Which of the following statements are true in the context of respiration in whales and dolphins ?

The correct option is (A), (C), (D).

Understanding Respiration in Cetaceans (Whales and Dolphins):
Whales and dolphins belong to the class Mammalia. Despite living continuously in aquatic environments, their respiratory mechanism is mammalian rather than fish-like. They rely on atmospheric oxygen and must surface to breathe.

Key Respiratory Adaptations:
1. Lungs as Respiratory Organs: Whales and dolphins possess well-developed lungs to inhale oxygen from the air and exhale carbon dioxide. They do not possess gills.
2. Blowholes (Nostrils): Their nostrils, called blowholes, are evolutionarily positioned on the upper surface of their heads. This positioning enables them to take in air quickly and efficiently upon reaching the surface without raising their entire head out of the water.
3. Surfacing for Air Exchange: Because they cannot extract dissolved oxygen from water, they regularly surface to expel stale air and take in fresh air.
4. Extended Breath-holding Capability: Whales and dolphins have specialized physiological adaptations, such as high concentrations of myoglobin in muscles and high hemoglobin levels in blood, allowing them to submerge and stay underwater for extended periods between breaths.

Statement Evaluation:
- Statement (A): They breathe air through lungs like other mammals. (True)
- Statement (B): They breathe underwater using gills. (False – Gills are respiratory organs of fish, not marine mammals.)
- Statement (C): Their nostrils (blowholes) are located on the top of their heads for easy surfacing. (True)
- Statement (D): They can remain underwater for long periods without breathing. (True)

Hence, statements (A), (C), and (D) are correct, which corresponds to the option (A), (C), (D).