Mammals, which evolved on land, made their way into water some millions of year ago to take advantage of what the sea has to offer. The term ‘marine mammals’ includes all mammals that live in either marine or freshwater environments and would be more correctly called aquatic mammals but they are almost always found in marine environments.
Three distinct lineages of mammals have evolved to live in water:
- Cetacea – whales, dolphins and porpoises
- Carnivora – seals, sea lions, walruses and sea otters
- Sirenia – dugongs and manatees
Life in water
Marine mammals all have certain characteristics that make them similar to each other and different to other marine animals such as fish.
The most significant is the fact that they breathe air rather than water, and so, they must return to the water’s surface every so often to breathe. They give birth to live young that suckle for milk, have glands such as sweat glands and mammary glands, and their bodies are heated internally by their own metabolism.
Life in water is a whole lot different than it is for land dwelling mammals. Marine mammals have evolved a range of adaptations that makes it possible to not only survive, but to thrive in their aquatic home.
The inability to breathe all the time means marine mammals have developed ways to survive for long periods on one breath of air; living in water removes the influence of gravity and so increasing in size is not an issue. Whales have evolved into the world’s largest animals and the blue whale (Balaenoptera musculus) that currently graces our oceans is the largest animal that has ever existed.
Water provides more resistance than air so marine mammals have evolved extremely streamlined bodies in order to move efficiently through the water. Beneath the surface layers of water light levels are very low.
Species that spend time at depth have accessory senses to help them ‘see’ in the dark, such as sonar. Water absorbs heat away from the body much faster than air does so marine mammals have thick layers of insulation to prevent themselves from getting hypothermia.
Whales, dolphins and porpoises
Whales, dolphins and porpoises are the most adored of the marine mammals. They belong to an order of animals called Cetacea and are split into two suborders: odontoceti (toothed) and mysticeti (baleen). The cetaceans evolved from an even-toed ungulate, similar to pigs and cows.
Mysticetes or the baleen whales, such as humpback whales and blue whales, are toothless filter-feeders and are mostly migratory. Many species travel between the tropics for breeding and polar regions for feeding each year.
The distribution of Odontocetes is more variable between species. Some inhabit large ranges while others stay within small areas. Orca, for example, can complete extensive migrations following food or other resources while some dolphins are only found along one patch of coast.
Seals and sea otters
Seals, including sea lions, leopard seals and elephant seals, can be divided into ‘true’ seals and ‘eared’ seals. Worldwide there are 18 species of ‘true’ seals, 14 species of eared seals and one species of walrus. Otter’s belong to the Mustelid family and are the only animals from this family to make its home in water although many other mustelids are very competent swimmers.
Dugongs and Manatees
Dugongs and manatees, also known as sea cows, are herbivores that feed on sea grass. They belong to the order Sirenia and have evolved from elephants. Dugongs are found in Australia and manatees can be found in the Americas and Africa.
They are slow moving and gentle animals but are under threat due to the loss of sea grass cover in coastal areas and are often hit by ships. Manatees were mistaken for mermaids by early sailors to the Americas. How, I’m not quite sure seeing as they look incredibly different to any picture of a mermaid I’ve ever seen.
Hunter or hunted?
Almost all marine mammals have been the victim at some point in time of human hunting. Booms in hunting industries following the industrial revolution led to the demolition of many marine mammal populations. Blubber was highly sort after for a number of uses and many animals were also hunted for their furs.
Many marine animals are however great hunters themselves and often find themselves at the top of the food chain. Whales and dolphins are some of the most intelligent of the world’s animals and have evolved some amazing strategies to catch prey. In addition to great intelligence, many marine mammals are fantastic swimmers and can reach speeds of up to 45km/h or 30 miles/hour.
Last edited: 13 December 2015