The mesopelagic zone of the northeast Pacific Ocean is an important foraging habitat for many predators, yet few studies have addressed the factors driving basin-scale predator distributions or inter-annual variability in foraging and breeding success. Understanding these processes is critical to reveal how conditions at sea cascade to population-level effects. To begin addressing these challenging questions, we collected diving, tracking, foraging success, and natality data for 297 adult female northern elephant seal migrations from 2004 to 2010. During the longer post-molting migration, individual energy gain rates were significant predictors of pregnancy. At sea, seals focused their foraging effort along a narrow band corresponding to the boundary between the sub-arctic and sub-tropical gyres. In contrast to shallow-diving predators, elephant seals target the gyre-gyre boundary throughout the year rather than follow the southward winter migration of surface features, such as the Transition Zone Chlorophyll Front. We also assessed the impact of added transit costs by studying seals at a colony near the southern extent of the species’ range, 1,150 km to the south. A much larger proportion of seals foraged locally, implying plasticity in foraging strategies and possibly prey type. While these findings are derived from a single species, the results may provide insight to the foraging patterns of many other meso-pelagic predators in the northeast Pacific Ocean.

Summary 1.To gain insight into the foraging behaviour of deep diving seals, we developed a long-term jaw-motion recorder, which successfully measured the feeding attempts of four post-breeding female northern elephant seals for 55-68 days during migration in the north-east Pacific Ocean. 2. Using the jaw-motion recorders in conjunction with satellite tracking data, we first reveal the three-dimensional fine-scale distribution of deep foraging activity in the north-east Pacific Ocean. 3. A large number of jaw-motion events (23 817-58 766 during 2925-4178 dives, per seal) were observed with diel patterns suggesting their dependency on small mesopelagic prey. Calculations using at-sea field metabolic-rate and the photographs concurrently obtained by the headmounted camera indicated feeding on small mesopelagic prey (10-20 g) including lantern fish (F. Myctophidae). 4. The foraging behaviour of the northern elephant seal contrasts with echolocating toothed whales, which make fewer feeding attempts, suggesting the whales forage more selectively. We hypothesize that the continuous diving mode exhibited by this seal could be attributed to their reliance on small prey and their less efficient 'passive sensors' for prey search, that is, their vision or whiskers to detect prey.

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Several taxa of animals fast completely from food and water during energy-intensive periods such as lactation, breeding, and development. In elephant seals, these behaviors are sustained by high adiposity, high rates of fat mobilization, and reduced oxidation of carbohydrates and proteins. Adiposity and the regulation of lipolysis directly affect lactation energetics, milk composition, and mating success. Long-term fasting induces changes in regulation of lipolysis and lipid metabolism that influence fatty acid (FA) availability and the onset of insulin resistance. Hypoinsulinemia and elevated circulating FAs are also associated with several unique features of carbohydrate metabolism, including elevated plasma glucose, gluconeogenesis, and Cori cycle activity as well as high rates of pyruvate and tricarboxylic acid cycling. Glucose-lactate pools and triacylglycerol-FA cycles may be linked via glyceroneogenesis and this may be an important pathway influencing both fat and carbohydrate metabolism. Together, these features allow a sustained, high intensity, fat-based metabolism without substantial accumulation of ketoacids.

SUMMARYNorthern elephant seals exhibit the rare strategy of fasting and lactating concomitantly. We investigated hormonal regulation of glucose clearance in northern elephant seals using glucose tolerance tests (GTT) performed early in lactation and again just prior to weaning. For comparison, identical measurements were made on separate females late in the molt fast. Serial blood samples were used to assess glucose clearance and hormone responses for 3 h post glucose injection. Plasma glucose remained elevated at the end of the sampling period in all groups. Glucose clearance rates were not significantly different among test groups. A significant insulin response was observed in early lactation, no significant response was observed late in lactation and an intermediate response was observed late in the molt fast. The insulin response to a glucose load decreased with adipose tissue proportions. Plasma glucagon decreased significantly following GTT in early and late lactation, although the magnitude of the depression was small in comparison to other species. Hypoinsulemia may be critical to facilitate net lipolysis late in lactation. Consistently low glucose clearance among test groups suggests insulin insensitivity within peripheral tissues. Glucagon suppression independent of insulin release suggests modification of the typical insulin-glucagon counter-regulation. These findings suggest that metabolic features of diabetic-like conditions may be adaptive in the context of long-term fasting.

In many species, empirical data suggest that temperatures less than 1 month before breeding strongly influence laying date, consistent with predictions that short lag times between cue and response are more reliable, decreasing the chance of mismatch with prey. Here we show in European starlings (Sturnus vulgaris) that mid-winter temperature ca 50–90 days before laying (8 January–22 February) strongly (r2 = 0.89) predicts annual variation in laying date. Mid-winter temperature also correlated highly with relative clutch size: birds laid later, but laid larger clutches, in years when mid-winter temperatures were lower. Despite a high degree of breeding synchrony (mean laying date 5–13 April = ±4 days; 80% of nests laid within 4.8 days within year), European starlings show strong date-dependent variation in clutch size and productivity, but this appears to be mediated by a different temporal mechanism for integration of supplemental cue (temperature) information. We suggest the relationship between mid-winter temperature and breeding phenology might be indirect with both components correlating with a third factor: temperature-dependent development of the starling's insect (tipulid) prey. Mid-winter temperatures might set the trajectory of growth and final biomass of tipulid larvae, with this temperature cue providing starlings with information on breeding season prey availability (though exactly how remains unknown).

A fundamental feature of the life history of true seals, bears and baleen whales is lactation while fasting. This study examined the mobilization of fatty acids from blubber and their subsequent partitioning into maternal metabolism and milk production in northern elephant seals (Mirounga angustirostris). The fatty acid composition of blubber and milk was measured in both early and late lactation. Proportions of fatty acids in milk and blubber were found to display a high degree of similarity both early and late in lactation. Seals mobilized an enormous amount of lipid (~66 kg in 17 days), but thermoregulatory fatty acids, those that remain fluid at low temperatures, were relatively conserved in the outer blubber layer. Despite the stratification, the pattern of mobilization of specific fatty acids conforms to biochemical predictions. Long chain (>20C) monounsaturated fatty acids (MUFAs) were the least mobilized from blubber and the only class of fatty acids that showed a proportional increase in milk in late lactation. Polyunsaturated fatty acids (PUFAs) and saturated fatty acids (SFAs) were more mobilized from the blubber, but neither proportion increased in milk at late lactation. These data suggest that of the long chain MUFA mobilized, the majority is directed to milk synthesis. The mother may preferentially use PUFA and SFA for her own metabolism, decreasing the availability for deposition into milk. The potential impacts of milk fatty acid delivery on pup diving development and thermoregulation are exciting avenues for exploration.

Costs of reproduction are an integral and long-standing component of life-history theory, but we still know relatively little about the specific physiological mechanisms underlying these trade-offs. We experimentally manipulated workload during parental care in female European starlings (Sturnus vulgaris) using attachment of radios and/or wing clipping and assessed measures of workload, current breeding productivity, future fecundity, and survival (local return rate) in relation to treatment. Females with wing clipping and radio attachment paid a clear cost of reproduction compared with all other treatment groups: they had lower future fecundity and lower return rates despite having lower current breeding productivity. We then measured 13 physiological traits, including measures of aerobic/metabolic capacity, oxidative stress and muscle damage, intermediary metabolism and energy supply, and immune function. Our results show that the cost of reproduction in females with wing clipping and radio attachment was associated with lower oxygen-carrying capacity (lower hematocrit and hemoglobin levels), lower energy reserves (plasma nonesterified fatty acid and triglyceride levels), decreased immune function (lower haptoglobin levels), and elevated levels of oxidative stress (higher levels of dROMs [reactive oxygen metabolites] and lower levels of the endogenous antioxidant uric acid). Our study provides evidence that costs of reproduction involve a widespread decline in physiological function across multiple physiological systems consistent with long-standing ideas of cumulative "wear and tear" and allostatic load.