Research indicates that females in many species of the fruit fly have multiple mates, which give the males motivation to ensure that their sperm are successful when in competition with the sperm of other males.
But in species where females only ever mating with one male, males experience a type of 'paranoia' in the presence of another male, which doubles the length of time when they mate with a female.
Scientists at the University of Liverpool set to find out why.
They found that once a male had been in contact with another male, it subsequently increased the length of time it mates with a female by 93 percent even though the risk of the female being fertilised by another male was remote.
The study suggests a number of reasons for this apparent 'paranoid' behaviour. One possible explanation is that females do occasionally mate with more than one male, and the male responds to the possibility of this rare event by changing its reproductive behaviour.
Another reason could be that the presence of a competitor prompts the 'fear' that a male is unlikely to obtain another mate and it therefore increases its reproductive efforts in order to keep the one female fertile with its sperm for the female's entire lifespan.
"This takes us a step closer to understanding the differences between males that have evolved within species where individual females mate with many males and those have just one partner," said Dr Anne Lize, from the University's Institute of Integrative Biology.
The research is published in Biology Letters.