Which of the following is a characteristic of both mammals and birds?

Correct — C. Warm-blooded is the trait both classes genuinely share. Mammals and birds generate their own body heat from metabolism and hold a near-constant internal temperature whatever the air outside is doing.

Fish, amphibians and reptiles are classed as cold-blooded: their body temperature tracks their surroundings.

None of the other three holds for mammals and birds together — two of them belong to one class only, and the remaining one describes no class in particular. Option (c) is the shared characteristic.

Why the others are wrong

• (a)
  Pigmented skin — Pigmented skin identifies nothing about these two classes. Melanin colours the skin and scales of fish, amphibians and reptiles as well, so it separates no group from another.
• (b)
  Pneumatic bones — Pneumatic bones are hollow and air-filled, linked to a bird's air sacs and part of what makes flight possible. A mammal's bones carry marrow, not air, so this is birds only.
• (d)
  Viviparity — Viviparity — bearing live young — is a mammal trait, and even there the egg-laying monotremes are the exception. No bird is viviparous: every bird lays eggs.

Warm-blooded means endothermic and homeothermic: the animal makes its own heat and holds its core temperature nearly steady.

Only two vertebrate classes do this — Mammalia and Aves — and they evolved it independently, from separate ancestries.

Both also have a four-chambered heart, which keeps oxygenated and deoxygenated blood apart and supports the high metabolic rate that constant body heat costs.

That pairing is what SSC tests: what mammals and birds share, and what belongs to just one of them.

Key facts

• Mammals and birds are the two vertebrate classes classified as warm-blooded.
• Fish, amphibians and reptiles are classified as cold-blooded.
• Pneumatic bones are hollow and air-filled and are found in birds, not in mammals.
• All birds lay eggs, while most mammals bear live young — the monotremes are the egg-laying exception.

Common traps

• Choosing viviparity because it feels like the more advanced trait, when no bird gives birth to live young.
• Treating pneumatic bones as a general vertebrate feature rather than part of the bird's flight apparatus.

No directly related past PYQ was found.