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First published online January 31, 2007
Journal of Experimental Biology 210, 715-721 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.000646

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Sexual dimorphism of extensor carpi radialis muscle size, isometric force, relaxation rate and stamina during the breeding season of the frog Rana temporaria Linnaeus 1758

Carlos A. Navas^1,* and Rob S. James²

¹ Departamento de Fisiologia, Instituto de Biociências, Universidade de São Paulo, Rua do Matão–Travessa 14 No 321, CEP 05508-900, São Paulo, SP, Brasil
² Department of Biomolecular and Sport Sciences, Faculty of Health and Life Sciences, Coventry University, CV1 5FB, UK

^* Author for correspondence (e-mail: navas{at}usp.br)

Accepted 18 December 2006

Mating success of individual male frogs within explosive breeding species can depend on their ability to compete for a mate and to hold onto that mate during amplexus. Such importance of amplexus has resulted in the evolution of sexual dimorphism in the morphology and contractile characteristics of the anuran forelimb muscles used during amplexus. The aims of our study were to use an explosive breeding frog (Rana temporaria) during the breeding season to compare extensor carpi radialis (ECR) muscle length, mass, isometric activation times, relaxation times, absolute force, relative force (stress) and fatigue between male and female frogs. We found that ECR muscle mass and length were greater (tenfold and 1.4-fold, respectively), absolute tetanic muscle force and relative tetanic force (stress) were greater (16-fold and 2.2-fold, respectively) and relaxation times were slower in males than in females. Male ECR muscles incompletely relaxed during fatigue tests and showed less fatigue than female muscles. These sex differences are likely to be beneficial to the male frogs in allowing them to produce relatively high absolute muscle forces for prolonged periods of time to hold onto their mate during amplexus.

Key words: amplexus, contractile, fatigue, sex, morphology, tetanus

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