Like most mammals, male whales have a penis and females have a vagina; however, the males keep their genitals hidden in a cavity. They mate underwater, lying belly-to-belly with each other, and can mate either while stationary or swimming. Both males and females mate with multiple whales to increase chances of reproduction.

Whales reach sexual maturity when they are 8 to 10 years old, and females give birth once every few years. The exact gestation time varies based on species; while humpback whale calves take 11 months, sperm whale calves can take up to 15. They are ready to swim from birth and nurse until they are 6 months old. Before the female is ready to mate again, she must rest for another six months after her calf is weaned. However, more social species of whales nurse for longer periods of time. Sperm whales nurse up to two years. Complete weaning can take additional time.

While whales are generally social and friendly with each other, pods can become very tense once males are battling for the affections of a female. There is very rarely serious injury done during these fights, however. The sound of their calls changes subtly, often attracting the attentions of other male whales that arrive to reproduce and lower the chances of inbreeding.