Effects of Sex, Seasonal Period, and Sea State on Calf Behavior in Hawaiian Humpback Whales (Megaptera novaeangliae)

Abstract

Ontogeny of behavior in young humpback whale (Megaptera novaeangliae) calves likely reflects preparation for adulthood, including courtship and reproductive activities, predator avoidance, and prey capture. Reproductive strategies differ for males and females, with males competing aggressively for females, while females focus their energy on raising calves; thus, certain behaviors may develop differently in each sex. In addition to these forces driving behavioral development, ambient conditions, such as Beaufort sea state, may also impact behaviors by requiring adaptations to different environments, some of which are louder or more energetic. Herein, we examine the roles of sex, seasonal period, and sea state on Hawaiian humpback whale calf behavioral development. We used underwater video recordings to document when calves were (1) at the surface without their mothers, (2) in physical contact with or in close proximity to (within 5 m) of their mothers, (3) playing, (4) milling, (5) interacting with divers, or (6) vocalizing (social sounds). We analyzed footage of 199 groups (1,485.5 min) in which a calf was present using linear mixed effects models. Sex of the calf was determined in 107 groups (64 females, 43 males). Results indicate that males played or were surface active significantly more often than females, and that calves were at the surface without their mothers significantly more often during January and February than March, and significantly more during the end of January than the beginning of February, indicating that spatial proximity to the mother varies. There were no significant findings characterized by sea state though trends were evident. Behavioral differences by calf sex may be attributable to differences by sex in adult social roles—that is, males may need a higher level of fitness and ability to compete for access to females. Greater mother/calf separation midseason may drive development of motor skills, independence, and fitness in preparation for migration.