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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.
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: | 9781400864720 |
|---|---|
| Publisher: | Princeton University Press |
| Publication date: | 07/14/2014 |
| Series: | Princeton Legacy Library , #382 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 410 |
| File size: | 27 MB |
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Read an Excerpt
Behavior and Ecology of the Northern fur Seal
By Roger L. Gentry
PRINCETON UNIVERSITY PRESS
Copyright © 1998 Princeton University PressAll rights reserved.
ISBN: 978-0-691-03345-7
CHAPTER 1
Introduction
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 period, the sale of northern fur seal pelts alone repaid the cost of the Alaska purchase.
Because of public interest, support for northern fur seal science has also been unusually good. The species was thoroughly described before 1900 (Steller, 1749; Veniaminov, 1839; Bryant, 1870–71; Scammon, 1874; Allen, 1880; Elliott, 1882; Nutting, 1891; Stejneger, 1896; Jordan, 1898; Townsend, 1899). In 1911, research became a treaty obligation of four nations, organized under an international commission that published extensive comparative data (Lander, 1980a,b). For many years, fur seal research was a line item in the U.S. federal budget. The population database that resulted is the longest and most diverse for any species of marine mammal.
A detailed analysis of the behavior of the northern fur seal is long overdue. This species has had a disproportionate and sometimes inappropriate influence on pinniped research and management. Mainly because of the extensive data on it, the northern fur seal is often used as a representative otariid in compendia of mammalian biology, even though in many ways it is not a typical otariid at all. Similarly, assumptions that are used to model the northern fur seal's population dynamics have been incorrectly applied to other species (York, 1987a). I hope that the details of its behavior will help specialists decide when the use of northern fur seal data is appropriate and when it is not.
This book is intended for general readers and for specialists alike. It discusses general trends and results in the text, and considers the finer details that will be of interest to specialists in the section of notes at the back of the book, referenced in the text by numbers in brackets [ ]. This work is not intended as a review of all northern fur seal literature, most of which relates either to the kill for pelts or to population dynamics, both of which are outside the scope of this work.
In this book I seek to accomplish two goals. The first is to answer specific questions about how northern fur seal behavior changes with population size and sex ratio. Toward that end, in this chapter I will introduce the species, its history of contact with humans, and the problem that led to the creation of the present study. I will outline the study questions and the research program that was put into place to answer them. My second goal is to apply the general principles and approaches of this work to the otariids as a whole.
This chapter introduces the pinnipeds as a group and the otariid family in particular. The relevance of northern fur seals to studies on other otariids will be taken up again in chapter 15, after the details of the northern fur seal work have been presented and synthesized. By describing this species in detail, this book contributes to ongoing attempts to compare the world's otariids (Nutting, 1891; Sivertsen, 1954; Gentry et al., 1986a; Croxall and Gentry, 1987; Boness and Majluf, in prep.).
CHARACTERISTICS OF THE PINNIPEDIA
Antiquity and Taxonomy
The Pinnipedia are amphibious mammals with feet modified as paddles (literally, wing-footed mammals). The suborder includes the true seals (family Phocidae), eared seals (family Otariidae), and walrus (family Odobenidae). The true seals swim with undulating motions of the rear flippers driven by their back muscles. They move caterpillar-like on land on their bellies, and have no external ear pinna. The otariids swim with their fore-flippers, using large pectoral muscles. They walk or run on all fours. They can outrun a human on slippery rocks, and can climb nearly vertical cliffs. The otariids are named for their external ear pinna, a short, vestigial, cone-shaped appendage. The walrus has some characteristics of both phocids (swims with rear flippers and lacks an external ear pinna) and otariids (walks on all fours). All pinnipeds bear a single, precocial young on land or ice, and forage at sea (or in some lakes). All pinnipeds, except some of the otariids that have underfur, use blubber as the main means of thermoregulation. On average, pinnipeds are larger than other mammals (range 50–2,000 kg).
Pinnipeds are derived from ursoid (bearlike) stock. About 23 million years ago, the ancestor of all modern pinnipeds was still partly terrestrial, and swam using all four limbs. It had some of the characteristics of all modern pinnipeds, including the walrus.
Amphibious Traits
Pinnipeds are mostly aquatic mammals with some specializations for a terrestrial life, not the reverse. A single example from the visual system will show this difference. The pinniped eye is adapted to low light levels, consistent with feeding at night or at depth (Walls, 1942; Landau and Dawson, 1970). They also have evolved a fishlike eyeball and cornea, shaped to resist water flow (Jameson, 1971). In air, they overcome the strong astigmatism that the distorted cornea causes by closing the pupil to a pinpoint. This technique gives California sea lions similar visual acuity in air and water (Schusterman, 1972). However, it only partly compensates because when the pupil opens as light dims, more of the distorted cornea is exposed (Schusterman, 1972; Lavigne and Ronald, 1975). Thus, aquatic adaptations in vision seem primary, and mechanisms that compensate for them in air seem secondary.
The otariids retain more extensive ties with land than the other pinnipeds. The walrus mates at sea and may suckle at sea or on land (or ice). Most phocids also mate at sea, but all of them suckle on shore. By contrast, the otariids suckle and mate on land. At least four species can also mate at sea, but not exclusively and usually to escape high temperatures. Also, otariids hear better in air than phocids do (walrus hearing has not been carefully measured; Richardson, 1995).
Pinniped Mating Systems
The true and eared seals are similar in that females produce a single young and males play no parental role. They differ in whether females feed during lactation, whether mating occurs on land, in the extent of polygyny and sexual dimorphism, in the number of sites used for mating, in time to weaning, and in other factors. Several authors have postulated evolutionary routes by which these differences arose (Nutting, 1891; Bartholomew, 1970; Stirling, 1983; Boness, 1991). Athumbnail sketch of current thinking follows.
Like land mammals, otariid females feed during lactation. Otariid mothers transfer less energy than phocid mothers to their young daily, but they lactate longer and so deliver more total energy (Oftedal et al., 1987). Nevertheless, otariid pups grow slower than phocid pups because of the metabolic cost of fasting during their mothers' feeding absences. Thus, the otariid lactation strategy is energetically more expensive and less efficient than the phocid strategy (Oftedal et al., 1987). Because of their relatively high energy demands, otariids can rear young only on land sites that are near extremely productive marine areas. Few such sites exist. Therefore, many animals share them; all otariids breed in a few dense colonies unless their numbers are abnormally small. Apparently, concentrations of females have given a few males the opportunity to monopolize mates, with the result that both polygyny and sexual dimorphism for size (males 2–4 times larger than females) have evolved in all the otariids (Bartholomew, 1970).
Most phocid females suspend feeding while lactating. They forage widely before parturition, store energy as fat, and deliver it to the young until weaning without replenishing fat stores by feeding. Limited energy stores in phocid mothers force lactation to end at 4–28 days (Bowen et al., 1985), compared to 4–42 months in otariids (Gentry et al., 1986a; Oftedal et al., 1987). Because phocids are not dependent on concentrated marine production for feeding during lactation, they may mate in many dispersed locations (Stirling, 1983). Apparently, dispersed mating prevents males from monopolizing females. Therefore, most phocids are not nearly as polygynous or sexually dimorphic as the otariids (Stirling, 1983), although three are. Accounting for these exceptions among the phocids has been an ongoing effort in pinniped research.
Litter size in pinnipeds is one. Thermal stress on homeotherms in cold water favors large body size (de Vries and van Eerden, 1995). Therefore, pinnipeds have a single large young, rather than multiple smaller ones. Twins do occur, but because of energy limitations of the mother they are weaned when still too small to survive (Doidge, 1987).
CHARACTERISTICS OF THE OTARIIDAE (GRAY 1859)
Antiquity and Taxonomy
The oldest known otariid was present by 11–12 million years ago (Kellogg, 1925; Repenning and Tedford, 1977; Berta and Deméré, 1986), It was a fur seal that very closely resembled modern forms, but its dental anatomy shows it was not a direct ancestor (Repenning and Tedford, 1977). The sea lions are the youngest otariids, having arisen in the late Pliocene or more recently (Repenning and Tedford, 1977). The earliest fossils were found in California, suggesting that the otariids evolved around the North Pacific. They apparently spread into the Southern Hemisphere and diversified about 5 million years ago.
Two types of otariids, fur seals and sea lions, were once thought to be distinguishable from each other by the presence (fur seals) or absence of underfur for thermoregulation. This distinction becomes less clear as more is learned about their behavior and molecular biology, although the names are still in wide use. The fur seals tend to be smaller (maximum 250 kg) than the sea lions (maximum 1,000 kg). The scientific and common names of all the seals discussed in this book are given in the Appendix.
Trophic Relations
Otariids are generally high-level consumers, taking fish, cephalopods, and sometimes crustaceans (krill: Boyd et al., 1994). The current thinking is that the fur seals tend to be feeding generalists, taking mostly small, surface-schooling fishes over the continental shelf, or small or juvenile phases of mesopelagic fish and squid associated with the Deep Scattering Layer (DSL) beyond the shelf break (Croxall et al., 1985; Gentry et al., 1986a; Boness and Majluf, in prep.). Sea lions, on the other hand, tend to specialize in large or adult stages of higher-trophic-level prey species found on continental shelves. Sea lions also are known to feed on fur seals (Gentry and Johnson, 1981; Harcourt, 1992,1993). Both sea lions and fur seals may occasionally prey on marine birds and are themselves preyed upon by sharks and killer whales.
Worldwide Distribution
The otariids' feeding proclivities have allowed them to establish successful populations from about 60 north to about 65 south latitude. They may land on but never breed on ice like the walrus and some phocid seals do. Three species (northern and antarctic fur seals and Steller sea lion) are subpolar breeders. The two fur seals take shallow prey that is highly concentrated but seasonal. They migrate to the high-latitude breeding areas in spring, wean pups at 4 months of age, and migrate back to lower latitudes when winter comes. The Steller sea lion does not feed on the same prey species and migrates in only part of its range (Schusterman, 1981a).
Two species (Galapagos fur seal and Galapagos sea lion) live exclusively near the equator. They exploit prey in areas of local upwelling that are surrounded by vast expanses of unproductive warm water. Neither species migrates. The time to weaning differs by species, as discussed under "Maternal Strategies," below.
All other otariids live in temperate waters, mostly in the Southern Hemisphere. They exploit prey that are usually widespread and dispersed or are concentrated at seamounts, continental shelf breaks, convergences, or at other unique features. No temperate otariid is entirely migratory, but the males of one species (California sea lion) are an exception, Most wean by one year, with several important exceptions, as discussed under "Maternal Strategies," below.
Only three species (northern and Guadalupe fur seals and California sea lion) still inhabit the ancestral waters of the North Pacific Ocean. Otariids probably never invaded the North Atlantic Ocean.
The largest otariid populations (1–2 million animals or more) presently are the northern (Loughlin et al., 1994) and antarctic fur seals (Boyd, 1993) and the Cape fur seal (David, 1987a). Most populations of fur seals are increasing at moderate rates, except the northern fur seal, which is at a plateau. By contrast, most sea lion populations are stable at low levels or decreasing at various rates, except for the California sea lion, which is increasing (Merrick et al., 1996),
General Traits
The eared seals derive almost all their water from the food they consume. Seawater drinking does occur (Gentry, 1981a), but usually among territorial males at the beginning of the fast.
Otariids compare favorably with cats and the higher primates in cognition and learning ability. They can rapidly learn relatively complex tasks (Schusterman, 1966; Schusterman and Thomas, 1966), they have a good memory (Schusterman, 1981b), and show some higher-order abilities (Schusterman and Krieger, 1986; Schusterman and Kastak, 1993), including semantic comprehension (Schusterman et al., 1992). These skills may be associated with their need to find and exploit marine prey that change over time and space.
Unlike other mammals, growth may be indeterminate (Trites, 1991), at least in female fur seals. They stop growing only at old age, after the years of greatest fecundity (Lander, 1981–82; Perez and Mooney, 1985; York, 1987a), probably as a result of delayed epiphyseal closure (Versaggi, 1981). This growth is important because females of prime age (8–13 years) may have an important size advantage over younger females in social interactions, and perhaps can dive to deeper depths (chapter 12). Indeterminate growth has also been reported in the antarctic fur seal (Costa et al., 1988).
(Continues...)
Excerpted from Behavior and Ecology of the Northern fur Seal by Roger L. Gentry. Copyright © 1998 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents
Preface
Acknowledgments
List of Contributors
Pt. 1. Behavior at the Population Level
Ch. 1. Introduction
Ch. 2. Population Changes, 1974 to 1986
Ch. 3. Temporal Factors in Behavior
Pt. 2. The Mating System
Ch. 4. Behavior of Adult Males
Ch. 5. Male-Female Associations
Ch. 6. Behavior of Adult Females
Pt. 3. Processes Fundamental to the Mating System
Ch. 7. Site Fidelity and Philopatry
Ch. 8. Estrus and Estrous Behavior By Roger L. Gentry, John R. Holt, Carolyn B. Heath
Ch. 9. Ontogeny of Male Territorial Behavior
Pt. 4. The Maternal Strategy
Ch. 10. Female Attendance Behavior
Ch. 11. Neonatal Growth and Behavior By Roger L. Gentry, Michael E. Goebel, John Calambokidis
Ch. 12. Female Foraging Behavior: Inter- and Intra-Annual Variation in Individuals By Roger L. Gentry, Michael E. Goebel
Ch. 13. Female Foraging Behavior: Effects of Continental Shelf Width By Roger L. Gentry, Mark O. Pierson, Valery A. Vladimirov
Pt. 5. Summary, Comparisons, and Conclusions
Ch. 14. Synthesis
Ch. 15. Implications for Otariid Studies
Appendix. Scientific and Common Names of Species Referred to in This Work
Notes
Literature Cited
Author Index
Subject Index