Earliest bat shows flight developed before echolocation

Blogging on Peer-Reviewed ResearchTheir heads and bodies of bats have amassed an extraordinary array of adaptations that have make them lords of the night sky. Today, the thousand-plus types of bats make up a fifth of living mammal species. Richard Dawkins once described the evolution of bats as “one of the most enthralling stories in all natural history” and as of this week, the story has a clearer beginning.

OnychonycterisThe success of bats hinges on two key abilities: their mastery of flight, a feat matched only by birds and insects; and echolocation, the ability to navigate their way through pitch-blackness by timing the reflections of high-pitched squeaks. For evolutionary scientists, the big question has always been: which came first?

The ‘clawed bat’

Until now, fossil bats haven’t provided any clues for all of them show signs of both echolocation and flight. But a stunning new fossil, discovered by Nancy Simmons from the American Museum of Natural History is an exception and it provides a categorical answer to the long-running debate – the earliest bats could fly but could not echolocate.

The new creature hails from the Green River in Wyoming and is known as Onychonycteris, meaning “clawed bat”. Its fossils date back to about 52.5 million years ago and by comparing it to other prehistoric bats, Simmons found that it is the most ancient member of this lineage so far discovered. It acts as a ‘missing link’ in bat evolution, much like the famous Archaeopteryx hinted that birds may have evolved from dinosaurs.

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Moths mimic each others’ sounds to fool hungry bats

Impressionists are a mainstay of British comedy, with the likes of Rory Bremner and Alistair MacGowan uncannily mimicking the voices of celebrities and politicians alike. Now, biologists have found that tiger moths impersonate each other too, and they do so to avoid the jaws of bats.

A distasteful tiger moth, part of a syndicate of mimics. Some creatures like starlings and lyrebirds are accomplished impersonators, but until now, we only had anecdotal evidence that animals mimic each others’ sounds for defence.

Some harmless droneflies may sound like stinging honeybees, while burrowing owls deter predators from their burrows by mimicking the distinctive warning noises of deadly rattlesnakes.

In tiger moths, Jesse Barber and William Conner from Wake Forest University, North Carolina, have found the first hard evidence of acoustic mimicry in animals. Tiger moths are hunted by bats, which use ultrasonic clicks – echolocation – to home in for the kill.

Moths are tuned into the sounds of these clicks and respond with their own ultrasonic sounds, created by vibrating special membranes called ‘tymbals’ on their abdomens (see a Quicktime video of the tymbals in action) .

The sounds are multi-purpose – they may startle the bats, or jam their transmissions. But according to Barber and Conner, they also carry a message – they say “Don’t eat me, I won’t taste very nice.”

Mimicking moths

The duo worked with two species of bats – red bats (Lasiurus borealis), which eat butterflies and moths and big brown bats (Epstesicus fuscus), which dine mostly on beetles, but will take the occasional moth.

A bat captures a tethered moth.They raised bats in the lab and trained them to hunt live but tethered moths (right), while using high-speed video cameras to capture the split seconds of the attack (Quicktime video). They also recorded the various ultrasonic clicks used by both moths and bats.

Over five nights, Barber and Conner presented the bats with one of two species of foul-tasting tiger moths, the dogbane tiger moth (Cycnia tenera) and the polka-dot wasp moth (Syntomeida epilais).

The two species look very different, but they do absorb poisons from the plants they ate as caterpillars. All the bats learned to avoid the first species, and when they were shown the other, they avoided it too even though they had never seen it before (Quicktime video).

But they had heard it, or at least, a moth that sounded very much like it. To show that this was the case, Barber and Conner silenced the second species of moth by surgically removing their tymbals. The bats eagerly attacked the now-muted moths, although they quickly spat them out again in distaste.

Fake mimics

This was clear evidence of a type of impersonation called Mullerian mimicry, where two or more distasteful or dangerous creatures provide predators with the same warning. But the tiger moths also show Batesian mimicry, where a delicious and defenceless animal pretends to be a more noxious one.

Cycnia tenera, a distasteful tiger moth, and a member of a syndicate of mimics.Barber and Conner repeated their experiment and trained the bats on the unpalatable dogsbane moth. But then, they offered them the milkweed tiger moth (Euchates egle; right), a moth without any chemical defences and a tasty treat for a bat. It didn’t matter – the bats were put off by its pretense and avoided it like the others.

However, some bats did wise up. Red bats specialise in catching butterflies and moths, and three of them started to attack the vulnerable milkweed tiger. After a day, they had learned that these charlatans were in fact quite edible (Quicktime video).

When they were offered the unpalatable dogsbane tiger again, they avoided it (unless its tymbals were removed). They had quickly learned to tell the difference between the sounds of the impostors and the real deals.

The moths’ charades are likely to be a common strategy. There are, after all, 11,000 species of tiger moths worldwide, many of which live in overlapping geographical ranges. Other insects too, including hawkmoths and tiger beetles, respond to approaching bats with ultrasonic clicks. For all we know, the night skies around us are full of the cacophony of impostors and impressionists.

Reference: Barber & Conner. 2007. Acoustic mimicry in a predator-prey interaction. PNAS 104: 9331-9334.

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Related posts on bats:
Bats create spatial memories without making new brain cell
Bats: internal compasses and record-breaking tongues

And a post on the best mimic of all, the mimic octopus:
The mimic octopus – a master of disguise

Bats create spatial memories without making new brain cells

Neurogenesis in the hippocampus is not necessarily a source of new spatial memories.In an earlier post, I wrote about a study which used carbon-dating to show that our brains are mostly stuck with the same neurons they are born with. After birth, neurogenesis – the manufacture of new neurons – is completely absent in most of the brain.

There are only two exceptions, where new neurons are made. The first is the olfactory bulb, which governs our sense of smell. The second, the hippocampus, is involved in spatial awareness and memory. Why these regions alone should produce fresh neurons is unclear.

For the hippocampus at least, scientists thought they had an answer – the fresh neurons play a role in spatial learning and memory. They could allow mammals to learn about new places, routes and directions.

But Imgard Amrein and colleagues from the University of Zurich have found evidence that disputes this idea. When he looked at the hippocampuses of some of the most accomplished mammal navigators, the bats, he found a startling lack of neurogenesis.

Bat-brains

Bats need superb spatial awareness to effortlessly fly in three dimensions. Those that feed on fruit and nectar need especially good spatial memories, and indeed, their hippocampuses are relatively large compared to other mammals.

Bats are some of the best navigators among the mammals.Their memories allow them to remember where the tastiest or ripest food sources are. And they also remember the locations of plants they have recently visited so that they don’t arrive at restaurants with no stock.

Amrein searched for signs of new neurons in 12 species of bats using special antibodies. Some detected proteins that only appear when new cells are born. Others homed in on proteins used by newborn neurons when they migrate to new places.

As expected, these molecular trackers picked up new neurons in the olfactory bulb. But they found no neurogenesis at all in the hippocampus of 9 species, and only the faintest traces in the other three. Clearly, the bats don’t need new hippocampal neurons to learn where things are or to remember how to find them.

Flexibility vs consistency

While Amrein’s bats were few in number, they were also a diverse bunch. They hailed form different evolutionary groups and had diverse diets, territory sizes and ages. This makes it unlikely that these variations in these factors were secretly responsible the trends that Amrein saw.

Instead, he believes that the dearth of new neurons in bats reflects their relatively long lifespans. Humans, apes and monkeys are similarly long-lived, and we too have low levels of neurogenesis as adults.

In contrast, rats and other rodents have short and brutal lives. In order to avoid becoming food for a predator, their behaviour must be as flexible as possible. When threatened, their stream of new hippocampal neurons could allow them to rapidly plan an escape route or find new hiding places.

Bats, and certainly humans, have far fewer predators, and can afford to take things easier. In our long lives, fixed long-term mental maps are very useful and to produce them, we can sacrifice some flexibility in our spatial memories.

This may explain why people tend to rely on the same routes more and more as they age. Fortunately for us, bats show a similar trend. Their reliance on the same flight paths allows canny researchers to catch them in well-placed nets and study how their brains work.

More about bats: 
Moths mimic each others’ sounds to fool hungry bats
Bats: internal compasses and record-breaking tongues

More about neurogenesis:
No new brain cells for you – settling the neurogenesis debate

More about neurons:
Simple sponges provide clues to origin of nervous systems
Monkeys (and their neurons) are calculating statisticians
Non-coding DNA drove brain evolution by making nerve cells stickier
Maternal hormone shuts down babies’ brain cells during birth

 

Reference: Amrein, Dechmann, Winter & Lipp. 2007. Absent or Low Rate of Adult Neurogenesis in the Hippocampus of Bats (Chiroptera) PLoS ONE.

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Bats: internal compasses and record-breaking tongues

Bats are masterfully adapted to life in the night skies. Now, two new studies demonstrate previously unknown abilities in bat species – an inbuilt magnetic compass, and a tongue so big it has to be stored in the rib cage.

If you were a biologist looking for astounding innovations in nature, you could do much worse than to study bats. Bats are like showcases of nature’s ingenuity, possessing a massive variety of incredible adaptations that allow them to exploit the skies of the night.

Bats navigate their way at night with echolocation and a magnetic sense. They are the only mammal group capable of true flight and are one of only four groups of animals to have ever evolved the ability. As a result, they have spread across the globe with tremendous success. Today, one in every five species of mammal is a bat.

They are most famous for their incredible echo-location. By listening for the echoes of sound waves rebounding off solid objects, bats have been finding their way in the dark using a radar that humans have only managed to duplicate millions of years later.

Magnetism

Echolocation is a short-range skill. Over longer distances, the bats’ ability to control their signals and perceive their echoes weakens, and other navigational skills must be used.

Until now, it was unclear what these might be. Richard Holland and colleagues from Princeton University have changed all that by showing that a large North American species, the big brown bat (Eptesicus fuscus), finds its way home by using the Earth’s magnetic field as a compass.image-big-eared-townsend-fledermaus.jpg

Holland took several big brown bats 20km away from their roosts and tracked their way home with radio telemetry. Before they were released, the bats were acclimatised at sunset to a magnetic field that was rotated to face either east or west.

For 5km, the group exposed to the easterly oriented field flew east, and those exposed to the westerly oriented field flew west. They were clearly using a magnetic compass which had been calibrated during sunset.

After initially losing their bearings, many of the confused bats corrected themselves and found the right way home. Holland believes that they recognised that the direction they were flying in did not match their magnetic map, possibly through other cues such as the position of the stars.

Other species including turtles and homing pigeons are known to navigate with a magnetic sense, but this is the first time the ability has been shown in bats. It adds to the already impressive array of senses that these animals possess.

A wicked tongue

Bats are also known for the variety of different food sources they exploit. Some species specialise in snatching spiders from their webs using their peerless radar, others take fish from lakes and perhaps the most famous representative drinks the blood of other mammals.

And many species make a living by drinking the nectar of the bountiful flowers found throughout the tropics, like leather-winged equivalents of hummingbirds.

Hummingbirds have evolved highly specialised relationships with flowers, so that some flowers are only accessible to a single species with the correctly shaped bill. Until recently, no such partnerships were seen between flowers and bats, mainly because the bat’s soft facial tissues are less easily moulded by natural selection than the bird’s hard bill.

The tube-lipped nectar bat (Anoura fistula) from the Andean forests of Ecuador is a striking exception (see left; photo by Nathan Muchhala).

The tube-lipped nectar batThese cloud forests are home to a plant called Centropogon nigricans that has flowers 8-9cm long. No ordinary bat can feed from these.

Nathan Muchhala from the University of Miami discovered that the tube-lipped nectar bat manages it with a tongue that is 50% longer than its body. In terms of relative size, its tongue is second only to the chameleon’s.

But where does a 5.5cm long bat store an 8.5cm long tongue? In most mammals, the base of the tongue is attached at the back of the mouth. But in the nectar bat, it is stored in its rib cage and its base lies between its heart and its sternum. Muchhala believes that the bat’s tongue and the Centropogon’s flower co-evolved to extreme lengths over time.

He captured various local bats over four months and found pollen from Centropogon nigricans only on the fur of the tube-lipped nectar bats and not related species.

Due to their exclusive relationship, the bat gets a permanently reserved dining spot and the flower gets a dedicated pollinating service.

Holland, Thorup, Vonhof, Cochran & Wikelski. 2006. Nature 444: 702.

Muchhala. 2006. Nature 444: 701.

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