Fibre types in primary ‘flight’ muscles of the African Penguin (Spheniscus demersus)

Schroeder, K.L., Sylvain, N.J., Kirkpatrick, L.J. and Rosser, B.W.C. 2014. Fibretypes in primary ‘flight’ muscles of the African Penguin ( Spheniscus demersus).—ActaZoologica (Stockholm) 00:000–000.The African penguin (Spheniscus demersus)isanendangeredseabirdthatresideson the temperate southern coast of Africa. Like all penguins it is flightless,instead using its specialized wings for underwater locomotion termed ‘aquaticflight’. While musculature and locomotion of the large Antarctic penguins havebeen well studied, smaller penguins show different biochemical and behaviouraladaptations to their habitats. We used h istochemical and immunohistochemicalmethods to characterize fibre type composition of the African penguin primaryflight muscles, the pectoralis and supracoracoideus. We hypothesized the pecto-ralis would contain predominantly fast oxidative–glycolytic (FOG) fibres, withmainly aerobic subtypes. As the supracoracoideus and pectoralis both powerthrust, we further hypothesized these muscles would have a similar fibre typecomplement. Our results supported these hypotheses, also showing an unex-pected slow fibre population in the deep parts of pectoralis and supracoracoide-us. The latissimus dorsi was also examined as it may contribute to thrustgeneration during aquatic flight, and in other avian species typically containsdefinitive fibre types. Unique among birds studied to date, the African penguinanterior latissimus dorsi was found to consist mainly of fast fibres. This studyshows the African penguin has specialized flight musculature distinct from otherbirds, including large Antarctic penguins.BenjaminW.C.Rosser,DepartmentofAnatomyandCellBiology,UniversityofSaskatchewan,Saskatoon,SK,Canada S7N5E5.E-mail:ben.rosser@usask.caIntroductionPenguin species vary widely in habitat and body size, from thelarge emperor penguin of Antarctica to the diminutive littlepenguin of New Zealand and southern Australia, yet exem-plary aquatic agility is a hallmark of all members of the orderSphenisciformes. The modern penguin uses its wings forswimminganddivinginamannerthathasbeentermed‘aquatic flight’, as the behaviour is similar to airborne birds,although underwater agility has developed at the expense ofaerial flight (Bannasch 1994; Ksepka and Ando 2011). Onceancestral penguins became flightless, their morphology wasnot bound by the demands of supporting body mass in airand further modifications were developed to increase effi-ciency in the aquatic environment. Musculoskeletalspecializations of the penguin wing that have increased swim-ming efficiency include flattened bones, reduction of the distalmusculature used to make fine adjustments by airborne birds,large sesamoid bones at the elbow that stiffen the wing andincreased relative mass of muscles involved in the upstroke(Schreiweis 1982; Bannasch 1994; Ponganis et al. 1997).These structural adaptations facilitate movement throughwater by reducing drag, increasing functionality of the wing asa hydrofoil and incorporating the upstroke as a power-gener-ating stroke (Bannasch 1994; Watanuki et al. 2006).In birds, the primary muscles used to power flight are thepectoralis and supracoracoideus. The pectoralis is the largestmuscle in the body and is primarily responsible for driving thedownstroke of the wing. Performing the opposite action is thesupracoracoideus muscle, which raises the wing during