Twenty-eight 
THE SOUTH AUSTRALIAN NATURALIST December, 1952 
Some Natural Factors Affecting Rabbit 
Populations in Australia 
E. W. L. Lines, B.Sc. 
“The rabbit has a pleasant face 
Its private life is a disgrace . . 
The rabbit and the rat are ex- 
amples of the few mammals that 
succeed in maintaining large popu- 
lations in the fact of the efforts of 
civilised man to exterminate them. 
The rabbit competes directly with 
the cow and the sheep for fodder, 
and in spite of a much lower diges- 
tive efficiency it occupies large 
areas of grass lands to the almost 
complete exclusion of domesticated 
ruminants. It has been estimated 
that in its absence from 25 to 40 
per cent more sheep and/or cattle 
could be depastured in Australia. 
The adaptations the rabbit has 
made to achieve this success in the 
Australian environment are ex- 
tremely interesting. The basis of its 
success lies in its rapid occupation 
of any expanding food situation in 
the face of natural and even of 
such artificial controls as arc 
economic and practicable. To 
achieve this, it conserves through 
adverse times a breeding stock 
capable both of multiple births at 
short intervals and of gathering 
and processing the large amounts 
of nutrients needed for successful 
lactation. Conservation of breeding 
stock is effected by confining repro- 
duction to times of favourable food 
conditions and by an ability of at 
least a portion of the adults to sur- 
vive on fodder of very poor 
quality. Evans and Bishop (1) 
showed that in the absence of 
adequate vitamin E the pregnant 
rabbit resorbed the foeti. As vita- 
min E is derived mainly from 
green grass, in its absence preg- 
nancy goes into reverse and young 
are not born when they cannot be 
suckled. Dennis Page**) has shown 
that if insufficient food is available 
to provide milk for all the litter, 
the doe just produces as much milk 
as the diet affords above her main- 
tenance and the excess young die 
of starvation. This behaviour con- 
trasts with that of the cow which 
will use its own tissues to provide 
milk for its young. As a rough esti 
mate a rabbit would need to get 
the equivalent of first grade dairy 
pasture in order to rear a litter of 
six young, so that anything that 
interferes with her food supply at 
that time will rapidly reduce the 
number weaned. The nutritional 
demands of lactation probably 
account for the comparatively 
small si/e of kittens when they 
start to graze. 
These factors also tend to main- 
tain stabilised populations in 
localities where the food supply is 
reasonably steady, for the more 
adults there are, the more intense 
the competition for good grade 
food and the smaller the number 
reared per litter. The proportion 
of young and old rabbits, in those 
trapped after the breeding season, 
suggests that in fairly stable popu- 
lations, the average annual crop 
raised per doe is about four. 
Reduction in number of breeders 
in the winter, e.g. by trapping for 
sale, probably raised the reproduc- 
tive rate of the survivors several 
times, because reduced competition 
for high grade food enables large 
