title: Shifts in thermoregulatory strategy during ontogeny in harp seals (Pagophilus groenlandicus).
abstract: Heat balance can be difficult for young and/or small animals in polar regions because environmental conditions in combination with small body size or physiological immaturity can increase heat loss. We investigated how thermoregulatory patterns change with ontogeny in 5 age classes of harp seal (Pagophilus groenlandicus) from birth to post-molt to further understand the timing of thermoregulatory development in relation to their potential vulnerability to ongoing fluctuations in the extent and stability of Arctic pack ice. We measured changes in the amount, conductivity, and resistance of the seal pups׳ insulative layers (blubber and fur), the potential for endogenous heat-generation by shivering (muscle enzyme activity), and nonshivering thermogenesis (NST; brown adipose tissue (BAT) uncoupling protein 1 (UCP1) expression and mitochondrial density). There was no significant difference in blubber conductivity among age classes, though the amount of blubber insulation significantly increased from birth to weaning. Pelage conductivity was low (0.12±0.01Wm(-1)°C(-1)) except in 9-day old pups (0.40±0.08Wm(-1)°C(-1)); the significantly higher conductivity may signal the beginning of the molt, and this age group may be the most vulnerable to early water entry. Citrate synthase activity significantly increased (49.68±3.26 to 75.08±3.52μmolmin(-1)gwetweight(-1)) in the muscle; however it is unlikely that increasing a single enzyme greatly impacts heat generation. BAT of younger pups contained UCP1, though expression and mitochondrial density quickly declined, and the ability of pups to produce heat via NST was lost by weaning. While total thermal resistance did not differ, neonatal and early nursing animals gained the majority of their thermal resistance from lanugo (82.5±0.03%); however, lanugo is not insulative when wet, and NST may be important to maintain euthermia and dry the coat if early immersion in water occurs. By late nursing, blubber seems sufficient as insulation (75.87±0.01% of resistance after 4 weeks), but high conductivity of fur may be responsible for retention of UCP1 expression. Weaned animals rely on blubber insulation, and no longer need NST, as wetted fur is no longer a threat to euthermia.
authors.affiliation: School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, 905 North Koyukuk Drive, Fairbanks, AK 99775-7220, USA; Department of Biology, University of Alaska Anchorage, 3101 Science Circle, Anchorage, AK 99508, USA. Electronic address: lepearson@alaska.edu.
authors.forename: Linnea E
authors.initials: LE
authors.lastname: Pearson
authors.affiliation: Department of Biology, Adelphi University, 1 South Avenue, Garden City, NY 11530, USA. Electronic address: hliwanag@adelphi.edu.
authors.forename: Heather E M
authors.initials: HE
authors.lastname: Liwanag
authors.affiliation: Department of Fisheries and Oceans Canada, Maurice Lamontagne Institute, 850 route de la Mer, Mont-Joli, Quebec, Canada H5H 3Z4. Electronic address: Mike.Hammill@dfo-mpo.gc.ca.
authors.forename: Mike O
authors.initials: MO
authors.lastname: Hammill
authors.affiliation: Department of Biology, University of Alaska Anchorage, 3101 Science Circle, Anchorage, AK 99508, USA. Electronic address: jmburns@uaa.alaska.edu.
authors.forename: Jennifer M
authors.initials: JM
authors.lastname: Burns
mesh_terms: D002001:Adipose Tissue, Brown / Q000254:growth & development / Q000378:metabolism / Q000502:physiology; D000818:Animals; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism; D046048:Seals, Earless / Q000254:growth & development / Q000502:physiology*; D022722:Thermogenesis; D000071256:Uncoupling Protein 1
chemical_list: D007473:Ion Channels; D024101:Mitochondrial Proteins; D000071256:Uncoupling Protein 1
keywords: Blubber; Brown adipose tissue; Lanugo; Phocid; Thermal conductivity; Thermogenesis