Fur Seals: Maternal Strategies on Land and at Sea

by Roger L. Gentry (Editor), Gerald L. Kooyman (Editor)

Overview

The contributors to this volume have accomplished a breakthrough in our ability to collect data on ocean-dwelling mammals. In the first large-scale comparison of fur seals, they have employed quantitative methods and a special instrument called a Time-Depth-Recorder to study the strategies used by females in six species of cared seals to rear and wean their young in different environments.

Originally published in 1986.

The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Product Details

ISBN-13:	9780691610740
Publisher:	Princeton University Press
Publication date:	07/14/2014
Series:	Princeton Legacy Library , #64
Pages:	312
Product dimensions:	6.00(w) x 9.10(h) x 0.40(d)

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Fur Seals

Maternal Strategies on Land and at Sea

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By Roger L. Gentry, Gerald L. Kooyman

PRINCETON UNIVERSITY PRESS

Copyright © 1986 Princeton University Press
All rights reserved.
ISBN: 978-0-691-08400-8

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CHAPTER 1

Introduction R. L. Gentry and G. L. Kooyman

INTRODUCTION

Marine mammal science has always been thwarted by the difficulty of collecting data on free-ranging animals at sea. This problem has hindered research on cetaceans (whales and porpoises) somewhat more than on pinnipeds (seals, sea lions, and walruses) because, except in rare circumstances, all data on cetacean biology must be collected at sea. Since pinnipeds mate and rear young on land where measurements are more easily made, some aspects of their behavior and biology are relatively well known. However, just as the pelagic lives of cetaceans remain obscure, those aspects of pinniped biology that occur at sea, such as feeding ecology, diving physiology, predator avoidance, and causes of mortality, remain little known. It is essential to collect data at sea if we are ever to obtain for marine mammals the breadth of information that is now available for some terrestrial groups.

The problem of gathering data on pelagic animals has been partially solved with the advent of new technology. A special instrument, attached to marine mammals, has been developed that continuously records time and pressure. Pressure can be translated into depth, so the instrument is called a Time-Depth-Recorder, or TDR. Up to 2 weeks, this instrument records the time of day, depth, and duration of all dives; the amount of time spent at any given depth; and the interval and kind of activity (resting, swimming, and grooming) between dives. These data allow us to make many new inferences about the behavior, physiology, and ecology of marine mammals at sea. The TDR also enables us to study the pelagic behavior of fur seals, which already have known histories on land, and to compare different species closely. The ability to obtain this information and to make such comparisons constitutes a major breakthrough for marine mammal research.

The authors of this book have applied this new technology to the study of eared seals (family Otariidae), a group whose land-based activities are, by marine mammal standards, under extensive study. In this book we integrate the on-land and at-sea behavior for each of six species. This integration in itself is a unique step forward for marine mammal studies because such integration was not possible for even one species until the TDR was developed. We also make detailed comparisons among the species studied. These comparisons allow us to see broad patterns in the life histories of fur seals and enable us to discuss some of the factors that shape these patterns.

Background Information on Eared Seals

The different species of fur seals are remarkable for their similarity in size, morphology, social behavior, and ecological role (as top carnivores). Their mating systems are also very similar. All otariids are polygynous, gregarious, and sexually dimorphic in body size (Bartholomew, 1970). Adult males range from two to four times larger than adult females, depending on the species. Males mate multiple times over a 2- to 3-year period, usually between the ages of 9 and 14 years, whereas females may bear one young per year from ages 3 to 20 years. Females wean their young at ages varying from 4 months to 3 years. This long neonatal dependency demands that the female alternate between feeding at sea and suckling onshore, and it creates longlasting social groupings on land, some of which never disband.

Otariid life histories are far more homogeneous than those of "true" seals (family Phocidae). Most phocids lack sexual dimorphism in size, but some are extremely dimorphic and others have reversed dimorphism (i.e., females are larger). Most phocids have adult sex ratios of approximately 1:1 and are solitary; others are highly polygynous and gregarious (Stirling, 1975, 1983). Finally, the phocids have a greater range in adult body size and occupy more diverse habitats than otariids. Phocids are similar to otariids in that females may bear a single young each year. However, phocid females fast throughout suckling and wean at 1 to 8 weeks. After weaning, social groupings, if they form at all, break down.

Fur seal life-history patterns are shaped by two major environmental factors: seasonality and the degree of environmental uncertainty. The seasonal factor is relatively straightforward. Otariids breed from approximately lat. 60° N to 60° S. Two species, the northern fur seal Callorhinus ursinus and the Antarctic fur seal Arctocephalus gazella, breed at the ends of this range where waters are cold and productive but highly seasonal. They occupy these sites only in summer and autumn and migrate in winter. Most other otariids live in temperate latitudes where seasons are less extreme. Temperate fur seals usually depend on upwelling systems or on productive ocean currents that provide food throughout the year. Two species, the Galapagos fur seal A. galapagoensis, and the Galapagos sea lion Zalophus californianus wollebaeki, live at the equator, where seasonal changes are perceptible but not profound. These species, and the Peruvian populations of the South American fur seal A. australis, depend on upwelling plumes around islands or on a thin band of coastal upwelling. These small areas of upwelling are surrounded by vast areas of warm, equatorial water.

Environmental uncertainty also varies in the area inhabited by fur seals and may have a more profound effect on life-history patterns than does seasonal change (Orians, 1969; Colwell, 1974; Wilbur et al., 1974). One major source of uncertainty is a meteorological and oceanographic phenomenon termed an "El Nino/Southern Oscillation" (hereafter abbreviated EN) that affects the entire equatorial Pacific Ocean (Cane, 1983). Its oceanographic effects in Peru, Ecuador, and Chile include intermittent suppression of upwelling and productivity around islands and along mainland coasts. When productivity is suppressed the fish on which otariids depend die, migrate, or descend to unreachable depths (Barber and Chavez, 1983). Under such conditions female otariids cannot find sufficient food and their young die (Limberger et al., 1983; Barber and Chavez, 1983). In severe ENs several cohorts of young may die along with some adults. EN events are known to have occurred since at least the 1700s and recur on average once every 4 years, although the interval varies from 2 to 10 years (Cane, 1983). Furthermore, no two ENs are alike in severity, timing, spatial distribution, or biological consequences (Barber and Chavez, 1983). Therefore, the timing and severity of food shortages for fur seals are unpredictable, though they are likely to occur within a female seal's reproductive lifetime (ca. 15 years).

Sea surface temperatures can be used as an index of the environmental uncertainty that exists for otariids. Figure 1.1 shows the mean annual sea surface temperatures for the Bering Sea and Chicama Beach, Peru. These locations represent the most extreme environments (subpolar and tropical) inhabited by otariids, and are at rookeries or close to them. The overall mean temperatures differed greatly, but variance around the means was similar. The sites differed most in the rapidity of change around the overall mean. In the Bering Sea the mean annual temperatures changed relatively little between successive years, describing a long sine wave having a period of 13 to 14 years. However, the Peruvian temperatures changed very quickly from one extreme to the other in an unpredictable pattern (EN years coincide with peaks in the graph). Computer modeling would be required to quantify the differences in these graphs.

The environmental uncertainty imposed by ENs on otariids may not be restricted to the equator, but may be graded according to latitude and severity of the event. Warm water from the equator flows poleward along land masses via the California and Peru countercurrents. In the severe EN of 1982-83, female feeding patterns and pup mortality among northern fur seals breeding at lat. 34° N were markedly different than in previous years (Antonelis and DeLong, 1985). No measurable effects were demonstrated on northern fur seals breeding in the Bering Sea (lat. 57° N), but measurable effects on water temperature and fish distribution were reported at lat. 50° N. The direction of current flow makes a similar effect south of the equator also likely. Furthermore, a phenomenon analogous to the EN has been reported for the equatorial Atlantic (Merle, 1980; Hisard, 1980; Shanon, 1983) and may affect fur seals of the temperate south Atlantic Ocean.

To summarize, fur seals face a gradient of seasonal change from very large in the subpolar areas to very slight in the tropics. They face a reverse gradient (or at least seals at the ends of the range face reversed extremes) in the predictability of their food supply while rearing young. Seals at one extreme (subpolar) begin a sharply defined breeding season with the high likelihood that sufficient food will be available for a brief period. Seals at the other end begin a longer, more equable season, but face the possibility that catastrophic food declines may occur before the young are independent. These opposing alternatives are shown graphically in Figure 1.2, which is a modification of Figure 7 in Stearns (1976). We show only the subpolar and tropical extremes because information is lacking on the extent of environmental uncertainty for temperate fur seals; presumably they would be intermediate between these extremes in both seasonality and environmental uncertainty.

The Central Issue

The purpose of this book is to show what suites of adaptive responses females have developed for producing the maximum number of young in their lifetime, given the different ways their environments fluctuate. To address this issue we compared a wide range of maternal behavior, using approximately forty variables, for each of the six species we studied. Attendance behavior — the pattern in which females deliver nourishment to their growing young — was observed from shore. Attendance included such measures as the number and duration of visits to shore from birth to weaning, the number and duration of trips to sea, changes in trip duration as a function of the pup's age, and suckling frequency while ashore. We view these parameters as components of the maternal strategy on land. Some direct results of female attendance were also measured, such as growth rates of suckling pups and the weight of the pup at weaning.

Using TDRs we measured pelagic behavior among foraging mothers. The measures included the transit times between shore and feeding areas, depth and duration of feeding dives, relationship between depth of dive and time of day, repetition rate of dives within bouts, number of feeding bouts per day, and the occurrence of rest periods at the surface. We regard these parameters as components of maternal strategies at sea because they are central to maternal efforts to rear young. Finally, we measured the interval from birth to weaning, the timing of births relative to weaning the previous young, and the partitioning of suckling where competing, dependent young exist.

Although the measures we made are extensive, they do not include all the components that are usually considered in life-history theory (such as juvenile and adult mortality schedules, age at first reproduction, and reproductive life span; Wilbur et al., 1974; Stearns, 1977). We refer to our subset as maternal strategies. We define these strategies as combinations of short- and long-term options by which females produce the largest number of weaned offspring in their reproductive lifetime. Short-term options are those associated with rearing a single young to weaning and include attendance and pelagic behavior. Long-term options affect the frequency with which females attempt to rear and wean young. The terms "option" and "strategy" refer to adaptive responses shaped by natural selection over time; they do not imply that animals have cognitive awareness of their actions, or that systems are teleological.

Our results will show two different maternal strategies. Seals breeding at high latitudes, where seasonal change is extreme and the environment is highly predictable from year to year, have brief, nonvariable periods of neonatal dependency; these females always wean their young at the same age (ca. 4 months) and size (ca. 40% of adult female mass) each year. Seals breeding at low latitudes, where seasonal changes are small and environmental uncertainty is large, have longer, highly variable periods of neonatal dependency; these females wean their young at 40% of adult female mass but suckle them for 18 to 36 months to attain this size. Thus, females adjust the weaning age according to the vicissitudes of the environment. We will show that attendance behavior, diving behavior, and the energy content of the mother's milk all covary with the age at weaning. We will contend that environmental uncertainty is more important than seasonal change in shaping these suites of characters. These results are not consistent with the r- and K-selection theory of life-history strategy, which holds that predictable environments should lead to longer neonatal dependency. Instead, our results are in general agreement with the bet-hedging alternative in which juvenile mortality is variable (see Table 4 in Stearns, 1976).

These broad patterns of maternal strategies among fur seals are apparent only from detailed comparisons among all species. Therefore, we will delay discussion of comparative maternal strategies until Chapter 15, leaving Chapters 3 through 14 to set forth unique aspects of the overall comparison.

This work is the first detailed investigation of diving behavior in any group of marine mammals. Other diving studies have been conducted, but they were brief or involved only a few dives (Evans, 197l; Lockyer, 1977; Ichihara and Yoshida, 1972). Only Kooyman's ongoing studies of diving in Weddell seals exceed this work in scope (Kooyman, 1968, 1975, 1981). This is also the first systematic attempt to compare the behavior of closely related marine mammals, although the need for such comparisons was recognized earlier (Gentry, 1975).

EVOLUTIONARY HISTORY OF OTARIIDS

The ancestors of modern otariids, the Enaliarctids, evolved approximately 22 million years ago in the early Miocene. With little diversification or change, the Enaliarctids gave rise to the earliest true otariids beginning about 12 million years ago (Mitchell and Tedford, 1973; Repenning and Tedford, 1977; Repenning et al., 1979). The otariid stock began to diversify around 6 million years ago when the lineage leading to modern northern fur seals arose. Aside from the divergence 3 million years ago of a form leading to modern sea lions, the basic otariid stock continued to evolve in the direction of modern Arctocephalus (Repenning et al., 1979).

Because the earliest fossils are found in the north Pacific Ocean, the otariids are believed to have evolved there. About 5 million years ago the first fur seals crossed into the southern hemisphere concomitant with the closure of the Central American Seaway and the cooling of the central Pacific, which occurred when the warm Atlantic equatorial current was diverted northward as the Gulf Stream (Repenning et al., 1979). Sea lions appear to have crossed the equator about 3 million years ago.

Despite the otariids' origins in the north Pacific, their diversification has been greater in the southern hemisphere, where five Arctocephalus species and three genera of sea lions live. Two kinds of fur seals (one Arctocephalus species plus Callorhinus) live in the northern hemisphere, as do two genera of sea lions. A subspecies of one of the northern hemisphere sea lions (Zalophus) and one fur seal species also breed at the equator (Repenning et al., 1971, 1979).

(Continues...)

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Table of Contents

• FrontMatter, pg. i
• Contents, pg. vii
• List of Contributors, pg. ix
• Preface, pg. xi
• Acknowledgments, pg. xv
• Chapter 1. Introduction, pg. 1
• Chapter 2. Methods of Dive Analysis, pg. 28
• Chapter 3. Attendance Behavior of Northern Fur Seals, pg. 41
• Chapter 4. Feeding and Diving Behavior of Northern Fur Seals, pg. 61
• Chapter 5. Free-Ranging Energetics of Northern Fur Seals, pg. 79
• Chapter 6. Attendance Behavior of Antarctic Fur Seals, pg. 102
• Chapter 7. Diving Behavior of Antarctic Fur Seals, pg. 115
• Chapter 8. Attendance Behavior of South African Fur Seals, pg. 126
• Chapter 9. Diving Behavior of South African Fur Seals, pg. 142
• Chapter 10. Attendance and Diving Behavior of South American Fur Seals during El Niño in 1983, pg. 153
• Chapter 11. Attendance Behavior of Galapagos Fur Seals, pg. 168
• Chapter 12. Diving Behavior of Galapagos Fur Seals, pg. 186
• Chapter 13. Attendance Behavior of Galapagos Sea Lions, pg. 196
• Chapter 14. Diving Behavior of Galapagos Sea Lions, pg. 209
• Chapter 15. Synthesis and Conclusions, pg. 220
• Literature Cited, pg. 265
• Author Index, pg. 279
• Subject Index, pg. 283