The Mysteries of Chester Zoo's 'Twilight Zone' explained

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Problem: how to balance the needs of both the bats and visitors to the Twilight Zone at Chester Zoo by keeping them both comfortable. As a rule of thumb, nocturnal animal exhibits in zoos in the past have always been somewhat of a disappointment. Many times in the past, people have tried to peer into pitch-black glass cubes to catch glimpses of elusive dark-dwelling creatures, or into brightly lit ones, that only result in the animals doing their best to hide away in the dark parts.

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However, advancements in understanding of animal behaviour and design of artificial habitats means that the paying public may nowadays be able to see the night-living animals that so eluded us in the past. And what animal more defines this than the bat? Bats - order Chiroptera -are either classified as megabats (Megachiroptera) or 'microbats' (Microchiroptera). The 'microbats' are the only ones that use echolocation, contrary to most belief that this 'SONAResque' method of locating obstacles and food is common across the entire Chiroptera order.

The 'megabats' use eyesight and moonlight or starlight to see at night, though 'microbats' are capable of seeing with their eyes as well, usually at dawn or dusk. Chiefly insectivores, many bats eat fruit or a combination of the two, or as is the famous (and possibly best known example, Desmondus rotundus, the Vampire Bat, on the blood of other mammals. These two important facts about keeping bats means that quite often, if to replicate their natural environment and feeding habits, ideally a group of bats with similar feeding habits should be provided, and light akin to moonlight should be in the enclosure.

To see if these could be successfully implemented, I visited Chester Zoo to see the new bat habitat, labelled to visitors as the 'Twilight Zone'. From the outside, certainly, it is an unimpressive building, just a green-painted corrugated iron. Inside however, the atmosphere changes greatly. It was a dark, grey and horrible January Wednesday when I visited the zoo, but stepping into the Twilight Zone really was a change from outside -my eyes had to spend some time readjusting, and the carrot cake I had for dessert last night hadn't helped at all.

When it was possible to see further than my hand at arm's length, a sign on the wall advertised the three types of bat living in the Twilight Zone. Three ... .the tourist guide and the website had only advertised two. The Rodriguez Fruit Bat or Rodriguez Flying Fox), Pteropus rodricensis, is possibly the most important venture of the zoo's because in the wild, like many other endangered or now-extinct species, it survives in a very small, specific area of land - in this case the Island of Rodriguez, a part of Mauritius -the island once known to be the habitat of the extinct dodo.

This island is just east of Madagascar, in the Indian Ocean, and the bats live in a single valley -'Cascade Pigeon' -which is made of dense, mature rainforest trees. Frequent cyclones and vicious tropical weather mean it is dangerous for the bats to stay out in the open, which is why they live in the dense forest. However, this habitat is threatened by human deforestation, and the lack of trees main that there is also little food for the bats now in the wild.

It will be referred to as the Rodriguez Bat from now on in this article 1. 'The Seba's Short- Tailed Fruit Bat (Caronia perspicillata) is the smallest bat that the zoo keeps, and the only 'microbat', the two Pteropus both being 'megabats'. It can grow to have a six inch wingspan (15 cm) and a mass that can range from a tiny 5g to 22g for fully grown males. For the purposes of clarity in the rest of this report, it will be referred to as the Seba's bat 2.

The third bat type, (the one not noted on the Chester Zoo website funnily enough), is either called the Comoro Black Flying Fox, (below) or Livingstone's Bat, is the biggest of the bats the zoo has, with a fully grown adult able to have a wingspan of around four and a half feet (130cm) and a maximum weight in captivity of nearly a kilogram. (In the wild this is slightly lower; the higher quartile of weight being about 800g, though the wingspan can be much larger, up to one and a half metres).

The inside of the Twilight Zone at Chester Zoo certainly belies the rather ordinary exterior. Once one's eyes adjust to the darkness, it's clear that a lot of care went into making the area around the entrance resemble a cave. Artificial rock has been set up in a way that arches over one's head, and much like in real caves, it is necessary (or it feels necessary) to duck through some of the arches for fear of cranial injury.

Double plastic curtains stop the bats from escaping from the area, though a look round makes one wonder exactly why they would want to. Though the roof is still obviously corrugated iron, but the artificial rock continues around the sides of the warehouse, even creating an entire cave for the bats to roost should they want to. Shallow blue-white light illuminates the area, making it seem like moonlight (important for the two Pteropus), and when I first entered, I had to stand still for a while just taking all the sights in.

The Livingstone's bats are an impressive sight to see swooping around and blotting out the light, whilst the Seba's bats simply flit around so quickly you can barely see if they're there. Wires are hung up above the 'rock' cliffs for the bats to perch, and it is obvious many do so -the wires are almost full of the three different sorts of bat, hanging and letting out their various chirps -the higher squeaks from the Seba's and the bird- like caws from the Rodriguez bats. There is a second cave, and a very dense forest area in one corner, not for access to the visitors.

According to one of the staff on duty there, many of the Rodriguez bats stay there during the daylight hours, whilst the majority of the Seba's bats and the Livingstone's roosted in the cave. The second cave, the one the brave (or mad) members of the public are allowed to walk through is habitat to the chief male of the bat brood and his 'harem' of all the females. The forested area and all the other examples of vegetation are an attempt to make the bats more comfortable, and many of the trees are taken from the African islands the bats come from or ones near to that point as well.

The fruit trees themselves that the bats fed on, most particularly across the three bat types being Tamarinds, couldn't be kept in the UK because of the heat requirements -most of the other trees in the Zone don't require as much heat (they do not need to produce such sweet and ripe fruits). Also in the middle of the zone is a small lake, where the zoo keeps its' Mexican Blind Catfish (Prietella phreatophila), Southern Stingrays (Dasyantis americana) and Electric Eels (Electrophorus electricus).

None of these fish are prey or predators of the bats, and they require low light as well. The nocturnal activity of bats is mimicked by the zoo by slowly adapting the bats to slight changes of about an hour when they change the light intensities. The shallow blue-white light is similar to moonlight, so it represents the night time; there are gradients of filters that can be placed in, so when the lights are orange, red or pink, it can indicate the change from dawn to dusk.

Daylight bulbs are used rather than traditional filament or argon strip lighting during the 'daylight hours' so as not to confuse the bats should they be approached with real sunlight, though at that time they would normally he roosting. The normal day light routine is reversed for the bats, so the visitors to the zoo manage to see the bats when they're active. Staff on duty in the Twilight Zone use red light in their torches to examine the bats even during the 'night' time. This is because bats are unable to see red as well as humans are and the light will not damage their eyes so much.

According to the zoo staff, any of the bats in the Twilight Zone could be released into the wild with no problems about adapting, as bats rarely venture much further from their roosting caves than the length of the Twilight Zone. Often, the plants bearing the animals' preferred fruit will grow in greater abundance close to the caves, as the bats choose to fly short distances, and the fruits that grow closer to the caves have a higher chance of having their seeds placed in a similarly close radius.

Bat faeces, known as guano, are very vitamin rich and provide an excellent fertiliser, so where bats hunt most is where plant life is most likely to grow. (Another useful aspect of guano, as this reporter found out, that should it get on clothes, it is relatively easy to remove by simple machine washing. ) This is a clear indicator that the Twilight Zone has done well in replicating the natural environment of the three bats - signs of stress on an animal, such as repeated circling, or staring into middle distance, were not detected in any of the bats -they were alert, active and there were no fights over food or mates among the bats.

Has Chester Zoo succeeded in mimicking the bats' feeding habits as well though? The three species of bat are mostly herbivores, and fruit is the mainstay of their diet, mostly because of the glucose stored inside fruit which allows for quick bursts of respiration, all the bat needs to fly from tree to tree in search of more food. The favoured fruit of both Pteropus is the Tamarind fruit that grows on the tropical tree of the same name. The fruits themselves resemble broad beans, brown in colour (see right) and about three to ten inches long.

When mature, it has a brittle fibrous exterior, but a soft and sweet pulpy interior with about 12 seeds inside -ideal food for bats with a strong bite to break through the outer husk4. However, it is difficult to grow Tamarinds in Britain, so the zoo provides other sorts of fruit for the bats. These include bananas (unpeeled -bats generally have claws or they are able to bite open the skin of the banana; this is the same as the situation as they would be faced with in the wild -bananas are not unpeeled hanging from trees), grapes, grapefruit and melons.

The food is grown specifically for the zoo's needs, with no pesticides (as it should be in their natural habitat) and no visitors may take in any food for the bats. Unfortunately, this reporter wasn't aware of this until he'd bought a box full of grapes and asked one of the staff on duty. The bats are kept on a very tight regime of food, to represent the likelihood of getting their food in the wild -a lot of the times when the bats actually fly out to get their food is when the winds blow the scent of the food to them.

They can therefore expect to feed well twice a night. The zoo provides a hundred and fifty fruit bowls around the edges of the Twilight Zone for the bats to feed from with about 100g in each. These are meant to mimic the small scraps of fruit that may have been dropped on the ground -as such the Pteropus usually feed from these as they are more ground-mobile than the small Seba's bats. Often, echo locating bats will avoid the ground because of all the mixed signals they can get from the debris on the floors of forests or caves.

This is a reason why most moths that are hunted by insectivorous bats will land on the ground to try and avoid the bat. For the Seba's bats, the trees planted in the Zone and also branches hanging from the roof have sharpened points on which the larger pieces of hit such as grapefruit, banana and melon (pieces of honeydew mostly, not whole watermelons! ) are speared -this gives all the bats adequate opportunity to practice flying and grabbing onto a 'fruit tree' and eating off one of the branches.

This training for the wild became apparent as did the Seba's bat's incredible sense of smell when I was talking to one of the staff on duty, with the aforementioned box of grapes sealed and placed in a paper bag by my side for my own lunch rather than a 'midnight' snack for the bats. It was after talking for a while that it seemed the smaller bats were being friendlier than usual, flapping around my head. However, their real purpose became apparent as a 'thump' against my leg proved to be one of the bats having held onto the paper bag and proceeding to try and gnaw through it.

Having shooed him (or her) away, I was suggested another test. A grape of the zoo's provision was given to me, and I went into the cave where the dominant Seba lived along with his harem. I held the grape between my fingers in the near-pitch blackness, and after about a minute, I could hear and feel flapping wings around me, just before the small fruit was yanked out of my hand. That experiment, crude though it was, most likely shows that the bats know how to adapt to a new presentation of food and take it.

Unlike some animals in zoos who associate humans with food, the bats at Chester Zoo do not need humans to feed well. In fact, the only problems that would result from releasing them into their environments once they'd reversed their day-night sleeping pattern is the threat of predators such as hawks and snakes for the little Seba's bats, and cyclones and other hostile environmental factors for the Rodriguez bats. The Rodriguez bat breeding programme has also been a great success, with over a quarter of the 200 bats in European Zoos living in the Twilight Zone.

Though a seemingly small number, there are an estimated 1000 bats in the wild and any steps taken to protect the animals are important. Perhaps these free-flight caves are one of the best zoo exhibits in existence today. These bats behave exactly as they would in the wild, in a place we humans can see them and experience what it's like to be near them, without the dangers of predators, storms and -with zoos providing adequate medical care -disease.

Perhaps these aviary ideas will catch on with other animals -if so it would be a huge leap forward from the early days of zoos where animals were stuffed into much too-small cages. Already some zoos have exhibits where the visitors can pass through the natural habitat albeit protected, this is already heavy in practice with most aquariums, and this reporter has also seen this in a French zoo, where a steel observation bunker was built in the lion enclosure for people to peer the vision slits at the lions while actually in the enclosure.

Also, drive-through safari parks operate this method of bringing people closer to nature. So, begging the question, why not continue this idea on with more animals? Though there is a risk of people who do not respect the animals visiting the enclosures, why not start walkways through the flamingo enclosures? Swim tanks with the penguins and other safe water life? Many animals wouldn't dare hurt a human, (like the little fruit bats) so why keep them at more than arm's reach? Sources and reliability:

Much of the information garnered for this report came directly from the keepers at Chester Zoo, particularly the information concerning bat dimensions in the wild and what food is fed to them. Some information was found through simple observation of the bat behaviour in the Twilight Zone, or looking at the surroundings. Both of these were cross-referenced against the University of Michigan's zoology pages, which have been a very concise and interesting form of information. (Some other links provided may be from this following site)