During the mating season, which can occur at any time in the North Atlantic, right whales gather into "surface-active groups" made up of as many as 20 males consorting a single female. The female has her belly to the surface while the males stroke her with their flippers or keep her underwater. The males do not compete as aggressively against each other as male humpbacks. The female may not become pregnant but she is still able to assess the condition of potential mates. The mean age of first parturition in North Atlantic right whales is estimated at between 7.5 and 9 years. Females breed every 3–5 years; the most commonly seen calving intervals are 3 years and may vary from 2 up to 21 years due to multiple factors.