Explanation:
Running uphill requires more energy and effort than running on a flat surface. Small animals have a higher metabolic rate than larger animals, which means they are able to produce more energy in a shorter amount of time. This allows them to run uphill more easily than larger animals.

Metabolic rate:

Metabolic rate is the amount of energy that an animal is able to produce in a given amount of time. Small animals have a higher metabolic rate than larger animals because they have a higher surface area to volume ratio. This means that they have more skin relative to their body size, which allows them to absorb more oxygen and nutrients from the environment. They also have a higher heart rate and breathing rate, which allows them to transport oxygen and nutrients to their muscles more quickly.

Muscle efficiency:

Small animals have a higher muscle efficiency than larger animals. This means that their muscles are able to convert more of the energy they produce into movement. Larger animals have less muscle efficiency because their muscles are larger and more complex. This means that they require more energy to produce the same amount of movement as a smaller animal.

Body weight:

Body weight plays a role in an animal's ability to run uphill, but it is not the most important factor. While it is true that smaller animals have less body weight to carry, this would not be enough to explain why they are able to run uphill more easily than larger animals. Other factors, such as metabolic rate and muscle efficiency, are more important.

Oxygen requirement:

Small animals have a higher oxygen requirement than larger animals, which means that they need to breathe more frequently to maintain their oxygen levels. However, this does not affect their ability to run uphill more easily than larger animals. In fact, their higher oxygen requirement may actually be an advantage, as it allows them to transport oxygen to their muscles more efficiently.

Conclusion:

In conclusion, small animals are able to run uphill more easily than larger animals because they have a higher metabolic rate and higher muscle efficiency. These factors allow them to produce more energy in a shorter amount of time and convert that energy more efficiently into movement. While body weight and oxygen requirement may also play a role, they are not as important as metabolic rate and muscle efficiency.