Females typically breed every two or three years. The gestation period is 11.5 months. The peak months for birth are January and February in the Northern Hemisphere, July and August in the Southern Hemisphere. Females wait for one to two years before breeding again. Recent research on mitochondrial DNA reveals that groups living in proximity to each other may represent distinct breeding pools. Humpback whale births have been rarely observed. One birth witnessed off Madagascar occurred within four minutes. Humpback whales have been known to hybridize with other rorquals; there is a well-documented report of a humpback-blue whale hybrid in the South Pacific.