Playful, agile, smooth and compassionate: dolphins have fascinated and bewildered people for millennia. The Roman author Pliny the Elder, writing around 77 AD, called dolphins and whales fish in his book Natural History. While confusion still exists today, it is actually a wonderful testament to the diversity of the natural world.
Cetaceans are one of the few mammals, along with manatees, that spend their entire life in the water, never going out on land. How similar they are to fish is due to the fact that their torpedo-like body with fins and flippers is one of the fastest and easiest ways to move through the water. This is an example of convergent evolution: two separate lineages evolve with the same characteristics in response to the same environmental pressure.
The dolphin family includes a large number of different species, including ocean dolphins, river dolphins, killer whales, false killer whales and pilot whales. Today, semi-aquatic hippos are considered the closest living relatives of all cetaceans.
But in fact, cetaceans evolved from a small amphibian mammal about 50 million years ago. The earliest known cetacean ancestor, called Indohyus, was a bit like the modern mouse deer or Chevrotan with long, thin legs and hooves. It also had exceptionally thick bones, allowing it to enter the water and walk along the bottom, where most land animals would find it difficult to move. The first fully marine cetaceans appeared about 40 million years ago. They began to develop streamlined body shapes, fins instead of limbs, a thick layer of subcutaneous fat, and other aquatic devices. The first true members of the dolphin family appeared about 11 million years ago.
These ancestors are enough to classify all dolphins as mammals. But this connection goes much deeper, right down to the anatomy and genes of the dolphin. They carry clear physical traits of the mammalian class, including lung and milk production. So, without further introduction, in this article we will look at eight main characteristics common to dolphins and other mammals.
Dolphins breathe with their lungs
If there is one defining trait that nearly all fish and most marine organisms (even the so-called lungfish) have, it is the gills. These respiratory organs allow them to filter oxygen from the water in order to breathe properly. Dolphins, on the other hand, breathe exclusively with their respiratory tract and lungs, like any land animal. Although dolphins have several adaptations that help them hold their breath for extended periods of time in a high-pressure underwater environment (including oxygen-rich blood, collapsible lungs, and the ability to slow heart rate), they should eventually return to the surface after an average of about 10 minutes to get more oxygen. If they fail to get to the surface in time, they will drown.
Back breathing (which leads to the nasal passage) is the main breathing mechanism. This allows them to push stagnant air up in a giant column and then draw in fresh oxygen. When warm air from the body meets cold air outside, it condenses into a cloud that combines with body mucus and seawater around the opening to form a kind of jet. Since dolphins breathe exclusively through their air holes, they never need to worry about drawing water into their lungs when they swallow their prey. Dolphin lungs are also distinguished by their ability to exchange gases. While humans can only exchange 17% of the air in their lungs with each breath, dolphins can exchange 80% more efficiently.
Dolphins have a placenta
Although internal embryonic development is not unique to mammals (some fish also reproduce this mode), dolphins have a unique mammalian feature that no one else has: the placenta. This temporary organ takes nutrients from the mother’s bloodstream and delivers them to the unborn baby. It is an elegant way to ensure the survival of the developing offspring, but it also puts a lot of strain on the mother’s body.
When mammalian babies are finally born after a relatively long gestation period, they are almost always highly developed and partially grown (with the exception of marsupial mammals, which keep their undeveloped offspring in a pouch). The parents (usually only the mother, but sometimes the father as well) devote a lot of time and resources to caring for the cubs, both before and after birth.
Dolphins can produce milk
Like all mammals, female dolphins have mammary glands that produce milk for their young. Since calves do not have lips, many of them have to form a waterproof seal by rolling their tongues in straw to consume milk without penetrating the sea water. Breastfeeding can last for several years before the calf completely rejects breast milk. Because of its high calorie content, milk helps dolphins store the fat they need to survive in the water.
Calves are born with hair
It is almost universally true that mammals grow hair at some point in their lives. Hair provides warmth and insulation from the environment. It is also true that dolphins don’t really need hair; their oil under the skin provides a good replacement for hair insulation (as well as buoyancy and protection).
However, baby dolphins are born with small follicles or hair sticking out of their snouts. This is likely an evolutionary relic with no real purpose (other than perhaps helping them feel their mother), because most dolphins lose this hair quickly after birth and never grow a single strand of hair for the rest of their lives. However, the Amazon River dolphin is an exception. The mustache-like hair remains with him into adulthood, perhaps because it serves as a useful sensory tool while the dolphin rummages through the river mud in search of food.
Dolphins are warm-blooded
The terms “warm-blooded” and “cold-blooded” have long lost popularity among biologists (endothermic and ectothermic are now the preferred terms) because they paint a deceptive picture of real life biology. However, dolphins are inherently endothermic. They have the ability to maintain and control a healthy internal body temperature, regardless of the outside temperature, through several different mechanisms. Warm-blooded organisms have both higher metabolic rates for heat production and better means of retaining heat in the body. This is a useful trait found in modern mammals and birds.
The dolphin’s spine naturally curves up and down
Many people may not realize that the swimming movements of dolphins are clearly related to the ancestors of land mammals, whose limbs extend beneath their bodies. To lengthen the gate, land mammals have spines that naturally curve up and down. While fish can tilt their tail from side to side, dolphins have developed mammals’ natural up and down movement to swim. This also means that the tail fin is horizontal rather than vertical. The dolphin vertical tail has many uses. It is believed that they flap their tails in the water to stun prey and communicate with each other.
Dolphins have a mammalian skeleton
While the skeletal structure of a dolphin is obviously different in many ways from your typical land mammal, they also retain many common characteristics, including the presence of three middle ear bones and a vestigial pelvic bone, which may play a minor role in supporting reproductive organs.
Most notably, the bones of the dolphin’s pectoral (lateral) fins are roughly analogous to the human hand: they have a humerus (with a full hinge joint) on the shoulder, ulna and radius of the lower arm, and a complete arm structure with wrist bones and five phalanges. (fingers). Of course, the actual shape of these bones is greatly altered (the arms are much smaller and the fingers are longer), but given the fact that they are used for completely different reasons and in different environments, the similarity is all the more remarkable.
Another interesting fact about dolphin morphology is that embryos develop hind limb buds for a short time before they are reabsorbed into the body. This is because an important limb-forming gene (interestingly called Sonic Hedgehog, like a video game character) suddenly becomes inactive in the hind limbs during development, causing them to regress. However, in very rare cases, the hind limb does not regress, and the dolphin retains two small butts in adulthood.
Dolphins have larger brains and complex social organization
Although intelligence is hardly exclusive to mammals, mammals have relatively large brains compared to body size, which is thought to enable them to form complex social organizations, communicate in a variety of ways, and solve complex problems. The dolphin is considered one of the most intelligent life forms on the planet.
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