Institute of Wildlife Research,
Heydon-Laurence Building, A08,
University of Sydney,
Sydney, NSW 2006
Australia
Phone: +61 2 9351 7683
Fax: +61 2 9351 4119
E-mail: adivljan@bio.usyd.edu.au
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Age determination
and population age structure of the Australian flying foxes
My broad research interests are in the ecology of terrestrial vertebrates,
with a particular emphasis on ecology and conservation of the flying
foxes. More specifically, in my PhD I am looking at the age structure
of the flying fox populations, and how it affects the population
dynamics and survival of the species.
The main objective of my current study is to aid recovery plans
for the vulnerable Grey-headed Flying-fox (Pteropus
poliocephalus) by focusing on (1) methods of
ageing individuals, and developing age-based population models,
(2) investigating morphology and sexual dimorphism
in flying foxes, and (3) exploring the tooth morphology
and formation of growth layers in teeth. This research is summarised
below.
Investigating fluctuations in population numbers with time is one
of the main interests in population ecology. The factors behind
the increase or decrease in population numbers differ, but can all
be classed into 4 categories; populations increase due to births
and immigration and decrease due to deaths and emigration. Patterns
of mortality and reproduction in many animals are directly related
to their age. Therefore, detailed age-specific data can be important
in estimating the population growth rates and assessing the effectiveness
of different management options to ensure the sustainability of
the population.
My research is aimed at investigating the age structure of the Grey-headed
Flying-fox, and subsequent modeling of the population dynamics for
the species. The current view, based on limited data, is that flying
fox populations have a very low capacity for increase, even under
most ideal conditions. This is supported historically, as the animals
are known to have a low reproductive rate (giving birth to a single
young per year) and high maternal investment, both indicative of
relatively long-lived animals with low natural mortality rates.
Thus human induced mortality (electrocutions, entanglements in netting
and barbed wire, shooting in orchards, etc.) is potentially putting
the population at a risk, which has been observed by the recent
decline in numbers (35% in the past decade). Nonetheless, this information
needs support, and I am currently investigating the longevity of
bats in the wild, the average age of bats in the wild, and the age
when females reach maturity.
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| My study relies
on finding an accurate method to age P. poliocephalus. In my
honours year I have developed the most reliable ageing technique
to date, which involves counting the cementum growth layers
in the tooth roots of the Grey-headed Flying-foxes. The growth
layers (like the tree rings) form annually, as suggested by
a strong linear relationship between the number of visible cementum
layers on a cross-section of a tooth and the age of known-aged
captive individuals (Divljan et al. 2005- in
press). However the time and causes of layer formation are
still unknown and I am investigating these using a technique
involving tetracycline marking. Specifically, I am orally administering
tetracycline (an antibiotic and permanent marker) to bats that
come into care due to being orphaned or injured. The drug incorporates
into the bat's tooth upon its administration, leaving a permanent
mark that I can correlate with cementum growth and create a
time-line of growth. Also, I am using animals from different
approximate age groups with the aim of determining any possible
differences in tooth layering that might be attributed to the
age of the bats. |
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How old is this flying fox? Cross
section of a bat's tooth showing 6 cementum growth layers
(a layer is made up of one light and one dark band)
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Although tooth extraction provides absolute age of individuals,
the ethical issues and time associated with the procedure merits
research into other possible methods for aging live bats. These
are associated with comparative measurements of some external physical
characteristics of the animals, but can as such suffer from individual
variation among specimens. This is particularly true for cases where
age determination is based upon increase in size. I am currently
looking at other possible means of ageing flying foxes by taking
a number of external measurements of the skulls and bodies, as well
as assessing their teeth condition. These procedures are likely
to place individuals into broader general age classes, and possibly
infer any sexual dimorphism present in the species.
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Flying foxes (Megachiroptera
species) feed predominantly on the pollen and nectar of Myrtaceae
(particularly Eucalyptus) and Proteaceae trees, as well as on
fruits of native and introduced tree species. They are known
to be highly mobile and are capable of dispersing pollen and
seeds on two different scales: a small localised scale (20-500m)
and a large scale of up to 29.5 km. Consequently, they act as
pollen vectors and seed carriers, thereby playing an important
role in the natural forest ecosystems they inhabit. The Grey-headed
Flying-fox is Australia's only endemic flying fox, and is found
in coastal southeastern Australia, from Victoria to Miriam Vale
in Queensland, and inland to the western slopes. Historically
P. poliocephalus covered a great distributional range
with numbers being estimated in many millions. However recent
counts are showing a substantial decline in numbers, leading
to the animal being listed as vulnerable under both State and
Federal legislations. A problem in assessing the vulnerability
of flying foxes in the wild is that there is insufficient data
to construct adequate life-tables. Thus, a sufficient way of
aging the animals is necessary for the studies of the population
dynamics of the Grey-headed Flying-fox and subsequent protection
of the species. |
In addition, similarity between P. poliocephalus and
other members of this genus suggests that this study potentially
has wider implications in the research of mega-bats. At present,
I am also involved in a collaborative research project investigating
the ecology of Little Red Flying-fox, Pteropus scapulatus
in the Northern Territory, and the prevalence of the Hendra
virus in the P. scapulatus populations (Northern
Territory Disease Surveillance Operation). Primarily I am
focusing on accurate age determination in live bats using their
tooth morphology and determining the age structure of the wild
population.
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Divljan A, Parry-Jones K, Wardle GM (2004)
Age determination in the Grey-headed flying-fox (in press)
2005: Ageing the Grey-headed flying-fox: a study using tetracycline
(talk, first author with K. Parry-Jones) Flying Fox Information
and Conservation Network (FFICN)
2004: Age determination of the Grey-headed flying-fox (Pteropus
poliocephalus). (poster; first author; with K. Parry-Jones and
G.M. Wardle) 11th Australasian Bat Society Conference, Toowoomba,
QLD
2003: Age determination of the Grey-headed flying-fox (Pteropus
poliocephalus). (poster; first author; with K. Parry-Jones and
G.M. Wardle) Ecological Society of Australia, Armidale, NSW
2004 (July): Age determination and age structure of the Grey-headed
flying-fox (Pteropus poliocephalus).Royal Zoological Society,
University of Sydney, NSW.
2004: Postgraduate Research Support Scheme
2005: Wildlife Preservation Society's 2005 University Student Research
Grant
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