Reproductive System

Male Anatomy

Unlike most mammals, the entire reproductive system in males is internal, concealed in a genital slit. This adaptation enhances their hydrodynamics when swimming and helps minimize heat loss. When mating, males can deploy their penis through their genital slit.

In cetaceans, the penis originates from two fused structures, each attached to a vestigial pelvic bone that corresponds to the pelvis of terrestrial mammals. Whales having lost their hind legs over the course of evolution, their pelvic bones are no longer used for locomotion, but are likely useful for penile erection and control.

The testicles of cetaceans are kept warm in their abdominal cavity, though their body temperature is too high for sperm production. The testicles are therefore equipped with a network of blood vessels, the rete mirabile (Latin for “wonderful network”), which serves to channel heat toward the dorsal fin or dorsal crest.

The male’s penis emerges from the genital slit during copulation. Photo of harbour porpoise. © Sytske Dijksen

Impressive numbers

The reproductive system of whales is proportional to the overall size of these giants. The penis of some baleen whales can reach 3 metres long. Right whales have the largest testicles in the animal kingdom, reaching a combined mass of 1,000 kg, which corresponds to 2% of their total weight. Despite being generally smaller than their baleen cousins, toothed whales have testicles that are 7 to 25 times larger than those of land mammals of comparable size. But what is the advantage of having such heavy testicles? They enable the production of large volumes of sperm, which increases the probability of fertilization. The blue whale, the largest known animal on Earth, is estimated to ejaculate about 20 litres!

Female Anatomy

Like other mammals, female cetaceans have two ovaries, a uterus, a vagina and, during gestation, a placenta. In toothed whales, one of the two ovaries is larger and used more than the other.

The uterus is bicornuate, unlike that of humans, which is composed of a single cavity. The fetus develops in only one of these uterine horns. Whales do not menstruate: the blood in the wall of the endometrium is reabsorbed rather than expelled from the female if she has not been impregnated.

The vagina of cetaceans consists of multiple folds, the shape and number of which depend on the species. These folds may have evolved to limit the amount of water entering the vagina. They also seem to be the result of co-evolution between males and females.

Co-evolution of Reproductive Structures

The vagina of cetaceans is far from being a straight path for sperm cells. Rather, it consists of twists, dead-ends and folds that limit the advancement of sperm. These structures are believed to give females some degree of control over reproduction: by changing the angle of penetration, they may be able to prevent fertilization. Since gestation is a very energy-intensive process, it is in a female’s best interest to choose a male that will give her “quality” offspring.

A male with a penis adapted to the complex structure of the female vagina will be more likely to reproduce and pass its genes on to future generations. His offspring can inherit his penis, thereby favouring reproduction of the next generation and so on. In species where breeding competition is high, males have larger organs and larger pelvic bones, which increases their odds of fertilizing a female.