The antimicrobial peptides magainin 2 and PGLa, discovered in the skin of the African clawed frog, Xenopus laevis, exhibit marked synergism [Westerhoff, H. V., Zasloff, M., Rosner, J. L., Hendler, R. W., de Waal, A., Vaz Gomes, A., Jongsma, A. P. M., Riethorst, A., and Juretic, D., Eur. J. Biochem. 228, 257-264 (1995)], although the mechanism is not yet clear. They are believed to kill bacteria by permeabilizing membranes. In this study, we examined the interactions of these peptides in lipid bilayers. PGLa, like magainin 2, preferentially interacts with acidic lipids, forming an amphipathic helix. The peptide induces the release of a water-soluble dye, calcein, entrapped within liposomes. The coexistence of magainin 2 enhances membrane permeabilization, which is maximal at a 1:1 molar ratio. Fluorescence experiments using L18W-PGLa revealed that both peptides form a stoichiometric 1:1 complex in the membrane phase with an association free energy of -15 kJ/mol. Single amino acid mutations in magainin 2 significantly altered the synergistic activity, suggesting that precise molecular recognition is involved in complex formation. The complex as well as each component peptide form peptide-lipid supramolecular complex pores, which mediate the mutually coupled transbilayer transport of dye, lipid, and the peptide per se. The rate of pore formation rate is in the order complex >/= PGLa > magainin 2, whereas the pore lifetime is in the order magainin 2 > complex > PGLa. Therefore, the synergism is a consequence of the formation of a potent heterosupramolecular complex, which is characterized by fast pore formation and moderate pore stability.

SUMMARYAquatic animals use a variety of strategies to reduce the energetic cost of locomotion. Efficient locomotion is particularly important for breath-holding divers because high levels of exercise may quickly deplete oxygen reserves,leading to the termination of a dive. We investigated the swimming behavior of eight adult Weddell seals, which are proficient divers, in McMurdo Sound,Antarctica. A newly developed data logger was attached to free-ranging females at their own breeding sites to record swimming speed, depth, two-dimensional accelerations (stroke frequency and body angle) and temperature. All seals conducted multiple deep dives (the mean dive depth range for each animal was 223.3±66.5–297.9±164.7 m). Prolonged gliding while descending was observed with thinner females (N=5 seals). But the fatter females (N=3 seals) exhibited only swim-and-glide swimming,characterized by intermittent stroking and fluctuating swim speed, throughout their descent and ascent. The body angles of four of the seals were restricted to less than 30° by the location of breathing holes in the ice and the slope of local bathymetric features. Of these four, the three fatter seals adopted the stroke-and-glide method while the other thinner seal descended with prolonged periods of gliding. Prolonged gliding seems to be a more efficient method for locomotion because the surface time between dives of gliding seals was significantly less than that of stroking animals, despite their same stroke frequencies.

It is obvious, at least qualitatively, that small animals move their locomotory apparatus faster than large animals: small insects move their wings invisibly fast, while large birds flap their wings slowly. However, quantitative observations have been difficult to obtain from free-ranging swimming animals. We surveyed the swimming behaviour of animals ranging from 0.5 kg seabirds to 30 000 kg sperm whales using animalborne accelerometers. Dominant stroke cycle frequencies of swimming specialist seabirds and marine mammals were proportional to mass K0.29 (R 2 Z0.99, nZ17 groups), while propulsive swimming speeds of 1-2 m s K1 were independent of body size. This scaling relationship, obtained from breath-hold divers expected to swim optimally to conserve oxygen, does not agree with recent theoretical predictions for optimal swimming. Seabirds that use their wings for both swimming and flying stroked at a lower frequency than other swimming specialists of the same size, suggesting a morphological trade-off with wing size and stroke frequency representing a compromise. In contrast, foot-propelled diving birds such as shags had similar stroke frequencies as other swimming specialists. These results suggest that muscle characteristics may constrain swimming during cruising travel, with convergence among diving specialists in the proportions and contraction rates of propulsive muscles.

During their long migrations through the Pacific, northern elephant seals, Mirounga angustirostris, never haul out on land and they rarely spend more than a few minutes at a time at the surface. They are almost constantly making repetitive, deep dives, raising the question of when do they rest? One type of dive, the drift dive, characterized by a time-depth profile with a phase of lower than average descent speed is believed to be a resting dive. To examine the behaviour of seals during drift dives, we measured body position and three-dimensional diving paths of six juvenile seals. We found that seals rolled over and sank on their backs during the drift phase, wobbling periodically so that they resembled a falling leaf. This enabled seals to drastically slow their descent rate, possibly so that negatively buoyant seals can rest without ending up in the abyss. This reduces the work required to return to the surface to breath, and allows them time to rest, process food or possibly sleep during the descent phase of these dives where they are probably less susceptible to predation.

Animal-borne electronic instruments (tags) are valuable tools for collecting information on cetacean physiology, behaviour and ecology, and forenhancing conservation and management policies for cetacean populations. Tags allow researchers to track the movement patterns, habitat use andother aspects of the behaviour of animals that are otherwise difficult to observe. They can even be used to monitor the physiology of a taggedanimal within its changing environment. Such tags are ideal for identifying and predicting responses to anthropogenic threats, thus facilitating thedevelopment of robust mitigation measures. With the increasing need for data best provided by tagging and the increasing availability of tags, suchresearch is becoming more common. Tagging can, however, pose risks to the health and welfare of cetaceans and to personnel involved in taggingoperations. Here we provide ‘best practice’ recommendations for cetacean tag design, deployment and follow-up assessment of tagged individuals,compiled by biologists and veterinarians with significant experience in cetacean tagging. This paper is intended to serve as a resource to assist tagusers, veterinarians, ethics committees and regulatory agency staff in the implementation of high standards of practice, and to promote the trainingof specialists in this area. Standardised terminology for describing tag design and illustrations of tag types and attachment sites are provided, alongwith protocols for tag testing and deployment (both remote and through capture-release), including training of operators. The recommendationsemphasise the importance of ensuring that tagging is ethically and scientifically justified for a particular project and that tagging only be used toaddress bona fide research or conservation questions that are best addressed with tagging, as supported by an exploration of alternative methods.Recommendations are provided for minimising effects on individual animals (e.g. through careful selection of the individual, tag design and implantsterilisation) and for improving knowledge of tagging effects on cetaceans through increased post-tagging monitoring.

Summary1. Breath-hold divers are widely assumed to descend and ascend at the speed that minimizes energy expenditure per distance travelled (the cost of transport (COT)) to maximize foraging duration at depth. However, measuring COT with captive animals is difficult, and empirical support for this hypothesis is sparse. 2. We examined the scaling relationship of swim speed in free-ranging diving birds, mammals and turtles (37 species; mass range, 0AE5-90 000 kg) with phylogenetically informed statistical methods and derived the theoretical prediction for the allometric exponent under the COT hypothesis by constructing a biomechanical model. 3. Swim speed significantly increased with mass, despite considerable variations around the scaling line. The allometric exponent (0AE09) was statistically consistent with the theoretical prediction (0AE05) of the COT hypothesis. 4. Our finding suggests a previously unrecognized advantage of size in divers: larger animals swim faster and thus could travel longer distance, search larger volume of water for prey and exploit a greater range of depths during a given dive duration. 5. Furthermore, as predicted from the model, endotherms (birds and mammals) swam faster than ectotherms (turtles) for their size, suggesting that metabolic power production limits swim speed. Among endotherms, birds swam faster than mammals, which cannot be explained by the model. Reynolds numbers of small birds (<2 kg) were close to the lower limit of turbulent flow ($3 · 10 5 ), and they swam fast possibly to avoid the increased drag associated with flow transition.

The antimicrobial peptides magainin 2 and PGLa, isolated from the skin of the African clawed frog Xenopus laevis, show marked synergism [Westerhoff, H. V., Zasloff, M., Rosner, J. L., Hendler, R. W., de Waal, A., Vaz Gomes, A., Jongsma, A. P. M., Riethorst, A., and Juretic, D. (1995) Eur. J. Biochem. 228, 257-264]. We suggested previously that these peptides form a potent heterodimer composed of either parallel or antiparallel helices in membranes [Matsuzaki, K., Mitani, Y., Akada, K., Murase, O., Yoneyama, S., Zasloff, M., and Miyajima, K. (1998) Biochemistry 37, 15144-15153]. To detect the putative heterodimer by chemical cross-linking, analogues of magainin 2 and PGLa with a Cys residue at either terminus were synthesized. These cross-linking experiments suggested that both peptides form a parallel heterodimer in membranes composed of phosphatidylglycerol/phosphatidylcholine but not in either buffer or a helix-promoting 2,2,2-trifluoroethanol/buffer mixture. The isolated parallel heterodimers exhibited an order of magnitude higher membrane permeabilization activity compared with the monomeric species, indicating that the observed synergism is due to heterodimer formation.

To better understand the foraging behavior of diving animals it is important to monitor aspects of the animal's environment, including prey distribution, which may influence their behavior. However, prior to recent technological advancements, monitoring the distribution of prey immediately surrounding a diving animal had been impossible. We attached newly developed Digital Still picture Loggers (DSLs) to 8 free-ranging female Weddell seals Leptonychotes weddellii at breeding colonies in McMurdo Sound, Antarctica, from November to December 2000. The DSLs provided depth data and several thousand underwater images taken from the seals' perspectives. Some of these images clearly showed scattered prey-like objects directly in front of the seal. Using image processing software, the images were converted to a 256 gray-scale and the prey-like objects were identified according to their brightness ratio and counted. Finally, a 'prey index' was calculated for each image and the vertical prey distribution along the seals' dive paths were compared with the seals' dive depth. Seals frequently dived to depths greater than 250 m where the prey index was both higher and exhibited a wider range. We concluded that the seals' dive depths might be affected by the vertical distribution of prey, which appeared to be aggregated in shoals at deeper depths.