Altered physical and social conditions produce rapidly reversible mating systems in water striders

Lay Summary Changes in space and group size for water striders (aquatic bugs) induced striking, reversible changes in mating behaviors within days or even hours. In large groups, no male could dominate the rest. Instead, males harassed females, drove them into hiding, mated frequently, and typically guarded females for several hours after mating. In contrast, in small groups, an alpha male often emerged, driving other males into hiding, mating only occasionally, and rarely harassing or guarding females.