Pro and anti the Grey-headed Flying-fox – the discussion about these creatures ‘causing trouble’ in urban areas like Sydney's Royal Botanical Gardens has been going on.
Dr Anja Divljan, Technical Officer at the Australian Museum’s Vertebrates Collections undertakes research about the interwoven lives of humans and bats.
About one quarter of all mammals in the world are bats, which are often categorised in literature into Megachiroptera (megabats) and Microchiroptera (microbats). The megabats consist of a single family, the Pteropodidae or Old World fruit bats (flying-foxes). The earliest fossils resembling a megabat date back to Oligocene approximately 35 million years ago. Today, flying-foxes are the largest bats in the world generally weighing up to 1000 g and with a wing span of 1.7 m. They are found in the subtropical regions from Africa, India and Southeast Asia to the islands in the west and southwest Pacific Ocean, including Australia.
Flying-foxes do not have the ability to echolocate (produce ultrasonic pulses originating in the larynx) like all microbats. Instead, they rely on their well-developed eyesight and a sense of smell to locate food. All megabats are phytophagous - feeding primarily on ripe fruit, but also floral resources (pollen and nectar), bark, seeds and leaves. This makes them important pollinators and long-distance dispersers of tropical plants.
About the research project
My study species is the Grey-headed Flying-fox (Pteropus poliocephalus), which belongs to the genus Pteropus - the largest genus in the family Pteropodidae, which is composed of 57 currently recognised species. Most of these species live on islands, and their distribution and dietary requirements make them vulnerable to human activities. Many flying-foxes, including the Grey-headed Flying-foxes, are in decline as a result of habitat loss to agricultural clearing of forests, sustained pressure from hunting and/or culling of the animals that feed on fruit crops.
My research interest is primarily in the population ecology of the Grey-headed Flying-foxes, the only endemic flying-fox in Australia. In particular, I study the age structure of the Grey-headed Flying-fox populations, which will hopefully aid monitoring, and protecting the species in the future.
Why is this important?
The research to date suggests that Grey-headed Flying-foxes have evolved in an environment with low natural mortality, promoting life-history with slow maturation, and low reproductive rates. These bats reach the reproductive age at 2 to 3 years, give birth to one young per year after a long 6-month pregnancy, and can also live long, well into their twenties. These characteristics make the population numbers particularly sensitive to any changes in survival rates of young and adults.
Since the late 1700s, flying-foxes have been progressively confined to smaller natural habitats due to clearance of vegetation for agriculture and urban development. More recently, individuals have started moving into urban areas where they face increased mortality due to culling, electrocution on power lines, collisions, and entanglements in barbed wire and netting. Recent changes in global climate, leading to an increase in the occurrence of hot days with low humidity and temperatures over 40°C, can also impact flying-fox numbers. Although my research suggests that young bats are no more likely to die from electrocution or collision, they appear to be more susceptible to heat: if the temperatures reach 40°C and persist for a couple of days or longer, thousands from a colony can die in a single day.
With the loss of habitat, natural food resources and all the added mortality factors, a Grey-headed Flying-fox population decline of at least 35% was documented in a single decade (1989–1999), and a further reduction in the population size has been predicted. The species has been listed as vulnerable nationwide, and humans have to find a way to live with them, as flying-foxes are very important part of the Australian ecosystems. Their preferred food is Eucalyptus, Angophora, Melaleuca and Banksia pollen and nectar and native fruits. They fly very long distances in search of this food, thereby playing a critical role in long-distance pollination and dispersal of Australian native plants.
What did you do?
Flying-foxes live high in the tree canopy and although they are often present in large numbers (few to several thousands) at any roost site, they are very difficult to catch. Still, we have developed a successful mist-net method to capture individuals: two 13 m tall, thin aluminium poles with a long mist-net suspended between them on a pulley system. As the bats return home from their foraging early in the morning (about 3 – 6 am), they sometimes fail to see our dark net in front of the tree canopy and fly into it. We then quickly lower the net, take the bat out and put it in a canvas bag, where it hangs calmly until later in the morning, when we take its measurements and collect the data. This is repeated until we get about 20 to 30 bats, after which we process them one at the time in the order of capture. Most of my project was done in the Sydney’s Royal Botanic Gardens, where flying-foxes can still be seen roosting all year round in varying numbers.
Since I am interested in seeing how old the bats are, the most reliable way to reveal the age of a flying-fox is very similar to revealing the age of a tree: counting growths rings (but in their teeth). Hence, I have to anaesthetise each bat to extract a small tooth from it and count the rings in the root of the tooth. This way I found that flying-foxes can live 20 years or more in the wild, but the bulk of the population (at least in the urban areas, where I catch the animals) are less than 6 years old.
Once the bats are anaesthetised, I also take measurements and record information about their general condition and reproductive characteristics, as well as attach a metal band around their thumb. The band has a unique number, so that we can identify the animal if it ever gets recaptured or reported injured or dead in the future. The whole procedure lasts only several minutes per animal (even with a tooth extraction) and they wake up usually about 30 minutes later. Once all the bats are processed, we release them back into their roost site. To date we have processed and successfully released all captured flying-foxes, which is well over 1000 individuals.
But there are so many of them...
Naturally, flying-foxes are very social animals and establish large roosts in the canopy of rain- and mangrove-forests, riparian vegetation and melaleuca swamps. The choice and the characteristics of the roost locations are not fully understood yet, but we know that the roosts are connected and dynamic, as individuals move between the sites in search for food.
Recently there has been an increase in the occurrence and permanent occupancy of the sites in urban areas, and as a result people often think that the flying-fox numbers are increasing. At times, as much as 15% (around 60,000) of the total population can be found in the Sydney Basin alone – so it is no wonder people think there are so many of them. However, the estimates of the total number of Grey-headed Flying-foxes in the 1930s put their population in the millions, while the current number is closer to 400,000 individuals nationwide. This population decline is likely to continue unless the appropriate management and conservation actions are developed and implemented in the near future.
Why do flying-foxes settle in the urban areas?
The Grey-headed Flying-fox distribution stretches along Australia’s east coast, from Queensland to Victoria (and more recently South Australia), with bats being commonly seen in most major cities (Brisbane, Sydney, and Melbourne) and many other urban areas. Historically, the bats have been frequenting these (now) urban areas on a seasonal basis, as they migrate up and down the coast in different seasons in search of flowering and fruiting eucalypts and other native plants. However, more recently, a proportion of the animals are staying in the cities all year round and the main reason for it is quite simple: they are losing their natural roosting and feeding habitats through vegetation clearance and modification for housing and farming projects.
At the same time, the urban areas offer relatively reliable sources of food as people have planted many native and exotic plants that flower at different times of the year. Hence, the bats are able to quickly adapt to these city conditions and form permanent roosts, which can have detrimental effects on the vegetation they roost in. If the animals do not migrate seasonally, the trees may not have time to recover from the mechanical damage (defoliation and claw-marks in the tree bark) that a large number of permanently roosting bats can cause. This, as well as many other flying-fox traits (e.g. noise, smell, and feeding in orchards) is increasingly putting these animals in a direct conflict with humans. People usually want them either culled (because they see them as vermin) or just moved away from their immediate surroundings, without thinking about whether such relocations are possible and how they could be achieved. However, these issues are often very complex, given the importance of these animals for Australia’s ecosystems, their vulnerable status, and the logistics of how to control the movement of a large number of highly mobile animals in the first place (not to mention the cost associated with such a task).
Therefore, there is no one quick and easy resolution to the conflict, and management actions should take into consideration the biology, behaviour and ecology of the flying-foxes, and acknowledge that, like many other mammals (e.g. whales, rhinos, elephants), flying-foxes subjected to high rates of mortality may not be able to recover easily. In contrast, there are some encouraging aspects for the future survival of the species, such as their adaptability to new introduced food sources and environments, and their high mobility (that could enable them to migrate to areas that are more favourable).
How can we help and save flying-foxes?
Everyone can have a share in saving the fliers with very little effort.
First rule: Please do not touch the animal yourself as it takes training and experience to handle an injured flying-fox. If the animal is on the ground, you could cover it with a cardboard box to restrict its movement, while waiting for an animal rescuer to arrive. An animal hanging low should not be disturbed, and any children and/or pets should be kept away, until the bat is rescued.
Find more tips, contact details & further info here: