Scientific studies
With whales, the length of the penis can be an indication of the maturity of the organism. Sei whales for instance are divided into categories such as immature, pubertal and mature.[4] However, length is not the only factor as it is recorded in relation to the size of the whale's testes and its histology. For instance, research conducted off South Africa found that the longest penis of an immature categorized whale was 96 centimetres (38 in) and the smallest penis of a mature whale was 95 centimetres (37 in). By comparison the average blue whale penis is nearly 3 times the size of that of a sei whale.[4] Specimen measurements indicate that a blue whale measuring 21.6 metres (71 ft) in length had a vestigial mammary slit of 40.6 centimetres (16.0 in) width and 48.2 centimetres (19.0 in) length, with a penis measurement of 1.83 metres (6.0 ft). In another specimen in which the length of the whale was 20 metres (66 ft) and weighed 50.9 metric tons, the brain weighed only 3.636 kilograms (8.02 lb).[5]
"Blue whales have the largest penises on Earth. An erect blue whale penis is 12 inches (30 cm) in diameter and ten feet in length. It is fibroelastic like those of the blue whale's artiodactyl relatives. The retracted penis curves in an S-shaped loop and stays inside the body. When erect, it peeks out of the genital slit. Blue whales also have a distal spiral valve as a spermatic tube. Cetaceans do not have scrotums like terrestrial mammalian males do. Mammalian scrotums lower ambient temperature to keep sperm viable, so cetaceans had to find a way to compensate. And they did. Blood in veins that go through fins and the fluke loses heat. When that blood is sent directly to the testes, the sperm are cooled below body temperature and remain viable. The females have a long vulva with two nipples on each side to nourish newborns. They go in heat any time of the year, but the most common is during the migration to warmer waters for mating season."
–"Balaenoptera musculus", University of Wisconsin[6]