Question

Many species of reef fish change sex once during the course of their lives. In these fish, a bigger body size increases an individual's competitive ability. When strong male-male competition creates high variance in fitness among males, which one of the following scenarios is most likely?

• A. Fish start their reproductive life as females and become males at larger sizes
• B. Fish start their reproductive life as males and become females at larger sizes
• C. Female-female competition leads to male sterility
• D. Male-male competition does not influence the sequence of sex change in an individual

Hint:

Whichever sex gains the most reproductive success from being large should come second; here that is the male role, so the fish start as females.

Answer 1

The Correct Option is A

This question is about the size-advantage model of sequential hermaphroditism, first worked out by Ghiselin and later formalized by Warner. The model asks a simple question for a sex-changing species: does an individual gain more from being large while male, or more from being large while female? Whichever sex profits more from large body size should come second, so the animal starts life as the other sex.

1. Fish start their reproductive life as females and become males at larger sizes: When male-male competition is intense, only the biggest, most dominant males get to mate, so male reproductive success rises steeply with body size. A small male would get almost no matings, while a small female can still produce and fertilize eggs at a decent rate. It pays to spend the early, small phase of life as a female, then switch to male once large enough to win fights and hold a territory or harem. This matches the pattern described and is the correct choice.
2. Fish start their reproductive life as males and become females at larger sizes: This pattern, protandry, fits better when female fecundity rises sharply with size (bigger females carry many more eggs) while male mating success does not depend much on size. That is the opposite of what the question describes, since here it is male fitness that varies strongly with size.
3. Female-female competition leads to male sterility: The question describes competition between males, not between females, and nothing here points to sterility. This option does not follow from the given information.
4. Male-male competition does not influence the sequence of sex change in an individual: This directly contradicts the setup, since the size-advantage model is built entirely on how competition and size affect reproductive payoff for each sex, which in turn decides the order of sex change.

Because high variance in male fitness means only large males do well, the species is expected to be protogynous, starting as female and turning male at a larger size.

Let's summarize:

• When male reproductive success depends heavily on large body size, protogyny (female first) is favored.
• When female reproductive success depends heavily on large body size instead, protandry (male first) is favored.

So the correct scenario is that fish start life as females and change to males once they are large enough to compete successfully.