Mammals (Mammalia) are a class of vertebrates in which we find about 5,500 different species, of which only 130 species are marine mammals, the ones we will focus on. The name of the mammals is due to the mammal glands present in the females and which are used to feed the young with milk secreted by these glands. Among the marine mammal species, we find whales, dolphins, killer whales, porpoises, manatees, seals, walruses and otters.
In general, the characteristic body shape of marine mammals is hydrodynamic, possessing modified limbs in the shape of fins that allow them to propel themselves under water. The skin has adapted to live in the aquatic environment, being provided with a thick coat that isolates them from the cold of the water, and/or has a layer of fat that also allows them to expand their distribution to any sea or ocean of the world, including both poles. Marine mammals, unlike the rest of the marine species which obtain oxygen from the water to breathe, do so by breathing air, which is why they must come to the surface from time to time.
Since mammals originally developed on land, their spines are optimized for walking, so they move up and down with ease, but have less lateral mobility. Therefore, marine mammals usually swim by moving their spines up and down. In contrast, fish normally swim by moving their spine laterally. For this reason, fish usually have their caudal fin vertical, while marine mammals have it horizontal.
As a curious fact, marine mammals that live exclusively in the sea had to experience important adaptations in their sleeping habits in order to sleep without drowning. From studies carried out with bottlenose dolphins in aquariums and zoos and also with whales and dolphins in the wild, we can see that they developed two sleeping methods: either they stay static in two waters (vertically or horizontally) or they swim while they sleep in a state similar to napping
Marine mammals are placental viviparous animals. The development of offspring occurs within the body of the mother, and at birth they are fully developed.