At dusk the bats fly silently over our house in inner Sydney. Unlike birds, which coast through the air without apparent thought or work, the bat’s effort to stay aloft, with each wing-beat going from out wide to almost touching under its heavy body, is visible in the span. When I watch the bats, for reasons I can explain, I always think of David Chalmers, one of the world’s leading philosophers of consciousness.

I had the good fortune to sit next to David while watching Cathy Freeman run in the Sydney 2000 Olympics. Strangely, I don’t remember much of Freeman’s race. What I remember is David explaining in layperson’s terms his area of interest, which is philosophy of mind, and, particularly, consciousness: how far is it possible to understand what it is like to be another living thing? “What is it like, for instance,” he said, “to see the world as a dog? Or a bat?” David’s work is at the intersection of neuroscience and philosophy. “Surely,” I said, “that is a question for poets and novelists. It is a question for the imagination.”

Maybe so, but the question “What is it like to be a bat?” is at the foundation of late-twentieth-century philosophy of consciousness. In a famous essay of that title, the philosopher Thomas Nagel argued that even if we knew everything about how a bat’s brain works, this knowledge could not render for us the subjective experience of what it is like to be a bat. The conundrum is sometimes expressed like this: Mary is a neuroscientist who doesn’t see in colour – only in black, white and grey. Nevertheless, Mary understands in enormous detail how the brain works to see colour. One day, Mary sees red for the first time. She understands then that all the objective knowledge she had could not give her the subjective experience of seeing red. The most sophisticated understanding of the grey matter and electrical impulses under our skulls can’t render the experience of consciousness. The neuropsychologist Paul Broks puts it this way: “When we see the brain, we realise that we are, at one level, no more than meat, on another, no more than fiction.”

The bats that fly over our house are native grey-headed flying foxes, on their way to find nectar, flowers and fruit. Chances are, they are coming from a roost or “camp” in Sydney’s Royal Botanic Gardens a couple of kilometres from here. At the moment, camp numbers are up to more than 22,000. This is causing problems for the trees, for the Botanic Gardens administration and, soon, for our beleaguered environment minister, Peter Garrett.

As I spoke with Tim Entwisle, executive director of the Botanic Gardens Trust, the flying foxes chattered in the trees outside his grand office. Though they ostensibly sleep during the day, Entwisle says, “there’s always some activity going on.” And there’s the rub. As they hang upside down, they jostle for position on a branch, their sharp claws shuffling and scraping off the tree’s bark, its new shoots and leaves. The gardens have lost 18 trees in this way in the last 12 years, some of them precious ancient specimens. “From a flying fox’s perspective,” Entwisle says, “this is a great spot in Sydney to fly out from and around the city.” But from his own, it’s a matter of “great concern”. Over the past two years the gardens have allocated the equivalent of two people full-time to write a 500-page report for Garrett in order to obtain the approval that’s needed to move the flying foxes, which are a listed vulnerable species, elsewhere.

When the problem arose in the 1890s, the director of the gardens invited the shooters’ club in. These days methods of moving them on are more inventive. They aim, as Entwisle puts it, to give the bats a “bad hotel experience”. The gardens trust already has permission to attempt to move bats out of particular trees. They have tried hanging plastic bags in the trees and installing a huge, inflatable balloon man, like those seen outside car dealerships, to wave its arms around. They have tried running strobe lights and smearing snake poo on the branches (“Not”, the gardens’ public relations spokesperson winced, “a pleasant experience for anyone involved.”) What Entwisle proposes now is to play unpleasant music to them at brief intervals at dusk, as they leave, and at dawn, as they return, to encourage them to find a new camp.

The music Entwisle wants to play was made by Rodney van der Ree of the botanic gardens in Melbourne. It was used successfully there in 2003 to move a large camp of flying foxes 4 kilometres downriver to a national park in Kew. Flying foxes do not use sonar to echo-locate. “Their hearing range is remarkably similar to ours,” says van der Ree. “They dislike what we tend to dislike.” So the CD has noises on it that are “loud, grating and obnoxious”. I listened to it. It is the computer-generated din of our civilisation: garbage-tin lids clanging, chainsaws starting up, excavators digging.

But, as Peggy Eby explained to me, it is our civilisation that has effectively invited them in. If anyone has a chance of understanding what it is like to be a grey-headed flying fox, it is Eby, who has written her PhD thesis on them and studied them – with radio telemetry and satellite tracking, from Cessna aeroplanes, and for government environment departments – for 20 years. Eby explained that in the 1970s, as inner-city areas were gentrified, people planted native gardens, hoping to attract native birds. Before that time working-class backyards in Balmain and Paddington had chooks, vegies or weeds. “But I am 100% sure,” Eby says, “that no one stopped to think they’d be attracting the flying foxes too.” The problem seems to be partly the noise, partly the perceived threat of disease (flying foxes can carry the lyssavirus and Hendra virus, though infection directly from a flying fox is extremely rare) and partly cultural. “Deeply embedded in our European culture is a fear of creatures of the night,” she says. “People mention vampires.” (This although the bloodsucking bats are a whole other sort – tiny, echo-locating and Latin American.)

Eby will be part of a team monitoring the relocation of the flying foxes, should it go ahead later this year. She tells me she admires them for their intelligence and spacial memory, for their methods of communicating information about flowering events to one another, and for the important role they play in pollination and seed dispersal. At dusk they leave the Botanic Gardens in two streams of flight, which follow linear elements in the landscape. “We believe they use them for navigation,” Eby says. “In the bush they usually fly along rivers or creeks. In the city, they use major thoroughfares like Anzac Parade instead. It is a silent, measured progression that can last 20 minutes or more. I have watched people on footpaths, in office blocks, in apartments, in cars, in hotels, in hospitals stop and watch; and I know that just for a moment their imaginations shift.”

On a recent morning my young daughter and I found a flying fox on the footpath outside our neighbour’s place, where a magnificent gum is in flower. The creature was still perfect, with a golden ruffle of fur and a surprisingly pretty face, more like a six-month-old kitten than a fox. My daughter got a stick and stretched its soft wings out wide. “Maybe it just got tired,” she said. Behind us lay a carpet of fuchsia pistils from the gum, which had a look of morning-after disarray, as if it had hosted a party.

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