104. Which of the following life history traits is most likely in a rodent species when snakes prefer to prey upon large, older individuals of the rodent species that grow continuously over their lifespan?
     (1) Early reproduction and slow growth rate
     (2) Delayed reproduction and fast growth rate,
     (3) Delayed reproduction and slow growth rate,
     (4) Early reproduction and fast growth rate,

Rodents are among the most diverse and adaptable mammals on Earth, inhabiting nearly every terrestrial habitat and evolving a wide range of life history strategies to cope with predation, resource availability, and environmental challenges. When a major predator, such as a snake, preferentially targets large, older individuals of a rodent species—especially if those rodents grow continuously throughout their lifespan—this selective pressure can have profound effects on the evolution of key life history traits. This article examines which combination of growth rate and timing of reproduction is most likely to evolve in such a scenario, and why.

Understanding Life History Traits

Life history traits are characteristics that influence an organism’s survival and reproduction, including age at first reproduction, growth rate, lifespan, and investment in offspring. These traits are shaped by natural selection in response to environmental pressures, especially predation.

In the context of predation by snakes that prefer large, older rodents, the evolutionary pressure is clear: individuals that grow large and live long are at greater risk of being eaten. This selective pressure can drive changes in how quickly rodents grow and when they begin to reproduce.

Predation Pressure and Its Effects

When predators such as snakes preferentially target large, older individuals, rodents that reach large sizes or live to old age have a lower chance of surviving and reproducing. This creates a selective advantage for individuals that adopt strategies to avoid becoming large or old, or that can reproduce before they reach a vulnerable size or age.

Why Do Snakes Prefer Large, Older Rodents?

Large, older rodents may be easier for snakes to detect, capture, or handle due to their size, reduced agility, or predictability. Alternatively, larger prey may provide more energy per capture, making them more attractive to predators. Regardless of the exact reason, the result is that large, older rodents are at a higher risk of predation.

How Do Rodents Respond to Predation Risk?

Rodents exhibit a variety of antipredator behaviors, including increased vigilance, avoidance, and changes in activity patterns. However, behavioral responses are only part of the story. Over evolutionary time, predation pressure can shape life history traits themselves, leading to adaptations that reduce the risk of being eaten.

Evaluating Life History Strategies

Given that snakes prefer large, older rodents, which life history strategy would be most advantageous for the rodent population? Let’s consider each option:

1. Early reproduction and slow growth rate

   • Analysis: Reproducing early reduces the risk of dying before reproducing, but slow growth means individuals remain small for longer. This could be beneficial if small size reduces predation risk, but it may not maximize reproductive output if individuals remain small and produce fewer or smaller offspring.

2. Delayed reproduction and fast growth rate

   • Analysis: Fast growth leads to large size quickly, but delayed reproduction means individuals may reach a size or age where they are more likely to be eaten by snakes before they can reproduce. This is the opposite of what would be favored by selection in this scenario.

3. Delayed reproduction and slow growth rate

   • Analysis: Both delayed reproduction and slow growth increase the likelihood that individuals will be eaten before reproducing, as they remain vulnerable for longer and may never reach reproductive age or size. This is the least advantageous strategy.

4. Early reproduction and fast growth rate

   • Analysis: Fast growth allows individuals to reach reproductive size quickly, and early reproduction ensures that they can produce offspring before they become large and old enough to be targeted by snakes. This maximizes the chances of successful reproduction before predation risk increases.

Why Early Reproduction and Fast Growth Are Favored

In environments where predators target large, older individuals, natural selection favors life history strategies that allow organisms to reproduce before they become vulnerable. Fast growth enables individuals to reach reproductive maturity quickly, and early reproduction ensures that they can pass on their genes before they are likely to be eaten.

This strategy is particularly effective if rodents grow continuously throughout their lifespan, as they will always be at risk of becoming large enough to attract predators. By reproducing early and growing quickly, rodents can maximize their lifetime reproductive success despite high predation pressure on large, older individuals.

Broader Implications and Examples

This pattern is observed in many prey species facing heavy predation on large or old individuals. For example, in fish populations where large adults are targeted by predators, individuals often mature earlier and grow rapidly to reproduce before they become vulnerable. Similar dynamics can occur in rodents, especially in habitats with high snake predation.

Rodents are known for their adaptability and diverse life history strategies, ranging from slow-growing, long-lived species in stable environments to fast-growing, short-lived species in unpredictable or predator-rich habitats. The specific combination of early reproduction and fast growth is a classic example of an r-selected strategy, which is favored in environments where mortality is high and unpredictable.

Contrasting with Other Strategies

• Delayed Reproduction: Delaying reproduction increases the risk of dying before reproducing, especially if predators target older or larger individuals.

• Slow Growth: Slow growth means individuals remain small and may avoid predation for longer, but it also delays reproduction and may reduce overall reproductive output.

• Fast Growth and Early Reproduction: This combination allows individuals to reproduce before they become large or old enough to be targeted by predators, maximizing their chances of passing on their genes.

Summary Table

Key Takeaways

• Predation by snakes on large, older rodents creates strong selective pressure for early reproduction and fast growth.

• Fast growth allows rodents to reach reproductive size quickly, reducing the time they are vulnerable to predation.

• Early reproduction ensures that rodents can produce offspring before they become large or old enough to be targeted by snakes.

• This combination maximizes lifetime reproductive success in environments with high predation pressure on large, older individuals.

Conclusion

When snakes prefer to prey upon large, older individuals of a rodent species that grows continuously over its lifespan, the most likely life history traits to evolve are early reproduction and fast growth rate. This strategy allows rodents to reproduce before they become vulnerable to predation, ensuring the survival of their genes in a challenging environment. Understanding these evolutionary dynamics provides valuable insights into the diversity of life history strategies in rodents and other prey species facing similar selective pressures.

In summary, the optimal strategy for a rodent population under predation by snakes targeting large, older individuals is to reproduce early and grow quickly. This approach maximizes reproductive success and is a classic example of how predation shapes the evolution of life history traits in nature.