For example, a male in a harem of four females has four times more reproductive success than any of his females because each individual female has only the harem's one male partner with whom to mate.
Just as in humans, in many sex-changing fishes, social context often determines an individual's behavior.
The bluebanded goby observes and reacts to the behavior of others around it, so as to optimally position itself within the social hierarchy. Other fish observe that individual's behavior and react to it, and so on.
Grober wanted to know if the same mechanisms that regulate female-to-male sex change also work in the other direction. Using saltwater aquariums in his lab, he designed an experiment that used isolated groups of all males and all females.
Each group had one fish that was significantly larger than the others to encourage one individual to achieve dominance over the rest of the group. Grober and others argue that while a larger fish is more likely to achieve social dominance over its smaller harem mates, size alone doesn't completely determine which fish is the most dominant.
His research group observed instances of aggressive behavior (approaches, flaring fins, head-standing) and submissive behavior (retreat or avoidance in response to aggressive behavior) in these groups. Taking all individual interactions into account, they could determine each fish's social status within the group.
The fish demonstrated male-to-female sex change in this experiment. But something else remarkable happened: One of the fish remained male, and it was the only fish to retain its dominance over the group.
For sex change in either direction, there seems to be one guiding social principle: If subordinate, be female. Because each group had only one dominant individual, each group ended up with only a single male.
Hormones also play a role in the sex changes of these fish.
In one experiment, Grober and colleagues set up pairs of female bluebanded gobies, so that one fish was clearly subordinate. When they loaded up the subordinate with a potent male hormone, 11-ketotestosterone, it was able to make her look like a male, but her female-typical social behavior didn't shift in a male direction. She continued to behave as a female.