1Marine mammals are characterised as having physiological specializations that 2 maximize use of oxygen stores to prolong time spent under water. However, it has 3 been difficult to undertake controlled studies to determine the physiological 4 limitations and trade-offs that marine mammals face while diving in the wild under 5 varying environmental and nutritional conditions. For the past decade, Steller sea 6 lions (Eumetopias jubatus) trained to swim and dive in the open ocean away from 7 the physical confines of pools participated in studies that investigated the 8 interactions between diving behaviour, energetic costs, physiological constraints, 9and prey availability. Many of these studies measured the costs of diving to 10 understand how they vary with behaviour and environmental and physiological 11 conditions. Collectively, these studies show that the type of diving (dive bouts or 12 single dives), the level of underwater activity, the depth and duration of dives, and 13 the nutritional status and physical condition of the animal affect the cost of diving 14 and foraging. They show that dive depth, dive and surface duration, and the type of 15 dive result in physiological adjustments (heart rate, gas exchange) that may be 16 independent of energy expenditure. They also demonstrate that changes in prey 17 abundance and nutritional status causes sea lions to alter the balance between time 18 spent at the surface acquiring oxygen (and offloading CO2 and other metabolic by-19 products) and time spent at depth acquiring prey. These new insights into the 20 physiological basis of diving behaviour furthers understanding of the potential 21 scope for behavioural responses of marine mammals to environmental changes, the 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Diving metabolism of Steller sea lions 3 energetic consequences of these adjustments, and the consequences of approaching 23 physiological limits. 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Diving metabolism of Steller sea lions 4The need to study diving metabolism 25Marine mammals are well known for being able to remain submerged for extended 26 durations. Early studies of marine mammals (mainly phocid or "true" seals) 27 investigated the anatomical features by which they managed to do so. These include 28 adaptations for withstanding the intense pressures experienced at depth and 29 greater relative on-board oxygen stores than their terrestrial counterparts, which 30 allows them to remain active during submergence breath-holding. For example, 31 elevated oxygen storage is present in both circulating haemoglobin and the 32 myoglo...

The diving capacity of marine mammals is typically defined by the aerobic dive limit (ADL) which, in lieu of direct measurements, can be calculated (cADL) from total body oxygen stores (TBO) and diving metabolic rate (DMR). To estimate cADL, we measured blood oxygen stores, and combined this with diving oxygen consumption rates (VO2) recorded from 4 trained Steller sea lions diving in the open ocean to depths of 10 or 40 m. We also examined the effect of diving exercise on O2 stores by comparing blood O2 stores of our diving animals to non-diving individuals at an aquarium. Mass-specific blood volume of the non-diving individuals was higher in the winter than in summer, but there was no overall difference in blood O2 stores between the diving and non-diving groups. Estimated TBO (35.9 ml O2 kg(-1)) was slightly lower than previously reported for Steller sea lions and other Otariids. Calculated ADL was 3.0 min (based on an average DMR of 2.24 L O2 min(-1)) and was significantly shorter than the average 4.4 min dives our study animals performed when making single long dives-but was similar to the times recorded during diving bouts (a series of 4 dives followed by a recovery period on the surface), as well as the dive times of wild animals. Our study is the first to estimate cADL based on direct measures of VO2 and blood oxygen stores for an Otariid and indicates they have a much shorter ADL than previously thought.

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Ecological speciation is the evolution of reproductive isolation as a direct or indirect consequence of divergent natural selection.Reduced performance of hybrids in nature is thought to be an important process by which natural selection can favor the evolution of assortative mating and drive speciation. Benthic and limnetic sympatric species of threespine stickleback (Gasterosteus aculeatus) are adapted to alternative trophic niches (bottom browsing vs. open water planktivory, respectively) and reduced feeding performance of hybrids is thought to have contributed to the evolution of reproductive isolation. We tested this "hybriddisadvantage hypothesis" by inferring growth rates from otoliths sampled from wild, free-ranging benthic, limnetic, and hybrid sticklebacks in two lakes. There were significant differences in growth rate between lakes, life-history stages, and among years (maximum P = 0.02), as well as interactions between most factors, but not between hybrid and parental species sticklebacks in most comparisons. Our results provide little evidence of a growth disadvantage in hybrid sticklebacks when free-ranging in nature. Although trophic ecology per se may contribute less to ecological speciation than envisioned, it may act in concert with other aspects of stickleback biology, such as interactions with parasites, predators, competitors, and/or sexual selection, to present strong multifarious selection against hybrids.

Marine mammal foraging behaviour inherently depends on diving ability. Declining populations of Steller sea lions may be facing nutritional stress that could affect their diving ability through changes in body composition or metabolism. Our objective was to determine whether nutritional stress (restricted food intake resulting in a 10% decrease in body mass) altered the calculated aerobic dive limit (cADL) of four captive sea lions diving in the open ocean, and how this related to changes in observed dive behaviour. We measured diving metabolic rate (DMR), blood O 2 stores, body composition and dive behaviour prior to and while under nutritional restriction. We found that nutritionally stressed sea lions increased the duration of their single long dives, and the proportion of time they spent at the surface during a cycle of four dives. Nutritionally stressed sea lions lost both lipid and lean mass, resulting in potentially lower muscle O 2 stores. However, total body O 2 stores increased due to rises in blood O 2 stores associated with having higher blood volumes. Nutritionally stressed sea lions also had higher mass-specific metabolic rates. The greater rise in O 2 stores relative to the increase in mass-specific DMR resulted in the sea lions having a longer cADL when nutritionally stressed. We conclude that there was no negative effect of nutritional stress on the diving ability of sea lions. However, nutritional stress did lower foraging efficiency and require more foraging time to meet energy requirements due to increases in diving metabolic rates and surface recovery times.