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Life history and reproductive ecology of a tropical rock wallaby, Petrogale assimilis

Life history and reproductive ecology of a tropical rock wallaby, Petrogale assimilis
Robyn Margaret Delaney


Department of Zoology, James Cook University, Douglas QLD 481, AUSTRALIA.


The rock wallaby, Petrogale assimilis, is a small macropodid living in isolated colonies in rocky outcrops in the wet-dry tropics of northern Australia, a region where the onset, magnitude and duration of the wet season are unpredictable. Eighty individual P. assimilis were caught in traps between one and 42 times during more 4000 trap days over 44 monthly field trips to Black Rock (19° 05'S., 144° 07'E.) between June 1986 and June 1990, as part of a longitudinal study of this species.

Petrogale assimilis was a typical macropodid for its size, with regard to its life history and reproductive ecology. Both sexes were capable of reproducing continuously; gestation was about the same length as the oestrous cycle (approximately one month); a single young was born and, a post-partum oestrus and embryonic diapause probably occurred. Pouch young remained permanently attached to the teat until 110 - 143 days (n=11). Permanent exit from the pouch occurred at 180 - 231 days (mean=201 days, n=25), and weaning occurred between 267 - 387 days (n=5). Sexual maturity occurred at a minimum age of 17.5 months in females and 23 months in males.

The growth of pouch young of P. assimilis at Black Rock was compared with that of captive animals maintained in ad libitum conditions at Macquarie University, Sydney. No significant differences in pouch young growth were found between these two groups of young despite the potentially different nutritional conditions of their mothers. Individual pouch young of P. assimilis varied in their rate of growth, although the data were too limited to examine the potential causes. Exponential and Gompertz growth curves were described for the head and pes length respectively of pouch young and were used to predict the age of pouch young of P. assimilis.

Body measurements were of limited use for predicting the age of older rock wallabies and cannot be used accurately for sexually mature animals. Petrogale assimilis exhibited only limited sexual dimorphism in its external morphology and growth during adult life with males slightly larger than females in the average length of the head (4%), pes (4%), tail (7%) and ear (3%). Sexual dimorphism was most pronounced in upper limbs (and presumably in their associated musculature) (11%) and in weight (15%).

Radiographs were taken of the teeth of rock wallabies older than pouch young to measure the eruption stage and position of the teeth. Regression curves which were fitted to data from known age animals were used to estimate the age of rock wallabies older than pouch young. Available data, albeit limited, suggested that the rates of molar eruption and movement are similar in both wild and captive P. assimilis. Molar progression was mostly limited to movement induced by wear. This movement occurred too slowly and with too much variability to be useful in ageing animals in which the head had ceased growing or which had full molar eruption. The use of molar eruption for determining the age of macropodids was limited to animals with less than full molar eruption (estimated mean age = 7 years). As P. assimilis appears to have a maximum life span of approximately 12 years, molar eruption was a useful tool for ageing animals during a major proportion of their adult life.

The influence of the seasons on changes in the body condition of adult rock wallabies probably resulted from the availability of water and quality of forage. Although below average annual rain fell in both years of my. study (1987 and 1988) which could be fully examined, the pattern of rainfall was quite different during each of these years and this pattern of rainfall influenced the body condition of adult rock wallabies. In 1987 rock wallabies lost weight (mean loss = 10%) and the packed cell volume of their blood decreased as the dry season progressed. In 1988, when a higher proportion of the year's rain fell during the dry seasons and green forage was available throughout the year, rock wallabies maintained weight and the packed cell volume of their blood increased.

The seasons were also crucial to the survivorship of pouch young. Pouch young born in the late dry season were twice as likely to survive total lactational dependence than those born during the wet or early dry seasons. However, the length and harshness of the late dry season is dependent on when the unpredictable wet season arrives. This may explain some of the loss of 46% of pouch young during total lactational dependence. If the wet season does not arrive, the mother may lose her pouch young early in pouch life when her investment is low, in the expectation that the next pouch young is more likely to survive to take advantage of the wet season flush. The cause of the mortality of young pouch young is unknown, although anecdotal evidence suggests that components of the mother's diet may be crucial.

There was considerable variation between mothers in the survivorship of pouch young during both total lactational dependence and late pouch life. However, I was able to identify only one maternal correlate of pouch young survivorship: that smaller-framed females were more successful in rearing young in late pouch life than their larger conspecifics. I was unable to examine the nfluence of other factors (such as maternal age and position in the dominance hierarchy) which have been shown to influence the survivorship of young in other mammals.

There was a negative correlation between the mother's success at rearing a pouch young through total lactational dependence and her success in rearing pouch young through late pouch life, although data are limited. Despite the individual variation in reproductive success observed, the ultimate survivorship of pouch young born during my study was very low (15%). Almost no young born during this period survived to sexual maturity. This result and the recent improvement in offspring survivorship, particularly young at foot, with better seasons suggests that infant survivorship occurs in pulses with a higher proportion of young from most females surviving during more favourable years.

The age structure of the population provides supporting evidence for the hypothesis that infant survivorship occurs in pulses. Previous research at Black Rock during the above average rainfall years of the early 1970's suggested that there were a high proportion of pre-reproductive animals present in the population. During my study, the age structure of the population was skewed towards very young animals (pouch young) and very old animals (full molar eruption). Almost no sub-adults or young adults were present at Black Rock. While the older animals could not be accurately aged, I speculate that these animals may be part of a cohort which was born during the above average rainfall years in the late 1970's and early 1980's.