A Dolphin’s Speed Is All in the Hips

SCIENCE

Believe it or not, how dolphins can swim so fast has been something of a riddle since the 1930s. Now, thanks to innovative experiments using “bubble curtains,” we know the marine mammals use their powerful tails to propel them around the turbulent sea. (National Geographic News)

Use our resources to better understand animals’ unique marine adaptations.

Bottlenose dolphins (like this one off the coast of Tasmania, Australia) are huge animals—almost 3 meters (10 feet) and more than 227 kilograms (500 pounds)—but their powerful tails can propel them more than 34 kilometers per hour (21 miles per hour). Photograph by Alan Jennison, National Geographic

Bottlenose dolphins (like this one off the coast of Tasmania, Australia) are huge animals—almost 3 meters (10 feet) long and weighing more than 227 kilograms (500 pounds)—but their powerful tails can propel them more than 34 kilometers per hour (21 miles per hour).
Photograph by Alan Jennison, National Geographic

Discussion Ideas

  • Read through our activity “Animal Adaptations in the Ocean.” Adapt its questions to the dolphin study. Do you think a strong tail is an adaptation? How does having a strong tail benefit the dolphin?
    • Dolphins’ muscular tails are an excellent adaptation that help them swim fast. Being able to swim fast allows dolphins, apex predators in the ocean, to better hunt their prey.
  • Look at the beautiful photo of the bottlenose dolphin above. Besides its muscular tail, can you spot any other physical adaptations that might allow it to swim fast?
    • The dolphin’s entire body is built for speed! Compare the dolphin’s shape to the ideal hydrodynamic form outlined by students designing a fish robot.
      • The dolphin’s extremely smooth skin reduces drag.
      • The dolphin’s narrow, rounded (bottle)nose reduces drag.
      • The dolphin’s streamlined form reduces drag.
      • The dolphin’s dorsal fin doesn’t directly help it swim fast, but it does help the animal to make sudden turns and stabilize its motion in turbulent water.
  • A “bubble curtain” is just what it sounds like—a harmless screen of tiny air bubbles being let out underwater and snaking their way to the surface. Read the Nat Geo News article. How did bubble curtains help researchers understand how dolphins swim?
    • Air bubbles are visible in water. Scientists coaxed dolphins to swim through the bubble curtains and watched the patterns created in their wake. Nat Geo’s Emerging Explorer Kakani Katija studies fluid mechanics and explains the pattern in easy-to-understand terms: “How many of you have flushed a toilet? Basically, if you take that whirlpool of fluid . . and you take the starting end, and the finishing end, and attach it to each other, that is a vortex ring.”
  • Nat Geo’s Emerging Explorer Kakani Katija studies the fluid patterns (vortex rings) created by much smaller animals—tiny plankton and jellies. Compare the vortices created by the dolphin and those created by the jellies—forward to about 2 minutes to see the jellies in Kakani Katija’s video. When are the vortices created in the dolphin video created? When do they appear with the jellies? Do the vortices have a similar shape?

  • The vortices are very similar—both are swirling rings of fluid.
  • In the dolphin video, the vortex appears after the dolphin’s tail (fluke) has made its final downward push and has risen for another pump.
  • In the jelly video, the vortex appears after the animal has expelled the (milk-colored) fluid from its body.

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