Body Temperatures of Rain Forest Mammals — Rudd 
475 
perature relate to the mammalian order and the 
adaptive habits of the species in question. With 
the exception of Nycticebus all 22 species de- 
scribed in this paper weigh less than 1 kg. Of 
these only one, Echinosorex, regularly exceeds 
500 g. Its position as a primitive insectivore 
prompted me to remove it from comparison 
with the remaining species. To determine 
whether consistent temperature differences were 
apparent among these small mammals, four 
125-g categories were created. The temperatures 
of animals of different species whose adult body 
size did not exceed 125 g were averaged to 
constitute the first class; those from 125-250g 
the second class, and so on. Mean results were 
as follows: Class I, 37.85°; Class II, 37.41°; 
Class III, 36.72°; Class IV, 35.87°. The last 
class included only one species (R. bower si ) . 
Only the first class meets the mean value of 
37.8° given by Morrison and Ryser (1952) as 
applicable to most mammals weighing under 
1 kg. Differences between means are not as 
great as temperature differences displayed by 
individuals over periods of time. Nonetheless, 
there is a consistent decline in mean tempera- 
ture as adult animal size increases. Most of 
the species included in the foregoing classes are 
rodents. About all one may conclude from these 
data is that no single figure precisely describes 
mean temperature values even among phylo- 
genetically closely related forms. 
Critical Internal Temperature 
In the absence of experimental data critical 
temperatures can only be estimated by indirect 
means. Apparently a lowering of 4-5° from 
normal is the maximum tolerable drop. Records 
of three subadult male Rattus sabanus may illus- 
trate. The first individual was heavily wetted 
and quiet at first capture but still in apparent 
good health. At this time its body temperature 
was 36.4°. In its next three captures the animal 
was very active and showed a mean temperature 
of 38.4° (38.0°-38.9°) . A second individual, 
quiet but in good health on its first three cap- 
tures, had a mean temperature of 35.9° (35.4- 
36.2). The animal was dry but obviously sickly 
on its fourth capture and at this time had a 
temperature of 31.3°. It was not subsequently 
recaptured. A third individual, heavily wetted 
and moribund at its first capture, had a body 
temperature of 31.4°. At its second capture six 
days later its temperature was 35.9°. 
Comparisons of Groups 
The mean temperature of all mammals in this 
study was 37.1°. Morrison and Ryser (1952) 
place the mean temperature of 56 species of 
mammals in the 15 g- 700 kg range at 37.8°. 
Irving and Krog (1954) place the value at 
38.6° for 22 species of arctic and subarctic 
forms. One might postulate a latitudinal rela- 
tionship with temperature. Since, however, 
large and consistent differences in temperature 
exist between groups it seems more reasonable 
to compare only equivalent taxonomic units. 
The mean temperatures of five groups are 
listed below. 
Insectivora (2 sp.) 33.2° 
Primates (3 sp.) 37.2° 
Chiroptera (2 sp.) 36.0° 
Rodentia 
Sciuridae (4 sp.) 39.8° 
Muridae (11 sp.) 37.1° 
In these instances it is clear that a single 
mean value cannot meaningfully represent the 
"typical” body temperature of a tropical mam- 
mal. Moreover, different species within these 
groups often consistently differ. The two gym- 
nures differ markedly, for example, and both 
differ from soricine insectivores (Morrison and 
Ryser, 1952). Tupaia has "squirrel-like” tem- 
peratures rather than the somewhat lower values 
found in other primates. The sciurids maintain 
a consistent "above average” temperature in 
relation to most mammals (as noted also in 
Irving and Krog, 1954 and Spector, 1956). 
Even among murid rodents mean values of the 
smaller species are 2° higher than the largest 
{Rattus bowersi ). The need to consider the 
adaptive requirements of the particular group 
seems obvious. How to judge what these adap- 
tive requirements are is, of course, a more dif- 
ficult task. 
ACKNOWLEDGMENTS 
Field investigations in Malaysia were sup- 
ported by National Institutes of Health Re- 
search Grant GM 11121-01. I acknowledge 
with particular thanks the help of my assistants, 
