Animals require a continuous supply of energy to sustain muscular activities such as movement, breathing, and maintaining body temperature. The primary energy currency of cells is ATP (adenosine triphosphate), which is produced by the oxidation of metabolic fuels.
Glycogen is a highly branched polysaccharide of glucose stored in:
During muscular activity, glycogen is rapidly hydrolyzed to glucose, which undergoes cellular respiration:
Glycogen provides a fast, readily available energy source, making it ideal for short bursts of intense muscular activity.
Hibernation is a state of prolonged dormancy in which an animal's body temperature, heart rate, and metabolic rate are drastically reduced to conserve energy during periods of food scarcity (typically winter).
Hibernating animals accumulate large deposits of fat (triglycerides) rather than relying on glycogen for the following reasons:
| Property | Fats (Triglycerides) | Glycogen (Carbohydrates) |
|---|---|---|
| Energy yield | ~9 kcal/g (38 kJ/g) | ~4 kcal/g (17 kJ/g) |
| Energy density | High | Low |
| Water content | Anhydrous (no water stored) | Stored with water (adds mass) |
| Suitability | Long-term energy reserve | Short-term, rapid energy |
Fats yield approximately twice the energy per gram compared to carbohydrates, making them far more efficient for sustaining an animal's reduced metabolic needs over months without food.
The following animals accumulate fat as their primary energy reserve before entering hibernation: