Aquatic Lifestyle Research Articles

Cetacean brains: How aquatic are they?

The adaptation of cetaceans to a fully aquatic lifestyle represents one of the most dramatic transformations in mammalian evolutionary history. Two of the most salient features of modern cetaceans are their fully aquatic lifestyle and their large brains. This review article will offer an overview of comparative neuroanatomical research on aquatic mammals, including analyses of odontocete cetacean, sirenian, pinniped, and fossil archaeocete brains. In particular, the question of whether a relationship exists between being fully aquatic and having a large brain is addressed. It has been hypothesized that the large, well-developed cetacean brain is a direct product of adaptation to a fully aquatic lifestyle. The current consensus is that the paleontological evidence on brain size evolution in cetaceans is not consistent with this hypothesis. Cetacean brain enlargement took place millions of years after adaptation to a fully aquatic existence. Neuroanatomical comparisons with sirenians and pinnipeds provide no evidence for the idea that the odontocete's large brain, high encephalization level, and extreme neocortical gyrification is an adaptation to a fully aquatic lifestyle. Although echolocation has been suggested as a reason for the high encephalization level in odontocetes, it should be noted that not all aquatic mammals echolocate and echolocating terrestrial mammals (e.g., bats) are not particularly highly encephalized. Echolocation is not a requirement of a fully aquatic lifestyle and, thus, cannot be considered a sole effect of aquaticism on brain enlargement. These results indicate that the high encephalization level of odontocetes is likely related to their socially complex lifestyle patterns that transcend the influence of an aquatic environment.

Sink or swim? Bone density as a mechanism for buoyancy control in early cetaceans

Previous analyses have shown that secondarily aquatic tetrapods, including whales, exhibit osteological adaptations to life in water as part of their complex buoyancy control systems. These structural specializations of bone span hyperostosis through osteoporosis. The past 15 years of paleontological effort has provided an unprecedented opportunity to examine the osteological transformation of whales as they make their transition to an obligate aquatic lifestyle over a 10-million-year period. It is hypothesized that whales manifest their osteological specialization in the same manner as extant semiaquatic and fully aquatic mammals. This study presents and analysis of the microstructural features of bone in early and late archaic cetaceans, and in a comparative sample of modern terrestrial, semiaquatic, and aquatic mammals. Bone histology was examined from the ribs of 10 fossilized individuals representing five early cetacean families, including Pakicetidae, Ambulocetidae, Protocetidae, Remintonocetidae, and Basilosauridae. Comparisons were then made with rib histology from nine genera of extant mammals including: Odocoileus (deer), Bos (cow), Equus (horse), Canis (dog), Lutra (river otter), Enhydra (sea otter), Choeropsis (pygmy hippo), Trichechus (sea cow), and Delphinus (dolphin). Results show that the transition from terrestrial, to semiaquatic, to obligate aquatic locomotion in archaeocetes involved a radical shift in bone function achieved by means of profound changes at the microstructural level. A surprising finding was that microstructural change predates gross anatomical shift in archaeocetes associated with swimming. Histological analysis shows that high bone density is an aquatic specialization that provides static buoyancy control (ballast) for animals living in shallow water, while low bone density is associated with dynamic buoyancy control for animals living in deep water. Thus, there was a shift from the typical terrestrial form, to osteopetrosis and pachyosteosclerosis, and then to osteoporosis in the first quarter of cetacean evolutionary history.

A MICROANATOMICAL AND HISTOLOGICAL STUDY OF THE PAIRED FIN SKELETON OF THE DEVONIAN SARCOPTERYGIAN EUSTHENOPTERON FOORDI

Sections of fore- and hindlimbs of a Paleozoic sarcopterygian (Eusthenopteron foordi from the Devonian) possess a thin cortical compacta and an extensive and relatively loose medullary spongiosa. Most long bones have no free medullary cavity. The smallest bones appear to have a proportionately thicker cortical compacta (although the trend is not statistically significant) and a free medullary cavity. The morphological synapomorphies of panderichthyids and stegocephalians that could be interpreted as suggesting a life in shallow water and possibly occasional excursions on dry land are absent in E. foordi. Thus, recent data on sarcopterygian morphology are congruent with recent paleoecological interpretations that E. foordi lived in a marginal marine or estuarine environment and had an aquatic lifestyle.

Sampling Daphnia's expressed genes: preservation, expansion and invention of crustacean genes with reference to insect genomes

BackgroundFunctional and comparative studies of insect genomes have shed light on the complement of genes, which in part, account for shared morphologies, developmental programs and life-histories. Contrasting the gene inventories of insects to those of the nematodes provides insight into the genomic changes responsible for their diversification. However, nematodes have weak relationships to insects, as each belongs to separate animal phyla. A better outgroup to distinguish lineage specific novelties would include other members of Arthropoda. For example, crustaceans are close allies to the insects (together forming Pancrustacea) and their fascinating aquatic lifestyle provides an important comparison for understanding the genetic basis of adaptations to life on land versus life in water.ResultsThis study reports on the first characterization of cDNA libraries and sequences for the model crustacean Daphnia pulex. We analyzed 1,546 ESTs of which 1,414 represent approximately 787 nuclear genes, by measuring their sequence similarities with insect and nematode proteomes. The provisional annotation of genes is supported by expression data from microarray studies described in companion papers. Loci expected to be shared between crustaceans and insects because of their mutual biological features are identified, including genes for reproduction, regulation and cellular processes. We identify genes that are likely derived within Pancrustacea or lost within the nematodes. Moreover, lineage specific gene family expansions are identified, which suggest certain biological demands associated with their ecological setting. In particular, up to seven distinct ferritin loci are found in Daphnia compared to three in most insects. Finally, a substantial fraction of the sampled gene transcripts shares no sequence similarity with those from other arthropods. Genes functioning during development and reproduction are comparatively well conserved between crustaceans and insects. By contrast, genes that were responsive to environmental conditions (metal stress) and not sex-biased included the greatest proportion of genes with no matches to insect proteomes.ConclusionThis study along with associated microarray experiments are the initial steps in a coordinated effort by the Daphnia Genomics Consortium to build the necessary genomic platform needed to discover genes that account for the phenotypic diversity within the genus and to gain new insights into crustacean biology. This effort will soon include the first crustacean genome sequence.

Phylogenetic relationships of Luziola (Poaceae: Oryzeae) and related genera from aquatic habitats

Luziola is a small aquatic genus consisting of nine American species and belonging to Poaceae (subfamily Ehrhartoideae, tribe Oryzeae). We surveyed 160 morphological characters and performed phylogenetic analyses to determine relationships within the genus and between it and its closest relatives, and to deduce the evolution of a number of characters in relation to the aquatic habit. Our results confirm that Luziola is a monophyletic group and that Zizaniopsis is its sister group among taxa examined but revealed only a few clades within Luziola, the best supported of which consists of Luziola fragilis Swallen, Luziola fluitans (Michx.) Terrell & H. Rob., and Luziola peruviana J.F. Gmel. We used this phylogenetic evidence to reconstruct the evolution of several characters hypothesized to be correlated to an aquatic life style, including reduction or absence of glumes, papillae arranged in horizontal rows on the lemmas, dumb-bell shaped silica bodies, and a change from hermaphrodite to unisexual florets.

A walk around the pond: insects in and over the water

A water strider darts across a pond, its feet dimpling the surface tension; a giant water bug dives below, carrying his mate's eggs on his back; hidden among plant roots on the silty bottom, a dragonfly larva stalks unwary minnows. Barely skimming the surface, in the air above the pond, swarm mayflies with diaphanous wings. Take this walk around the pond with Gilbert Waldbauer and discover the most amazingly diverse inhabitants of the freshwater world. In his hallmark companionable style, Waldbauer introduces us to the aquatic insects that have colonised ponds, lakes, streams and rivers, especially those in North America. Along the way we learn about the diverse forms these arthropods take, as well as their remarkable modes of life - how they have radiated into every imaginable niche in the water environment, and how they cope with the challenges such an environment poses to respiration, vision, thermoregulation and reproduction. We encounter the caddis fly larva building its protective case and camouflaging it with stream detritus; green darner dragonflies mating midair in an acrobatic wheel formation; ants that have adapted to the tiny water environment within a pitcher plant; and insects whose adaptations to the aquatic lifestyle are furnishing biomaterials engineers with ideas for future applications in industry and consumer goods. While learning about the evolution, natural history and ecology of these insects, readers also discover more than a little about the scientists who study them.

Comparative Genomics and Experimental Characterization of N-Acetylglucosamine Utilization Pathway of Shewanella oneidensis

We used a comparative genomics approach implemented in the SEED annotation environment to reconstruct the chitin and GlcNAc utilization subsystem and regulatory network in most proteobacteria, including 11 species of Shewanella with completely sequenced genomes. Comparative analysis of candidate regulatory sites allowed us to characterize three different GlcNAc-specific regulons, NagC, NagR, and NagQ, in various proteobacteria and to tentatively assign a number of novel genes with specific functional roles, in particular new GlcNAc-related transport systems, to this subsystem. Genes SO3506 and SO3507, originally annotated as hypothetical in Shewanella oneidensis MR-1, were suggested to encode novel variants of GlcN-6-P deaminase and GlcNAc kinase, respectively. Reconstitution of the GlcNAc catabolic pathway in vitro using these purified recombinant proteins and GlcNAc-6-P deacetylase (SO3505) validated the entire pathway. Kinetic characterization of GlcN-6-P deaminase demonstrated that it is the subject of allosteric activation by GlcNAc-6-P. Consistent with genomic data, all tested Shewanella strains except S. frigidimarina, which lacked representative genes for the GlcNAc metabolism, were capable of utilizing GlcNAc as the sole source of carbon and energy. This study expands the range of carbon substrates utilized by Shewanella spp., unambiguously identifies several genes involved in chitin metabolism, and describes a novel variant of the classical three-step biochemical conversion of GlcNAc to fructose 6-phosphate first described in Escherichia coli.

From terrestrial to aquatic habitats and back again — molecular insights into the evolution and phylogeny of Hydrophiloidea (Coleoptera) using multigene analyses

The phylogenetic relationships within Hydrophiloidea have been a matter of controversial discussion for many years and the supposedly repeated changes between aquatic and terrestrial lifestyles are not well understood. In order to address these issues we used an extensive molecular data set comprising sequences from six nuclear and mitochondrial genes. The analyses accomplished with the entire data set resulted in largely congruent tree topologies concerning the main branches, independent from the analytical procedures. However, only Bayesian analyses yielded sufficient high posterior probabilities, whereas bootstrap support values for most nodes were generally low. Our results are only partially congruent with hypotheses based on morphological analyses. Spercheidae were placed as the sister group of the remaining hydrophiloid subgroups. Hydrophiloidea excluding Spercheidae split into two clades: the ‘helophorid lineage’ comprising the small groups Epimetopidae, Hydrochidae, Georissidae and Helophoridae, and the largest family, Hydrophilidae. Within Hydrophilidae, Hydrophilinae do not form a monophylum. The predominantly terrestrial Sphaeridiinae were placed as a subordinate clade within this subfamily. Furthermore, our data suggest a single origin of the aquatic lifestyle in Hydrophiloidea, with numerous secondary changes to terrestrial habits and tertiary changes to aquatic habitats within Sphaeridiinae.

Histology of selected tissues of the leopard seal and implications for functional adaptations to an aquatic lifestyle

The microscopic anatomy of the cardio-respiratory system, digestive system, kidney, lymphatic system and integument was investigated in the leopard seal, Hydrurga leptonyx, by examining histological sections of tissues collected from leopard seals in Antarctica and New South Wales, Australia. The majority of the tissues had similar histological features to those described in terrestrial mammals and other pinniped species, particularly phocid seals. Differences noted included readily identifiable Purkinje cells within the endocardium, muscular rather than cartilaginous reinforcement of the smaller airways, a single capillary layer within the alveolar septa, limited and variable keratinization of the oesophageal epithelium, few lymphoid follicles within the lamina propria of the gastrointestinal tract, and an absence of a sporta perimedullaris musculosa described in the kidney of cetaceans and some pinniped species. Adaptations of the lung, spleen and integument, similar to those described in other pinnipeds, including reinforcement of the pulmonary terminal airways, prominent pulmonary interlobular septa, ample smooth muscle in the capsule and trabeculae of the spleen, increased thickness of the epidermis, well-developed dermal sebaceous glands, and a thick blubber layer, appear to confer upon the leopard seal advantages related to its aquatic lifestyle.

Données microanatomiques sur la conquête de l'environnement terrestre par les vertébrés

Microanatomical data and the conquest of land by vertebrates. The compactness profile of femoral cross-sections and body size of 46 species of lissamphibians was studied to assess the relationship between lifestyle (aquatic, amphibious or terrestrial) and bone compactness. Two tests that incorporate phylogenetic information (permutational multiple linear regression incorporating phylogenetic distances and concentrated changes tests) show that the return to an aquatic lifestyle is associated with an increase in the compactness of the femur and an increase in body size. However, amphibious taxa cannot be distinguished from terrestrial ones on the only basis of size or compactness. Mathematical equations obtained from our data using logistic regression with phylogenetic weighting are used to infer the lifestyle of four early stegocephalians from the Permian and the Triassic. The results are generally congruent with prevailing palaeontological interpretations, which suggests that this method could be applied to infer the lifestyle of early taxa whose lifestyle is poorly understood. To cite this article: M. Laurin et al., C. R. Palevol 5 (2006) .

Morfologia de osteodermos de crocodilomorfos do Sítio Paleontológico de Peirópolis (Bacia Bauru, Cretáceo Superior)

Osteoderms are dermal bony plates situated over the epiderm and covered by queratin layer. Until now are recognized at the Paleontological Site of Peirópolis three distinct crocodylomorph species, the Peirosauridae Peirosaurus termini Price, 1955 and Uberabasuchus terrificus Carvalho; Ribeiro & Ávilla, 2004 and the Trematochampsidae Itasuchus jesuinoi Price, 1955. The peirosaurids have nostrils frontally positioned within the skull and are regarded as terrestrial crocodylomorphs. The trematochampsids differently from the peirosaurids have the nostrils positioned at the dorsal portion of the skull, similar to the extant crocodiles with aquatic habits. The osteoderms morphological analysis of P. tormini, U. terrificus and I. jesuinoi corroborate, with the cranial evidences, for the terrestrial or aquatic lifestyle of these animals.

The ancestor’s tale A pilgrimage to the dawn of evolution

The ancestor’s tale A pilgrimage to the dawn of evolution

The evolution of long bone microstructure and lifestyle in lissamphibians

The compactness profile of femoral cross-sections and body size of 105 specimens of 46 species of lissamphibians was studied to assess the effect of lifestyle (aquatic, amphibious, or ter- restrial). Several tests that incorporate phylogenetic information (permutational multiple linear re- gression incorporating phylogenetic distances, logistic regression using phylogenetic weighting, concentrated-changes tests) show that the return to a fully aquatic lifestyle is associated with an increase in the compactness of the femur and an increase in body size. However, amphibious taxa cannot be distinguished from terrestrial ones solely on the basis of size or compactness. Body size and compactness profile parameters of the femur exhibit a phylogenetic signal (i.e., closely related taxa tend to be more similar to each other than to distantly related taxa). Mathematical equations obtained from our data by using logistic regression with phylogenetic weighting are used to infer the lifestyle of four early stegocephalians. The results are generally congruent with prevailing paleontological interpretations, which suggests that this method could be applied to infer the lifestyle of early taxa whose lifestyle is poorly understood.

Comparative study of thoracic structures of adults of Hydrophiloidea and Histeroidea with phylogenetic implications (Coleoptera, Polyphaga)

External and internal structures of adults of Hydrophiloidea and Histeroidea were examined. Skeletal and muscular features of Helophorus aquaticus and Margarinotus brunneus are described in detail. Morphological data are presented as a list of characters and data matrix, and analysed together with other characters of adults, characters of larvae, and characters related to reproduction, habitats and feeding habits. The results of the analysis (characters unweighted) of the full dataset unambiguously support the monophyly of the following clades: [Scarabaeoidea (represented by a genus of Lucanidae and a genus of Scarabaeidae)+Hydrophiloidea+Histeroidea], [Hydrophiloidea+Histeroidea], Histeroidea, [Sphaeritidae+Histeridae], Hydrophiloidea, Hydrophiloidea (excluding Helophoridae), Hydrophiloidea (excluding Helophoridae and Hydrochidae), [Epimetopidae+Georissidae], and [Spercheidae+Hydrophilidae]. The monophyly of all histeroid and hydrophiloid families and of Hydrophilidae (represented by hydrophilines and sphaeridiines) excluding Berosus is also supported. The placement of Scarabaeoidea is in contrast to a taxonomic treatment as a lineage with the same rank as Staphyliniformia. Hydraenidae are not closely related to Hydrophiloidea. The clade comprising Hydrophiloidea and Histeroidea is well supported, but mainly by larval features correlated with predacious habits. The position of Spercheidae implies that a considerable number of seemingly plesiomorphic features of the head are due to reversal and specialized feeding habits (filter feeding). Histeridae show a highly derived pattern of thoracic features with unusual muscular modifications, a long horizontal, dorsal part of the mesopleuron, widely separated metacoxae and a strongly simplified metafurca. Hydrophiloidea are well supported by character transformations of the thorax and other body parts, which are probably related to the invasion of the aquatic habitat in the adult stage, e.g. surface modifications related to the ventral plastron. Georissidae+Epimetopidae are characterized by derived features, which may be the result of a secondarily terrestrial or semiterrestrial life style (partly reduced ventral pubescence), and by a weakly sclerotized mesonotum, mesofurca and metafurca. Hydrochidae, Georissidae and Epimetopidae show a considerable number of autapomorphies, whereas Helophoridae are probably close to the groundplan of Hydrophiloidea. The adaptations to an aquatic life style and the specific habits are very different in Hyrophiloidea and the aquatic groups of Adephaga.

Paleobiology: Fossils' ear design hints at aquatic lifestyle

Science NewsVolume 164, Issue 11 p. 173-173 Of Note Paleobiology: Fossils' ear design hints at aquatic lifestyle First published: 02 July 2009 https://doi.org/10.1002/scin.5591641113AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume164, Issue1113 September 2003Pages 173-173 RelatedInformation

Phylogeny of Hydradephagan Water Beetles Inferred from 18S rRNA Sequences

Several families in the beetle suborder Adephaga have an aquatic life style and are commonly grouped in the “Hydradephaga,” but their monophyly is contentious and relationships between and within these families are poorly understood. Here we present full-length 18S rRNA sequence for 84 species of Hydradephaga, including representatives of most major groups down to the tribal level, and a total of 68 species of the largest family, Dytiscidae. Using a direct optimization method for the alignment of length-variable regions, the preferred tree topology was obtained when the cost of gaps and the cost of nucleotide changes were equal, and three hypervariable regions of 18S rRNA were downweighted by a factor of five. Confirming recent molecular studies, the Hydradephaga were found to be monophyletic, indicating a single colonization of the aquatic medium. The most basal group within Hydradephaga is Gyrinidae, followed in a comb-like arrangement by families Haliplidae, Noteridae, Amphizoidae, and Hygrobiidae plus Dytiscidae. Under most alignment parameters, Hygrobiidae is placed amid Dytiscidae in an unstable position, suggesting a possible data artifact. Basal relationships within Dytiscidae are not well established, nor is the monophyly of subfamilies Hydroporinae and Colymbetinae. In contrast, relationships at the genus level appear generally well supported. Despite the great differences in the rates of change and the significant incongruence of the phylogenetic signal in conserved vs hypervariable regions of the 18S rRNA gene, both contribute to establish relationships at all taxonomic levels.

A killer appetite: metabolic consequences of carnivory in marine mammals

Among terrestrial mammals, the morphology of the gastrointestinal tract reflects the metabolic demands of the animal and individual requirements for processing, distributing, and absorbing nutrients. To determine if gastrointestinal tract morphology is similarly correlated with metabolic requirements in marine mammals, we examined the relationship between basal metabolic rate (BMR) and small intestinal length in pinnipeds and cetaceans. Oxygen consumption was measured for resting bottlenose dolphins and Weddell seals, and the results combined with data for four additional species of carnivorous marine mammal. Data for small intestinal length were obtained from previously published reports. Similar analyses were conducted for five species of carnivorous terrestrial mammal, for which BMR and intestinal length were known. The results indicate that the BMRs of Weddell seals and dolphins resting on the water surface are 1.6 and 2.3 times the predicted levels for similarly sized domestic terrestrial mammals, respectively. Small intestinal lengths for carnivorous marine mammals depend on body size and are comparatively longer than those of terrestrial carnivores. The relationship between basal metabolic rate (kcal day −1) and small intestinal length (m) for both marine and terrestrial carnivores was, BMR=142.5 intestinal length 1.20 ( r 2=0.83). We suggest that elevated metabolic rates among marine mammal carnivores are associated with comparatively large alimentary tracts that are presumably required for supporting the energetic demands of an aquatic lifestyle and for feeding on vertebrate and invertebrate prey.

ARE PINNIPEDS FUNCTIONALLY DIFFERENT FROM FISSIPED CARNIVORES? THE IMPORTANCE OF PHYLOGENETIC COMPARATIVE ANALYSES

It is widely assumed that adaptations to an aquatic lifestyle are so profound as to produce only obvious differences between pinnipeds and the remaining, largely terrestrial carnivore species (“fissipeds”). Thus, comparative studies of the order Carnivora routinely examine these groups independently. This approach is invalid for two reasons. First, fissipeds are a paraphyletic assemblage, which raises the general issue of when it is appropriate to ignore monophyly as a criterion for inclusion in comparative studies. Second, the claim that most functional characters (beyond a few undoubted characteristic features) are different in pinnipeds and fissipeds has never been quantitatively examined, nor with phylogenetic comparative methods. We test for possible differences between these two groups in relation to 20 morphological, life-history, physiological, and ecological variables. Comparisons employed the method of independent contrasts based on a complete and dated species-level phylogeny of the extant Carnivora. Pinnipeds differ from fissipeds only through evolutionary grade shifts in a limited number of life-history traits: litter weight (vs. gestation length), birth weight, and age of eyes opening (both vs. size). Otherwise, pinnipeds display the same rate of evolution as phylogenetically equivalent fissiped taxa for all variables. Overall functional differences between pinnipeds and fissipeds appear to have been overstated and may be no greater than those among major fissiped groups. Recognition of this fact should lead to a more complete understanding of carnivore biology as a whole through more unified comparative tests. Comparative studies that do not include monophyletic groups for phylogenetically based comparative tests should be reconsidered.Corresponding Editor: S. Edwards

The evolution of cost efficient swimming in marine mammals: limits to energetic optimization

Mammals re–entered the oceans less than 60 million years ago. The transition from a terrestrial to an aquatic lifestyle required extreme morphological and behavioural modifications concomitant with fundamentally different locomotor mechanisms for moving on land and through water. Energetic transport costs typically reflect such different locomotor modes, but can not be discerned from the fossil record. In this study the energetic challenges associated with changing from terrestrial to aquatic locomotion in primitive marine mammals are examined by comparing the transport, maintenance and locomotor costs of extant mammals varying in degree of aquatic specialization. The results indicate that running and swimming specialists have converged on an energetic optimum for locomotion. An allometric expression, COTTOT= 7.79 mass−0.29(r2= 0.83,n= 6 species), describes the total cost of transport in J kg−1m−1for swimming marine mammals ranging in size from 21 kg to 15,000 kg. This relation is indistinguishable from that describing total transport costs in running mammals. In contrast, the transitional lifestyle of semi–aquatic mammals, similar to that of ancestral marine mammals, incurs costs that are 2.4–5.1 times higher than locomotor specialists. These patterns suggest that primitive marine mammals confronted an energetic hurdle before returning to costs reminiscent of their terrestrial ancestry, and may have reached an evolutionary limit for energetic optimization during swimming.

Energy intake and utilisation by nursing bearded seal (Erignathus barbatus) pups from Svalbard, Norway.

In this study we measure energy intake via milk in nursing bearded seal (Erignathus barbatus) pups and determine how this energy is allocated into metabolism and storage of new tissues. This was accomplished using longitudinal mass gain records and the doubly labelled water technique on nursing pups in combination with cross-sectional data on changes in milk composition from bearded seal mothers. The pups (n = 3) were all less than a week old at the start of the experiments. Pups gained 3.3 +/- 0.4 kg.day-1 of which 50% was fat, 14% protein and 36% water. Average daily water influx for the pups was 69.5 +/- 9.0 ml.kg-1. day-1. Average CO2 production during the study period was 0.99 +/- 0.10 ml.g-1.h-1, which corresponds to a field metabolic rate of 642 +/- 67 kJ.kg-1. day-1, or 6.0 +/- 0.5 times the predicted basal metabolic rate according to Kleiber (1975). The pups drank an average of 7.6 +/- 0.5 kg of milk daily. This corresponds to a daily energy intake of 154 +/- 8 MJ, 47 +/- 14% of which was stored as new body tissue. Despite this high energy intake bearded seal pups do not get as fat as do other nursing phocids. This is in part due to their larger body size but also due to their very active aquatic lifestyle and the lower and more consistent fat content of the milk compared to other phocid species. Bearded seal mothers forage during lactation and may also be involved in teaching their pups to feed independently. All these data suggest that the lactation strategy of bearded seals differs from the phocid norm.