Question

In sperm competition, what is "second-male advantage"? a. the observation that when females mate with two males, each male fertilizes the same number of eggs b. the observation that when females mate with two males, the second male fertilizes most of the eggs c. the observation that females routinely mate with at least two males before laying eggs or becoming pregnant d. the observation that accessory fluids prevent matings by second males-for example, by forming copulatory plugs

Short answer

In sperm competition, the "second-male advantage" is the observation that when females mate with two males, the second male fertilizes most of the eggs. The correct answer is (b).

Step-by-step solution

Option A

This option suggests that when females mate with two males, each male fertilizes the same number of eggs.

Option B

Option B states that when females mate with two males, the second male fertilizes most of the eggs.

Option C

This option explains the second-male advantage as the observation that females routinely mate with at least two males before laying eggs or becoming pregnant.

Option D

In option D, the second-male advantage is described as an observation that accessory fluids prevent matings by second males—for example, by forming copulatory plugs. The correct definition is:

Second-male advantage

The observation that when females mate with two males, the second male fertilizes most of the eggs. So the correct answer is \( \boxed{\textbf{(b)}} \).

Key concepts

Sperm Competition

Sperm competition occurs within species where females have the opportunity to mate with multiple males within a short period, resulting in the sperm from different males competing to fertilize the same set of eggs. This biological phenomena plays a significant role in the evolutionary process. Imagine a race where millions of tiny competitors, the sperm cells, are vying to reach the finish line - the egg for fertilization.

One intriguing outcome of sperm competition is the 'second-male advantage.' This refers to situations where the sperm of the second male to mate with a female has a higher probability of fertilizing her eggs than the sperm of the first male. Researchers have found various mechanisms that give rise to this advantage, such as sperm displacement, where the second male's sperm physically removes or displaces the first male's sperm, and differences in sperm quality and quantity. Understanding how these microscopic races are won or lost can shed light on the complex mating strategies and reproductive behaviors in the animal kingdom.

Sexual Selection

Sexual selection is a mode of natural selection where individuals with certain heritable traits are more likely to succeed in the mating game. Charles Darwin first introduced this concept, explaining how some traits evolve not for survival advantage, but for a reproductive one.

There are two main types of sexual selection: intersexual selection, where members of one sex choose mates from the other (think of a peacock's dazzling feathers), and intrasexual selection, where there is competition among members of the same sex (like elk antler battles). Sperm competition can be seen as a form of intrasexual selection, where the 'battle' takes place after mating. The traits that ensure sperm success are often less visible than peacock feathers, but they are nonetheless powerful drivers of evolutionary change.

Fertilization Patterns in Animals

Fertilization in animals can be internal or external and each method has developed its own unique patterns and strategies to maximize reproductive success. In external fertilization, often seen in aquatic environments, eggs and sperm are released into the water where fertilization occurs openly. This strategy typically involves the production of large quantities of gametes to increase the chances of successful fertilization.

On the other hand, internal fertilization, common in terrestrial animals, often involves more complex mating rituals and mechanisms for ensuring sperm transfer, like copulatory plugs mentioned in the exercise. This strategy may involve fewer gametes but requires more precise delivery systems. The patterns of fertilization are deeply influenced by the environment, behavior, and physiology of the species, which over time can lead to sophisticated and diverse reproductive strategies.

Reproductive Strategies

Reproductive strategies are the methods by which individuals make sure their genes are effectively passed on to the next generation. These strategies can be shaped by various ecological and social pressures and can include mating systems, parental care behaviors, and mechanisms of gamete transfer.

Reproductive strategies often involve trade-offs, such as the quantity of offspring versus the amount of investment in each. For example, producing many offspring may increase the chances of genes being passed on, but the caregiver's ability to nurture each offspring may decrease, affecting their survival rates. In the context of sperm competition, reproductive strategies might evolve to improve the chances of a male’s sperm succeeding over competitors'. This could result in an arms race of adaptations, from improved sperm mobility to behaviors that increase mating opportunities.