Protandry is nature’s way of assuring males are sexually mature and ready to mate the moment females arrive on the scene. It tends to assure reproductive success. I first heard the word when I was studying Pacific salmon, but since then I’ve run across protandry in many species.
In salmon, the males travel upstream first. They find a spawning ground, establish their territory, and wait for the females to arrive. In many bees, such as mason bees, the males are the first to hatch. They have a day or so to mature as they hang around nest openings and wait for the females to hatch.
Females choose the sex
The females of many bee species, including honey bee queens, can decide whether to lay a female (fertilized) or male (unfertilized) egg. So a tube-dwelling bee such as the mason bee can lay female eggs deep within the tube and place the males close to the opening. This specific placement of the eggs allows the whole protandry thing to work.
Protandry literally means “males first.” And, yes, the opposite condition or protogyny exists in some species as well.