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Patterns of distribution and use of rabbit warrens in 540 acres (219 ha) of subalpine habitat during 1963 and 1966-71 are described. The numbers of rabbits and of burrows in which they live can fluctuate independently of each other. It is demonstrated in a marked population that careful measurement of changes in burrow use can be used to estimate changes in the numbers of rabbits using the burrows. The most favourable habitat for rabbits in subalpine regions is raised areas on the floor and sides of valleys in open alpine grasslands, where there are projecting granite tors and moraine-like materials.

Rabbits in subalpine areas in south-eastern Australia show a marked avidity for sodium salts during the spring and early summer months. During that period rabbit populations may be controlled by allowing access to soft wooden meranti pegs impregnated with sodium chloride and sodium luoroacetate (poison 1080). If the pegs are left out as permanent bait stations they produce long-term effective control. The method should be applicable to other areas in Australia where soil sodium is very low.

During severe drought in north-western New South Wales populations of rabbits in sandy habitats are restricted to certain warrens, which are located in light sandy and gravelly soils in protected and elevated positions close to food supplies in major drainage channels and swamps. Warrens in less favourable localities become completely covered by drifting sand. After rain the unfavourable areas become more favourable and are recolonized by rabbits which open the old, buried warrens. Foxes are less able to excavate the permanently occupied warrens in the more favourable habitats, apparently because of their depth. The more evanescent warrens in less favourable areas show evidence of far more predation by foxes on nestling rabbits. The importance of these findings for rabbit control in the arid zone is discussed.

This paper describes a dramatic fall in rabbit populations in a large area of semiarid north-western New South Wales, due to a severe drought, and the increase in numbers which followed. The reduction in numbers differed markedly in different land systems. The areas which supported rabbit populations throughout the study were limited to the proximity of swamps in sandy habitats, and close to drainage channels in stony habitats; populations became extinct over large areas of sandy habitats. In the stony habitats populations increased very rapidly in the 12 months following the breaking of the drought, whereas in the sandy habitats it took c. 3 y before a noticeable increase. The difference was probably a result of differences in survival, due mainly to the effects of predation and to the availability of open warrens. In sandy habitats drift sand buried untenanted warrens very rapidly. The environment affected not only rabbits but also their main predators, the fox and the cat. No cats and very few foxes were observed in stony habitats. The main activities of both species were confined to refuge areas in sandy habitats. The most significant correlation between indices of rabbit numbers and indices of use of rabbit warrens was the number of active burrows per unit area of habitat.

Native Australian and introduced species of animals are adapted to severe sodium deficiency. Effects of such a deficiency are aggravated by pregnancy and lactation, and it has been shown that mechanisms controlling sodium homeostasis in wild rabbits are subject to profound stress.

Aerial surveys can be used to measure the distribution of rabbit warrens over large areas of arid and semiarid Australia. Clumped patterns of distribution of rabbit warrens found by ground surveys are also detected in aerial surveys, which further show that the highest density of warrens occurs in habitats containing shales or other fine-grained rocks overlain by recent sediments, or those closely related to drainage systems. A good picture of rabbit distribution is obtained when about 25% of the area has been overed by the aircraft.

Female domestic rabbits living with vasectomized males in large compounds (3/16 ac in area) exhibited periods of sexual attraction and mating, together with other behaviour such as aggression towards other females, digging, burrow interest, and nest building at intervals of about 6 days or multiples thereof. The behavioural cycles occurred during pseudopregnancy (of about 18 days) and were not a manifestation of an underlying inherent sequential cycle or a typical mammalian oestrous cycle. Behaviour with a strong sexual component culminating in mating and ovulation was observed at the beginning (or end) of each of 54 pseudopregnancies. Similar behaviour was also grouped about either the sixth or twelfth days of pseudopregnancy or both. This behaviour was extremely variable in both incidence and the number of components involved. Sexual components occurred less frequently and only once culminated in mating and ovulation. The behaviour observed on the sixth and twelfth days of pseudopregnancy was not reflected in measures of physiological activity such as numbers of graafian follicles, diameter of graafian follicles or corpora lutea, or weight changes in ovaries or uteri. The histology of the ovaries, uterus, and vagina and dimensional changes such as ovarian and uterine weights, number and diameter of graafian follicles, and diameter of sets of corpora lutea showed no detectable difference from those reported by other workers during pseudopregnancy for caged female rabbits isolated from males. The incidence of 6-day and 12-day behaviour was highest in the autumn-winter months when sub-optimal reproductiveconditions prevailed and lowest in spring-summer. Some possible causal mechanisms and the adaptive significance of the sixth-day and twelfth-day periods of sexual behaviour are discussed.

Evidence is presented for a reproductive cycle in the confined wild rabbits (Oryctolagus cuniculus (L.) ) at Albury. Females become sexually attractive and mate at intervals of 7 days or multiples of 7 days. The cycle appears to continue on into pregnancy, and is not interrupted by resorptions. With increase in density, fecundity decreases. There is an increase in the rate of resorption and an apparent suppression of oestrous behaviour, resulting in a lower number of young born per female. Resorption varies seasonally, with the highest litter loss occuring during winter. There is no apparent relationship between dominance per se and fertility. There is an obvious relationship between age and fertility. Older females have larger and more litters than younger females. There is no apparent direct relationship between dominance and rate of resorption. Resorption decreases with age. Young dominant females resorb at the same high rate as, and produce similarly to, the rest of their age group. Litter size increases from approximately 4.0 at the beginning of the season to about 6.0 at the end. There is no relationship between length of period of gestation and either age or size of litter. The incidence of seasonal breeding in the wild rabbit is explained by postulating an annual cycle in male fertility correlated with the solar cycle, with a single peak in spring, and a bimodal annual cycle in female fertility with a minor peak in the autumn and a major peak in the spring. These cycles are greatly affected by nutritional changes in pastures under varying rainfall and climatic regimes.

Coprophagy is a normal feature in the biology of the wild rabbit (Oryctolagus cuniculus) in Australia. The evidence presented shows that coprophagy occurs normally during the daily period of inactivity when rabbits are below ground. When feeding activities commence in the afternoon on the surface, coprophagy ceases. Evidence suggests that the passage of normal pellets continues throughout the night, the formation and ingestion of soft pellets recommencing when the rabbit population as a whole again moves below ground into the warrens, in the early hours of daylight. Kittens commence ingesting soft pellets as soon as they leave the nests to feed on green grass, although suckling continues for some time. There appears to exist a seasonal influence on the normal daily rhythm in eating of faeces. In winter less time is spent in ingesting soft pellets and more in green-feeding than in autumn and spring. It is suggested that coprophagy assumes its greatest importance to the animal during summer when rabbits are below ground for a greater proportion of the day than at any other season. During summer, also, green feed is scarce and the standing pastures are dry. It is evidently during this season that rabbits are forced to obtain from their food all the nutriment possible. Observations show that the switchover from the production of one type of pellet to the other may be either immediate or delayed.