title: Summing the strokes: energy economy in northern elephant seals during large-scale foraging migrations.
abstract: BACKGROUND
  The energy requirements of free-ranging marine mammals are challenging to measure due to cryptic and far-ranging feeding habits, but are important to quantify given the potential impacts of high-level predators on ecosystems. Given their large body size and carnivorous lifestyle, we would predict that northern elephant seals (Mirounga angustirostris) have elevated field metabolic rates (FMRs) that require high prey intake rates, especially during pregnancy. Disturbance associated with climate change or human activity is predicted to further elevate energy requirements due to an increase in locomotor costs required to accommodate a reduction in prey or time available to forage. In this study, we determined the FMRs, total energy requirements, and energy budgets of adult, female northern elephant seals. We also examined the impact of increased locomotor costs on foraging success in this species.
  
  
  RESULTS
  Body size, time spent at sea and reproductive status strongly influenced FMR. During the short foraging migration, FMR averaged 90.1 (SE = 1.7) kJ kg(-1)d(-1) - only 36 % greater than predicted basal metabolic rate. During the long migration, when seals were pregnant, FMRs averaged 69.4 (±3.0) kJ kg(-1)d(-1) - values approaching those predicted to be necessary to support basal metabolism in mammals of this size. Low FMRs in pregnant seals were driven by hypometabolism coupled with a positive feedback loop between improving body condition and reduced flipper stroking frequency. In contrast, three additional seals carrying large, non-streamlined instrumentation saw a four-fold increase in energy partitioned toward locomotion, resulting in elevated FMRs and only half the mass gain of normally-swimming study animals.
  
  
  CONCLUSIONS
  These results highlight the importance of keeping locomotion costs low for successful foraging in this species. In preparation for lactation and two fasting periods with high demands on energy reserves, migrating elephant seals utilize an economical foraging strategy whereby energy savings from reduced locomotion costs are shuttled towards somatic growth and fetal gestation. Remarkably, the energy requirements of this species, particularly during pregnancy, are 70-80 % lower than expected for mammalian carnivores, approaching or even falling below values predicted to be necessary to support basal metabolism in mammals of this size.
authors.affiliation: University of California Center for Ocean Health/Long Marine Lab, 100 Shaffer Rd., Santa Cruz, CA 95060 USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Maresh
authors.affiliation: Department of Polar Science, Graduate University for Advanced Studies, Midoricho Tachikawa, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Adachi
authors.affiliation: Department of Polar Science, Graduate University for Advanced Studies, Midoricho Tachikawa, Japan ; National Institute of Polar Research, Midoricho Tachikawa, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Takahashi
authors.affiliation: National Institute of Polar Research, Midoricho Tachikawa, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Naito
authors.affiliation: Department of Biology, Sonoma State University, Rohnert Park, USA.
authors.forename: D E
authors.initials: DE
authors.lastname: Crocker
authors.affiliation: Department of Fisheries & Wildlife, Marine Mammal Institute, Oregon State University, Newport, USA.
authors.forename: M
authors.initials: M
authors.lastname: Horning
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, USA.
authors.forename: T M
authors.initials: TM
authors.lastname: Williams
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, USA.
authors.forename: D P
authors.initials: DP
authors.lastname: Costa
keywords: Accelerometer; Aerobic dive limit; Body size; Disturbance; Field metabolic rate; Foraging; Hypometabolism; Locomotion; Pregnancy