This humpback whale and its calf were found playing off the coast of Tonga.

Whales and dolphins are surprisingly like us. These marine mammals boast remarkable adaptations, complex social behavior, and resounding voices of their own.

This World Oceans Day, we’re celebrating some of the largest animals to ever live by taking a look at a few of their incredible behaviors.

Stressed Narwhals

When reacting to a threat, most animals react in one of two ways: They go still and slow their heart rate, or their heartbeats accelerate to prepare for a speedy retreat.

Strangely, the narwhal, a deep-diving Arctic whale known for its unicorn-like tooth, fall into neither of these categories.

When stressed, narwhal heart rates plummet to four beats per minute—but they also take off on a high-speed dive to escape the danger, according to a study published in 2017 in the journal Science.

Because the narwhal's muscles aren't receiving oxygen fast enough, these muscles quickly run out of gas, sharply decreasing the narwhal's dive time. Along with that, their blood flow lessens, increasing risk of the bends.

"What I don’t understand is how these tissues are able to maintain function," study leader Terrie Williams, a wildlife ecophysiologist at the University of California, Santa Cruz, said in a previous interview.

In the lab, this paradoxical reaction can kill other animals. Scientists aren’t sure how these low-oxygen bouts affect narwhal health, but as humans keep encroaching on their habitat, these marine mammals may have to rely on desperate dives more often.

Whispering Whales

Humpback whales are far better known for their bellowing "songs" that can travel up to six miles, but the animals also have quiet voices.

Biologists have observed humpback calves “whispering” to their mothers in tones that can be heard only within 330 feet—a strategy to prevent predators, such as orcas, from eavesdropping. (Read about a mysterious new humpback whale song caught on tape.)

Calves also want to avoid adult male humpbacks, which may try to separate the calf from its mother to mate.

This shortened time with its mom could ultimately weaken the calf, which needs to drink 50 gallons of milk a day to fuel up for its long migration.

Matriarchal Orcas

Orcas don't generally leave their mothers—they swim with their family group for their entire life.

As the largest dolphin species, orcas live in a matriarchal society that's relatively rare in the animal kingdom. Mothers educate their calves, for instance, by holding a salmon in her mouth while the juveniles chew on it. Later in life, those calves may retain a taste for that species of salmon.

The 7,000-pound behemoths also discipline their young, showing displeasure by hitting their tails on the water, making strong head movements, and emitting unusual noises with their teeth. (Also read about the first known case of orca infanticide.)

Researchers have found evidence that female orcas can live up to 90 years to serve as a kind of library of information, directing their groups or pods to where they can find food when fish are scarce.

"The pioneers of killer whale ecology have long felt that matriarchs serve as repositories of traditional ecological knowledge that can help these whales survive through years of low prey abundance," whale biologist Rob Williams, a Pew Fellow in marine conservation, said in a previous interview.

Jumping Dolphins

Jumping is a well-known behavior among dolphins, but why they do it has been a matter of scientific debate—in part because different dolphins use jumps in different ways.

For instance, dusky dolphins off Kaikoura, New Zealand, will jump to "ask" their podmates for help corralling fish.

A 2006 study also found evidence that bottlenose dolphins jump as a broader means of social communication.

David Lusseau, formerly at the University of Otago, spent 137 days decoding the jumps of dolphins of New Zealand’s Doubtful Sound. (See 10 intimate portraits of dolphins.)

“Side-flops occurred when the dolphins finished a behavioral bout and started to travel, while [tail slapping on the water's surface] occurred when the dolphins instigated a behavioral bout after traveling,” Lusseau wrote in the study.

“This non-vocal communication can take place over a few meters to hundreds of meters.”

Vocalizations can be heard from miles away, but jumps are heard and seen only over short ranges—thereby avoiding unwanted signaling to prey, predators, or rivals.

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