Suchergebnisse

The age-structures of rabbit populations at 4 climatically different sites in South Australia were found to have altered significantly since the introduction of Spilopsyllus cuniculi (Dale) as a vector of myxomatosis. The ratio of young of the year to older rabbits in summer was greatly reduced. There was evidence of decreases in size of all observed populations and (for 1 population) in ability to increase rapidly in favourable years. The ratio of young of the year to older rabbits was a convenient and sensitive index of changes in population structure; for analytical purposes, the log transformation (ln [ratio] or ln [ratio + 1]) was biologically and statistically sound. Since the fleas became established, field-strain viruses may have become more virulent and morbidity rates may have increased, but the factors enabling rabbit fleas to enhance myxomatosis as a cause of mortality are unknown. The apparent virulence may have increased because of selection for viruses of high virulence, which are best suited for transmission by S. cuniculi, and possibly because the dose of virus transmitted by fleas is considerably greater than that by mosquitoes or other agents.

Rabbits given an excess of air-dry food but only small amounts of drinking water had a lower food intake and lost more weight than those given food and water freely. They lost weight because they had less digesta in the gut and less water in the skin, and had catabolized more tissue. Their response to water shortage was similar to that seen in other grazing mammals. With low food intake, gut contents fell and the digestibility of food rose, while the daily mass of and water loss in faeces fell; the moisture content in faeces remaining the same. Dehydrated rabbits produced relatively small amounts of urine, up to 1.9 M urea, which was twice as concentrated as that of rabbits given water freely. The rabbits were not able to reduce their requirement for water to less than 55% of their total intake of food and water. As pasture species in mediterranean-type environments become relatively dry in summer and contain only 10 to 5% water, a shortage of water in natural pastures in those areas may, therefore, limit the amount of food which rabbits can use, and the ability of rabbits to survive in summer would, therefore, depend upon the availability of succulent, drought-resistant perennial vegetation.

Shortage of water in natural pastures led to a sharp decline in a large rabbit population in arid, northeastern
South Australia. The pastures were dry and some rabbits drank at springs and water troughs.
Further from water, rabbits climbed trees and shrubs to obtain succulent leaves and twigs. Rabbits
provided with water maintained their weight and apparently survived better than those which did not
drink. It seems unlikely that the rabbits lost weight because the water shortage reduced the amount of dry
food they could eat. In caged rabbits, water shortage limits food intake but also results in low gut fill;
whereas the wild rabbits had the normal amount of digesta in their guts. It is more likely that, as the
pastures became dry, rabbits ate woody twigs and bark which were moist enough to meet their water
requirements but contained too little digestible energy for maintenance. The water shortage apparently
arose because rabbits were numerous and had eaten out the succulent pasture plants. Normally, it takes a
long drought to reduce arid-zone plants to dry straw, and overgrazing is probably the usual cause of a
lack of adequate water for rabbits.

Rabbits could be controlled by various combinations of warren ripping, poisoning and fumigation, without the need for clearing roadside Eucalyptus spp.

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.