Diet and Nutrition

• Main Authors: Hindell, MA, Walters, A
• Format: Book Part
• Language: English
• Published: CRC Press 2016
• Subjects: Biological Sciences; Ecology; Marine and Estuarine Ecology (incl. Marine Ichthyology); Hydrurga; Hydrurga leptonyx; Leopard Seals; Orca; Orcinus orca; Ursus maritimus
• Online Access: https://www.crcpress.com/Marine-Mammal-Physiology-Requisites-for-Ocean-Living/Castellini-Mellish/p/book/9781482242676; http://ecite.utas.edu.au/106605

Diet and nutrition are fundamental in shaping the life histories and reproductive strategies of all animals but particularly so for marine mammals. This is because, despite their diverse evolutionary origins, all marine mammals returned to the sea from terrestrial antecedents to exploit the abundant resources available in the marine environment. In so doing, they have retained some of the traits of their land-based predecessors which dictate many aspects of their feeding biology. Primary among these is their need to breathe air, requiring them to spend time on the surface away from the prey fields. The tension between the need to obtain oxygen at the surface but to feed, hunt, and even digest food underwater in the absence of available oxygen is a fundamental challenge for marine mammals. Further, while cetaceans and sirenians can give birth and nurse young at sea, pinnipeds need to be on land (or ice) for reproduction, further influencing their relationship with marine-based food resources. During breeding and lactation pinnipeds either do not forage at all (many of the phocids) or they forage dose to the breeding sites (many of the ottariids); both strategies have their own implications for diet. The diverse phylogenetic origins of marine mammals also offer scientists an important opportunity to better understand the complex interplay between diet (or "energy in") and how marine mammals go about their aquatic lifestyles (or "energy out"). Biology has a long tradition of employing a comparative approach to understanding general underlying principles, and the marine mammals with their differing evolutionary histories, but convergent lifestyles are particularly valuable in this regard. By comparing and contrasting the diets and nutrition of marine mammals, we can learn a lot about how these influence life-history strategies. As a group, marine mammals have a wide range of diets, from exclusive herbivory for the sireniens (manatees and dugongs), through grazers on lower trophic levels (mysticete whales), those that exploit mesopelagic fish and squid (most of the odontocete whales and pinnipeds), up to top predators that take other marine mammals (killer whales, Orcinus orca polar bears, Ursus maritimus and leopard seals, Hydrurga leptonyx ). It is important to note that most of these dietary types are used by several orders of marine mammals. For example, both phocid seals and mysticete whales have evolved filter feeding as a mechanism for collecting small zooplankton. This convergence in diets and feeding techniques is a good example of how species with diverse phylogenetic origins are able to exploit the same abundant resources available in the marine environment. With the exception of the sirenians, all marine mammals are carnivorous , and indeed are likely to have had carnivorous terrestrial antecedents. In this chapter, we review the diets of all the major families of marine mammals; this includes the sirenians, mysticete, and odontocete whales, as well as the three families of pinnipeds (ottarids, phocids, and odobenidae), but not sea otters ( Enhydra lutris ) or polar bears as these families are not exclusively marine. The overall scope of the chapter is to detail the range of diet types and, in so doing, highlight the diversity of taxa using each diet and their morphological convergence. We begin with an overview of foraging strategies as well as the morphological and behavioral adaptations and implications this has for their life history and broader ecology. As marine mammals are a particularly challenging group of organisms for dietary studies (due in large part to the difficulties in observing and accessing them in the ocean), we then include a brief review of the techniques that scientists use to quantify (with varying degrees of taxonomic and temporal resolution) what marine mammals eat.