Biology
Mate Guarding and Sperm Competition in Damselflies
Quick fact
In many damselflies, the last male to mate fathers over 90% of the offspring, thanks to specialized structures that physically remove sperm from previous rivals.
Why this is interesting
You might think that after mating, the male's job is done—but in the world of damselflies, the real battle has just begun. Why would a male risk his life to stay glued to his mate?
Read the full explanation
Understanding Mate Guarding and Sperm Competition in Damselflies
Imagine a dance floor where every male wants to ensure he's the only partner. In damselflies, after a male successfully mates, he faces a problem: the female can store sperm from previous males and use it later. To prevent his sperm from being outcompeted, he engages in mate guarding—physically staying attached to the female or flying in tandem with her to deter other males. This behavior is a direct response to sperm competition, the battle between sperm from different males to fertilize the eggs. The female's ability to store sperm means that the timing and occupancy of the last mating are crucial.
A deeper explanation
The mechanism behind this is twofold: sperm displacement and female sperm storage. Male damselflies possess structures on their aedeagus (the intromittent organ) that physically scoop out sperm from the female's storage organ (spermatheca) before depositing their own. This occurs during copulation, followed by a period where the male guards the female as she lays eggs. The result is last-male precedence—the last male to mate fertilizes the majority of eggs. This strategy is driven by sperm competition, which is a form of post-copulatory sexual selection. The intensity of guarding correlates with the risk of encountering rival males and the female's reproductive value. This system exemplifies how sexual selection does not end at copulation; it continues at the gamete level and through behavioral adaptations that maximize reproductive success.