ancestors arising from a sister to Chriacus, which may be a large late survivor of a smaller common ancestor.

Figure 1. Hypothetical bat ancestors arising from a sister to Chriacus, which may be a large late survivor of a smaller common ancestor.

Phylogenetic Analysis
Here (Fig. 2) bats nest with Panprimates, specifically: Chriacus, Palaechthon and Ptilocercus in order of increasing distance to bats. Essentially bats were derived from small, tropical arboreal mammals with an omnivorous diet.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

Figure 2. Bat origins cladogram. Here Onychonycteris and Pteropus represent bats.

The Family Tree of Bats
Here (Figure 2) Chriacus is the closest sister taxon to bats and Ptilcercus (Fig. 2) is a close second. Fossil mammals are rarely used in phylogenetic analyses of bat origins. Most workers prefer molecule analysis. Others have mixed bat and mice genes to get mice with longer limbs.

Colugos are sisters to Ptilocercus, the extant pen-tailed tree shrew. Formerly tree shrews were associated with primates.

Arboreal Chriacus was considered close to the ancestor of the Artiodactylia (hooved mammals and whales). And that is why the long list of  Laurasiatherian mammals (see above) comes into play.

The Hands of Bats
Baby bats have short fingers, so they more closely resemble little colugos. The “hands” of adult bats have become so transformed that they can no longer be used to support the body in a typical mammalian manner. In the only other flying vertebrates, pterosaurs and birds, a bipedal phase enabled their “hands” to rise off the substrate and in time, become wings. The same is hard to imagine with bats because nothing about their anatomy suggests that bat ancestors were ever traditional bipeds. However, all bats hang by their feet, so they may be considered inverted bipeds — leaving their hands free to develop into something else.

Like birds and pterosaurs, bat hand/wings fold up for compact storage between deployments. The bat wrist folds and rotates to a much greater extent than in any other mammal and the metacarpals spread much more widely. As bat embryos develop, their metacarpals are widely abducted. Finger bones develop within the round buds that all tetrapod embryos have, but in bats there is no cell death between the digits to free them from one another. Thus the fingers remain webbed.

The Hind Limbs of Bats
In similar fashion, the hind limbs of bats no longer operate like those of typical mammals. The pelvic openings and femora permanently splay the limbs in a lizard-like configuration. Together with a loose ankle joint, bats use this configuration to hang inverted with soles oriented ventrally. The question is: did the hind limbs lose their traditional abilities before or after the arrival of wings?

Comparisons to Birds and Pterosaurs
Pterosaurs and birds have similar pectoral girdles. Their scapulae are braced by immobile coracoids and anchored by close bony connections to their ribs and vertebrae. They flap their arms/wings principally with huge pectoral muscles anchored on huge sternal plates and keels.

In bats, however, there is no huge sternum and no coracoid to lock the scapula in place. Instead bats essentially flap their shoulder blades from spine to side, pivoting them on the proximal clavicles articulating with the narrow shallow sternum. Giant back muscles anchored on low wide vertebrae and broad flat ribs provide the power. Yes, the pectoral muscles are massive, but in essence bat arms/wings ‘go along for the ride’ as the scapulae swing back and forth through huge arcs.

Muscle attachments aside, broad ribs increase stability and decrease mobility in the thorax and vertebral column. Decreased thoracic mobility appears to be a preadaptation for flight, as demonstrated by birds and pterosaurs.

Comparisons to Ptilocercus (Pen-Tailed Tree Shrew)
Like bats, the carpals (wrist bones) of Ptilocercus are able to rotate laterally much more so than is typical for other mammals. This facilitates hanging from and climbing down tree trunks head first, as in bats. Some civets also do this, but colugos never do. Ptilocercus has been observed climbing inverted on horizontal branches, as in colugos and bats. Like bats, Ptilocercus can spread its metacarpals, to such an extent that finger #1 opposes #5. This permits branch grasping in a fashion more typical of primates than carnivores. With such hands, Ptilocercus stalks and pounces on its insect prey, then shoves the meal into its mouth. At times Ptilocercus sits on its haunches to feed at leisure while holding prey. Nandinia, the palm civet, has similar habits. Bats no longer capture prey in this manner in trees, but continue to do so in the air.

Like bats, the femora of Ptilocercus are able to spread widely. Pen-tailed tree shrews are better adapted to belly-crawling and tree-clinging than to running and leaping. The ankles are similarly loose and permit rotation of the feet, soles down, but not to the same extent seen in bats. While the toes in civets and Ptilocercus are able to oppose one another for branch grasping, this ability is not as developed as in primates. In bats this ability is lost. Ptilocercus and some civets are plantigrade or flat-footed, as in bats and other primitive mammals.

Like bats, the long tail of Ptilocercus is not fur-covered (except at the tip). Like bats, Ptilocercus gives birth to one pup (rarely two) at a time. Like bats and Nandinia, Ptilocercus is nocturnal. Like bats, Ptilocercus changes its body temperature to fit climatic conditions, but not to the same degree. Civets are generally solitary. Ptilocercus sometimes nests in groups. Bats are typcially communal.

Hypotheses for the Development of Wings in Bats.
Post-dusk and pre-dawn Nandinia and Ptilocercus feed by creeping up on resting prey, whether birds, eggs or grasshoppers. With stealth, rather than speed, they grab their prey with their “hands” before shoving their meals into their mouths.

Given these phylogenetic starting points, we should expect a hypothetical pre-bat to do the same, but in a more specialized manner. If this pre-bat had proportions midway between Pteropus and Ptilocercus, it would have a larger scapula than Ptilocercus, double the arm length, four times the hand length, a thirty-percent longer leg, half the length of tail and an overall increase in claw size. At this point the pre-bat would cease using its fore and hind limbs in traditional locomotion to become a sit-and-wait predator. Inverted it might stand almost motionless, locked onto rough tree bark by feet in which the metatarsals are reduced and the toes lengthened so as to conform more closely to the irregular substrate, like those of bats. This configuration is also used by nursing bats to attach themselves to their mother. After waiting for an insect to come within range, the pre-bat would extend elongated fingers to cage the prey item before attacking with its teeth.

The ability of bats to enter torpor, and thus to remain motionless for long periods of time, as well as their general inability to walk in a traditional fashion supports this “sit-and-wait” hypothesis. If valid, the legs lost their traditional abilities before the onset of flight.

Finger 2 in bats is much shorter than 3-5, which supports the “finger cage” hypothesis. As in the hands of Ptilocercus, bats and humans, as fingertips 3-5 touch a flat surface, fingertip 2 remains elevated. Thus in the wings of Pteropus and Icaronycteris only digits 1 and 2 retain claws and they are much shorter. Essentially bats fly with only digits 3-5.

At some point in the genesis of bats the skin between the pre-bat’s fingers was not diminished during embryogenesis and the enlarged hand snare became complete. Of course, the fingers would have to be kept together during the sweep forward. Otherwise they would act like twin parachutes, slowing the adduction of the hands and betraying their imminent arrival by the advancing gust they would produce – unless they moved very slowly.

Flight as a Means to Escape Predators
Provided with such hands, a pre-bat would not only have sufficient membrane to drop and glide, but the distal development of those membranes could provide thrust if flapped. Flapping is not an option for the colugo, Cynocephalus, with its extended proximal membranes and smallish hands. It can only glide and does so very well. Nandinia has no gliding membranes whatsoever, but it has been observed free-falling from trees over and over in a spread-eagle configuration, apparently in play. This technique might also be used to avoid aerial and arboreal predators, such as birds, snakes and army ants. Ptilocercus has not been observed falling from trees, but its diminutive size would preclude damage if falling into leaf litter. If a predator approached our hypothetical pre-bat, and traditional forms of escape (i.e. running and leaping) were no longer in its forte, survival would depend on dropping and finding another safer perch. Flapping and the continuous development of the ability to fly, of course, would open up grand new vistas of unoccupied niches. The Big Bang of Eocene bat evolution that followed the origin of bats is a testament to that.

 

References
Cope ED 1882. Synopsis of the Vertebrata of the Puerco epoch. Proceedings of the American Philosophical Society 20:461-471.
Gunnell, GF and Simmons NB 2005. Fossil evidence and the origin of bats. Journal of Mammalian Evolution 12: 209-246 (2005).
Mac Intyre GT 1962. Simpsonictis, a new genus of viverravine miacid (Mammalia, Carnivora). American Museum Novitates 2118:1-4.
.
Matthew WD 1937. Paleocene faunas of the San Juan basin, New Mexico. Transactions of the American Philosophical Society, new series 30: 1-510.
Simmon NB, Seymour KL, Habersetzer J, Gunnell GF 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451 (7180): 818–21. doi:10.1038/nature06549. PMID 18270539.
Simpson GG 1935. New Paleocene mammals from the Fort Union of Montana. Proceedings of the U. S. National Musem 83: 221-244.

wiki/Onychonycteris