372 
PACIFIC SCIENCE, Vol. XX, July 1966 
DISCUSSION AND CONCLUSIONS 
Caum (1922) found that sugar cane by itself 
was an inadequate diet for rats, for those he had 
kept on a strict cane diet showed symptoms of 
malnutrition and partial starvation. Yet in the 
Hamakua study sugar cane was the preferred 
food of R. exulans in cane fields as well as in 
gulches and of R. rattus in cane fields. R. rattus 
in gulches did not display this strong attraction 
toward sugar cane, presumably because other 
preferred foods were more easily available. 
A study of rats inhabiting gulches adjacent 
to cane fields on the Island of Kauai (Spencer, 
1938) showed results similar to mine: sugar 
cane comprised 26% of the food materials of 
the gulch-inhabiting R. exulans , but was absent 
from the diet of R. rattus in the same habitat. 
Caum (1922) hypothesized that rats feed on 
sugar cane only incidentally or in order to ex- 
pose and feed upon the caneborers infesting 
the stalks. These hypotheses seem very unlikely, 
however, as the data showed that too many rats 
were attracted to sugar cane and consumed too 
much of it for it to be an incidental food item; 
moreover very few insects of any kind were 
found with the ingested sugar cane. 
Doty (1945) stated, "the availability of pro- 
tein foods is the limiting factor controlling the 
increase of rats in cane fields and adjacent waste 
areas.” Protein foods such as insects, lower in- 
vertebrates, and animal flesh were available, 
with insects forming a large part of the diet of 
Mus in the selected cane fields. Rats in the same 
habitat did not utilize these sources of protein 
as much as did Mus but, instead, fed heavily 
on grass stalks, which are also a source of crude 
protein. Hosaka (1957) reported average crude 
protein (green weight basis) of Panicum maxi- 
mum as 1.2% and of P. purpurascens as 1.8%. 
My findings indicate that the various rodents 
tend to satisfy their nutritional requirements in 
different ways. 
In cane fields rodent populations are not 
necessarily limited only by the availability of 
protein foods, but also by other environmental 
conditions, and perhaps by behavioral and phys- 
iological traits as well. The diets, and hence 
the prosperity, of rats depend, therefore, largely 
upon the materials available to them, which in 
turn may influence their choice of habitat, and 
upon their abilities to exploit these materials. 
In cane fields Mus and R. exulans were the 
predominant species and R. rattus was present 
in small numbers, but only a single R. norvegi- 
cus was found. In gulches R. exulans and R. 
rattus were the prominent species. In areas of 
human habitation only R. rattus and R. norvegi- 
cus were examined, primarily to augment in- 
adequate samples from the other habitats. R. 
rattus thrived in gardens and orchards, but R. 
norvegicus was abundant only near houses, live- 
stock pens, poultry coops, or slaughter houses. 
These differences in the species composition of 
rodents within each of the three habitats may 
reflect differences in utility, preference, or avail- 
ability of food sources as well as in selection 
of cover. Eskey (1934) captured 56% R. rat- 
tus 3 6% R. norvegicus, and 8% R. exulans 
inside and within 50 ft of buildings; 64% R . 
rattus, 16% R. norvegicus, and 20% R. exulans 
were caught 51-500 ft from buildings; and 
72% R. rattus, 9% R. norvigicus, and 19% 
Ft exulans were trapped more than 500 ft from 
buildings. 
Spencer (1938) found that R. rattus pre- 
ferred wild foods and R. norvegicus domestic 
foods, and that R. exulans was intermediate in 
preference between wild and domestic foods, 
but inclined toward domestic foods. However, 
the present findings show that R. exulans pre- 
fers wild foods; R. rattus uses both wild and 
domestic types, but is inclined toward wild 
foods, and R. norvegicus prefers domestic 
foods. Calhoun (1962) found that Norway 
rats took garbage more readily than the com- 
mercially prepared food left in feed troughs 
of penned rats, showing prominent selection 
between kinds of domestic foods. These find- 
ings indicate that while R. rattus can easily 
adapt itself to most habitats, R. norvegicus in 
particular and R. exulans to a lesser degree are 
rather limited in their use of habitats and food 
sources. Schein and Orgain (1953) found that 
rats generally preferred foods that promoted 
gain in body weight and avoided foods not 
useful to them. Under present conditions in the 
Hamakua district, the most versatile of the 
three species of rats appears to be R. rattus, 
which utilizes a wide variety of domestic and 
wild food sources and adapts itself readily to 
field habitats as well as to domestic environ- 
ments. R. exulans, however, is restricted to field 
and gulch environments and depends wholly on 
