Cost of migration and migratory timing in Western Australian humpback whales

Abstract

Migratory humpback whales (Megaptera novaeangliae) cover the cost of reproduction in low-latitude breeding grounds with stored energy accumulated from polar feeding grounds. The ability to accumulate sufficient energy reserves during feeding periods is vital for key life history stages during migration, including mating, calving, and lactation. This study aimed to investigate the relationship between migration timing and body condition of Western Australian humpback whales. We used unmanned aerial vehicles to measure body condition (residual of body volume vs. length) in 2017 and 2021. Morphometric measurements were obtained from 460 individuals (71 calves, 83 juveniles, 235 adults, and 71 lactating females) during the northbound (toward breeding grounds) and southbound (toward feeding grounds) migration between May and November. Body condition decreased by 23 and 13 percentage points for juveniles and adults, respectively. The body condition of juveniles was shown to be correlated with migration timing for their northern migration, with individuals in better body condition migrating to the breeding grounds earlier. While stored energy is vital for humpback whales to successfully complete their vast migration to-and-from breeding grounds, we found no evidence that body condition affects the migration timing for adults, lactating females, and calves.