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Selective factors in the origin of the mammalian diaphragm

Published online by Cambridge University Press:  08 April 2016

John A. Ruben ,
Albert F. Bennett  and
Frederick L. Hisaw
John A. Ruben
Affiliation:
Zoology Department, Oregon State University, Corvallis, Oregon 97331
Albert F. Bennett
Affiliation:
School of Biological Sciences, University of California, Irvine, California 92717
Frederick L. Hisaw
Affiliation:
Zoology Department, Oregon State University, Corvallis, Oregon 97331
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Abstract

The origin of endothermic homeothermy and of high metabolic rate in mammals is currently believed to be the result of early (Mesozoic) selection in advanced cynodont therapsids and/or early mammals for either (1) enhanced thermoregulatory capacity or (2) increased powers of endurance and stamina. Selective factors underlying the origin of specialized respiration/ventilation-support systems in mammals are possible indices of the validity of these two hypotheses. One such support structure is the diaphragm, a specialized muscle that facilitates lung ventilation. We tested capacity for maintenance of resting metabolic rate, thermoregulation, and for extended, intense exercise in laboratory rats (Rattus rattus) in which diaphragm function had been completely ablated. The results were virtual elimination of aeroboic scope (active metabolic rate — resting metabolic rate) but resting metabolic rate was unaffected. Thermoregulatory capacity was unimpaired to at least 8° below lower critical temperature. These and other data suggest that the origin of the mammalian diaphragm, as well as mammalian metabolic rates, may have been related to selection for greater levels of sustainable activity rather than for functions associated with thermoregulation.

Type
Articles
Information
Paleobiology , Volume 13 , Issue 1 , Winter 1987 , pp. 54 - 59
Copyright
Copyright © The Paleontological Society 

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