Rhinoceros horns are not made of bone; they are composed of which biological material that also forms human hair and nails?

Introduction:
Many people assume that horns are simply extensions of bone, but different animals have horns made from different materials. Rhinoceros horns are especially interesting because they are not bony structures. This question checks whether you know the specific protein that forms rhino horns and also appears in human hair, nails and the outer layer of skin.

Given Data / Assumptions:
- The options are bones, keratin, collagen and cartilage. - We are asked about the main structural material, not minor components. - The focus is on rhinoceros horns, not on antlers of deer or horns of cattle. - Basic biology and zoology facts are assumed to be known.

Concept / Approach:
Rhinoceros horns are made of densely packed keratin fibres. Keratin is a tough fibrous protein that also forms human hair, nails and animal hooves. Unlike deer antlers, which are true bone, rhino horns are more like a compacted mass of hair like material. Remembering this difference between bone and keratin based horns is the key to answering correctly.

Step-by-Step Solution:
Step 1: Recall that many textbooks clearly state that rhinoceros horns are made of keratin. Step 2: Connect keratin with familiar structures such as hair, nails and hooves, which reinforces the association. Step 3: Eliminate bones and cartilage because they are more typical of skeletal structures and joint tissues, not rhino horns. Step 4: Collagen is a protein found in connective tissues, but it is not the primary material forming the horn shaft, so keratin remains the best answer.

Verification / Alternative check:
Standard zoology and conservation sources explain that rhino horn is composed entirely of keratin and does not contain a bony core. Even microscopic examinations show tightly packed keratin fibres similar to compressed hair. This independent evidence supports the choice of keratin as the correct material.

Why Other Options Are Wrong:
Bones: Deer antlers are bone, but rhinoceros horns do not have a bone core and are not bony extensions. Collagen: Important structural protein in skin and connective tissue, but it is not the main ingredient of the horn shaft. Cartilage: Provides flexible support in joints and some body parts such as the nose and ear, but rhino horns are not cartilage structures.

Common Pitfalls:
Learners often group all horns and antlers together and assume they have similar composition. This leads to the wrong choice of bones. Another mistake is confusing collagen and keratin because both are structural proteins. A useful memory trick is to associate keratin with hair, nails, feathers and horns like those of rhinoceroses, while collagen is more linked to tendons, ligaments and skin strength.

Final Answer:
A rhinoceros horn is made of the fibrous protein Keratin.