In a natural population of rabbits, studied for 5 years, 307 dispersal movements by young rabbits were detected. Most movements occurred at the beginning of summer and more males moved than females. With the exception of one year there was no significant relationship between the ages of dispersing rabbits and the distances moved. Three rabbits, less than 1 month old, moved more than 1500 m. During a drought year the orientation of the movements was towards Lake Urana. Adult rabbits rarely changed warrens.
A 3-yr study (1967-70) in a 280-ha enclosure on the Riverine Plain, where the rabbit population had not returned to its pre-myxomatosis high level. Rabbit warrens were concentrated in sandy Callitris columellaris woodland. Although myxomatosis still caused significant (4-14%) mortality of young rabbits, the main population control was predation by feral cats (Felis catus) and birds of prey. The effectiveness of predation since myxomatosis is probably due to cessation of trapping and poisoning, measures which destroyed relatively more predators than rabbits.
Shot samples of rabbits were taken every eight weeks for two years at four sites in the Southern Tablelands of New South Wales. Age structure differed between sites. The two sites at which rabbits were controlled by poisoning had the youngest age structures; the site where rabbits were not controlled had the the oldest age structure, the lowest density, the least myxomatosis and the highest production of young per female. The seasonal percentages of males in the samples were inversely proportional to the percentages of lactating females. Rabbits that were recovering from myxomatosis had higher infestations of Graphidium strigosum than rabbits that had active myxomatosis or no symptoms. At one of the sites, rabbits with myxomatosis were shot on every sampling occasion. Differences between sites, in duration and timing of reproduction, were very small compared with differences between rabbit populations from climatically different regions of Australasia; annual production of young per female older than six months varied between sites from 23.7 to 26.5 compared with the range of 15-53 for Australasia. The percentage of pregnant females and fertile males was highest in late spring and fell to zero for 1-2 months in summer. This seasonal pattern of reproduction is intermediate between the pattern found in some high-rainfall Australasian environments, where more than 30% of the females are pregnant in all months, and the patterns in arid, semi-arid and subalpine environments, where no females are pregnant for 3-6 months of the year.
We analysed monthly records of the presence or absence of myxomatosis in the 59 Pasture Protection Board Districts of New South Wales for 1959-64 and 1980-86. These periods respectively precede and follow the introduction of the European rabbit flea, Spilopsyllus cuniculi (Dale), into New South Wales. Throughout New South Wales during the two periods, myxomatosis was reported more frequently in summer than in winter. The seasonal trend was more pronounced on the western and central plains than on the slopes, tablelands or coastal regions. The incidence of myxomatosis was positively related to rainfall on the western and central plains and slopes, but not on the tablelands and coast. After the introduction of the rabbit flea, the reported incidence of myxomatosis increased more on the tablelands than in the other regions.
Capture-recapture estimates of the density of adult rabbits on an aeolian sand ridge near Mitchell in Queensland varied from four to seven per hectare over 18 months. The density of adult plus young rabbits peaked at 15 per hectare. Adult weights were high and the weights of adult males showed little seasonal variation. Young rabbits grew at a constant rate of 9.1 g day-1 until they reached a body weight of 1400 g. In the breeding season following a dry period there was a general synchrony of successive conceptions throughout the breeding season. For rabbits weighing more than 1400 g the sex ratio was biased towards males; for rabbits of less than 750 g there were more females than males. Immature rabbits dispersed more frequently than adults, and immature males dispersed more frequently than immature females.
Rabbit warrens in a semiarid environment of New South Wales were concentrated in those areas where impact penetrometer readings indicated friable soil to a depth of at least 75 cm. Isolated warrens in areas with few warrens were found in restricted patches of favourable soil. The absence of warrens from areas with shallow soil was considered to be due to high soil temperatures. The distribution of the rabbit in Australia was examined in relation to deep soil temperatures. It was suggested that the application of bituminous coating on ripped warrens may be a useful technique for the control of rabbits in the arid zone.
A 5-year-old rabbit warren in a 33-ha enclosure had 150 entrances and a total tunnel length of 517 m; it was estimated that 10.35 m*3 of soil had been excavated from the warren. Penetrometer readings indicated the presence of a hard layer in the soil at 600-700 mm. The depth of the warren was limited by this hard layer, the rabbits not having excavated below 700 mm. The hard layer had a low permeability, and flooding of the warren resulted in the deaths of nestling rabbits.
A site of 280 ha at Urana had four discrete rabbit populations. In September 1977 fleas, Spilopsyllus cuniculi, were used to successfully introduce a highly virulent strain (Lausanne) of myxoma virus into two of the populations; the other two were untreated. Eight weeks after virus introduction, rabbits infected with the Lausanne strain were found on the two largest warrens in one of the untreated areas. A field-strain epidemic reached most of the other warrens in the untreated areas in the summer months. Eighteen weeks after the introduction of the Lausanne strain, 1% of the 738 susceptible animals on warrens in the Lausanne-strain areas, 3% of the 328 on warrens in the field-strain areas, and 14% of the 29 on peripheral warrens that escaped myxomatosis had survived. The low rainfall, 25% of average in the second half of 1977, affected pasture production and led to the deaths of almost all rabbits born late in the season. Repeated annual introductions of the Lausanne strain in non-drought years will be necessary to determine the effectiveness of this procedure in reducing recruitment to the breeding population.
A very attenuated strain of myxomatosis was introduced annually into two rabbit populations from 1978 to 1980, to protect susceptible rabbits from the effects of the field strains of myxomatosis. Within 2 years one population had increased by a factor of eight and the other by a factor of 12. The results indicate that myxomatosis is still an important factor in suppressing those populations of rabbits. After 4 years of introducing a highly virulent strain of myxomatosis into two populations the number of adult rabbits had not declined.
Shot samples of rabbits were collected over several years at 11 sites in Australia and one in New Zealand. At any one site, pregnancy rates and litter sizes vary both with age of mother and with time of year. Few rabbits become pregnant before the age of 19 weeks; pregnancy rate increases until the full adult rate is achieved about 27 weeks old. Litter size is also affected by age, adult litter size being reached by females at about 43 weeks old. Rates and timing of reproduction vary greatly from site to site in response to local conditions. The annual production of young per fully adult female is highest at Wairarapa, N.Z. (53 young) and lowest in the semiarid (17) and subalpine (15) environments. At six of the sites cohorts of marked rabbits provided estimates of seasonal survival rates; survival improves with age but becomes relatively constant in rabbits aged more than 24 weeks; survival patterns differ between sites. Life tables were constructed combining the information on reproduction and survival. Their possible usefulness is discussed.
