This paper was presented at the Australian Mammal Society Conference, held at James Cook University in Townsville in September, 1995.

 
Author and friend

Short-nosed bandicoots (Isoodon macrourus : Peramelidae) were capture-mark-recaptured at four sites around Rockhampton, Central Queensland. Sites varied with respect to precipitation and ground mulching, factors which influence the food supply of bandicoots.

At the site with augmented precipitation and thick mulch cover, females weighed up to 1.8 kilograms and produced young throughout the year, while at drier sites females were much lighter in weight and bred irregularly. Neither scrotal area nor weight of males varied significantly between sites.

Head lengths and developmental characters of the pouch young were compared with those of captive animals recorded by Hall (1990). Pouch young grew at an accelerated rate in females at the site with augmented precipitation and thick mulch cover. At the drier sites pouch young either conformed to Hall's data or grew more slowly. The only litters reduced or terminated were those growing slowly.

Weight and timing of reproduction in females and developmental rates of pouch young are linked to precipitation and mulch cover, probably through food supply. Developmental stages previously recorded reflect the slowest successful growth rates. Diet of captive and many wild bandicoots may be minimal and severely limit reproduction.

The northern short-nosed bandicoot, Isoodon macrourus (Marsupialia:Peramelidae) (Gould, 1842) occurs in coastal areas of Australia from the Kimberleys through the monsoonal tropics of the north to the Hawkesbury River in New South Wales (Gordon, in Strahan, 1983). Information on the reproductive ecology of I. macrourus is known from 4 wild and 2 captive populations.

Kemper et al. (1990) found births occurred between September and April on the Mitchell Plateau of Western Australia. Friend (1990) recorded bandicoots breeding from late August to mid April at Kapalga, near Darwin. Conflicting data have been recorded around Brisbane. Both Gordon (1974) and Hall (1983) found year-round breeding in wild bandicoots. Gemmell, however, recorded breeding in wild bandicoots from July to March (Gemmell, 1982). Captive outdoor populations in Brisbane displayed seasonality of breeding (Gemmell, 1982, 1988). Near Newcastle in New South Wales Gordon (1971) found seasonal breeding which varied slightly both temporally and spatially.

Mean litter size varied from a high of 3.7 (in captive populations) (Gemmell, 1982, p. 189) to a low of 2.5 (for wild populations) (Kemper et al., 1990).

These data suggest that cues initiating breeding in I. macrourus appear to differ depending on latitude, with sharply seasonal breeding in the wet-dry tropics in response to rainfall, and somewhat reduced seasonality in the subtropics, perhaps in response to photoperiod (Friend, 1990).

The Rockhampton climate is subtropical, with neither heavy monsoonal nor regular winter rains. If breeding by I. macrourus is correlated with rainfall, it may differ between naturally and artificially watered areas within Rockhampton. Therefore the reproduction of I. macrourus was studied at four sites in Rockhampton which received similar amounts of natural rainfall, but different amounts of artificial watering.

Four sites were selected within an area of seven square kilometres in North Rockhampton (23o19'40"S, 150o31'30"E). Site 1 was open woodland on clay soil and received only natural rainfall. Site 2 was a small gardened area, mulched with woodchips and watered using sprinklers. The third site was an experimental orchard, mulched with straw and trickle irrigated. Site 4 was a heavily watered and mulched public park.

Permit to Take No.T00947

Wire traps were used to trap all sites on 3 consecutive nights per month. Later in the study the fourth site was trapped for another three consecutive nights each month, about two weeks after the main trapping to monitor the development of pouch young.

On first capture all adults were individually marked, weighed and measurements taken of head, ear, manus, pes and tail lengths. Female pouch condition was noted, especially for signs of a recent litter, and measurements made of pouch young where practical. Measurement of crown-snout head length was taken for one individual of each litter since Hall (1990) found this to be the most reliable indicator of age of pouch young. Hall's information on developmental stages of pouch young (1990) was used to generate a checklist (Gemmell and Johnston, 1985; Hall, 1990; Mackerras and Smith, 1960).

All sites were assumed to receive the same amount of monthly rainfall as recorded by the Rockhampton office of the Bureau of Meteorology. Artificial sprinkling and soil moisture content were measured at all sites. Thus sites being sprinkled were recorded as having natural plus artificial rainfall; those not sprinkled received only natural rainfall.

Young in pouch

At Site 1 (clay soil, unwatered) litters were produced in March (3) and September (1,2,3). None of the four females exceeded 1.2 kilograms and weights remained relatively constant throughout the study.

At Site 2 (woodchipped, lightly sprinkled) one female produced a litter of three in September. Again, neither female exceeded 1.2 kilograms and their weights remained relatively constant for the period of the study.

At Site 3 (straw mulched and trickle irrigated) females weighed between 1.15 and 1.7 kilograms and gained weight around mid July after heavy rain. One female produced a litter of four in late July and another of five in September. The second female produced a litter of 3 in late April but ejected these offspring in late May. She produced a litter of one in late June which grew successfully until at least five weeks of age and a third litter (2) in September.

At the fourth site (heavily watered and mulched) four females were resident. The females almost always were either carrying a litter, or showed signs of suckling a nested litter. Female Number 6, weighed approximately 1.8 kilograms and this is the heaviest wild-caught female Isoodon macrourus recorded (in Strahan, 1983). The other females were lighter than those from the trickle irrigated site, but bred regularly throughout the study period. Ten litters, of between 3 and 6 young, are known to have been produced here over the eight months of study; a total of 41 young, of which only four are known to have died before weaning. These females were trapped frequently and development of their young closely followed.

Survival of pouch young between captures was high. On only one occasion did a female lose any pouch young within the 3 day trapping session.

Developmental stages and rates were found to vary from those reported by Hall (1990). The female weighing about 1.8 kilograms from the heavily watered site produced litters with apparently accelerated growth rates (Table 1). In one ten day period her litter of four developed at a rate equivalent to twenty six days. These young were initially estimated to be about twenty days old; upon recapture ten days later they were estimated at about forty six days of age. Her subsequent litters did not show such rapid development but still developed at a faster rate than Hall's study would predict.

In contrast, a female from the trickle irrigated site produced a litter in late April. At first capture the head length of these young suggested they were 10 days old. Upon recapture twenty three days later their estimated age was thirty days. Thus their development was slower than that recorded by Hall. This female had been captured several times previously and this was the only occasion when a litter was ejected. As another example of slow growth and development, one litter of four from the trickle irrigated site developed slowly until the litter was reduced to two, after which growth and development continued at a rate similar to that of Hall's animals.

Despite time constraints and the lack of replication, intersite comparisons suggest differences amongst treatments which merit future consideration. The numbers of animals captured were small so no statistical analyses were done.

Isoodon macrourus is reported to feed primarily on insects and other invertebrates but will also eat plant material (Gordon, 1974, 1983). Insect abundance (Wolda, 1978) and litter arthropod populations (Levings and Windsor, 1982) vary seasonally in the tropics, and are positively correlated with rainfall. Higher rainfall results in greater abundance of the bandicoots' food, and thus increased body weight. In this study differences amongst sites in precipitation were positively correlated with body weights.

The body weight of females is a recognised factor in mammalian reproductive condition (Perry, 1971). Females of many mammalian species do not commence breeding until they have attained a species-specific body weight (Perry, 1971). Thus onset and cessation of reproduction are strongly affected by nutrition. In the wet-dry tropics, fluctuations in rainfall and growth of vegetation govern mammalian reproduction (Perry, 1971, p. 170). In this study differences amongst sites in precipitation were positively correlated with female reproductive success.

Even a mildly cool temperature combined with food scarcity is a serious challenge to small female mammals (Bronson, 1985). Seasonal breeding in a captive population of bandicoots in Brisbane (Gemmell, 1988) when wild populations nearby exhibited year-round breeding (Gordon, 1974; Hall, 1983) suggests some inadequacy of the diet supplied to the captive animals. If the captive population were nutritionally stressed, cooler temperatures in late autumn / winter could prevent females breeding. The appearance of seasonal breeding may have been attributed solely to short daylength. Another reproductive response to nutritional stress might be small litter size.

Similarly, growth and development of young depend on available resources and may be influenced by the same factors that control breeding. Little has been recorded on the growth and development of pouch young in wild populations, compared to captive populations of I. macrourus. Merchant (1990) has found both retarded and accelerated growth in individuals within litters in captivity. The great variation in development of pouch young in this study is probably due to nutritional factors. As one female produced a litter which grew the equivalent of 26 days during only 10 days this suggests that most wild (and some captive) populations do not develop at the maximum rate. Nutritional stress is likely to result in a slower rate of development. In regards to the only litter ejected in this study poor growth rate, rather than trap trauma, may have been the crucial factor in that ejection.

Females at relatively dry sites produced their first litters two months after heavy rainfall, suggesting an indirect link between rainfall and breeding, perhaps via food supply. Litters were produced throughout the study period at the heavily watered site. Females may breed only when their weights are above a critical minimum.

Growth rates of pouch young were also found to reflect maternal nutrition, with apparently accelerated development at the heavily watered and mulched site. At drier sites the developmental rate of young was consistent with Hall's findings. The hypothesis that food supply affects reproductive condition and development of young needs to be tested explicitly.

Table 1 : Development of litters of one female from the heavily watered site, showing disparate growth rates.

6 Bibliography

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