Problem 3 - Animal Behavior: An Evolutionary Approach

Question

Here's another way of looking at the effect of differences between males and females on the probability that their putative offspring are truly their genetic offspring. If a female lays a fertilized egg or gives birth to a baby, this offspring will definitely have 50 percent of her genes. In contrast, a male that mated with this female may or may not be the father of that offspring. Thus, the argument goes, males have less to gain by parental behavior. But imagine a hypothetical species in which males have, say, a 40 percent chance of siring the offspring of any given mate. Further, imagine that there are two hereditarily different male phenotypes in the population, a paternal type and a nonpaternal type. The average paternal male mates with two females (each with an average of 10 eggs), whereas the average nonpaternal male mates with five females (which enables him to fertilize 40 percent of 50 eggs in all). In addition, let's say that the paternal male boosts the survival chances of the eggs under his care to 50 percent, whereas the unprotected offspring of nonpaternal males have a 10 percent survival rate. Which behavior is adaptive here? Show your math. What point does this example make about the evolution of male parental care?

Step-by-Step Solution

Answer

Paternal behavior is adaptive, resulting in more surviving offspring. Male parental care increases offspring survival, favoring evolutionary development.

Step 1: Calculate Offspring Sired by Paternal Male

An average paternal male mates with 2 females, each with an average of 10 eggs. Since the male fertilizes 40% of the eggs, we calculate the number of offspring sired as follows:\[\text{Offspring sired by paternal male} = 2 \times 10 \times 0.40 = 8 \text{ offspring}\]

Step 2: Calculate Surviving Offspring of Paternal Male

The paternal male increases the survival of these offspring to 50%. Therefore, the number of surviving offspring is:\[\text{Surviving offspring of paternal male} = 8 \times 0.50 = 4 \text{ offspring}\]

Step 3: Calculate Offspring Sired by Nonpaternal Male

The nonpaternal male mates with 5 females, each with an average of 10 eggs. He fertilizes 40% of these eggs, so the number of offspring sired is:\[\text{Offspring sired by nonpaternal male} = 5 \times 10 \times 0.40 = 20 \text{ offspring}\]

Step 4: Calculate Surviving Offspring of Nonpaternal Male

The survival rate of these unprotected offspring is 10%. Therefore, the number of surviving offspring is:\[\text{Surviving offspring of nonpaternal male} = 20 \times 0.10 = 2 \text{ offspring}\]

Step 5: Compare Survival Outcomes

Comparing the surviving offspring, the paternal male has 4 surviving offspring, while the nonpaternal male has 2. Thus, the parental behavior is more adaptive in this scenario.

Step 6: Evolutionary Implication

The example suggests that male parental care can be adaptive by increasing the survival rate of offspring. This can drive the evolution of parental behavior as it leads to more surviving offspring than a non-caring approach.

Key Concepts

Male Parental Care

Male parental care refers to the involvement of male individuals in activities related to the upbringing of their offspring. This is an important concept in evolutionary biology because it plays a significant role in the survival and fitness of the offspring.
The exercise at hand gives us a scenario where paternal males and nonpaternal males show different parental strategies. By calculating the number of surviving offspring, we can compare the efficiency and advantages of each strategy.

• Paternal males have fewer mates but increase their offspring's survival rate by providing care.
• Nonpaternal males mate with more females but provide no care, resulting in lower offspring survival rates.

These findings underscore the fact that in some species, male parental care can significantly boost offspring survival, thus having a potential advantage over non-caring strategies.

Adaptive Behavior

Adaptive behavior in evolutionary terms refers to traits or actions that increase an individual's reproductive success and chances of survival. Parental care is an example of an adaptive behavior, where males invest in the survival of their offspring to ensure their genes are passed on to the next generation.
In the given scenario, even though paternal males fertilize the same number of eggs as nonpaternal males, their act of providing care results in a higher number of surviving offspring. This demonstrates the adaptiveness of such behavior as:

• It directly increases the chance of offspring survival.
• It improves reproductive success despite having fewer mates.

Hence, adaptive behavior like male parental care can evolve when it markedly improves the reproductive success of the caring males, leading to its increase in frequency within the population over time.

Reproductive Strategy

Reproductive strategy encompasses the set of behaviors or patterns an organism employs to pass on its genes to future generations. Differences in reproductive strategies between paternal and nonpaternal males highlight a significant trade-off between quantity and quality of offspring.
Paternal males invest more time and resources into a smaller number of offspring, ensuring higher survival rates for each offspring. In contrast:

• Nonpaternal males focus on mating with more females and producing more offspring without investing in their care.
• This can result in a greater number of offspring initially, but with much lower survival rates as observed in the example.

Through evolutionary pressures, strategies that lead to better survival and reproduction over time become prevalent. Some species thus develop strategies focusing on high-survival rather than high-reproduction rates. This balance between competing strategies can influence the evolution of species over generations.