December, 1952 THE SOUTH AUSTRALIAN NATURALIST 
Twenty-nine 
litters to be raised. Reducing corn- I 
petition for food by selective trap- 
ping of bucks, will have a similar 
effect and this may be the basis of 
the limited success of Rodier’s pro- 
posal to release the bucks and kill 
the does only. It seems reasonable 
to conclude that the rapid build 
up of the rabbit populations after 
commercial trapping is due, more 
to the above factors than to delib- 
erate preservation of breeding 
stock as so often alleged against the 
trappers. 
The sheep and the cow digest 
poorer pastures much more effici- 
ently than do rabbits, and to meet 
this situation it has evolved several 
interesting adaptions. In the 
ruminant, otherwise undigestable 
plant material (e.g. cellulose) is 
made available by fermentation in 
the paunch before digestion. The 
rabbit lacks an anterior fermenting 
sac but has a very large caecum, 
where analagous processes go on, 
however to reclaim the nutrients 
set free there, this material must 
be returned for digestion, which 
the rabbit does by swallowing the 
fermented material again. This 
phenomenon was discovered and 
investigated by A. Eden in England 
in 1 940 1 3 ) . He found that the rab- 
bit produces alternate batches of 
“hard” and “soft” faeces, the 
former contain much undigested 
fibrous material and constitute the 
ordinary dung, whilst the latter 
are swallowed whole as excreted. 
Eden showed that this practice 
improved the digestibility of both 
energy and protein to a useful ex- 
tent, and that the soft faeces origi- 
nated from the caecal contents. Up 
to half the stomach contents may 
be faecal pellets and some evidence 
suggests that the amount of faeces 
eaten is limited only by the total 
bulk which the gut can handle. 
Apparently the undigested resi- 
dues from the small gut are sorted 
into two fractions as they reach the 
colon, a fibrous fraction which 
passes downwards whilst the fines 
and part of the fluid are squeezed 
out and shunted into the caecum. 
Where the small gut joins the 
colon, there is a muscular sac — the 
sacculus — which may be concerned 
in this sorting. Even with this 
mechanism in action the rabbit 
still loses about 50 per cent more 
of the fodder in the faeces than 
does the ruminant. This becomes 
important on poorer grade pastures 
for whilst digestion of 60 per cent 
of an ad lib. diet will fatten a 
sheep, recovery of only 40 per cent 
from the same material would 
prevent a rabbit from processing 
enough fodder to get its mainten- 
ance. In order to get enough 
nuitrients from poorer pastures the 
rabbit selects the more digestible 
parts of the pasture particularly 
the growing points of perennial 
grasses and digs up their roots to 
get at the juicy underground 
storage organs. Such seTction re- 
duces the nett nutrients in the pas- 
ture more than it does the bidk 
and also seriously reduces the pro- 
ductivity of the perennials, which 
may be the explanation of the 
farmers’ lore that “rabbits spoil 
more than they eat.” From the 
nutritional difficulties experienced 
by ruminants on most of our 
tropical pastures during the dry 
season it well may be a nutritional 
barrier which has prevented the 
rabbit occupying them as it has 
those of our wheat belt. 
Another animal which appar- 
ently depends on caecal fermenta- 
tion to make nutrients available is 
the Koala, which Minchen (4) 
showed feeds its young on the 
ejected content of that organ. 
