The theme of structure and function runs throughout biology, and is of the utmost importance for reproduction. Some of the strangest biological structures have been discussed in this chapter. They are each designed to perform a specific duty and require a specific shape.
Let's start with the testicles. Most mammals have testicles that hang outside of their body in a sac called the scrotum. The scrotum is a structure built for function. It likely exists to keep the testicles at an optimal temperature for sperm production. As sperm prefer a cooler temperature than the rest of the body, the testicles allow them to "hang-out" in a more comfortable climate. The structure of the sac also allows them to be pulled in closer if they get too cold.
Next, we have the penis and the vagina. As discussed above, the purpose of the penis is to deposit sperm inside the female during internal fertilization. The vagina is the receiving runway for the sperm. It is necessary that the penis and the vagina have a lock and key type of fit. They must be complementary for copulation to succeed. Argentine lake ducks even have corkscrew shaped penises and vaginas that spiral in opposite directions to make a perfect match.
The uterus of a female is a structure built for function as well. In an unfertilized female, the uterus is a small organ. Then, when necessary, it can miraculously grow and expand into a large cavity to accommodate the developing baby (or babies). It must be tough enough to protect the fetus from the outside, and tough enough to protect the mother from the fetus. Remember, some of these babies have hooves. Ouch.
Sperm must be tiny so that they can be produced cheaply in mass quantities and so that they can fit inside the egg easily. They must have a strong tail for swimming, and a head full of enzymes for breaking into the egg. Gametes must be haploid so that when they meet they can create a diploid zygote. Eggshells must be thick for protection, but thin enough to break out of. We could go on and on. The structures involved in reproduction are all perfectly suited for their functions.