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First published online August 17, 2006
Journal of Experimental Biology 209, 3448-3456 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02396

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Isometric contractile properties of sexually dimorphic forelimb muscles in the marine toad Bufo marinus Linnaeus 1758: functional analysis and implications for amplexus

Douglas Lee Clark and Susan E. Peters^*

Department of Biology, The University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, NC 28223, USA

^* Author for correspondence (e-mail: sepeters{at}email.uncc.edu)

Accepted 20 June 2006

It has been shown in the bullfrog, Rana catesbeiana Shaw 1802, that certain forelimb muscles in males have different contractile properties when compared with females, which may result from adaptation for amplexus. We extended this study to a distantly related species, Bufo marinus Linnaeus 1758, by testing the isometric contractile properties of three muscles, abductor indicus longus (AIL), and flexor carpi radialis (FCR) (both dimorphic muscles), and extensor carpi ulnaris (ECU) (non-dimorphic control). In males the dimorphic muscles had greater wet mass and cross-sectional area than in the females, and also produced significantly greater isometric force. As in bullfrogs, however, the maximum tetanic force per cm² of muscle cross-section did not differ between the sexes. In spite of this similarity in maximum force, the two dimorphic muscles were much less fatigable in the males than in the females. Lower fatigability in males correlated with exceptionally elongated relaxation times that maintained high levels of force between stimulus trains. This sustained force was negligible in the females, suggesting that this feature may allow males to maintain amplexus for prolonged periods. The same sustained force response was observed in the earlier study of Rana catesbeiana. Because this response is similar in Bufo and Rana, muscular properties correlated with amplexus may be shared across anurans by inheritance of this response from a common ancestor.

Key words: muscle, anura, amplexus, Bufo marinus, Rana catesbeiana

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