C S I R O
P U B L I S H I N G
Wildlife Research Volume 27, 2000 © CSIRO Australia 2000
A journal for the publication of original scientific research in the biology and management of wild native or feral introduced vertebrates
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Published by CSIRO PUBLISHING for CSIRO Australia and the Australian Academy of Science
Wildlife Research, 2000, 27, 21–37
Population dynamics and life history of the mahogany glider, Petaurus gracilis, and the sugar glider, Petaurus breviceps, in north Queensland Stephen M. Jackson Department of Zoology and Tropical Ecology, James Cook University, Townsville, Qld 4811, Australia. Present address: Healesville Sanctuary, PO Box 248, Healesville, Vic. 3777, Australia.
Abstract. Mahogany gliders, Petaurus gracilis, and sugar gliders, Petaurus breviceps, were trapped in an area of open woodland in north Queensland between 1994 and 1996 to examine their population ecology and life history. This study area contained two smaller areas, one consisting of continuous habitat and the other an area of fragmented habitat adjacent to the continuous habitat. Within the continuous area, the mahogany glider had an average density of 0.24 ha–1 whereas the sugar glider had an average density of 0.27 ha–1. In contrast, the density of mahogany gliders in the fragmented habitat averaged only 0.16 ha–1 whereas the density of sugar gliders was higher, at 0.46 ha–1. Both the mahogany glider and the sugar glider showed sexual dimorphism in their weight, head length and head width. The average body weight of both species fluctuated throughout the year with no consistent pattern. The mahogany glider showed a distinct breeding season, with births being recorded between April and October, whereas the sugar glider showed no pattern, with births being recorded during all months except February and April. During the study, all adult females of both species were observed to have bred, with an average litter size of 1.55 and a natality rate of 2.09 for the mahogany glider, and an average litter size of 1.83 and a natality rate of 2.14 for the sugar glider. The sugar glider was able to raise two litters of young within a single year whereas a second litter was raised by the mahogany glider only if the first litter was lost.
Introduction Since its rediscovery in 1989, very little information has been collected on the life history of the mahogany glider, Petaurus gracilis, with the available information being limited largely to Van Dyck (1993) and more recent work on its diet, home-range, social organisation and habitat requirements by Jackson (1998, 2000a, 2000b). It has a very limited distribution, occurring between the Hull River near Tully in north Queensland and approximately 30 km south of Ingham (a north–south distance of approximately 110 km). Throughout this limited distribution, the mahogany glider lives sympatrically with the sugar glider, Petaurus breviceps, which has a very wide distribution, occurring along the east coast of Australia from Tasmania to north Queensland and across northern Australia. The sugar glider is also found outside Australia, occurring in New Guinea and numerous surrounding islands (Flannery 1995a, 1995b). The ecology of the sugar glider has been studied extensively in southern and central eastern Australia (Smith 1980; Goldingay 1984; Henry and Suckling 1984; Suckling 1984; Henry 1985; Quin 1995) whereas in northern Australia it has been studied little or not at all. © CSIRO 2000
The aim of this study was to compare the demography, reproductive biology and longevity of the mahogany glider and the sugar glider in sympatry. Differences in the population densities of the mahogany glider and the sugar glider in both continuous habitat and fragmented habitat were also examined. Methods Study site The site chosen to study the mahogany glider contained several different vegetation types and was located approximately 25 km south of Cardwell in north Queensland on the eastern side of the Cardwell Range, at an elevation of approximately 10 m (Fig. 1; Jackson 1998). This area contained two zones that were immediately adjacent to each other. The first was a 100-ha area of continuous habitat located at Mullers Creek (18°268139S, 146°078159E). The second zone was located immediately south of Mullers Creek at Porters Creek (18°268579S, 146°078359E). This was an area of discontinuous or fragmented habitat that was composed of the buffer strips along Porters Creek and its tributaries, an area of approximately 45 ha, and was surrounded by a plantation of the introduced Caribbean pine, Pinus caribaea. The climate in the region has two distinct seasons. The wet season commences in November/December and ends in March/April, with the highest average monthly temperatures occurring in December (32.4°C) 10.1071/WR98044
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mixture of creamed honey and rolled oats wrapped in greaseproof paper. A honey/water solution (approximately 1 : 8) was sprayed as far as possible up the tree above the trap bracket, on the bracket and bait to attract gliders to the tree and the trap. The honey/water solution was resprayed each day to keep the bait and trail fresh. Handling, marking and measurements
Fig. 1.
The location of the Mullers Creek and Porters Creek study area.
prior to the highest rainfall in January (462 mm). The dry season commences in April and continues until November, during which time the lowest average monthly rainfall and lowest average maximum temperatures occur in July (29 mm and 24.8°C, respectively). The mean yearly rainfall is 2166 mm. Trapping censuses Demographic and life-history information was obtained from live-trapping (capture–mark–recapture) in both the continuous habitat at Mullers Creek and the adjacent fragmented habitat at Porters Creek. Wire cage traps, 20 × 20 × 56 cm in size (Mascot Wire Works, Enfield, NSW), were used to catch gliders. These traps were attached to wooden brackets fixed approximately 4 m above the ground (on trees with a diameter at breast height greater than 20 cm) using elastic straps and with the entrance facing the trunk of the tree. Brackets for the traps were made using two pieces of timber 920 mm long and 75 or 100 mm wide, and screwed together in a ‘T’ shape. These were supported with a crossbracket of hardwood that was set at 45° to the other two pieces. In the continuous habitat at Mullers Creek a 100-ha grid was established with 11 transects by 11 traps (with traps placed 100 m apart). In the fragmented habitat at Porters Creek, transects began 200–300 m into each end of the corridors and ran parallel to the creekline, with 100 m between traps. These trapping grids were designed to target animals that were resident in the corridors and not ‘visitors’. The area enclosed by the traps in the buffer strips was approximately 45 ha. Full details of the trapping grids can be found in Jackson (1998). A trial trapping session was conducted in the continuous habitat at Mullers Creek from December 1994 to February 1995, during which the entire grid was trapped. Because of the limited number of traps available (80), and because of the time required to move and service traps, only the central area at Mullers Creek was trapped after this initial trapping period (an area enclosing 63 ha). The remaining trapping area was divided into two during each trapping session, with Transects 1–5 (40 traps) then 6–10 (40 traps) being trapped. These grids were trapped every eight weeks from February 1995 until December 1996. The fragmented habitat at Porters Creek was trapped from December 1995, and every eight weeks until December 1996. During each trapping session, traps were set over 3 or 4 nights (except in October 1996, when only 2 nights of trapping were undertaken in transects 1–5 because of torrential rain) and baited with a
Traps were inspected from dawn until all traps were cleared, which was generally by mid-day. Captured animals were restrained in cloth bags, sexed, measured, and weighed (to ±1 g) with Salter scales. The following measurements were taken with Vernier callipers for all new animals caught: (1) snout to vent; (2) vent to tail tip (from the cloaca to the last vertebrae of the tail tip); (3) snout to tail tip; (4) head length (from occiput to snout tip); (5) head width (maximum width across the zygomatic arches); (6) length of right tibia; (7) length of right pes, not including the claw; (8) length of testis; (9) width of scrotum; and (10) length of lower incisors. All three body-length measurements (snout–vent, vent–tailtip and total length) were taken to allow a more accurate measurement of total body length. Individuals that were recaptured during subsequent trapping sessions were weighed and the head length, head width and tibia length recorded only. A condition index was determined for both the mahogany glider and sugar glider following that derived by Krebs and Singleton (1993). The estimation of a condition index involved three steps: (1) estimating the regression between skeletal size (tibia length, head length, head width and total body length were used) and body weight; (2) using the most significant regression to predict body weight from the appropriate skeletal size for each individual; and (3) estimating the condition of each individual by taking the ratio of the observed to the predicted body weight. Gliders were tagged in each ear with a hamster ear-tag (Sieper and Co., NSW) to which Scotchlite reflective tape (3M Australia Pty Ltd, Victoria) was attached. Reflective tape of various colour combinations was used so that individuals could be identified at night while spotlighting (Jackson 2000a). Although ear-tags generally enabled identification of each individual, occasionally both ear-tags were lost so implant chips were used to verify identification. A passive integrated transponder (PIT tag) (Life chip, Animal Electronics ID Systems) was placed under the skin between the scapulas using a single-use syringe. The implant chips were read with a Pocket Reader (Destron–Fearing, Spain). Age estimation The age of individual gliders was determined by using a combination of parameters modified from Alexander (1981), Suckling (1984) and Quin (1995) (Table 1). Parameters included a combination of tooth wear, patagium colour, weight and reproductive condition. Three methods were used to determine tooth wear: (1) the categories of tooth wear of Alexander (1981), based on the degree of flattening of the upper incisors when viewed from the anterior; (2) the categories used by Suckling (1984), which use the degree of wear and proportion of dentine exposed on the upper incisors when viewing the ventral surface; and (3) the colour and wear of the lower incisors and the presence of lateral cracks. Young animals have white lower incisors with no lateral cracks, whereas older animals have teeth that are more discoloured and develop increasing number of lateral cracks with age. In old animals part of the lower incisors can be completely chipped off. The patagium colour was used following Quin (1995), Suckling (1984) and personal observations of captive squirrel gliders, Petaurus norfolcensis. As in other studies (e.g. Quin 1995; Suckling 1984) body weight was a useful indicator of age until approximately 18 months of age for both species. The weights used to determine age (Table 1) for the mahogany glider were derived by multiplying the weights used by Quin (1995) for squirrel gliders by the ratio of the difference of the weights of adult squirrel and mahogany gliders. Weights of sugar gliders to determine age were calculated by multiplying the weights used by Suckling (1984) for
Biology of the mahogany glider and the sugar glider in north Queensland
Table 1.
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Age-estimation parameters of mahogany gliders and sugar gliders at Mullers Creek and Porters Creek Modified from Suckling (1984) and Quin (1995)
Parameter
Estimated age (years) 3
Mahogany gliders Weight of males Weight of females
330 g
>370 g >330 g
Sugar gliders Weight of males Weight of females
70 g
>80 g >70 g
Mahogany gliders and sugar gliders Wear of upper incisors
None to slight
None to slight
Slight to moderately heavy
Moderately heavy to very heavy
Wear of lower incisors
White, no cracks
Slight discoloration, lateral cracks slight
Orange discoloration, lateral cracks obvious. Occasionally chipped teeth in old animals.
Patagium colour
White
Cream-yellow
Yellow
Frontal gland (males)
Not developed
Pouch
Small and shallow with fine white hairs; teats 1 mm long
Yellow-orange
Partially to well developed. Larger and deeper than in females that had not bred. Yellow/orange hairs with black scale. Teats >1 mm.
sugar gliders in southern Australia by the ratio of the weights of adult southern sugar gliders and those of sugar gliders in the present study. If pouch young of either species were present, the head length of each pouch young was recorded and an approximate month of birth was estimated using regression estimates from Smith (1979). This was done by finding the ratio of the difference of head length between the adult mahogany gliders in this study and adult squirrel gliders, and between the adult sugar gliders in the present study and the sugar glider in Smith (1979). This was done because sugar gliders in this area are smaller than those studied by Smith (1979) in southern Australia. The head lengths of mahogany gliders and sugar gliders in the present study were then adjusted by this ratio and an approximate age (and month of birth) was determined from the squirrel glider or sugar glider regressions respectively. The reproductive status of male and female animals could also be used to gain an indication of age. Females younger than one year of age usually had not previously bred and had very small teats ( 0.05). During 1995, when few sugar gliders were caught, the condition indices were very erratic; however,
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during 1996, when larger numbers of sugar gliders were caught, the condition indices of both male and female sugar gliders closely mirrored each other, with a decrease in body condition in the wet season and a higher condition in the dry season (Fig. 8b). In both the mahogany glider and the sugar glider, changes in body weight and body condition appear to be a result of changes in food availability and reproduction. Reproduction The mahogany glider had a distinct breeding season, with pouch young being present between April and October, except for a single observation in December 1996 (Fig. 9a). Births were calculated to have occurred from April to September in 1995 and April to October in 1996 (Fig. 9b). The peak birth time appears to be from April to August/September. The time to weaning is estimated to be approximately 4–5 months for the mahogany glider (on the basis of the presence of young in the pouch and the occurrence of young in the trapped population). After weaning,
Fig. 8. Yearly variation in (a) mean body weight and (b) condition index of adult male and female sugar gliders during 1995 and 1996 (m, males; h, females). Standard errors are shown.
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Fig. 9. Mahogany glider: (a) seasonal variation in adult female reproductive condition, and (b) estimated months of all recorded births during 1995 and 1996. The number of individuals is shown at the top of the column. Solid bars represent litters of two, clear bars represent litters of one.
both sexes appeared to disperse from their parents’ home range within a year. The sugar glider, in contrast, had pouch young present during all months of the year, with a peak in August and October (Fig. 10a). Births were calculated to have occurred during all months except February and April (Fig. 10b) with a peak from June to September. All adult female mahogany and sugar gliders that were caught more than 2–3 times were found to have bred during both 1995 and 1996 as all had bred
by the time they were 24 months old. A number of sub-adult (age
