New Zealand Journal of Ecology (1985) 8: 55- 82

Population ecology of rabbits in the Wairarapa, New Zealand

Research Article
J. A. Gibb  
A. J. White  
C. P. Ward  
  1. Ecology Division, Department of Scientific and Industrial Research,, Private Bag, Lower Hutt, New Zealand

An aged sample of over 17,000 rabbits (Oryctolagus cuniculus) from the Wairarapa, in the North Island of New Zealand (area 2535 km', 41 °10'S), was autopsied between September 1965 and September 1967. The sample represented about two-thirds of all rabbits shot by the Pest Destruction Board. Information included: date; property; age; sex; body-weight; if male testes scrotal or not, if female – whether pregnant / lactating, number and size of healthy / resorbing embryos.
About 52% of the rabbits shot were males. Percentages of rabbits in the 1st, 2nd, 3rd and> 3rd year age— classes were: males 52.2, 30.7, 9.6,7.5; females 60.9, 27.9, 6.9, 4.3. Young up to at least 3 months old were under-represented in the sample. The age distribution of the samples differed between years.
About 10% of young conceived May-November survived at least 6 months, compared with about 0.1 % of those conceived in other months. Relative changes in numbers through the year of 1st, 2nd and 3rd year, and of older rabbits, are illustrated. The average annual survival rate beyond age 6 months was 0.4t.
About 33% of males aged 4-6 months had scrotal testes, rising to 62% of those> 12 months old. Some males were found with scrotal testes in every month. Few females < 4 months old were pregnant. Above this age, over 90% were pregnant in September-November, and fewer than 50% in April, May and June.
Pre-natal mortality of embryos may be adaptive in matching litter-size at birth to environmental conditions. In some months pre-natal mortality affected at least 28% of pregnant females; it was more frequent among older than younger females. Mean monthly litter-size at birth ranged from 4.17 in June to 6.55 in October. Females aged 10-12 months were more productive than younger or older females. On average, about 45 young were born annually per adult female.
Both sexes gained in weight until> 3 years old. Adult females weighed more than adult males, the difference depending on the proportion pregnant.
Overseas, dates of breeding vary widely even at similar latitudes and productivity varies with the length of the breeding season. In Spain, where the rabbit evolved, summer rainfall is sparse and the arrival of autumn rain is erratic. Rabbits breed opportunistically in Spain and in parts of Australia, but rarely in New Zealand. Litter-size of Oryctolagus seems not to vary latitudinally, unlike that of Sylvilagus spp. in America.
The first summer was on average 1.5°C warmer than the second. Females were heavier and more of them were pregnant in the first summer, but they had smaller litters; males were lighter and fewer had scrotal testes. Productivity was similar in the two years.
An average annual kill by the Pest Destruction Board of about 15,000 rabbits of mixed ages probably accounted for less than 20% of the adult population. This is unlikely to have reduced the size of the population a year later, though it may have reduced average survival.