To keep warm in extremely cold environments, polar bears have an insulating layer of which tissue under their skin?

Introduction / Context:
Adaptations of animals to their habitats are a fundamental topic in biology and environmental science. Polar bears live in very cold Arctic environments, where maintaining body heat is essential for survival. The question focuses on which specific tissue under the skin helps polar bears retain warmth and insulate their bodies against freezing temperatures. Understanding this helps illustrate how structural adaptations support survival in extreme climates.

Given Data / Assumptions:

• The animal in question is the polar bear, living in Arctic conditions.
• The question asks about a layer present under the skin that keeps the animal warm.
• Options are muscle, fat, cartilage, and hair.
• Standard biological knowledge of mammalian anatomy is assumed.

Concept / Approach:
Mammals in cold regions often have a thick subcutaneous layer of adipose tissue, commonly referred to as fat or blubber. This layer acts as an excellent insulator because fat conducts heat poorly and thus reduces heat loss from the warm core of the body to the cold external environment. While polar bears also have dense fur, the question specifically mentions a layer under the skin, which points to fat rather than hair. Muscle tissue generates heat through activity but is not primarily an insulating layer. Cartilage is a supportive connective tissue found in joints and certain structures, not a general insulator. Therefore, the correct answer is the layer of fat beneath the skin.

Step-by-Step Solution:

Verification / Alternative check:
A simple verification is to compare polar bears with whales and seals, which are also known to have thick layers of blubber for insulation. Common biology textbooks and documentaries emphasise that polar bears have both thick fur and a substantial layer of fat. Remembering this combination confirms that fat is the correct tissue mentioned in the question as the under skin insulating layer.

Why Other Options Are Wrong:

• Muscle: While active muscles can generate heat, they do not form a continuous insulating layer; their main role is movement and posture.
• Cartilage: Found in joints, nose, and ear structures, cartilage provides support and flexibility, not thermal insulation for the whole body.
• Hair: Polar bears do have dense fur for insulation, but hair is present outside the skin. The question explicitly asks about a layer under the skin, which refers to fat.

Common Pitfalls:
Some candidates may quickly choose hair because animal fur is widely associated with warmth. However, the wording "under their skin" should guide careful readers towards fat. Another pitfall is to assume that muscle bulk alone is responsible for warmth, ignoring its limited role as insulation. Training yourself to read key phrases such as "layer under the skin" can prevent these misunderstanding in similar questions.

Final Answer:
Polar bears keep warm with a thick insulating layer of Fat under their skin.