Tree Mice

          The first of the mammals arose from the ashes of the Permain extinction at the same time as the first of the dinosaurs. The reptiles rose faster – and came to predominate. They became the larger, swifter predators while the early mammals shrank away to occupy a niche they still occupy today – the mouse niche. They spent their lives in hiding, always alert, ready to dash for shelter, venturing out under the cover of darkness. The mammals would live this way for two hundred million years, throughout the Mesozoic Era.
          Those rodent-like creatures were similar to the frogs – ready to leap away just before the raptors of the day pounced. But they were also similar to the dinosaurs – both were warm blooded. So they were better able to forage in the chill of the night than their cold-blooded, web-footed ancestors.
          The tree frogs took to the trees to escape the terrestrial predators. So did the early mammals. They were small enough to concealed themselves under dark leafy recesses by day. They kept in touch with each other through the shadows or the dark of night as did the tree frogs, by quiet chirping. With their sensitive ears, they listened for one another, and for their insect prey dropping onto nearby leaves – and for their predators slithering through the branches.
          Over the millions of years of their early existence, these tree mice evolved improvements in their survival strategies. Some of them became gliders. A membrane like that of the flying squirrel extended along their flanks, connecting hands and feet. They sailed from one grove of trees to the next at dusk or by moonlight – never touching the ground.
          Among those gliders another alteration appeared. A single tree mouse was born with the gliding membrane extended into the space between its digits. This membrane is present early in the embryonic stages of all the animals that evolved from fish. In dryland animals it is usually resorbed well before birth. But the timing was delayed in that particular mutant tree mouse – it was born with some webbing still in place between its fingers, like a frog.
          This aberrant, clumsy-fingered pup was slow growing, and needed extra help from its mother to survive. But when it was finally independent enough to leap and glide, the finger-webbing provided it with an advantage. When it opened its hand to reach for a landing, it found it could steer itself through the air – as did the flying frogs, gliding on their webbed feet. The new flying ability out-weighed the clumsy-fingered disadvantage, so this mutant tree mouse survived to pass the improvement of webbed fingers down to its progeny.
          The pace of evolution is too gradual to see from one generation to the next. The amount of webbing did not change between the fingers of these newer tree mice for thousands of years. But over that time the webbed-fingered individuals came to make up an ever-larger proportion of their lineage. One day another mutant was born with slightly longer fingers, and a proportionally greater surface area of webbing between them. More thousands of years (and many thousands of generations) later, most of the gliding tree mice in her lineage had those larger gliding hands.

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          The tree mice continued to perfect their survival strategies, always just a jump ahead of their predators. The dinosaurs had by then evolved their own capacity for gliding flight and pursuit. But the next step for the tree mice would not be a change in anatomy, but a change in behavior. They would begin evolving within their own heads.
          These were social animals – they lived to communicate. They chirped back and forth whenever they were awake, even when gliding. They listened for answering calls, and recognized each of the respondents who called from close by or from far off through the woods. But some of sharpest-hearing individuals among them noticed a fainter answering call, which came from a stranger.
          This unidentified return call always sounded immediately after their own call, often so soon after as to be superimposed in time – as though the caller was right next to them – where there were only bare branches. Finally, these attentive callers recognized that the response depended upon the direction they were looking when they called.
          These tree mice did not realize they were listening to the sound of their own voice. But they did come to associate the fast echo with the tree-top topography around them. They heard a soft repeat of their call from the leafy boughs off to one side; a stronger, delayed reflection from the hard trunk out to the other side; and no echo at all returned when they called straight ahead through the clearing in the trees. As evening stretched into night, these animals recognized that the same echoes come back from the same objects – even after their visible forms had dissolved into darkness.
          When they took to the air, chirping and listening for the reflected sound, they found the echo helped to guide them between obstructions. They learned to treat the calls of others of their group as distractions to be filtered out. As they perfected their chirp-guided navigation, the dusk faded. They learned that they could sail through the evening and into complete darkness, guided only by reflected sound.
          These air-borne mammals were evolving within their own brains. The brain controls the motions of a moving animal in response to sensory in-puts. The part of the brain that controls those motions is next to the visual cortex. That cortex is a flat screen that displays the moving image of the visual field carried back from the retina by the optic nerves. These first mammalian flyers learned to send audio information to this part of the brain that guides flight around obstacles, using the same neural circuitry that responds to vision.
          The capacity to form a two-dimensional image using non-visual senses had evolved before – with the predators of the mammals. The pit vipers see their warm-blooded prey in front of them by tracking infrared radiation, through heat-sensing pits in their snouts. Their brains visualize this in-put on a two dimensional field, the same way they see in-put from their eyes. This capacity would arise again when the cetaceans took over the oceans from the dinosaurs. The whales and dolphins would guide their movements through the water using audio sonar. And finally, even a few birds would learn to guide their flight by echolocation.

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          The ability to see by sound through the dark complemented the ability of the tree mice for flight. Those two skills improved in parallel over the eras. Finger bones lengthened to the lengths of the animals’ bodies, with the webbing expanding to a flying hand as long as the wingspan of a bird. Echolocation improved as the audio frequency of the chirps increased. Then their echo-vision came to reveal a previously unknown food source for them – insects on the wing at night.
          A whole new lifestyle became available to these animals, driving the completion of their evolution from tree mice – to bats. They perfected powered nighttime flight and moved into hundreds of previously unexploited niches. And as they explored this new life style, they discovered new communications, from an unexpected source. Their ears again noticed hypersonic chirping from a stranger. This time, it was coming from animals in an entirely different phylum – from their insect prey.
          The tiger moths are protected from predation because they are distasteful. Visual predators can learn to avoid them by their stripes. But their coloration cannot be seen in the dark. So they have developed an audio warning to communicate their protected status to the bats.
          Bat vision contains a dimension that is unknown to animals that see by light. The sun produces the light that day-flying creatures see by. Those animals perceive depth through parallax cues. They compare their fields of view from moment to moment, or between one eye and the other, and they notice how much more slowly the distant objects pan across the field than do the nearby ones.
          However, the bats themselves produce the sound that they see by. Since they are the source of the signals that guide them, they are more actively involved in how they see. They can perceive depth not only passively, by tracking relative movements
of near and far objects across their field of view, but also actively.
          As objects come closer, the sound the bats broadcast echoes back to them more quickly. They compute the time it takes their own sound to reflect back, and add this information as a depth cue to the texture their vision. This provides them a richer sense of depth than the light seers will ever know.
          When a tiger moth hears the approaching calls of hunting bats, it calls back on the same frequency. When bats see a tiger moth – drawn in reflected sound on their visual field – the first thing they notice is that the insect is out of focus. The bats loose depth perception on their target, because some of the return sound that they hear is not their own, and the extra chirps confuse their depth calculations. Their attack misses badly.
          If the bats persist long enough to finally get a bite on a tiger moth, they taste the poisonous flavor of the insect and drop it back into the air. The repellent flavor is in the wings, and if the leading fore-wing edge is not broken, the tiger moth can fly away from the attack. The bats learn that when they hear this moth chirping to them, it is a warning call – from a creature best left alone.

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          A silent swarm of Tufted Bats materializes from the forest in the dim predawn of the present day. Like a flurry of shadows they scurry back and forth along the river bank – touching down on the trees for a second or two then bursting back into the air. They are selecting the day’s roosting spot. In an instant the cloud of bats contracts, then disappears. The animals have landed, one after the other, to position themselves single-file, nose-to-tail, on the underside of a branch over the water.
          The line of resting bats remains in plain sight as the tropical sun ascends the sky. Its members sit motionless, sleeping lightly, until one of them shifts position, perhaps in response to some perceived threat. Then the bat behind the first shifts as well, to realign directly behind the individual in front of it. This shift in position propagates down the line as each successive bat scuttles sideways to re-establish the nose-to-tail order.Tufted Bats at rest.
          From a bird’s-eye-view through the leaves, the row of bats appears to be a single creature slithering across the trunk. Its segments slide to the side one-after-another – the tail section the last to come to rest – like an arboreal viper setting an ambush position. The resemblance is enough to startle the occasional kestrel or civet cat.
          Should a threat persist and move closer, the entire group of bats explodes in unison back into the air. They swarm together so closely that a flying predator would struggle to pick out a single target. In a few seconds they have all faded away through the foliage.
          The behaviors of the bats have been evolving evermore complexity throughout the Cenozoic era, over the last sixty five million years. They have advanced through the air in parallel with the day-time expansion of the feathered dinosaurs. Both lineages have survived from prehistoric times, in the same space but at different times of day, filling the sky right up through the present.
          The bats are now an integral a part of the biosphere, as are the birds. Modern forests have co-evolved with the flying mammals. The insectivores among them have modified the populations of the nocturnal insects, which are the main herbivores in the forest. Many of the trees now flower at night, and depend on the nectar bats for their pollination. Fruiting trees in the tropics depend on fruit bats for the dispersal of their seeds. Some of these “flying fox” fruit bats have wingspans six feet in length.
          The bats and the birds trade dominance of the air between the night and the day. Small birds are subject to predation by bats that attack them while they sleep on their perches, or during their nighttime migrations thousands of feet up in the air. Then, when the light rises in turn, small bats become subject to predation by visual hunters, such as the Bat Falcon.
          The bats have diversified to fill thousands of niches, to become the second most populous of the mammalian orders in number of species (behind the rodents). Their numbers of individuals rival those of the birds as the most populous of the larger animals.

Tree Mice, notes. The mammals and the dinosaurs differentiated from their presumptive amphibian forebears some 250 million years ago. There are now more than 1,200 different species of bats (3 of which are vampires); they make up a fifth of all the species of the mammals. The pitch of the bat’s chirp has risen over evolutionary time (Speakman, 2001). By now their echolocation calls are too high (ultrasonic) for humans to hear. Higher frequencies allow for greater chirp rates, and finer spatial discrimination. Moths that communicate with bats, such as Tiger Moths, use those frequencies (Corcoran & Laner, 2012). The birds that echolocate still use audible frequencies (Bniklof et al, 2013) suggesting that their echolocation is a recently evolved trait. The greatest diversity of the bats is in the tropics. In the rain forests of Indonesia and South Asia, they share the treetops with animals that still glide, such as the calugo, and Wallace’s Flying Frog. The bats, like the birds, gave up the use of their arms for movement along the branches – for the capacity to fly. Milestones in their evolution of night flight came about largely between their ears. Their brains have evolved the capacity to process the time differential between the broadcast and return of their echolocation call. That differential is in milliseconds. The calculation runs continuously while they are on the wing, and provides the depth in their visual field. The visual cortex is a part of the brain that renders visual in-put into a two dimensional field of
view. However, it is a likely locus for the processing of other sensory in-put for two-dimensional representation. E.g, the visual cortex is the region that becomes activated in the brains of echolocating humans (Thaler et al, 2011) as they move forward guided by sound. By day and at dusk, predatory birds such as the tropical Bat Falcon prey on bats. The extent of bat predation on birds is less well known, since it happens in the dark. Predatory bats in the tropics prey on small resting birds. They recognizing them by the distinct change in the echo caused by the texture of their feathers. In the old world, the Greater Noctule Bat preys on small flying birds (Fukui et al, 2013) in midair during their nocturnal migrations.

Brinklof, S et al (2013) Echolocation in oilbirds and swiftlets. Frontiers in Physiology 4, 123

Corcoran, A J, Laner, W E (2012) Sonar jamming in the field: effectiveness and behavior of a unique prey defense. Journal of Experimental Biology 215, 4278-87

Fukui, D et al (2013) Bird predation by the birdlike noctule in Japan. Journal of Mammology 94, 657-61

Speakman, J R (2001) The evolution of flight and echolocation in bats: another leap in the dark. Mammal Review 31, 111-130

Thaler, L et al (2011) Neural correlates of natural human echolocation in early and late blind echolocation experts. PLoS ONE 6, e20162

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