Bats: compasses, tongues and memories

In a (very) loose tie-in with the recent release of the Dark Knight, it's Bat Weekend at Not Exactly Rocket Science, where I'll be reposting a few old but relevant pieces. If you were a biologist looking for astounding innovations in nature, you could do much worse than to study bats. They are like showcases of nature's ingenuity, possessing a massive variety of incredible adaptations that allow them to exploit the skies of the night.

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 and enjoyed tremendous success. Today, one in every five species of mammal is a bat. None of them beat up criminals, but some have internal compasses, others have record-breaking tongues, and others have unique spatial memories.

An inbuilt magnetic compass

Bats 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. But this sonar 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 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.

Holland took several big brown bats 20km away from their roosts and tracked them with radio telemetry as they flew home. Before they were released, the bats were acclimatised at sunset to a magnetic field that was rotated 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 tongue so long it's stored in the ribcage

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 (which are an integral part of its sonar powers) 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. These 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.

Batty memories

Throughout our lives, our brains are mostly stuck with the neurons we are born with. After birth, neurogenesis - the manufacture of new neurons - is completely absent in most of the brain, with a couple of exceptions - the olfactory bulb, which governs our sense of smell, and the hippocampus, which is involved in spatial awareness and memory.

It's unclear why these regions alone should produce fresh neurons but for the hippocampus at least, scientists thought they had an answer. There, the fresh neurons are thought to play a role in spatial learning and memory, allowing mammals to learn about new places, routes and directions.

But in bats - some of the most accomplished of mammalian navigators - Imgard Amrein and colleagues from the University of Zurich found evidence that disputes this idea. 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. 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.

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.

References: Holland, R.A., Thorup, K., Vonhof, M.J., Cochran, W.W., Wikelski, M. (2006). Navigation: Bat orientation using Earth's magnetic field. Nature, 444(7120), 702-702. DOI: 10.1038/444702a

Muchhala, N. (2006). Nectar bat stows huge tongue in its rib cage. Nature, 444(7120), 701-702. DOI: 10.1038/444701a

Amrein, I., Dechmann, D.K., Winter, Y., Lipp, H., Baune, B. (2007). Absent or Low Rate of Adult Neurogenesis in the Hippocampus of Bats (Chiroptera). PLoS ONE, 2(5), e455. DOI: 10.1371/journal.pone.0000455