Sperm from different species vary in size and shape. Bull and human sperm, for example, have paddle-shaped heads, rodent sperm have hook-shaped heads, and the heads of chicken sperm are spindle-shaped and almost difficult to distinguish from the midpiece. The images shown below of rat, bull and chicken sperm are all at the same magnification.
In a rat, there’s a mating ritual. The female rat will approach the male head-to-head. She will wiggle her ears, she will wiggle her whiskers, she will nibble at him, and finally she’ll turn and run away. If the male chooses not to pursue her, she may return and, as one leading rat sexologist puts it, ‘kick him in the face’. This tends to do the trick.
The male gives chase, catches the female and climbs on top of her, at which point only two key preparations remain to be completed.
First, so that the female’s low-slung genitalia can be reached from above, her hindquarters will bend upward in a reflexive arching of the back called lordosis. Second, so that the male may take advantage of this invitation, his penis will emerge from its hiding place under the abdominal fur. And then they copulate.
