New research reveals that male cleaner fish aggressively nudge their mates to watch their diet, as a way to prevent their female partners from turning into rival males.
Cleaner wrasse () are hermaphrodites that are all born female but can turn into males when they become the biggest fish in their group.
If given the choice, males will prefer to mate with a virgin female.
Sometimes, they will go so far as to hang around females that are about to mature.
This and follow-up experiments revealed male punishment was most severe for the loss of high-value clients (a bigger plate) and when the females were large.