Development of Non-lethal Methods for the Evaluation of Energetic Reserves in Humpback Whales (Megaptera novaeangliae)

Abstract

Cetaceans have been suggested as ideal ocean sentinels for the evaluation of ecosystem health because they are long-lived, higher trophic foragers, often with high site-fidelity. The body condition of cetaceans provides an integrated picture of foraging success and energy expenditure and thus represents an effective ‘sentinel parameter’ for ecosystem monitoring. In addition, the body condition of an individual can serve as a predictor of reproductive success and offspring survival. The ability to monitor the body condition of cetaceans is therefore of direct relevance for conservation biology. Despite the value of this parameter, there is currently a lack of reliable, standardized, non-lethal methodologies for its evaluation across species and age-categories. The introduction chapter describes the natural history of the humpback whale (Megaptera novaeangliae) in the context of its Antarctic ecosystem and its energetic requirements. It sets out that the species is among the most vulnerable in a changing environment due to it being a specialised feeder depending on Antarctic krill. Krill is the keystone of the Antarctic ecosystem and is presently facing numerous threats. Humpback whales are capital breeders that spend summers feeding in high latitude feeding grounds, building up their energy reserves reflected in their body condition. During winter they migrate to tropical waters to reproduce, a journey that is fuelled by the energy stores previously accumulated. The seasonal dynamics of intensive feeding and lipid deposition, followed by high energy activities that are accompanied by fasting and lipid mobilisation, render the species ideal for the study of possible biomarkers of fluctuating energy stores. This thesis focuses on the evaluation of fluctuating adiposity of southern hemisphere humpback whales. The second chapter presents a critical review of the available approaches of assessing cetacean body condition. Currently the methodologies can be divided into four groups, namely: Blubber Measures, Body Composition, Body Morphometry, and Biochemical and Chemical Biomarkers. The last category is dedicated mainly to new developments and explores the emerging methodologies. This chapter describes and analyzes their applications as well as their respective strengths and weaknesses. The chapter concludes by advocating research priorities that fill the critical technical research needs for a reliable, non-lethal and standardized methodology that is applicable across species and age-categories and for large numbers of individuals. In response to the research need identified in chapter two, chapter three investigates adipocyte metrics as a potential biomarker of adiposity in humpback whales. Blubber is composed of adipocytes, which are filled with lipids according to the individual energy reserves. This means that a change in total energy reserve impacts the volume of individual adipocytes. Two adipocyte metrics were assessed histologically: the two-dimensional adipocytes area and the relative proportion of adipocyte space in the blubber tissue which is termed the adipocyte index. Both adipocyte metrics exceed the sensitivity of the most widely used proxy for energetic reserves, blubber lipid percentage, thus validating the implementation of adipocyte area and the adipocyte index as energetic reserve biomarkers. This published work therefore advocates the inclusion of these measures, obtained from superficial blubber biopsies, in long-term monitoring programs as a means of improving evaluation of fluctuating energy reserves. Following the molecular techniques’ potential to assessing body condition identified in the second chapter, chapter four explores the RNA extraction from humpback whale blubber for downstream applications. Good quality RNA is a prerequisite for successful downstream applications, however its extraction can be difficult, especially if the starting material is high in lipids. This chapter describes the strategic approach taken for method optimization and the difficulties encountered. The fifth chapter provides a theoretical basis for the use of blubber proteins “adipokines” as biomarkers of adiposity. The main physiological role of blubber is energy storage; however, the tissue is highly integrated into the overall physiological and metabolic control systems of mammals. This mechanism is carried out through the secretion of proteins, synthesized in the adipose tissue and actively involved in the lipid dynamics, with a profound effect on food intake and energy expenditure in model species. The different concentrations of these proteins, such as leptin and adiponectin inform the hypothalamus of the amount of energy stored. However, very little is known about the operation of adipokines in animals that experience extreme fluctuations in their energy reserves while at the same time using adipose tissue as insulation as cetaceans do. This chapter explores the possibility that the gene expression profile of selected adipokines may provide insight into the fluctuating energetic health of cetaceans. This chapter also outlines trialled approaches and advocates future priorities. The key findings and implications of this work are discussed in the concluding chapter. In summary, this study makes clear the importance of the energy reserves measurement and its ecological scope. It identifies the priorities for the improvement and standardization of the current methodologies and highlights the importance of new developments, pointing towards possible areas for further development. It proposes a simple and easily applicable methodology that improves precision and accuracy of the current method. In addition, it proposes advances in the optimization of the extraction of good quality RNA from blubber for downstream application. Finally, it explores the gene profile of the adipokines leptin and adiponectin, leaving the theoretical bases and some progress in the study of adipokines gene expression from humpback whale blubber.