Allometric Relationships among the Shoulder Muscles in the Chiroptera

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Abstract
Several empirical determinations of the power requirements of different sized birds suggest that large birds require relatively more power to fly than small birds do. However, large birds do not appear to have relatively more flight musculature than small birds. Presently available information indicates that the same is true of different sized bats. To gain more insight into the allometry of the shoulder musculature in bats this study presents the results of linear regression analyses comparing the dry masses of 23 bat shoulder muscles with body mass for 29 species of bats. The results of the study suggest that there is an overall trend toward positive allometry among the shoulder musculature, and that five of the 23 muscles have regression line slopes which are significantly greater than the isometry value of 1.0. The relatively low degree of positive allometry seen in most of the shoulder muscles is probably due primarily to the fact that large bats generally adopt behaviors which require relatively less power than those of smaller bats, allowing them to function without a significant increase in the relative size of the flight musculature. Osteological specializations related to increasing the efficiency of the flight musculature of large bats may also be involved in maintaining a low level of positive allometry. The effects of the windstream in aiding wing elevation during the first part of the upstroke may be greater in large bats than in small ones, and may contribute to the reduced positive allometry of some of the upstroke musculature. Those muscles whose actions during the upstroke cannot be aided by windstream forces all show a relatively high level of positive allometry.

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