Question

Theory predicts that when both terrestrial and freshwater animals evolve higher metabolic rates, they can be expected to evolve greater challenges to maintaining water balance. Explain why this is so for both terrestrial and freshwater animals.

Short answer

Metabolic rates directly correlate to water loss in animals. In terrestrial animals, higher metabolic rates increase body temperature, resulting in more water loss through evaporation and causing difficult in maintaining water balance. They handle this problem by adaptations like reducing water loss or increased water ingestion. For freshwater animals, higher metabolic rates increase ion excretion, posing a challenge in maintaining ion balance. They deal with this by reducing ion loss or actively uptaking ions from their environment.

Step-by-step solution

Understanding the Correlation between Metabolic Rates and Water Balance

Metabolic rate dictates how fast an animal uses energy. Higher metabolism increases body temperature, resulting in increased water loss through processes such as sweat and respiration. This presents a challenge in maintaining water balance as more water is lost from the body.

Water Balance in Terrestrial Animals

For terrestrial animals, the primary challenge is the reduction of water loss. In a higher metabolic rate scenario, the body temperature is increased, leading to increased evaporation. This consequently causes these animals to lose more water, posing a challenge in maintaining their water balance. They must ingest more water or decrease water loss through adaptations like reduced surface areas, improved water reabsorption in the kidneys, and sourcing water from food metabolism.

Water Balance in Freshwater Animals

Freshwater animals, on the other hand, live in an environment where water is abundant, but the challenge comes from the ion concentration difference between their body fluids and the freshwater environment they inhabit. Higher metabolism leads to increased excretion, altering their ion balance. Therefore, to maintain water balance, these animals must actively uptake essential ions from their environment, often against the concentration gradient, or reduce ion loss.