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 analyzed 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.
Specimen BF2 surface STL used in CFD
Oreochromis niloticus specimen BF2 (Royal Museum for Central Africa), female, mass = 63 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF2_F_63g_SC_CFD_avg.stl
Specimen BF5 surface STL used in CFD
Oreochromis niloticus specimen BF5 (Royal Museum for Central Africa), male, mass = 100 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF5_M_100g_SC_CFD_avg.stl
Specimen BF6 surface STL used in CFD
Oreochromis niloticus specimen BF6 (Royal Museum for Central Africa), male, mass = 103 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF6_M_103g_SC_CFD_avg.stl
Specimen BF8 surface STL used in CFD
Oreochromis niloticus specimen BF8 (Royal Museum for Central Africa), male, mass = 125 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF8_M_125g_SC_CFD_avg.stl
Specimen BF10 surface STL used in CFD
Oreochromis niloticus specimen BF10 (Royal Museum for Central Africa), female, mass = 63 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF10_F_63g_SC_CFD_avg.stl
Specimen BF11 surface STL used in CFD
Oreochromis niloticus specimen BF11 (Royal Museum for Central Africa), female, mass = 54 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF11_F_54g_SC_CFD_avg.stl
Specimen BF13 surface STL used in CFD
Oreochromis niloticus specimen BF13 (Royal Museum for Central Africa), male, mass = 86 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF13_M_86g_SC_CFD_avg.stl
Specimen BF17 surface STL used in CFD
Oreochromis niloticus specimen BF17 (Royal Museum for Central Africa), female, mass = 48 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF17_F_48g_SC_CFD_avg.stl
Specimen BF18 surface STL used in CFD
Oreochromis niloticus specimen BF18 (Royal Museum for Central Africa), female, mass = 51 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF18_F_51g_SC_CFD_avg.stl
Specimen BF19 surface STL used in CFD
Oreochromis niloticus specimen BF19 (Royal Museum for Central Africa), male, mass = 95 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF19_M_95g_SC_CFD_avg.stl
Specimen BF20 surface STL used in CFD
Oreochromis niloticus specimen BF20 (Royal Museum for Central Africa), female, femass = 57 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF20_F_57g_SC_CFD_avg.stl
Specimen BF22 surface STL used in CFD
Oreochromis niloticus specimen BF22 (Royal Museum for Central Africa), male, femass = 115 g, left-right assymetry removed, scaled isometrically to the same mean body volume (excluding the head)
Tilapia_BF22_M_115g_SC_CFD_avg.stl
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): reference model.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head width +10%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head width +20%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head width +30%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head width +40%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head height +10%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head height +20%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head height +30%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head height +40%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head length +10%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head length +20%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head length +30%.
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Oreochromis niloticus model for sensitivity analysis (Fig. 2 in the article): head length +40%.