What are marine mammals?

The concept of marine mammals is very broad and does not refer to a specific taxonomic group. Within these animals, we include:

• The cetaceans : whales , dolphins and porpoises.
• The pinnipeds : seals, sea lions and walruses.
• The sirenians : manatees and dugongs.
• Some otters : sea otter and sea cat.
• The polar bear or white bear , although it is not an aquatic animal, can be considered within marine mammals, since it spends most of the year on sea ice and is adapted for life in the sea.

Of these groups, cetaceans and sirenians spend their entire lives in the water, while pinnipeds and otters are part of their life on land. As a consequence, cetaceans and sirenians are the most adapted to marine life.

Marine mammals are a very charismatic megafauna of aquatic environments. However, they have a long history of commercial exploitation by humans, for fat, meat, oils, skin or ivory. This has made many of these populations vulnerable or in danger of extinction . For this reason, the vast majority of marine mammal species are protected from this exploitation and have the support of some environmental groups.

As examples of marine mammals, in the main image of the article we can see a whale, in the bottom of this section a manatee and in the last image dolphins.

Findings and fossil studies tell us that the earliest ancestors of marine mammals lived in the ancient Sea of ​​Tethys in Earth’s past (more than 70 million years ago). These ancestors gave rise to the ancestors of the marine mammals that exist today (although very different).

Although the evolutionary processes that allowed them to adapt to the marine environment are not known, it is known that they are not a monophyletic group (that is, the different groups arose from different terrestrial ancestors ). This is based on the study of their anatomical patterns, their fossils and their molecular similarities. In cetaceans, it is believed that it was an artiodactyl (pigs, cows, …) distantly related to hippos. In sirenians, a proboscidean brother of current elephants, while in pinnipeds, an ascendant common to bears and mustelids (weasels, skunks and otters). Later, the three groups adopted similar physical characteristics, due to their need to adapt to life in the sea, something known as evolutionary convergence.

In their process, marine mammals were acquiring different morphological and functional adaptations that allowed them to live in the new environment. To understand the adaptation process, it is necessary to know that the marine environment has very different physical properties from the terrestrial environment and, therefore, an animal that wants to live in the sea must adapt to it.

To interpret the adaptation process, it is necessary to be clear about some concepts related to the characteristics of the aquatic environment. The first thing is to know that the density of water is three times greater than that of air and the viscosity , about 60 times greater at similar temperatures. These two properties influence friction, since they are forces opposite to the movement of the body in water. Another important factor is that in the marine environment, the pressure , a force that is exerted on a body and tends to compress it, is greater than in the terrestrial environment, approximately 1 atmosphere more for every 10 meters of depth. The thermal conductivity It is also greater in water than in air, that is, the transfer of heat from a body to the outside and the light energy is attenuated at greater depths.

Given these conditions, marine mammals must adapt to them. Some of the adaptations of marine mammals to make life in water are the following:

• Hydrodynamic adaptations : fish-like fish-like bodies, limbs and tails transformed into fins, disappearance of fur or reduction to reduce resistance to swimming or shortening of the length of their necks.
• Thermoregulatory adaptations : otter fur as an insulator to water, endothermic or homeothermic (internal heat generation) or thick layers of fat under the skin.
• Reproductive adaptations : lips capable of vacuuming to avoid milk losses during lactation or highly concentrated milk to minimize losses to the environment.
• Respiratory adaptations : large respiratory surfaces that allow them to carry out a more efficient gas exchange, increased lung capacities due to the position of the diaphragm in the body or expulsion of air at the surface (instead of inhaling it) to avoid embolism at high depths .

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