Form and function of the fibularis brevis muscle in some passerine birds

Abstract

The morphology of the fibularis brevis muscle was analysed in 29 passerine species. As expected, most of them showed onl y one point of origin, the head arising crani all y from the f ibul a and th e adj acent tibiotarsus. However, Panurus hiarmicus, Regulus regulus. Parusater. P. caeruleus. Cerrhiafamiliaris and C. hrach)'dacty/a showed an extra point 01' ori gin, with fl eshy fibres ari s ing from the outer cnemial crest of the tibiotarsus. The occurrence of such a tibi al head was compared with the foraging postures ofth e spec ies and a corre lation was fo und between the presence of two fully developed heads in fih u/aris hrn is and a greater propensity in lhe spec ies for hang ing and/or creeping. The musc ul ar requirement impon ant to these acti ons is for fl ex ing 01' the leg jo ints. Hence, Ihe res ults seem lo indicate that the presence of a tibi al head in jibu/aris hreris might be relaled to tarsometatarsal tl ex ion. The mechanics of the s ituation seems 10 support this inte rpretation, as the torque of the musc ul ar fo rce is inc reased, which enhances the effec ti veness of the flex ing acti on of the muscle. This adaptive hypothes is is tested by the outgroup comparison method.