Collaborative Research: Foraging Ecology and Physiology of the Leopard Seal (Hydrurga leptonyx)

 

BACKGROUND AND RATIONALE

Polar apex predators are especially vulnerable to global climate change (Derocher et al. 2004), and a growing body of polar research is focused on predicting causal associations between ecosystem health and climate change using marine mammals as sentinel species (Moore and Huntington 2008, Burek et al. 2008, Costa et al. 2010a, Constable et al. 2014). While establishing the relationship between sentinel species and the environment is of the utmost importance, there are virtually no data on many sentinel polar predators. Determining the physiological constraints and energetic needs of individual species in polar ecosystems is crucial for understanding food web dynamics and the potential effects of climate change on ecosystem functioning (Murphy et al. 2012, Hobday et al. 2013, Murphy et al. 2013). Under the framework of using marine mammals as sentinel species, the main goal of this Antarctic experienced research team is to gain an understanding of the foraging ecology and physiology of an apex Antarctic predator, the leopard seal (Hydrurga leptonyx). By gathering data on foraging and dive behavior, diet, habitat use, and energetics of leopard seals, we will be able to relate foraging behavior with physiological performance and determine physiological limits for this species. The estimated physiological limits combined with habitat modeling will help us understand how leopard seals may respond and cope with a changing Antarctic environment.

The leopard seal is unique, as it is one of only three species of marine mammals that routinely feed on both ectothermic and endothermic prey. Leopard seals are responsible for more predation on endothermic prey than any other pinniped and are important apex predators in Antarctic food webs, exerting top down control on Antarctic fur seals and other marine vertebrates (Boveng et al. 1998, Forcada et al. 2009, Schwarz et al. 2013). Leopard seals have a diverse diet, consuming a wide range of prey types, including krill, cephalopods, fish, seabirds, and other seals (Walker et al. 1998, Casaux et al. 2009). Although evidence is limited, it appears that some leopard seals may opportunistically switch between prey types in relation to prey availability (Siniff and Stone 1985, Hocking et al. 2013, Meade et al. 2015). In contrast, other work has suggested that leopard seals are prey specialists and have associated prey capture tactics for their target prey (Rogers and Bryden 1995, Hiruki et al. 1999, Hocking et al. 2013, Krause et al. 2015). Regardless of whether individuals are specialists or generalists, leopard seals directly influence Antarctic food web dynamics and structure at multiple trophic levels. Because leopard seals target diverse prey types, this may offer leopard seals an advantage in the face of a changing Antarctic environment, especially compared to other more specialized predators such as crabeater seals, Weddell seals, emperor penguins, and Adelie penguins. 

The leopard seal is also unique among pinnipeds (seals, sea lions, fur seals, and walruses) because it uses a unique hybrid form of underwater locomotion. Individual leopard seals appear to use both hindflipper propulsion (like phocids—true seals) and foreflipper propulsion (like otariids—sea lions and fur seals) when swimming (Pierce et al. 2011). In contrast, most pinnipeds use one primary type of underwater propulsion with specific muscular adaptations (longissimus dorsi in phocids; pectoralis major in otariids) that reduce energetic costs associated with swimming and diving. As a phocid, leopard seals should primarily use hindlimb propulsion when swimming and diving; however the forelimb bones and associated musculature in leopard seals are larger and better developed compared to other phocids (Bryden and Felts 1974). The leopard seal’s ability to use both hindlimb and forelimb propulsion while feeding may be tied to its generalist diet. Individual leopard seals may use foreflipper propulsion for speed and high maneuverability when tracking large-bodied endothermic prey but may rely on more energy-efficient hindflipper propulsion when consuming small ectothermic prey. The ability to use multiple forms of flipper propulsion to pursue and capture prey may allow leopard seals to more readily adapt to the changing Antarctic environments and effectively feed on multiple prey types. Conversely, maintaining both forms of locomotory musculature is likely energetically expensive and may place additional stress on leopard seals as the environment (and their prey base) shifts.

                                                                                                                                                                              

OBJECTIVES

Does the leopard seal’s diverse diet and multiple modes of underwater propulsion make it the ultimate predator or constrain its adaptability in a changing world? To address this question, this project will address two complementary objectives:

 

1.       We will examine the foraging behavior, diet, and habitat use of leopard seals. We will deploy satellite tag and time-depth recorders on leopard seals to obtain dive profiles and movement data across multiple seasons. In conjunction with the biologging data, we will use genomic and carotenoid sampling and stable isotope analysis to assess diet, trophic position, and habitat use of each individual across multiple temporal scales. We will also use these data to examine foraging strategies used by individual seals. We will use oceanographic data to develop habitat models that will be used to assess and describe home range and core foraging areas, habitat use/ preference, and preferred foraging habitat characteristics for leopard seals. Additionally we will incorporate the oceanographic data into climatic models to predict how core foraging habitats will change under different climatic scenarios. To date, little is known about the foraging behavior of free-ranging adult leopard seals, and there are no data regarding how these predators locate or exploit prey resources. Without such information it is difficult to predict how leopard seal populations are likely to respond to changing environmental conditions. Examination of leopard seal foraging behavior and habitat use will build on previous work and provide a greater understanding of the foraging ecology of large apex polar predators.

 

2.       We will investigate the physiological adaptations that allow leopard seals to be apex predators and determine to what extent leopard seals are working at or near their physiological limit. The foraging strategies identified for individual leopard seals in Objective 1 will be related to physiological variables (e.g., blood volume, muscle oxygen stores, and metabolic characteristics) to better understand the link between foraging behavior and physiology. We will determine the metabolic profile of the leopard seal’s major swimming muscles (e.g., pectoralis major, longissimus dorsi). We will investigate whether their foreflipper swimming muscles (i.e., pectoralis major) reflect the cellular physiology commonly seen in other otariids and if their muscles for hindflipper propulsion (i.e. longissimus dorsi) parallel the metabolic characteristics measured in other phocids. Combining the individual physiological profiles with the foraging behavior will provide new insights into the mechanisms that have allowed leopard seals to be effective apex predators in the Antarctic ecosystem.

 

The two objectives and associated hypotheses of this research project (outlined below) are to collect new information on leopard seal foraging behavior and physiology, develop models of leopard seal habitat use and habitat selection that incorporate information on oceanographic features thought to influence the abundance and distribution of their preferred prey, determine the physiological limits for this apex predator, and explore how these animals will be affected by large-scale oceanographic and climatic changes. This project will be the first to integrate environmental and oceanographic data with the foraging ecology and the cellular physiology of one of the world’s most unique apex predator. This type of integration of environmental, organismal, and cellular research, has been a hallmark of our research team (Kanatous et al. 1999, Trumble et al. 2003, Costa et al. 2004, Costa and Sinervo 2004, Crocker et al. 2006, Costa et al. 2010a, Hückstädt 2012a).