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Reproduction occurs in different ways in the animal kingdom which range from egg hatching to cell division. In mammals, reproduction happens when the female carries a zygote in its womb within a defined period, where it develops, and later birth occurs. The period between fertilization and birth is known as the gestation period or pregnancy. The gestation period varies across the different mammalian species. In humans, the gestation period is normally nine months or 270 days. However, the human gestation period is dwarfed by that of many other mammals.

The African Elephant (645 days)

The African Elephant (Loxidanta) is the genus given to the two species of elephants that inhabit Africa which is the larger African Bush Elephant and the relatively smaller African Forest Elephant. The African elephant is known to be the largest living terrestrial animal on earth with weights exceeding 10,000 pounds. With a gestation period of 645 days, these animals also have the record of having the longest gestation period of any mammal. The female elephants are usually in estrus (in heat) for about seven days where they attract males to mate and begin the 22-month period of gestation.

The Sperm Whale (535 days)

The Sperm whale (Physeter macrocephalus), is a marine mammal and is the largest toothed predator on earth. With sizes exceeding 60 feet in length and an average weight of 63 tons, the sperm whale is the largest of all toothed whales. The sperm whale also has the longest gestation period of all marine mammals amounting to 525 days after which the female gives birth to a 13-foot newborn calf.

Walrus (456 days)

The walrus is a huge marine mammal which is primarily located in the North Pole. The walrus is identified through massive tusks that they possess. With males having weights reaching over 4,000 pounds, these marine mammals are among the largest flipped mammals and are second only to the elephant seal. The female walrus becomes sexually mature at the age of four and mates with the dominant males. The gestation period of these mammals lasts about 15 months after which a 100-pound calf is born.

Black Rhinoceros (450 days)

The black rhino is one of the two rhinoceros species that inhabit Africa which is currently critically endangered. The black rhino is distinct from its white counterpart through the shape of its lip where the black rhino has a hook-shaped lip. The species is further divided into seven subspecies defined by their area of habitation. The black rhino is one of the largest terrestrial mammals in Africa with sizes exceeding 12 feet in length with an average weight of 3,000 pounds. The black rhino has the second-longest gestation period of all African terrestrial mammals lasting 16 months after which a calf is born.