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Behavior and Ecology of the Northern Fur Seal

Behavior and Ecology of the Northern Fur Seal

Roger L. Gentry
Copyright Date: 1998
Pages: 408
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  • Book Info
    Behavior and Ecology of the Northern Fur Seal
    Book Description:

    Covering the behavior and ecology of the northern fur seal, this book is a model long-term study of marine mammals, one that tests theory through both observation of undisturbed behavior and manipulative experiments on individuals. Here Roger Gentry draws on nearly two decades of research on three different islands to show how behavior among these seals changes with population size, sex ratio, and environment, to explain the behavior of the population beginning with individuals, and to generalize the results to other members of the eared seal family. In so doing, he offers one of the most comprehensive studies of its kind on any marine mammal species to date.

    Gentry shows that the species is driven by very different behavioral traits than have been assumed for it in the past. His book analyzes behavior on scales of hours to lifetimes, investigates the mating system, considers processes that underlie the mating system (site fidelity, behavioral estrus, and the development of territoriality), and addresses specific aspects of maternal strategy (female attendance behavior, pup growth, seasonal influences, and the effects of continental shelf width). Gentry contributes to knowledge about marine mammals by providing a very specific basis for interspecies comparisons, and he suggests a link between population trend and environmental regime shifts. He also guides the debate over seal mating systems from an interpretive to an empirical or experimental basis.

    Originally published in 1997.

    ThePrinceton Legacy Libraryuses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback 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.

    eISBN: 978-1-4008-6472-0
    Subjects: Biological Sciences

Table of Contents

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  1. Front Matter
    (pp. i-vi)
  2. Table of Contents
    (pp. vii-viii)
  3. Preface
    (pp. ix-x)
  4. Acknowledgments
    (pp. xi-xiv)
  5. List of Contributors
    (pp. xv-2)
  6. PART ONE: Behavior at the Population Level

    • CHAPTER 1 Introduction
      (pp. 5-37)

      This book reports on a 19-year study of the northern fur seal,Callorhinus ursinus(Linnaeus 1758), a medium-sized eared seal (Pinnipedia, family Otariidae) that inhabits the Pacific Ocean from about 35 to 60 north latitude. The study was designed to answer specific behavioral questions about a long-term decline in fur seal numbers.

      The northern fur seal has been of uncommon importance to society for the past 250 years because of the luxurious, durable underfur it produces. To obtain furs, towns were founded, people were enslaved, battles were fought, sealing fleets sailed, and an international treaty was struck. In a 5-year...

    • CHAPTER 2 Population Changes, 1974 to 1986
      (pp. 38-63)

      This chapter describes in detail how the St. George Island population of northern fur seals changed in the first 14 years after the kill of males ended in 1972 (chapter 1). Our data cover the period 1974 through 1988. During these 14 years, the male population increased as predicted, but the female population unexpectedly continued to decline. These changes had profound effects on the size and shape of breeding aggregations on central breeding areas, the sex ratio, and the size and distribution of nonbreeding males on their traditional landing areas.

      Unlike the remainder of this book, this chapter focuses on...

    • CHAPTER 3 Temporal Factors in Behavior
      (pp. 64-86)

      The previous chapter showed that some behavioral tendencies of individuals can be inferred from the behavior of the herd at large, specifically that groups reflect the spacing tendencies of their members. There are other cases in which the behavior of individuals cannot be predicted well from the behavior of the population. As this chapter will show, individuals are sometimes more narrow and specific in their behavior than is the population they form. The difference between individuals and the population becomes important whenever humans manipulate seal populations for some specific purpose.

      One way to differentiate between the behavior of the population...

  7. PART TWO: The Mating System

    • CHAPTER 4 Behavior of Adult Males
      (pp. 89-112)

      If male northern fur seals are to mate, they must defend space among females. A few mate at sea (Baker, 1989) or on landing areas but so infrequently that they make little genetic contribution to the population. Males are territorial. They use aggressive behavior and topography to delineate borders around nonoverlapping domains within which they have exclusive access to females (Steller, 1749; Veniaminov, 1839; Elliott, 1882; Jordan, 1898; Osgood et al., 1915; Bartholomew, 1953; Bartholomew and Hoel, 1953; Kenyon, 1960; Bychkov and Dorofeev, 1962; Peterson, 1965, 1968; Johnson, 1968; DeLong, 1982). Peterson (1965) described territoriality in northern fur seals in...

    • CHAPTER 5 Male-Female Associations
      (pp. 113-131)

      The document that established this study (Anon., 1973) questioned whether the population failed to recover from the herd reduction program because past kills had created a suboptimal sex ratio that affected the population through the ways males and females associate. Our study was not designed to answer this question directly because stopping the kill of males at St. George Island was expected to reduce the sex ratio, not increase it to its former level. However, the behavioral study could describe the determinants of male-female associations, and from that, judge whether a suboptimum sex ratio was likely to have developed in...

    • CHAPTER 6 Behavior of Adult Females
      (pp. 132-150)

      The previous chapter concluded that in the northern fur seal mating system, males are behaviorally dominant over females but have an ephemeral effect on the shore aggregation and virtually no effect on where, when, or how often females come ashore. The timing, location, and persistence of shore colonies depend on female, not male, preferences. Therefore, understanding this mating system largely depends on understanding the factors that shape the behavior of individual females. The present chapter examines these factors through long-term observation of individuals and experiments on captives.

      Predictability in the time and place of female gathering, the key factors upon...

  8. PART THREE: Processes Fundamental to the Mating System

    • CHAPTER 7 Site Fidelity and Philopatry
      (pp. 153-166)

      Site fidelity (repeated return to a non-natal site over years) and philopatry (returning to the natal site) are related phenomena. Together they explain the tendency of northern fur seals to persist on the same central breeding and landing areas for centuries, or conversely, to avoid colonizing new islands (two in 200 years; chapter 1). They also explain the resilience these areas have to human disturbances. Both sexes show site fidelity and philopatry, but the latter has not been documented well in either sex because it requires following animals from birth to adulthood.

      Persistence in land use can be seen on...

    • CHAPTER 8 Estrus and Estrous Behavior
      (pp. 167-197)

      The postpartum estrus is central to the northern fur seal’s mating system (Sadlier, 1969). Estrus directly affects the timing and synchrony of breeding and the efficiency of reproduction, and indirectly affects male reproductive strategies, sexual dimorphism, and the type and extent of polygyny that can exist. Estrus and embryonic diapause (Sandell, 1990) account for the annual timing of reproduction. The synchrony of estrus among females gives males a focus for mate competition (Bartholomew, 1970; Ralls, 1977; Stirling, 1983) and enables males to monopolize females, thereby contributing to the evolution of sexual dimorphism for size.

      Understanding estrus was very important to...

    • CHAPTER 9 Ontogeny of Male Territorial Behavior
      (pp. 198-216)

      Young male northern fur seals do not possess the behavioral or physiological traits that are required for prolonged territorial defense. Since male territoriality brings the sexes together for mating, the ontogenetic changes leading to territoriality carry a high selective premium. As young males grow and age, they change the ways they relate to space, their location, food, and each other. Learning these behavioral changes requires social contact and frequent practice. The ontogenetic changes in females are less profound than in males. Females join the breeding group at age 2 years, mate at 3–5 years, and thereafter maintain the same...

  9. PART FOUR: The Maternal Strategy

    • [PART FOUR Introduction]
      (pp. 217-218)

      The chapters in this section address four different aspects of the northern fur seal’s maternal strategy. A maternal strategy is a system for extracting energy from the environment and delivering it to the young as the basis of their growth and survival to nutritional independence. Maternal strategies were a concern in this project because of the possibility that past human disturbance had altered a vital element in the system that produces new recruits. We looked at maternal strategies from several viewpoints.

      Otariid maternal strategies are unusual among mammals in that they involve frequent shuttling between a highly productive marine area...

    • CHAPTER 10 Female Attendance Behavior
      (pp. 219-231)

      Female attendance behavior, alternating feeding trips to sea with suckling visits to shore, is one of the more flexible portions of otariid maternal strategy [1], The particular challenge of northern fur seal females is to wean pups by a given date, perhaps at a given mass, by varying the rate or amount of energy delivered consistent with local foraging opportunities. Attendance behavior is their main means of regulating energy delivery, as it is for otariids that are not time limited. Attendance is under the direct control of the foraging mother, whereas maintenance metabolism, energetic content of food, and other factors...

    • CHAPTER 11 Neonatal Growth and Behavior
      (pp. 232-242)

      One measurable result of the maternal strategy is the rate at which neonates grow. Until pups begin taking solid food, which occurs only after weaning in this species, growth is based entirely on the energy in the mother’s milk. Growth is the end product of the total energy gained by the mother from prey (including its abundance, size, and energy content), minus the metabolic costs of acquiring (diving effort) and delivering it (attendance behavior—transit to and from foraging areas), minus maternal and pup maintenance metabolism, minus the pup’s metabolic cost of fasting in its mother’s absence.

      Relative to birth...

    • CHAPTER 12 Female Foraging Behavior: Inter- and Intra-Annual Variation in Individuals
      (pp. 243-259)

      A major question when the St. George Island Program started was whether a changed relationship between the northern fur seal and its prey base caused the herd to fail to recover from the herd reduction program of 1956–68 (Anon., 1973). A pelagic research program was to have addressed this question, but it was terminated in 1975. As a substitute, we initiated the use of Time-Depth Recorders(TDRs)(Kooyman et al., 1976, 1983a) to measure maternal diving behavior, arguably the most important link between fur seals and marine food resources. These instruments could not reconstruct pre-1956 diving behavior, but they...

    • CHAPTER 13 Female Foraging Behavior: Effects of Continental Shelf Width
      (pp. 260-276)

      The final aspect of the northern fur seal maternal strategy considered here is the effect that the local foraging environment has on diving behavior (using diving as a proxy for foraging). Foraging behavior has been broadly compared across taxonomic lines (Costa 1991a,b, 1993), in different maternal strategies (Antonelis et al., 1990a), and in a search for global trends (Gentry et al., 1986a). In these comparisons, the taxonomic status of the seals, their diet and maternal strategies, climate, and other factors may differ. The recent trend is toward comparing the same species in different environmental settings (examples, Costa et al., 1989;...

  10. PART FIVE: Summary, Comparisons, and Conclusions

    • CHAPTER 14 Synthesis
      (pp. 279-302)

      This chapter summarizes the natural history, mating system, and maternal strategy of the northern fur seal from information presented in earlier chapters. It reviews the research project that produced this information and answers some of the key questions posed by the St. George Island program, largely from the behavioral standpoint. It also reviews some possible causes for the long-term decline in fur seal numbers that started the research project reported here (including some information that does not appear in earlier chapters). Finally, this chapter discusses some implications of the present work for those that manage fur seal populations.

      In this...

    • CHAPTER 15 Implications for Otariid Studies
      (pp. 303-314)

      A brief history of research on the eared seals may be useful to those who are unfamiliar with this field, and as a prelude to the following summary of the present work’s contributions to the field. Prior to 1953, researchers tended to make qualitative observations on single species (example; Hamilton, 1934, 1939). Many species were recovering from sealing; they were at low numbers and poorly known. Bartholomew (1953) introduced the use of marked animals to address particular questions and thus ushered in the modern era of research. Throughout the 1950s and 1960s, studies of increasing sophistication were conducted (examples: Rand,...

  11. Appendix: Scientific and Common Names of Species Referred to in This Work
    (pp. 315-316)
  12. Notes
    (pp. 317-352)
  13. Literature Cited
    (pp. 353-378)
  14. Author Index
    (pp. 379-382)
  15. Subject Index
    (pp. 383-392)