title: Hydrodynamic drag constrains head enlargement for mouthbrooding in cichlids.
abstract: Presumably as an adaptation for mouthbrooding, many cichlid fish species have evolved a prominent sexual dimorphism in the adult head. Since the head of fishes serves as a bow during locomotion, an evolutionary increase in head volume to brood more eggs can trade-off with the hydrodynamic efficiency of swimming. Here, the differences between males and females in three-dimensional shape and size of the external head surfaces and the effect thereof on drag force during locomotion was analysed for the Nile tilapia (Oreochromis niloticus), a maternal mouthbrooder. To do so, three-dimensional body surface reconstructions from laser scans and computational fluid dynamics simulations were performed. After scaling the scanned specimens to post-cranial body volume, in order to theoretically equalize propulsive power, the external volume of the head of females was 27% larger than that of males (head length + 14%; head width + 9%). These differences resulted in an approximate 15% increase in drag force. Yet, hydrodynamics imposed important constraints on the adaptation for mouthbrooding as a much more drastic drop in swimming efficiency seems avoided by mainly enlarging the head along the swimming direction.
authors.affiliation: Evolutionary Morphology of Vertebrates, Ghent University, K.L. Ledeganckstraat 35, 9000 Gent, Belgium Department of Biology, Universiteit Antwerpen, Universiteitsplein 1, 2610 Antwerpen, Belgium sam.vanwassenbergh@ugent.be.
authors.forename: Sam
authors.initials: S
authors.lastname: Van Wassenbergh
authors.affiliation: Evolutionary Morphology of Vertebrates, Ghent University, K.L. Ledeganckstraat 35, 9000 Gent, Belgium.
authors.forename: Nuno Zavattieri
authors.initials: NZ
authors.lastname: Potes
authors.affiliation: Evolutionary Morphology of Vertebrates, Ghent University, K.L. Ledeganckstraat 35, 9000 Gent, Belgium.
authors.forename: Dominique
authors.initials: D
authors.lastname: Adriaens
mesh_terms: D000818:Animals; D023681:Cichlids / Q000033:anatomy & histology / Q000502:physiology*; D005260:Female; D006257:Head / Q000033:anatomy & histology / Q000502:physiology*; D057446:Hydrodynamics; D008297:Male; D013550:Swimming / Q000502:physiology*
keywords: drag reduction; fish; head shape; sexual dimorphism; streamlining; swimming