Cetacean

Cetaceans are a diverse group of marine mammals belonging to the infraorder Cetacea.

These animals include whales, dolphins, and porpoises. They have a cosmopolitan distribution, meaning they live in all of the Earth's oceans. Some species even live in fresh or brackish water in rivers. Cetaceans are defined by a fully aquatic life cycle. They possess streamlined, fish-like bodies for efficient movement. They are exclusively carnivorous, meaning they eat only other animals.

To move through the water, cetaceans use a unique method of propulsion. They are nektonic animals, which means they can swim actively against currents. Instead of moving tails side-to-side like fish, they move them up and down. Their tails have evolved into horizontal, paddle-like structures called flukes.

Their forelimbs have transformed into flippers used for steering and stability. Over time, their hindlimbs have disappeared. Most cetaceans only retain vestigial remains, such as small parts of the pelvis and femurs. Many species also have a dorsal fin on their backs to help with directional stability.

Scientists divide the approximately 90 living cetacean species into two distinct parvorders. The first is Odontoceti, known as toothed whales. This group contains 75 species, such as dolphins, belugas, and sperm whales. They use teeth to capture prey like fish, squid, or even other marine mammals. The second group is Mysticeti, or baleen whales. This group includes 15 species, such as the blue whale and humpback whale. Instead of teeth, they have baleen, which are keratin bristles used to sieve small invertebrates from the water.

These two groups are thought to have diverged about thirty-four million years ago.

Anatomy in cetaceans is highly specialized for life in the ocean. Most species have a thick layer of fat called blubber. This provides thermal insulation and helps maintain a streamlined shape. In larger species, blubber can reach a thickness of up to 10 centimeters. Their respiratory system is also adapted for surfacing. Their nostrils have moved to the top of the head to form blowholes. This allows them to breathe while keeping most of their body submerged.

Their skeletons are often composed of dense cortical bone to provide stability in the water.

Cetaceans are known for their high intelligence and complex social behaviors. They often use song-like communication to interact with one another. Many species also use echolocation, a biological sonar system. They produce sounds and use a fatty mass in the head called a melon to focus them.

This helps them navigate and find prey in dark or murky water. Some species, like the sperm whale, have exceptionally large brains. The sperm whale holds the record for the largest brain mass of any animal on Earth. In mature males, this mass can average 7.8 kilograms.

Despite their marine lifestyle, genetic and fossil evidence links cetaceans to land animals. They are part of the order Artiodactyla, which includes even-toed ungulates. Specifically, they are most closely related to hippopotamids, or hippos. This shows how much their bodies have changed through evolution. The blue whale provides a massive example of this biological success. It is the largest animal known to have ever existed. It can reach a maximum length of 30 meters and weigh up to 173 tonnes.

Today, cetaceans face many environmental challenges caused by human activity. Since the Industrial Revolution, commercial whaling has targeted them for meat, blubber, and oil. While the International Whaling Commission has halted most commercial whaling, some hunting continues. Other modern threats include underwater noise pollution and entanglement in nets. They also face ship strikes from propellers and plastic pollution in the oceans. Anthropogenic climate change and food scarcity from overfishing further impact their survival. The impact varies by species, ranging from minimal to functional extinction, as seen with the baiji.