Problem 5 - Campbell Biology

Question

In which of the following species should natural selection favor the highest proportion of juxtamedullary nephrons? (A) a river otter (B) a mouse species living in a temperate broadleaf forest (C) a mouse species living in a desert (D) a beaver

Step-by-Step Solution

Answer

C) a mouse species living in a desert

Step 1 - Understanding Nephrons

Nephrons are the functional units of the kidney that filter blood and form urine. There are two types: corticomedullary nephrons, which are mainly in the cortex, and juxtamedullary nephrons, which have long loops that deeply penetrate the medulla. Juxtamedullary nephrons are more efficient at concentrating urine.

Step 2 - Function of Juxtamedullary Nephrons

Juxtamedullary nephrons conserve water by producing concentrated urine. This is particularly beneficial in habitats where water is scarce.

Step 3 - Analyzing the Species

Evaluate the habitat of each species: (A) River otters live in aquatic environments with ample water. (B) Mice in temperate forests have moderate water availability. (C) Desert mice live in environments with very limited water. (D) Beavers live in aquatic environments with abundant water.

Step 4 - Identify the Species with Water Scarcity

Among the listed species, desert mice face the most limited water resources and thus need to conserve water most efficiently.

Step 5 - Conclusion

Natural selection would favor the species that needs to conserve the most water. Therefore, favoring more juxtamedullary nephrons helps desert mice survive in their arid environment.

Key Concepts

Juxtamedullary Nephrons

Kidneys have tiny structures called nephrons that help filter blood and produce urine. One special type of nephron is the juxtamedullary nephron. These are different from the regular nephrons because they have long loops of Henle that extend deep into the kidney's medulla. This long loop is particularly important because it helps concentrate urine. By doing so, juxtamedullary nephrons help conserve water, which is crucial for animals that live in environments where water is not easily available. This adaptation allows the body to retain as much water as possible while still getting rid of waste.

In summary, juxtamedullary nephrons are essential for water conservation due to their ability to produce highly concentrated urine.

Water Conservation in Kidneys

Water conservation is a vital function of the kidneys, particularly in animals living in arid environments. The kidneys conserve water by concentrating urine. This means removing water to form a small volume of urine that has a high concentration of waste products. Juxtamedullary nephrons play a key role in this process. The long loops of Henle in these nephrons use a mechanism called the countercurrent multiplier system.

This system incredibly enhances the kidney's ability to reabsorb water:

• The descending limb loses water but not salt.
• The ascending limb loses salt but not water.

This interaction allows very concentrated urine to be produced, conserving water efficiently. The extent of water reabsorption can adapt depending on the hydration status of the animal.

Desert Adaptation in Animals

Animals living in deserts face extreme water scarcity and high temperatures. These challenging conditions have led to several fascinating adaptations. One of the primary adaptations in desert animals is the development of an efficient kidney with a high proportion of juxtamedullary nephrons:

• These nephrons enable the animal to produce very concentrated urine, minimizing water loss.
• Another adaptation includes behavior changes, like being active during cooler parts of the day and finding shelter during the hottest times.
• Desert species may also have physiological changes, such as lower rates of urination and specialized ways to store and metabolize water from their food.

Each of these adaptations helps desert animals conserve water and survive in the harsh, dry conditions of their habitat.