KEEPING WARM 
pacity correlated nicely with the preva- 
lent weather conditions. Heat-generat- 
ing capacities, measured in terms of 
both the maximum level of heat produc- 
tion and the ability to sustain thermo- 
genesis, increased as ambient tempera- 
tures dropped in the late fall, reached 
their peak in January, and then declined 
in the early spring. Birds sampled in the 
warmer months of April through Octo- 
ber exhibited only marginal abilities to 
cope with cold. 
To investigate the mechanisms under- 
lying the striking seasonal variation in 
thermogenic capacities in goldfinches, 
we looked for three types of physiologi- 
cal adjustments that might enhance 
these capacities in winter: an increase in 
the ability of the heat-generating tissues 
to utilize oxygen and to oxidize meta- 
bolic fuels for the production of ATP; an 
increase in the supply of triglyceride 
and glycogen to thermogenic tissues; 
and a seasonal shift in the relative im- 
portance of triglyceride and glycogen as 
fuels for shivering. 
The pectoralis muscles of birds serve 
both as the major flight muscles and as 
the primary site of shivering thermogen- 
esis. We first investigated whether the 
mass of these muscles varied seasonally 
since an increase could indicate a 
change in the total amount of thermo- 
genic tissue. We found that the pecto- 
ralis masses, which make up 20 percent 
of the body mass, do not vary with the 
seasons. We then tested the oxidative 
ability of this muscle, that is, its ability 
to use oxygen to catabolize fuel. (Ca- 
tabolism is a metabolic process involv- 
ing the breakdown of the complex mate- 
rials of living tissue and the release of 
energy.) An analysis of the activities of 
two enzymes in the tricarboxylic acid 
cycle — a metabolic pathway that re- 
quires oxygen and metabolic derivatives 
of either glycogen or triglyceride to pro- 
duce ATP — did not show a seasonal 
change. The oxidative capacity of the 
goldfinch pectoralis muscles was, how- 
ever, among the highest found in verte- 
brate muscle. We believe that the lack 
of seasonal variation in oxidative capac- 
ity or muscle mass in goldfinches can be 
explained by the basic characteristics of 
avian flight muscle. In some species, 
flight may require an elevation of meta- 
bolic rate to more than ten times the 
basal level. Therefore, the evolution of 
flight may have necessitated the devel- 
opment of pectoralis muscles with oxi- 
dative capacities that are not only ade- 
quate to sustain long flights but also 
ample for prolonged thermogenesis. 
Our second area of exploration was 
the possibility that the seasonal change 
in thermogenic capacity was related to a 
yearly variation in availability and mo- 
bilization of fuels for shivering thermo- 
genesis. We found that body masses of 
Under experimental conditions, 
goldfinches captured in winter can 
endure more cold than those caught 
in summer. The difference is largely 
due to seasonal variation in the 
capacity to store and use metabolic 
fuels involved in heat production. 
Mena el j Kop«ak lor tne Cornell Laboratory of Ornithology 
goldfinches fluctuated from 12 or 13 
grams in summer to 15 or 16 grams in 
winter. The body component responsible 
for most of this variation was triglycer- 
ide, which ranged from less than 0.5 
grams in summer to nearly 2 grams in 
winter. Laboratory tests confirmed that 
triglyceride was mobilized from storage 
depots and oxidized by the birds during 
cold stress, and the resultant decline in 
the triglyceride content of birds tested 
under conditions of severe cold (-94°) 
and moderate cold (14°) accounted for 
most of the heat production. The sea- 
sonal variation in the triglyceride con- 
tent of goldfinches thus closely paral- 
leled the yearly change in thermogenic 
capacity, evidence that the ability to 
store triglyceride and to mobilize it rap- 
idly plays an important role in the suc- 
cess of goldfinches in cold climates. 
Our studies of substrate utilization by 
shivering goldfinches clearly indicated, 
however, that the seasonal shift in tri- 
glyceride storage was not the only im- 
portant factor determining endurance 
during cold stress. Goldfinches do have 
significant triglyceride reserves in sum- 
mer, but in the -94° tests, they fail to 
use these reserves before going hypo- 
thermic. Most important, when gold- 
finches tested at —94° in winter do 
finally succumb to the cold, they still 
have considerable triglyceride reserves 
remaining in the storage depots. The 
failure to thermoregulate when triglyc- 
eride reserves appear adequate may be 
related to a complex series of hormonal 
and biochemical factors that influence 
mobilization, transport, and oxidation of 
fatty acids. 
To understand further the role of 
fuels in limiting the thermogenic capaci- 
ties of goldfinches, we turned to an 
examination of the use of carbohydrates 
by shivering muscles. Vertebrates nor- 
mally store only a fraction of their total 
energy' reserves as the carbohydrate gly- 
cogen. Glycogen has about one-half as 
much energy per gram as does triglycer- 
ide and, in addition, must be stored 
61 
