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

Step 1: Picture two reproductive value curves.
Imagine plotting expected reproductive success against body size separately for a male and for a female of this species. Sex change theory says an individual should be the sex with the higher curve at small sizes, then switch to the other sex once its curve overtakes.

Step 2: Work out which curve is steeper here.
The question tells us male-male competition is intense and creates high variance in male fitness. That means a handful of large, dominant males get almost all the matings, while small males get close to none. So the male reproductive curve is flat and low at small sizes, then rises steeply at large sizes.

Step 3: Compare with the female curve.
Female reproductive success usually rises more gradually with size, since egg production scales more smoothly with body size and does not depend on winning fights. So at small sizes, the female curve sits above the male curve.

Step 4: Decide the order of sex change.
Since the female curve is higher at small size and the male curve is higher at large size, an individual gains the most lifetime reproductive success by starting as a female and switching to male once it grows large enough to win competitive contests.

Step 5: Rule out the alternatives.
Starting as a male and switching to female late (option B) would apply only if the female curve rose steeply with size instead, the reverse of this setup. Sterility from female-female competition (option C) is not mentioned anywhere. Saying competition has no effect on sex change order (option D) contradicts the entire logic of the size-advantage model.

Step 6: Conclude.
\[ \boxed{\text{Fish start life as females and become males at larger sizes}} \]