Body condition and migration timing of east Australian humpback whales

In order to exploit seasonally favourable habitats for feeding and breeding, humpback whales Megaptera novaeangliae undertake one of the longest migrations in the animal kingdom. Stored energy is crucial for a successful migration, but few studies have investigated the relationship between migration timing and body condition in baleen whales. Using unmanned aerial vehicles, we quantified the body condition of east Australian humpback whales. We collected data on 513 individuals (48 calves, 166 juveniles, 251 adults, and 48 lactating females) during their northbound and southbound migrations between June and October 2020. For adults and juveniles, we explored the loss of body condition between migration direction (north versus south) as well as the relationship of migration timing (day of year) and body condition. We found a significant loss in body condition between the northbound and southbound migrations for both adults (9.8%) and juveniles (18.3%). However, migration timing did not influence body condition for either reproductive class. Cow/calf pairs were analysed using relative calf length (percentage of maternal length) as a proxy for days postpartum. We found a positive curvilinear relationship between migration timing and calf body condition. However, lactating females showed no relationship between migration timing and body condition. Whilst body condition is important for capital breeding whales, the lack of a correlation found for adults and juveniles suggests that body condition is not the main driver of migration timing from feeding or breeding grounds. However, calf body condition may be a significant factor for the migration timing of cow/calf pairs.

Automated summary

In order to exploit favorable habitats, humpback whales undertake long migrations. This study investigated the relationship between migration timing and body condition in east Australian humpback whales. It found a significant loss of body condition during migrations for adults and juveniles, but migration timing did not influence body condition. However, calf body condition may be a significant factor for the migration timing of cow/calf pairs.