Bumblebees engage in a reproductive cycle that culminates in mating for the next generation of queens. While the busy workers and the founding queen never mate again, the future of the species rests entirely on the young males, known as drones, and the newly produced virgin queens. This process occurs only at a single point in the annual colony cycle and involves a unique form of sperm storage that sustains the new queen for her entire life. This brief, late-season encounter is the sole mechanism by which the species renews itself each year.
The Seasonal Context of Bumblebee Reproduction
The lifespan of a bumblebee colony is annual, a cycle beginning with a single queen that emerges from hibernation in the spring. This founding queen establishes a nest and raises her first brood of sterile female workers alone, foraging for food and tending to the young. As the colony grows throughout the summer, these workers take over the duties of foraging and nest maintenance, allowing the queen to focus solely on laying eggs.
The colony’s goal is to produce the next generation of reproductives before the cold weather arrives. In late summer or early fall, the queen initiates a significant shift in egg production, marking the transition to the reproductive phase. She begins laying unfertilized eggs that develop into males (drones) and fertilized eggs that are reared into new queens (gynes).
The emergence of these reproductive individuals signals the end of the original colony’s existence. The founding queen and her workers will soon die, often around the time of the first hard frost. Mating must ensure that the newly emerged queens are fertilized before they locate a suitable site for their winter hibernation.
The Specific Roles of Drones and New Queens
Drones are males that develop from eggs the queen did not fertilize, a genetic system known as haplodiploidy. Because the male develops from an unfertilized egg, he possesses only one set of chromosomes, meaning he has a mother but no biological father.
The new queens, by contrast, are diploid females, having developed from fertilized eggs that contain a full set of chromosomes. These young queens leave the nest to forage and build up fat reserves. Their sole biological purpose is to mate with a drone from a different colony and then hibernate until the following spring.
Once mature, the drones leave the nest and never return, dedicating their short lives to finding a virgin queen. They perform no work for the colony. This division of labor and the temporary nature of the reproductive individuals ensure that mating occurs externally, promoting genetic diversity.
The Mating Ritual and Sperm Storage
The mating ritual begins when a drone leaves his colony and patrols a specific flight path, often near flowers or prominent landmarks. He deposits a species-specific scent pheromone along this circuit to attract a newly emerged queen. When a virgin queen flies into this territory, the drone attempts to intercept her, and the pair will often fall to the ground to copulate.
This physical act can be quite lengthy. The duration is thought to be a mechanism to ensure successful sperm transfer and to prevent the queen from mating with other males immediately afterward. The male dies shortly after, having fulfilled his only function.
The new queen, however, stores the transferred sperm in a specialized internal organ called the spermatheca. This organ is designed to keep the sperm viable for an extended period. She will carry this single lifetime supply of sperm through her hibernation and use it to selectively fertilize eggs one by one when she starts her own colony the following spring.