Adaptations for swimming in marine mammalsSpec B3.3.10
In short
Marine mammals such as dolphins and whales are adapted for swimming by a streamlined body that reduces drag, forelimbs modified into flippers for steering, and a tail with a horizontal fluke moved up and down for propulsion. Changes to the airways, such as a blowhole that closes, allow periodic breathing at the surface between dives.
Marine mammals, such as the bottlenose dolphin (Tursiops truncatus), evolved from land mammals but are adapted to swim and dive.
| Adaptation | How it helps |
|---|---|
| Streamlining | A smooth, tapering body with no external ears and little hair reduces drag (water resistance). |
| Forelimbs form flippers | The bones of the forelimb are enclosed in a stiff, flat flipper used for steering and balance. |
| Tail forms a fluke | A wide, horizontal tail fluke is moved up and down by powerful back muscles to push the animal forwards; fish tails move from side to side. |
| Hind limbs reduced | In whales and dolphins the hind limbs are absent externally, improving streamlining. |
| Changes to the airways | The nostrils have moved to the top of the head as a blowhole, closed by a muscular flap during dives; air can be exchanged rapidly at the surface, so the animal breathes periodically between dives. |
When asked to compare with fish, say that the fluke moves up and down (vertical movement) whereas a fish tail fin moves from side to side.
Quick check
HL only What is the difference between motile and sessile organisms?
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Motile organisms can move from place to place; sessile organisms are fixed in one place but can still move parts of their bodies.
HL only Which bands of a sarcomere shorten during contraction?
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The I band and the H zone shorten; the A band stays the same.
HL only What two roles does titin have in the sarcomere?
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It helps the sarcomere recoil after stretching and prevents overstretching.
HL only What does a motor unit consist of?
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One motor neuron, the muscle fibres it stimulates and the neuromuscular junctions connecting them.
HL only Which two bones meet at the hip joint?
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The femur and the pelvis.
Written and checked against the IB Biology HL specification · Updated October 2026