Elephant Seals: Population Ecology, Behavior and Physiology.
This book reports the results of a conference on elephant seals held in Santa Cruz, California on 20-21 May, 1991. The conveners of the conference were able to bring together the principal elephant seal researchers from around the world. In fact, the degree to which the key researchers were involved with this project as authors or co-authors no doubt made it difficult for the Ecology editor for book reviews to find a suitable reviewer. Be that as it may, I found the volume extremely enjoyable and informative.
The book is divided into four parts and included 22 separate chapters by 40 authors. The primary focus of the volume was directed at three questions: 1) why are some of the populations of the southern species declining, while the abundance of the northern species is increasing, 2) how does reproduction and survival vary among colonies within a species and between species, and 3) how does the diving behavior, feeding ecology, and physiology of the two species compare? The overall intent was to provide a detailed summary of current knowledge of certain aspects of elephant seal life. Here the reader (or anyone interested in purchasing the book) needs to interpret "certain aspects" to mean population and feeding ecology, diving behavior, and physiology and "current" to mean up until 1990 or 1991. The reader seriously interested in comparative behavior, morphology, or anatomy will be disappointed. The reader interested in what has transpired over the last four years will have to look for it in the published literature.
My favorite chapter in the section regarding population ecology was Chapter 4 (Hindell et al.) as it directly addressed the possible causes of the decline of the Kerguelen stock at least until the mid-1980s and the current decline of the Macquarie Island stock. As has been noted for observed declines in abundance of northern fur seals in the 1970s and the current decline of Steller sea lions, "whatever factor is causing the decline appears to be operating on the younger age classes, possibly before sexual differences in foraging patterns develop." Further, the authors considered evidence regarding causative factors for the observed population dynamics beyond the narrow confines of single-species models by incorporating hypotheses related to competition and large-scale changes in the physical and biological environment. Finally, the author's recommendations regarding future research will be extremely relevant to graduate students interested in the population dynamics of long-lived vertebrates or the incorporation of environmental data into hypotheses regarding factors that affect the distribution and abundance of marine vertebrates.
Regarding the section on behavior and life history, I found Chapter 10 by Deutsch et al. to be the most enjoyable. The objective of the research was to determine how reproductive effort varies with age and sex in the northern species. The authors noted that mating effort in male northern elephant seals approaches that of parental effort in females. Comparing these results with data from four other phocid species, they concluded that this was only true for the most polygynous seal species (i.e., the two species of elephant seals). Finally, the authors' attempts to explain the pattern of constant reproductive effort with age for parous females and males older than five years were not only very interesting, but also cried out for further experimentation.
In addition, Chapter 6 is a fascinating synthesis of biological processes and how they impact the conservation of this species. Boyd et al. presented a lucid integration of recent information on southern elephant seal 1) population size and structure, 2) energetic requirements contrasted by age and sex, 3) diet, and 4) potential interactions with commerical fisheries. The authors derived a convincing range of estimates of the total biomass of squid and fish consumed annually by the South Georgia population of southern elephant seals. In addition, they estimated that the average capture rate per dive of fish or muscular squid by male elephant seals was 0.26 kg and for females was 0.15 kg. They further speculated that, because males demand a greater proportion of the total resources than females and likely require richer foraging grounds than females, males may be more responsive to changes in environmental conditions.
The final 11 chapters on diving and foraging (Part III) and physiological ecology (Part IV) summarize recent reports regarding 1) foraging behavior, 2) diving physiology, 3) endocrine changes in newborn seals, and 4) fasting physiology. The efforts of all the authors to synthesize this information regarding species, age, reproductive status, and gender makes these sections valuable additions to the volume. Not to be overlooked is Chapter 12 by Hill. In this chapter, Hill summarizes a technique for geolocation, which requires only an accurate determination of dawn and dusk on a daily basis.
However, despite all of the fascinating detail in the book, there were a few disappointments. For one, the number of references to unpublished data in several of the chapters was unwarranted and leaves the reader wondering why these data weren't ever published (e.g., one chapter has 14 such references). Also, I expected to find a discussion about future interactions between northern elephant seals and humans or perhaps even a few predictions about when density dependent growth will first be detected. However, such topics seem to have been carefully avoided. Finally, there were altogether too many statements regarding specific differences in a parameter value (e.g., see section on age at first breeding in Chapter 8), where that difference was not statistically supported. This approach creates two problems: 1) some of these authors (or other researchers) will cite these conclusions as being from the "peer-reviewed" literature, which will confound the evaluation of future literature (perhaps we can all agree not to cite as fact any statement that is not properly substantiated) and 2) the "naive" reader will likely be far too accepting of several conclusions that a more judicious reader would reject or at least question.
In spite of these problems, this volume is a valuable contribution to the literature regarding the ecology, physiology, and behavior of two closely related species of vertebrates. The editors and authors have made a serious (and successful) effort to contrast the results of research on elephant seals with other phocids, and where appropriate, other vertebrates. For while elephant seals are interesting in their own right, their value to the fields of ecology, physiology, and behavior is related to the number of hypotheses that can be tested. Based on the information presented in this volume, elephant seals have been and will continue to be a key species in attempts to synthesize theory related to the ecology and evolution of long-lived vertebrates.
DOUGLAS P. DEMASTER NATIONAL MARINE MAMMAL LABORATORY Alaska Fisheries Science Center, NMFS 7600 Sand Point Way, NE Seattle, Washington 98115