We constructed an experimental system that can observe bovine sperm motility in three dimensions, simultaneously from the vertical and horizontal directions. The comparison of the experiments conducted using cover glass and square tubing indicates that the effect of the cover glass on the sperm motility is not negligible. Based on this result, we investigated a three-dimensional trajectory of a motile sperm in the square tubing by simultaneously observing through two high-speed cameras. In

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... ion, we investigated the effect of different viscosities of the surrounding fluid on the sperm motility. The experimental results indicate that the increasing viscosity causes a decline in the sperm motility. Furthermore, we focus on the width of the sperm head and the flagellar shape. At low viscosity, the time variation of the head widths is relevant with the two directions and varies periodically. In contrast, at high viscosity, an almost constant value is maintained and periodical time variations are not observed for both directions. Moreover, a difference in the viscosity is observed for the tortuosity of the flagellum shape. These results suggest that the three-dimensional flagellar structure of the bovine sperm is a helix whose cross-sectional shape is an ellipse with quite a large aspect ratio. Further, the results state whether the rotation or nonrotation of the sperm is dependent on the balance between the viscous resistance acting on the sperm and the torque due to the flagellar shape. The experimental results obtained will be useful in clarifying the mechanics of sperm motility under their actual environmental conditions at high viscosity.