Finally, they mated. Roy and her colleagues then made female tree frogs listen to tapes of various males played through different speakers in the lab. The females called after hearing the largest, heaviest males, and moved toward that speaker. The rarity of female calls may be one reason why they weren't noted before. Although Roy recorded male calls on each of 148 nights of observation, she heard females' croaks on only 15 days.
Females play a more active role and are no longer passive partners in a system dominated by males.