28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 143 



muscle, while not powerful, can continue contraction for a consider- 

 able period if supplied with oxygen and fuel. The flight muscles tend 

 to parallel the performance in the air, while the muscles of the lower 

 extremities are related to their use for locomotion or for acquiring 

 food. Most important of the muscles is the heart, since it is the 

 driving force that circulates oxygen and nutrients and removes waste 

 products. A small heart limits activity, while a large one can maintain 

 it much longer. 



We had previously found no difference between the sexes in the 

 relative heart weight (Hartman, 1955), but in the present survey we 

 observed that in 19 species the heart in the male was relatively heavier 

 than in the female; if more data were available the list might be 

 increased. This difference occurred in small or relatively small birds, 

 with the single exception of G alius. In some species the males are more 

 active than the females, but we cannot say whether this was so in the 

 19 species in which a difference was observed. Groebbels (1932) 

 noted a difference in heart size between the sexes in some species. 



In my earlier paper (Hartman, 1955) evidence suggesting a sea- 

 sonal difference in heart weight was presented, but the number of 

 individuals was too small to make the findings significant. This short- 

 coming was obviated in the present work in the case of a few species 

 by the inclusion of a sufficient number of individuals. Birds collected 

 in January and February possessed hearts larger than those obtained 

 during June, a finding contrary to the suggestive evidence in the 

 former paper. Such a difference could be explained by the higher 

 rate of metabolism in the cooler weather. In keeping with this inter- 

 pretation we find that certain species living in Florida possess larger 

 hearts than the same species from Panama. Since many Cathartes 

 aura migrate to the south in winter, the season when our Panama birds 

 were collected, the question has been raised as to the identity of some 

 of these birds. Whether they were migrants or not, there were very 

 significant differences in the hearts and pectoralis plus supracora- 

 coideus muscles between the two groups. 



If altitude is a factor in increasing the heart size, why do not more 

 species show the effect ? Some of those that have larger hearts at the 

 higher altitude are not very active (e.g., cuckoos). Perhaps additional 

 data would extend the difference to more species. Stieve (1934) com- 

 pared heart size in Lagopus mutus mutus and L. lagopus lagopus, 

 both from the same altitude (400 to 700 m.). The alpine grouse 

 (L. m. mutus) at this low level has a distinctly larger heart than the 

 moor grouse. Therefore in these instances it is a species difference 



