Dolphins are known as marine mammals and belong to the cetacean family, including whales and porpoises.
Unlike whales, however, all dolphin species possess teeth and are equipped with echolocation, which helps them find food and navigate the ocean in areas where light is limited or nonexistent.
Although dolphins live purely in the ocean, their ancestral history points to a more land-based past.
Over 50 million years, these animals have evolved from land-dwelling mammals with limbs used for walking and hunting into the ocean-bearing marine mammals we know today as dolphins.
Their spine and forelimb bones, found in their flippers, still contain some of the bone structure and shape these mammals possessed when their ancestors walked on land.
When observed, the forelimb bones still contain individual digits like fingers which appear to have been used for walking and grasping prey and other objects.
The spine’s structure also indicates a body designed for running instead of swimming.
This can often be observed when comparing a dolphin’s vertical up-and-down swimming motions with a fish’s horizontal (snake-like) left and proper swimming motion.
In this sense, dolphins and humans can be seen arching their backs up and down during certain swimming motions.
In addition to bone structure, another clear indicator is their mammalian lifestyle.
Unlike fish, dolphins cannot breathe underwater and do not possess gills.
They must come up for oxygen to breathe.
They are also warm-blooded and give birth compared to most species of fish that lay eggs.
To help them survive in the water, dolphins have also developed blubber and biological regulators that allow them to stay warm in cold climates and enable vital organs to receive blood in areas of extreme cold.
Dolphins have also developed a semi-sleep state, allowing half of their brain to remain active during rest periods.
This helps prevent dolphins from falling completely asleep and drowning.
If a human fell asleep in the water, they would likely drown by remaining in a semi-sleep state. Dolphins can rest while staying partially alert.
Diversity and Species
Currently, 17 known genera of dolphins are divided into 40 species.
Depending on the dolphin species, they can range significantly in size and weight.
Maui’s dolphin is the smallest and rarest dolphin species, which can measure around four ft. in length and weigh less than 100 pounds.
On the other end of the spectrum is the killer whale (the largest dolphin species) which can grow to over 30 ft. long and weigh more than 22,000 pounds.
Regarding anatomical features, all dolphins have teeth, flippers, a tail, flukes, and a single blowhole used for breathing.
Dorsal fin sizes may vary significantly from one species to the next, and some species may have a small hump or bump on their back instead of a dorsal fin.
Teeth shape, structure, and size vary as some species may have significantly more teeth than others, and some dolphins may only carry teeth on the lower jaw.
Interestingly, although all dolphins have teeth, not all species use them to break their prey apart or tear flesh.
Some species use their teeth to grasp their prey and keep it from escaping but will consume their food whole without chewing.
Most dolphin species consume a diet consisting of fish, squid, and crustaceans; however, dolphins such as the killer and false killer whales may also consume marine mammals such as seabirds and seals, walruses, and other cetaceans.
Body and skull composition diversify among species, such as the long-beaked dolphin and the Chilean dolphin.
As the name suggests, the long-beaked dolphin has a slender aero-dynamic body and a long beak or snout.
In contrast to the long-beaked dolphin, the Chilean dolphin has a stockier body and a bulbous/triangular head, which appears to lack a beak.
Dolphins also vary significantly in terms of color and skin tone.
Color variations can differ between various species, from white to black and different shades of gray and pink.
For instance, the adult Chinese white dolphin (indo-pacific humpback dolphin) has a white or pinkish skin tone.
Comparatively, the killer whale is mainly black with a white colored underbody.
And the bottlenose dolphin varies from light to dark grey with a light-colored lower snout and belly.
Some dolphin species may also have spotted or striped patterns across various body parts.
All species of dolphin are marine mammals, and because they are mammals, they are warm-blooded (requiring body heat to stay warm), give birth to their young (instead of laying eggs), produce milk (to feed their young), and require oxygen to breathe (they cannot breathe underwater).
To breathe, dolphins must rise to the surface of the water, where they can then exhale and take in another deep breath before diving under the water.
It is estimated that some dolphin species hold their breath for up to 15 minutes before coming to the surface to take on fresh oxygen.
Habitat and range
Dolphins can be found swimming in all of the world’s major oceans, from the polar climates near the north and south poles to the center of the equator.
While a few species are known to inhibit freshwater environments (4 – 7 species), most dolphins found in the ocean are saltwater dolphins.
Those that do live in freshwater environments can be found primarily in the Amazon, Brahmaputra, Ganges, Indus, Tocantins, and Yangtze rivers, as well as other locally connecting river systems.
While not true of all species, many smaller dolphin species live in and around the equator. In comparison, the larger dolphin species may live further from the equator, and some species may even reside in various polar regions worldwide.
Habitat may also be broken down into coastal and offshore species.
In some cases, smaller species of dolphins can be found aggregating in and around shallow coastal waters to maintain their food supply and avoid being attacked by larger predators such as sharks which can be found living further out to sea.
Those that do live further out in the ocean can often be found aggregating into large pod sizes, which helps them deter would-be predators from trying to attack.
Larger dolphin species, on the other hand, tend to require larger food supplies. They may consume a diet consisting of deep-sea fish, crustaceans, and various sea life, which live further out in the ocean and at greater depths, so they are unlikely to be found living in coastal waters, at least not for long periods.
Giant dolphins such as killer whales are so large that most sharks wouldn’t attempt to attack them, even in small pods.
In theory, blubber thickness and overall physical size can also significantly affect where various dolphin species are likely to be found.
In addition to the dolphin’s overall size (which helps it disperse body heat), the more blubber and lipid concentration a dolphin has, the better it is at maintaining body heat in colder climates.
Interestingly, a dolphin’s lipid concentration (the number of lipids found in its fat) can be more detrimental to keeping the dolphin warm than how thick its blubber is.
For example, a dolphin with a thick layer of blubber but a low level of lipid concentration may have a more challenging time staying warm than a dolphin with a thin layer of fat but a higher level of lipid concentration.
To adapt to certain types of environments, dolphins such as the killer and false killer whales have adapted a diet that consists of marine mammals such as sea birds, seals, and even other cetaceans in addition to their regular diet.
However, most species maintain a diet consisting of fish, squid, and crustaceans.
Natural and Human Threats
Over the centuries, dolphins have faced several threats from humans, such as poachers looking to sell their meat for consumption by humans at restaurants and local stores.
In some cases, dolphin meat would be sold to butchers and stores that chop up the meat and sell it as tuna or other types of food.
While most cetaceans were hunted for their meat and oil consisted of whales, dolphins were not exempt from being tracked.
Poaching
Today, the practice of hunting whales and dolphins has become outlawed in many countries, and the increase in alternative resources has significantly stifled the whaling industry that still exists and hunts these animals in the countries that still allow poachers to hunt and kill whales and dolphins.
While dolphin hunting has diminished considerably, these dolphins still face threats from occasional poachers looking to sell their meat.
Fishing net Entanglements
In addition to poaching, dolphins also face threats from accidental entanglements in fishnets from fisheries that lay ocean nets intended to capture large schools of fish.
In some cases, dolphins see a large school of fish balled into a tightly knit group and assume its easy prey.
When they charge into the ball of fish, they end up getting stuck in the fishing net.
Since dolphins are mammals, they require oxygen to breathe, and if they end up stuck underwater for an extended time, they can drown.
In cases where dolphins get caught in these nets, some end up drowning because they cannot rise to the water’s surface in time to get sufficient oxygen to their lungs.
Dams and Water Systems
Dolphins also face threats from the construction of dams and other water systems that have forced certain species to fragment and become cut off from other dolphins and food supplies.
Significant fragmentation can lead to difficulty in providing dolphins with adequate reproduction opportunities and make it harder for them to find good food sources.
Water and Noise Pollution
Another issue that has plagued dolphins in recent years is water and noise pollution.
As more and more boats and submarines take to the ocean, the combination of large engine noises, sonar, and other artificial ambient sounds is believed to be interfering with the dolphin’s biological and auditory systems (such as echolocation), causing dolphins to get confused and lose the direction or become nauseated and disoriented due to too many ambient noises.
While the idea of dolphins being affected by artificial sounds is primarily a theory, at this point, it is worth looking into and identifying any possible causes artificial equipment may play in how they may affect a dolphin’s sense of direction and biological systems.
Over the last few decades, water pollution from toxic waste and oil spills has claimed the lives of thousands of dolphins, fish, and seabirds, among other aquatic species.
Even though steps have been put in place to try and minimize these issues, dolphins nonetheless still face them, and these issues are not likely to disappear overnight.
Natural Threats
When it comes to more natural threats, some species of dolphins face threats from larger predators such as sharks, and on rare occasions, dolphins have been spotted being chased and attacked by hungry killer whales.
Highly endangered species of dolphins that live in small aggregates (often near shallow rivers and banks) may face threats from natural diseases and illnesses which can wipe out an entire endangered colony.
In these cases, conservation societies may watch endangered species very closely to try and prevent these issues from rising.