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Fur seals: maternal strategies on land and at sea
TREE vol. 1, no. 5, November
1986
-Monitoring MarineMammals Fur Seals:Maternal Strategieson Land and at See edited by Roger L. Gentry and Gerald L. Kooyman, Princeton University Press, 1986. f40 hbk, f74.50 pbk (xviii + 291 pages) /SBN 0 691 08399 1 The study of marine mammals has largely concentrated on animals ashore: on pinnipeds when they haul out on land or ice to breed or rest, and on whales lying dead on whaling station slipways or stranded on beaches. Much of what they do when at sea remains a mystery, and a frustrating one to biologists who are interested in the whole life history of their study animals. The development of electronic devices over the last ten years has opened up exciting opportunities for the remote recording of many aspects of the pelagic behaviour and physiology of marine mammals. This book marks a turning point in the study of diving behaviour. The title does not immediately reveal the pivotal element of the book. It is centred around the development of Time-Depth Recorders (TDRsl by Jim Billuns, one of two people to whom the book is dedicated. The desian and caoabilities of TDFis are descyibed, together with the method of analysing the traces obtained. In simple terms, a timing circuit and a pressure transducer (a bent tube which straightens under pressure) makes a continuous trace on film of the time and the depth of the animal to which it was attached by a harness for a period of about two weeks. Such devices have been used on five of the eight species of fur seals, and one species of the closely related group, the sealions. These animals were chosen because the females return to land frequently and predictably during an extended lactation period, allowing retrieval of the TDRs. The book consists essentially of a set of scientific papers which describe the diving behaviour of each species, interleaved with chapters on their attendance patterns derived from conventional observations on behaviour. There is one chapter on the energy costs of lactation and foraging in the northern fur seal. The papers are set in a context of life history strategies via an introduction and a final synthesis. The chapters on individual species
Antarctic
fur sesl, Arctocephslus
gazella. Photograph
by British Antarctic Survey.
have been well edited to achieve uniformity in presentation which readily allows comparison, but the amount of information available on diving behaviour varies greatly between species. Where there is marked individual variation in diving behaviour, as in the northern fur seal, a larger sample size than seven would have aided confidence in the potentially fascinating conclusion that some individuals specialize in shallow and others in deep diving. The data on the South American fur seal is derived from one animal studied during the most extreme El NiAo event for 100 years. The temperate species were represented by two South African fur seals. However, I am glad the authors have not waited for full records on all species. They present more than adequate data to show the usefulness of the TDR eauioment and to achieve the main aim’ - to explore how females of closely related species produce the maximum number of young in their lifetime under different environmental conditions. The key chapter is the final synthesis. Its authors identify two fixed features of fur seal reproduction (one young per birth and a weaning weight of about 40% of maternal weight), and the more flexible traits such as dive trip duration, time ashore and length of lactation, all of which occur in each species but vary according to the environmental setting. The authors write of options being available to the seals, and that sub-sets of options form strategies which they define as ‘adaptive re-
sponses shaped by natural selection over time’. Although the usual caveats are expressed, assuring us that seals are not consciously planning their next move, the use of the words ‘options’ and ‘strategies’ implies that choices are available - an implication that is not supported by much of the data presented. Individual seals respond to the prevailing conditions, and in most cases there seems to be little latitude in the responses that are possible for individuals of each species. The use of ‘strategy’ here has been extended in a way which does not aid interpretation. Indeed, the arguments attain a certain circularity when the authors state that sub-polar and tropical strategies exist and then ask how environmental factors affect these strategies. This criticism of the conceptual framework does not, however, detract from the excellence of the biology presented. The comparative approach across several closely related species is a fruitful one. It allows a closer analysis of the elements involved in the process of evolution, and it certainly points to the areas we know too little about, in this case the prey available to seals and comparative energetics. Marine science has much to gain from a wider application of the technology, ingenuity and good knowledge of natural history demonstrated by the authors of this book.
Sheila M. Anderson Sea Mammal ResearchUnit, do British Antarctic Survey, High Cross,MadingleyRoad,Cambridge CB3OET.UK
137
1986
-Monitoring MarineMammals Fur Seals:Maternal Strategieson Land and at See edited by Roger L. Gentry and Gerald L. Kooyman, Princeton University Press, 1986. f40 hbk, f74.50 pbk (xviii + 291 pages) /SBN 0 691 08399 1 The study of marine mammals has largely concentrated on animals ashore: on pinnipeds when they haul out on land or ice to breed or rest, and on whales lying dead on whaling station slipways or stranded on beaches. Much of what they do when at sea remains a mystery, and a frustrating one to biologists who are interested in the whole life history of their study animals. The development of electronic devices over the last ten years has opened up exciting opportunities for the remote recording of many aspects of the pelagic behaviour and physiology of marine mammals. This book marks a turning point in the study of diving behaviour. The title does not immediately reveal the pivotal element of the book. It is centred around the development of Time-Depth Recorders (TDRsl by Jim Billuns, one of two people to whom the book is dedicated. The desian and caoabilities of TDFis are descyibed, together with the method of analysing the traces obtained. In simple terms, a timing circuit and a pressure transducer (a bent tube which straightens under pressure) makes a continuous trace on film of the time and the depth of the animal to which it was attached by a harness for a period of about two weeks. Such devices have been used on five of the eight species of fur seals, and one species of the closely related group, the sealions. These animals were chosen because the females return to land frequently and predictably during an extended lactation period, allowing retrieval of the TDRs. The book consists essentially of a set of scientific papers which describe the diving behaviour of each species, interleaved with chapters on their attendance patterns derived from conventional observations on behaviour. There is one chapter on the energy costs of lactation and foraging in the northern fur seal. The papers are set in a context of life history strategies via an introduction and a final synthesis. The chapters on individual species
Antarctic
fur sesl, Arctocephslus
gazella. Photograph
by British Antarctic Survey.
have been well edited to achieve uniformity in presentation which readily allows comparison, but the amount of information available on diving behaviour varies greatly between species. Where there is marked individual variation in diving behaviour, as in the northern fur seal, a larger sample size than seven would have aided confidence in the potentially fascinating conclusion that some individuals specialize in shallow and others in deep diving. The data on the South American fur seal is derived from one animal studied during the most extreme El NiAo event for 100 years. The temperate species were represented by two South African fur seals. However, I am glad the authors have not waited for full records on all species. They present more than adequate data to show the usefulness of the TDR eauioment and to achieve the main aim’ - to explore how females of closely related species produce the maximum number of young in their lifetime under different environmental conditions. The key chapter is the final synthesis. Its authors identify two fixed features of fur seal reproduction (one young per birth and a weaning weight of about 40% of maternal weight), and the more flexible traits such as dive trip duration, time ashore and length of lactation, all of which occur in each species but vary according to the environmental setting. The authors write of options being available to the seals, and that sub-sets of options form strategies which they define as ‘adaptive re-
sponses shaped by natural selection over time’. Although the usual caveats are expressed, assuring us that seals are not consciously planning their next move, the use of the words ‘options’ and ‘strategies’ implies that choices are available - an implication that is not supported by much of the data presented. Individual seals respond to the prevailing conditions, and in most cases there seems to be little latitude in the responses that are possible for individuals of each species. The use of ‘strategy’ here has been extended in a way which does not aid interpretation. Indeed, the arguments attain a certain circularity when the authors state that sub-polar and tropical strategies exist and then ask how environmental factors affect these strategies. This criticism of the conceptual framework does not, however, detract from the excellence of the biology presented. The comparative approach across several closely related species is a fruitful one. It allows a closer analysis of the elements involved in the process of evolution, and it certainly points to the areas we know too little about, in this case the prey available to seals and comparative energetics. Marine science has much to gain from a wider application of the technology, ingenuity and good knowledge of natural history demonstrated by the authors of this book.
Sheila M. Anderson Sea Mammal ResearchUnit, do British Antarctic Survey, High Cross,MadingleyRoad,Cambridge CB3OET.UK
137