Experimental manipulation reveals the importance of body size for monarch butterfly (Danaus plexippus) mating success

Body size is a critical factor for mating in many organisms. The size of individuals is especially important in systems with coercive mating behavior. When copulations are obtained through physical force, larger and stronger males should have an advantage. This forced mating behavior is common in monarch butterflies (Danaus plexippus), where males pin females on the ground and force them to mate. Indeed, this behavior is thought to favor larger-bodied males. Curiously, however, multiple field studies show that females frequently copulate with smaller males. Here we experimentally manipulate body size to investigate its influence on monarch mating success. We conducted a series of choice and no-choice mating trials containing various combinations of males and females of different sizes. We found that monarchs do, in fact, assortatively mate based on size. Large males typically mated with large females, and small males typically mated with small females. Unexpectedly, we found that this mating pattern was often determined by female choice, where females could reject both large and small males. These data suggest that despite the physical coercion by males, females may play a larger role in monarch mating success than previously realized. Together, these data highlight potential sexual selection dynamics in a coercive mating system and shed light on the reproductive behavior of this iconic butterfly.