The energetic cost of reproduction and growth in humpback whales

Abstract

Recent declines in reproductive output and calf survival among North Pacific humpback whales (Megaptera novaeangliae) underscore the critical need to quantify the energy requirements of mothers and their offspring. This dissertation examines energy allocation to reproduction and growth in humpback whales across their Hawaiian breeding and Southeast Alaskan feeding grounds. Using a comparative framework, I modeled pre- and post-natal energetic demands, incorporating key external and internal factors such as migration, maternal size and body condition, sex, and age. In Chapter 2, I estimated the energetic cost of gestation in humpback whales, encompassing both direct (tissue growth) and indirect costs (metabolic load). I incorporated multiple data streams, including morphometric measurements from historical whaling records (678 mother-fetus pairs) and non-invasive unoccupied aerial system (UAS) photogrammetry (987 mother-calf pairs), and tissue samples from an opportunistically collected placenta and a post-mortem specimen. I demonstrated that energetic costs increase exponentially over gestation, with over 95% of total costs incurred in the final 100 days. This illuminates a critical time period during which pregnant females endure extreme energetic demands while fasting on their southbound migration to the breeding grounds. In Chapter 3, I analyzed 2,410 UAS measurements of 1,659 individual whales, including 803 repeat measurements of 275 individuals, to assess the influence of maternal size and body condition on postnatal offspring investment. Larger females not only produced larger calves at birth, as found in Chapter 2, but their calves also gained body volume faster. Together, findings from Chapters 2 and 3 highlight the critical role of maternal size and condition in mitigating the energetic demands of reproduction, with mass-specific costs decreasing as maternal size increases. Consequently, smaller females face greater energetic trade-offs, potentially reducing maternal investment and leaving both mothers and their offspring more vulnerable to stressors. In Chapter 4, I estimated the energetic cost of growth from birth to 50 years of age by modeling the body length and mass of 1,503 known-age whales. I estimated that postnatal somatic growth requires 38 times more energy compared to prenatal growth, with calves allocating 6-8 times more energy to growth than mature adults. Notably, nearly 30% of a calf’s lifetime somatic growth costs occur within less than 2% of its lifespan. This necessitates that mothers draw extensively on their energy reserves to sustain both their metabolic demands and those of their rapidly growing offspring. These findings advance our understanding of energy allocation around parturition, highlighting the extreme energetic demands on reproductive females and their offspring. By quantifying these costs, this work sheds light on their vulnerability to energy imbalances during migration and enhances our ability to assess the impacts of anthropogenic disturbances and changing ocean ecosystems on life history strategies.