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| The
Grey-headed flying-fox, Pteropus poliocephalus
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The
grey-headed flying-fox, Pteropus poliocephalus (Figure
1) is of the order Chiroptera, suborder Megachiroptera, family
Pteropodidae, subfamily Pteropodinae, and genus Pteropus.
The species was first described in 1825 by Temminck.
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Figure 1. The grey-headed flying-fox, Pteropus
poliocephalus. (photograph by J. Welbergen)
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| General
characteristics |
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P.
poliocephalus is dark brown except for an orange/brown
mantle that fully encircles the neck and its fur extends right
down the legs to the toes (Hall & Richards, 2000). Adult
individuals weigh between 600-1000 g with a maximum-recorded
weight of 1070 g and have a wingspan exceeding one 1.5 metres
(Welbergen, 2005). P. poliocephalus is long-lived
and there are reports of individuals surviving in captivity
for up to 23 years (Pritchard, 2001) and a maximum age of
up to 15 years seems possible in the wild (Tidemann, 1999).
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| Behaviour |
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Roosting
Like most Pteropus species, P. poliocephalus forms
colonies (i.e. “camps” or “day roosts”)
during the day on the exposed branches, commonly of canopy trees
(Figure 2) (Eby, 1996; Mickleburgh et al., 1992; Nelson, 1963,
1965a, b; Ratcliffe, 1931, 1932; Tidemann et al., 1999). P.
poliocephalus colonies may contain thousands of individuals
(Nelson, 1965b; Tidemann et al., 1999). Ratcliffe (1931) reported
colony size estimates of over 200,000 in many colony sites and
such camps were found into the 1990’s (Eby, 1991). Recent
counts however have found few colonies over 20,000 individuals
(Eby et al., 1999) |
Figure 2.
Side view of a P. poliocephalus colony in northern
NSW, Australia
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Colonies
are formed in seemingly arbitrary locations (Tidemann et al.,
1999). Roost vegetation includes rainforest patches, stands
of Melaleuca, mangroves and riparian vegetation (Nelson, 1965b;
Ratcliffe, 1931), but colonies also use highly modified vegetation
in urban areas (e.g. Birt & Markus, 1999; Tidemann &
Vardon, 1997). P. poliocephalus exhibits a high seasonal fidelity
to traditional colonies and return annually to the same locations
(Augee & Ford, 1999; Lunney & Moon, 1997; Nelson,
1963). When undisturbed, colony locations can be stable for
several decades and fidelity to some sites may even have pre-dated
human settlement (Augee & Ford, 1999; Lunney & Moon,
1997; Nelson, 1963). In these colonies, mating, parturition
and rearing of the young takes place.
An
important determinant of colony size is food available within
nightly foraging distances, commonly within 20 kilometres
but up to 50 km (Eby, 1996; Parry-Jones & Augee, 1991;
Parry-Jones & Augee, 2001). Another important determinant
is the time of year with large aggregations of individuals
occurring mainly during the mating seasons (Nelson, 1965b;
Parry-Jones & Augee, 2001).
Foraging
At dusk the grey-headed flying-fox emerges from the
colony to forage for floral resources (i.e. blossom, pollen,
nectar and fruit) (Eby, 1991, 1995, 1996; Fujita & Tuttle,
1991; Spencer et al., 1991). The exact timing of the start
of the emergence reflects the outcome of a trade-off between
the risk of predation and the need for foraging (Welbergen,
2005). The emergence timing of males also depends on their
social status: bachelor males leave before harem holders and
the latter leave only when the last female in their harem
has left.
The
food sources usually associated with P. poliocephalus
belong mainly to the genera Eucalyptus, Angophora,
Melaleuca and Banksia, but in some areas
it also utilises a wide range of rainforest fruit (Eby, 1998;
Parry-Jones, 1987; Parry-Jones & Augee, 1991; Ratcliffe,
1932). P. poliocephalus is the only mammalian frugivore
to occupy substantial areas of subtropical rainforests and
so is of key importance for seed dispersal in those forests
(Eby, 1996; Mickleburgh et al., 1992). Most vegetation communities
used by P. poliocephalus produce foraging resources
in seasonal but annually irregular superabundant pulses, and
P. poliocephalus has adopted complex migration traits
in response to such ephemeral and patchy food resources (Eby,
1996, 1998; Nelson, 1965b; Parry-Jones & Augee, 1992;
Spencer et al., 1991). However, there are some temporally
and spatially reliable resources restricted to a small number
of coastal vegetation communities in northern New South Wales
and Queensland that may support smaller resident populations
(Eby, 1996). Cultivated orchard fruits are also taken but
apparently only at times when other food items are scarce
(Parry-Jones & Augee, 1991).
Territoriality
From January onwards males set up ‘mating territories’
in the colony that are actively maintained by aggression
and scent-marking (Figure 3) (Nelson, 1965a; Welbergen,
2005). The average territory size is about 3.5 body lengths
along branches (Welbergen, 2005). The neck of P. poliocephalus
contains androgen-sensitive sebaceous neck glands which
are enlarged in breeding-season males (Martin et al.,
1995) and are used frequently for scent-marking the entire
length of the branches within their territories (Welbergen,
2005).
In the colonies territorial interactions (i.e., chases,
biting, beating with wings) were always between resident
and intruding males, never between females (Welbergen,
2005; contrary to Nelson, 1965a). During the mating season
(April-May: Martin et al., 1993; Martin et al., 1995;
Martin et al., 1987) territory defence is especially evident
(Nelson, 1965a) when male mating territories may contain
up to five females. During this time fights can involve
up to 10 individuals fighting simultaneously (J. Welbergen,
pers. obs.). |
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Figure 3. Two fighting male P. poliocephalus.
(photograph by J. Welbergen) |
Intruding males fly around in circles of approximately 20
m diameter and attempt to crash-land onto the territory holder
or the territory that it is guarding. This behaviour is remarkably
similar to the intruder behaviour (sensu Richards, 1995) of
P. conspicillatus that I also observed in P.
poliocephalus at the feeding sites.
Males are also territorial in the feeding areas (J. Welbergen,
pers. obs.); however, this territorial behaviour does not
seem to function to obtain matings as matings at the feeding
sites are extremely rare (see below). In fact, during the
mating season at night, males are mostly separated from females
because the majority of males return to the colony approximately
two hours after they have started foraging (Welbergen, 2005).
The majority of behaviours that are displayed by the males
at night in the colony involve scent-marking, territorial
fighting and the uttering of territorial cries (sensu Nelson,
1964) (Welbergen, 2005). Only around dawn, several hours later,
are these males rejoined by females (Welbergen, 2005).
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Mating
Matings are generally observed between March and May (Martin
et al., 1993; Martin et al., 1995; Martin et al., 1987; Nelson,
1965a) but the most likely time of conception is April (Martin
et al., 1993; Martin et al., 1995; Martin et al., 1987) when
there is a concomitant peak in mating activity (Welbergen, 2005).
Mating is almost entirely restricted to the mating territories
in the colonies during the day; mating at night, outside the
context of the mating territories, is extremely rare as it has
only been observed once (J. Welbergen, pers. obs.) despite many
hours of observations on individual P. poliocephalus
in the feeding areas by others and myself (e.g. Eby, 1996; McWilliam,
1986; Nelson, 1965a). Mating is a highly stereo-typed and energetically
demanding behaviour (Welbergen, 2001; see also Markus, 2002
for mating behaviour in the closely related P. alecto).
Females have an active role in the initiation and termination
of copulations (Welbergen, 2002). Males mate repeatedly with
the same female during a mating session that can last up to
one hour, and have multiple mating sessions with the same female
over the course of several days (Welbergen, 2002).
Unambiguous extra-group copulations have been recorded (J. Welbergen,
pers. obs.); in all cases it was a neighbouring male that mated
with a female of a harem the holder of which had left due to
a disturbance (see also Kaiser, 2004). Matings occurred all
through the day but increased in frequency during sudden bouts
of mass-mating (J. Welbergen, pers. obs.) similar to what has
been observed in P. giganteus (Neuweiler, 1968).
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Gestation and parturition
P. poliocephalus breeds seasonally and the majority of
females reproduce once a year when they give birth to a single young
(Eby, 1995; Hall & Richards, 2000). Parturition usually occurs
in October (Eby, 1995; Nelson, 1965a) after a gestation period of
about 27 weeks (Martin et al., 1987; O'Brien, 1993). The young are
born altrical and are dependent on their mothers for temperature
regulation (Bartholomew et al., 1964). Females carry their young
for the first three weeks during their nightly foraging trips and
after this time young are left in the colony at night (Eby, 1995).
Young are capable of independent flight in January at which time
they commence foraging outside the colony (Eby, 1995). Young are
weaned between February-April (Eby, 1995; Martin et al., 1995; Nelson,
1965a; Pook, 1977; Welbergen, 2002).
Early observations on social organisation of P. poliocephalus
Variability in mating systems is a consequence of variability
in the characteristics of female dispersion (e.g. Clutton-Brock,
1989) and early observations (i.e. Nelson, 1965a) suggested that
in P. poliocephalus female dispersion varies both on a spatial and
temporal scale and social organisation is complex.
The spatial variability in female dispersion can be found within
the colony: Nelson (1965a) reported that harems (Figure 4) were
larger in the higher-density centres of colonies.
Figure 4. A P. poliocephalus harem
Males
with territories in the centre of the colonies were polygynous whereas
males with territories away from the centre were monogamous. Males
roosting near the periphery of the colonies rarely roosted with
females and conceivably engaged in some form of non-parental polyandry.
Nelson (1965a) also noted a higher degree of monogamy among males
that form mixed-sex groups with females that were still nursing
their young from the previous year. He further reported that males
set up territories around females suggesting some form of female
defence strategy. Female dispersion varied temporally between the
colony during the day and the feeding sites at night. At the feeding
sites, males also defended feeding territories (Nelson, 1965a) suggesting
that some form of resource defence strategy. Based on the above
information, the mating system of P. poliocephalus proved
difficult to categorise according to the scheme used by McCracken
and Wilkinson (2000). For convenience they categorised P. poliocephalus
as mating in multi-male/multi-female polygynous groups.
Bradbury suggested four criteria to distinguish leks from other
mating systems (Bradbury, 1981; Bradbury & Gibson, 1983): (1)
males aggregate at specific sites for display; (2) females can select
their mate(s); (3) there is no male parental care so that males
contribute nothing to the next generation apart from gametes; (4)
the only resource females find at these sites are the males themselves.
The mating system of P. poliocephalus seems to fulfil the
first three of these criteria (Welbergen, 2005): (1) males held
mating territories that are costly to maintain and the highest quality
males had the most central mating territories; (2) from observing
general spatial shifts in the composition of the primary study colony
and from observing the changes in the harem composition of individually
marked males, it became clear that towards the start of the mating
season females moved from the colony’s periphery into the
centre of the colony and into the mating territories of males forming
harems consisting of unstable female groups; (3) males do not provide
any parental care as they never roost next to their offspring and
any adult male – young interactions that could be construed
as such were never observed. Mating territories do provide a place
for females to roost and, strictly speaking, therefore criterion
(4) is not satisfied. However, since roosting space was found not
to be limiting in this species, the only essential resource in a
mating territory seems the male itself.
Since mating on leks is never random and reproductive success tends
to be skewed towards a limited number of males, it is of interest
to know what makes some males more attractive than others. Possible
cues for female preferences are display execution (including advertisement
of intrinsic male traits such as health, age or size) and/or locale
within the lek. The latter is cited as a major determinant of mate
choice on leks (e.g. Borgia, 1979; Emlen & Oring, 1977; Kokko
et al., 1999). Since in P. poliocephalus competition for
the central territories seems more severe in the more central territories
(Kaiser, 2004), males are expected to become aligned spatially in
order of increasing competitive ability towards the centre of the
colony. Indeed, in P. poliocephalus: male characteristics
that commonly covary with competitive ability such as body size
and body weight increased towards the centre of the colony (Welbergen,
2005). Thus it is possible that in P. poliocephalus, males
use the location of a male’s territory as an indication of
his quality. This would make the task of finding a suitable mate
in a colony that contains tens of thousands of individuals much
more efficient as it would enable a female to quickly discard a
large number of low-quality males. Thus, the locale within the lek
becomes the male’s ‘display execution’ and therefore
part of the male’s sexually selected extended phenotype.
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| Conservation
and threats |
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P. poliocephalus was once considered to be an abundant
species with numbers estimated in the many millions (Ratcliffe,
1931).
P. poliocephalus is exposed to several threatening processes,
the most serious of which is loss of foraging and roosting habitat
(e.g. Eby, 1995; Tidemann, 1999). European settlement, initiated
in 1788, engendered extreme habitat loss and fragmentation across
most of the P. poliocephalus range (Tidemann, 1999), with
diverse and mostly detrimental impacts on biodiversity (Saunders
et al., 1990).
Another serious threat is direct killing of animals in orchards
and harassment and destruction of roosts. P. poliocephalus
destroys commercial fruit in Queensland and New South Wales (Tidemann
& Vardon, 1997). As a consequence P. poliocephalus is
regarded as a pest species over much of its range (e.g. Hall, 1987).
In the past decade negative public perception of the species has
intensified with the discovery of three new diseases that are potentially
fatal to humans, Hendravirus, Menanglevirus and Australian Bat Lyssavirus
(Allworth et al., 1996; Fraser et al., 1996; Halpin et al., 1999),
so persecution has further intensified (Tidemann et al., 1999).
The exact number of animals destroyed is unknown, but estimates
as high as 100,000 annually have been made (Vardon & Tidemann,
1995). The impact is more substantial than direct deaths alone would
indicate, for a large proportion of animals shot on orchards are
pregnant and lactating females (Tidemann et al., 1997).
Competition with P. alecto (and perhaps P. scapulatus)
may also be a threat to P. poliocephalus. The distribution
of P. alecto has undergone a substantial southerly shift
since 75 years ago (Eby & Palmer, 1991; Nelson, 1965b; Ratcliffe,
1932; Welbergen, 2002) extending further into coastal areas inhabited
by P. poliocephalus (Webb & Tidemann, 1995). The two
species share roosts and diet plants, and there is tentative evidence
that under conditions of increased habitat loss, the southwards
expanding P. alecto becomes more competitive than P.
poliocephalus in obtaining the remaining resources (Eby et
al., 1999; Tidemann, 1999; Webb & Tidemann, 1995), although
it has not been directly assessed. Furthermore, hybridisation seems
possible (Webb & Tidemann, 1995) and I have observed several
inter-specific matings between male P. alecto and female
P. poliocephalus in the wild.
Temperature extremes have caused the death of tens of thousands
of Australian flying-foxes in the last decade alone causing some
of the most dramatic mass die-offs ever to have been recorded in
mammals (Welbergen, 2005; Figure 5). Such extremes selectively affect
the effective breeding population and recruitment of the species,
which further exacerbates their impact (Welbergen, 2005). Since
temperature extremes are expected to increase in the future (Meehl
& Tebaldi, 2004), mass die-offs will become more frequent and
widespread, and will occur at lower latitudes than previously. This
will undoubtedly increase the threat to the survival of the species
in addition to the anthropogenic factors that have already been
identified.
Figure 5.A: Distribution of P. alecto
(blue), P. poliocephalus (orange) and their current zone
of overlap (purple) in eastern Australia. Red numbered dots: locations
of past die-offs (unpublished data; Ratcliffe, 1932): 1: Dallis
Park area, 2002; 2: Helidon, 1905; 3: Ipswich, 1994; 4: Ipswich,
1999; 5: Mallanganee, 1913; 6: Bellingen, 2004; 7: Gordon, 2003;
8: Cabramatta, 2003. Horizontal blue lines show the southern latitudinal
extent of P. alecto in 1928, 1965 and 2004 (Eby & Palmer, 1991;
Nelson, 1965b; Ratcliffe, 1932). Inset shows the colonies that were
affected (red dots), unaffected (white ticks), and colonies from
which no information was availble (question marks) during a 12-1-2002
temperature extreme in northern NSW (Dallis Park area).
Ratcliffe
(1931), on the basis of anecdotal evidence, already believed that
the species had declined by 50% since pre-European times as a result
of clearing of native vegetation and competition with P. alecto.
In recent years however, direct evidence has been accumulating that
the species is indeed in serious decline (Eby et al., 1999; Hall
& Richards, 2000). Current estimates for the species are about
300,000 (Eby; pers. comm.) and it has been suggested that the national
population may have declined by as much as 30% in the last decade
alone (Richards, 2000).
To answer these growing threats, roost sites have been legally protected
since 1986 in New South Wales and 1994 in Queensland (Tidemann &
Vardon, 1997), and in 1999 the species was classified as vulnerable
to extinction in the Action Plan for Australian Bats (Duncan et
al., 1999). The species is now protected both at state as well as
federal level in Australia (Duncan et al., 1999; Eby et al., 1999;
Tidemann, 2003) (Table 1), and as of 2008 the species has been listed
as 'Vulnerable' on the IUCN Red List of Threatened Species (Lunney
et al. 2008).
Table
1. Conservation status of P. poliocephalus in
Australia.
| Jurisdiction |
Conservation
Rating Instrument |
Conservation
Status |
| National |
Environment
Protection and Biodiversity
Conservation Act 1999 |
Vulnerable |
| Victoria |
Flora and
Fauna Guarantee Act 1988 |
Not listed |
| New South
Wales |
Threatened
Species Conservation Act 1995 |
Vulnerable |
| Queensland |
Nature
Conservation
Act 1992 |
Common |
The
IUCN Action Plan for Old World Fruit Bats (Mickleburgh et al., 1992)
and the Action Plan for Australian Bats (Duncan et al., 1999) lists
lack of knowledge of the biology of Pteropus spp. as one
of the main threats to their survival. The lack of information on
the behaviour in colonies and at feeding sites severely limits our
understanding of P. poliocephalus, and this understanding
is particularly important now that it has become a prominent federal
conservation problem in Australia.
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| |
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