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Mating Walnut Flies: Sex Ratios and Copulation Duration

What are the advantages (for males) of long copulations?

• may function as a form of mate guarding
• may enable male to displace sperm from matings with other males

What are the disadvantages?

• diminishing returns in terms of number of eggs fertilized
• if reproductive success is increased with the number of mates a male has, he may be losing chances for other matings

As you may have already guessed, the relative benefits of long versus short copulations can depend on the operational sex ratio (OSR). If the OSR is female-biased, and there are plenty of females to go around, the male fertilizes the most eggs (potentially) by mating for a short time and then immediately mating with the next female. However, if the OSR is male-biased, this increases the time that it takes the male to find another unoccupied female, and it may be more advantageous for the male to prolong the copulation.

All of this assumes, importantly, that it is the male that controls copulation duration. It is not yet known for this fly which gender determines copulation duration.