Circadian coupling of mitochondria in a deep-diving mammal.

Regulation of mitochondrial oxidative phosphorylation is essential to match energy supply to changing cellular energy demands, and to cope with periods of hypoxia. Recent work implicates the circadian molecular clock in control of mitochondrial function and hypoxia sensing. Because diving mammals experience intermittent episodes of severe hypoxia, with diel patterning in dive depth and duration, it is interesting to consider circadian-mitochondrial interaction in this group. Here, we demonstrate that the hooded seal (Cystophora cristata), a deep-diving Arctic pinniped, shows strong daily patterning of diving behaviour in the wild. Cultures of hooded seal skin fibroblasts exhibit robust circadian oscillation of the core clock genes per2 and arntl. In liver tissue collected from captive hooded seals, expression of arntl was some 4-fold higher in the middle of the night than in the middle of the day. To explore the clock-mitochondria relationship, we measured the mitochondrial oxygen consumption in synchronized hooded seal skin fibroblasts and found a circadian variation in mitochondrial activity, with higher coupling efficiency of complex I coinciding with the trough of arntl expression. These results open the way for further studies of circadian-hypoxia interactions in pinnipeds during diving.

Structure-function relationships in the nasal cavity of Arctic and subtropical seals.

The heating and moistening of inhaled air, and the cooling and moisture removal from exhaled air, are crucial for the survival of animals under severe environmental conditions. Arctic mammals have evolved specific adaptive mechanisms to retain warmth and water and restrict heat loss during breathing. Here, the role of the porous turbinates of the nasal cavities of Arctic and subtropical seals is studied with this in mind. Mass and energy balance equations are used to compute the time-dependent temperature and water vapor profiles along the nasal passage. A quasi-1D model based on computed tomography images of seal nasal cavities is used in numerical simulations. Measured cross-sectional areas of the air channel and the perimeters of the computed tomography slices along the nasal cavities of the two seal species are used. The model includes coupled heat and vapor transfer at the air-mucus interface and heat transfer at the interfaces between the tissues and blood vessels. The model, which assumes constant blood flow to the nose, can be used to predict the temperature of the exhaled air as a function of ambient temperature. The energy dissipation (entropy production) in the nasal passages was used to measure the relative importance of structural parameters for heat and water recovery. We found that an increase in perimeter led to significant decreases in the total energy dissipation. This is explained by improved conditions for heat and water transfer with a larger complexity of turbinates. Owing to differences in their nasal cavity morphology, the Arctic seal is expected to be advantaged in these respects relative to the subtropical seal.

The roles of brain lipids and polar metabolites in the hypoxia tolerance of deep-diving pinnipeds.

Lipids make up more than half of the human brain's dry weight, yet the composition and function of the brain lipidome is not well characterized. Lipids not only provide the structural basis of cell membranes, but also take part in a wide variety of biochemical processes. In neurodegenerative diseases, lipids can facilitate neuroprotection and serve as diagnostic biomarkers. The study of organisms adapted to extreme environments may prove particularly valuable in understanding mechanisms that protect against stressful conditions and prevent neurodegeneration. The brain of the hooded seal (Cystophora cristata) exhibits a remarkable tolerance to low tissue oxygen levels (hypoxia). While neurons of most terrestrial mammals suffer irreversible damage after only short periods of hypoxia, in vitro experiments show that neurons of the hooded seal display prolonged functional integrity even in severe hypoxia. How the brain lipidome contributes to the hypoxia tolerance of marine mammals has been poorly studied. We performed an untargeted lipidomics analysis, which revealed that lipid species are significantly modulated in marine mammals compared with non-diving mammals. Increased levels of sphingomyelin species may have important implications for efficient signal transduction in the seal brain. Substrate assays also revealed elevated normoxic tissue levels of glucose and lactate, which suggests an enhanced glycolytic capacity. Additionally, concentrations of the neurotransmitters glutamate and glutamine were decreased, which may indicate reduced excitatory synaptic signaling in marine mammals. Analysis of hypoxia-exposed brain tissue suggests that these represent constitutive mechanisms rather than an induced response towards hypoxic conditions.

Thermal modeling of the respiratory turbinates in arctic and subtropical seals.

Mammals possess complex structures in their nasal cavities known as respiratory turbinate bones, which help the animal to conserve body heat and water during respiratory gas exchange. We considered the function of the maxilloturbinates of two species of seals, one arctic (Erignathus barbatus), one subtropical (Monachus monachus). By means of a thermo-hydrodynamic model that describes the heat and water exchange in the turbinate region we are able to reproduce the measured values of expired air temperatures in grey seals (Halichoerus grypus), a species for which experimental data are available. At the lowest environmental temperatures, however, this is only possible in the arctic seal, and only if we allow for the possibility of ice forming on the outermost turbinate region. At the same time the model predicts that for the arctic seals, the inhaled air is brought to deep body temperature and humidity conditions in passing the maxilloturbinates. The modeling shows that heat and water conservation go together in the sense that one effect implies the other, and that the conservation is most efficient and most flexible in the typical environment of both species. By controlling the blood flow through the turbinates the arctic seal is able to vary the heat and water conservation substantially at its average habitat temperatures, but not at temperatures around -40 °C. The subtropical species has simpler maxilloturbinates, and our model predicts that it is unable to bring inhaled air to deep body conditions, even in its natural environment, without some congestion of the vascular mucosa covering the maxilloturbinates. Physiological control of both blood flow rate and mucosal congestion is expected to have profound effects on the heat exchange function of the maxilloturbinates in seals.

Reduced immune responsiveness contributes to winter energy conservation in an Arctic bird.

Animals in seasonal environments must prudently manage energy expenditure to survive the winter. This may be achieved through reductions in the allocation of energy for various purposes (e.g. thermoregulation, locomotion, etc.). We studied whether such trade-offs also include suppression of the innate immune response, by subjecting captive male Svalbard ptarmigan (Lagopus muta hyperborea) to bacterial lipopolysaccharide (LPS) during exposure to either mild temperature (0°C) or cold snaps (acute exposure to -20°C), in constant winter darkness when birds were in energy-conserving mode, and in constant daylight in spring. The innate immune response was mostly unaffected by temperature. However, energy expenditure was below baseline when birds were immune challenged in winter, but significantly above baseline in spring. This suggests that the energetic component of the innate immune response was reduced in winter, possibly contributing to energy conservation. Immunological parameters decreased (agglutination, lysis, bacteriostatic capacity) or did not change (haptoglobin/PIT54) after the challenge, and behavioural modifications (anorexia, mass loss) were lengthy (9 days). While we did not study the mechanisms explaining these weak, or slow, responses, it is tempting to speculate they may reflect the consequences of having evolved in an environment where pathogen transmission rate is presumably low for most of the year. This is an important consideration if climate change and increased exploitation of the Arctic would alter pathogen communities at a pace outwith counter-adaption in wildlife.

Transcriptome analysis reveals a high aerobic capacity in the whale brain.

The brain of diving mammals is repeatedly exposed to low oxygen conditions (hypoxia) that would have caused severe damage to most terrestrial mammals. Some whales may dive for >2 h with their brain remaining active. Many of the physiological adaptations of whales to diving have been investigated, but little is known about the molecular mechanisms that enable their brain to survive sometimes prolonged periods of hypoxia. Here, we have used an RNA-Seq approach to compare the mRNA levels in the brains of whales with those of cattle, which serves as a terrestrial relative. We sequenced the transcriptomes of the brains from cattle (Bos taurus), killer whale (Orcinus orca), and long-finned pilot whale (Globicephala melas). Further, the brain transcriptomes of cattle, minke whale (Balaenoptera acutorostrata) and bowhead whale (Balaena mysticetus), which were available in the databases, were included. We found a high expression of genes related to oxidative phosphorylation and the respiratory electron chain in the whale brains. In the visual cortex of whales, transcripts related to the detoxification of reactive oxygen species were more highly expressed than in the visual cortex of cattle. These findings indicate a high oxidative capacity in the whale brain that might help to maintain aerobic metabolism in periods of reduced oxygen availability during dives.

Seasonal variation in the thermal responses to changing environmental temperature in the world's northernmost land bird.

Arctic homeotherms counter challenges at high latitudes using a combination of seasonal adjustments in pelage/plumage, fat deposition and intricate thermoregulatory adaptations. However, there are still gaps in our understanding of their thermal responses to cold, particularly in Arctic birds. Here, we have studied the potential use of local heterothermy (i.e. tissue cooling that can contribute to significantly lower heat loss rate) in Svalbard ptarmigan (Lagopus muta hyperborea) - the world's northernmost land bird. We exposed birds kept under simulated Svalbard photoperiod to low ambient temperatures (Ta; between 0 and -30°C) during three seasons (early winter, late winter, summer), whilst recording resting metabolic rate (RMR), core temperature (Tc) and several cutaneous temperatures. Leg skin temperature varied the most, but still only by up to ∼15°C, whereas body trunk skin temperature changed <1°C when Ta decreased from 0 to -30°C. At the same time, Tc increased by 0.9°C, concomitant with increased RMR. This was probably driven by the triggering of cerebral thermosensors in response to cooling of the poorly insulated head, the skin of which was 5.4°C colder at -30°C than at 0°C. Thermal conductance in winter was higher in yearlings, probably because they were time/resource constrained from acquiring a high-quality plumage and sufficient fat reserves as a result of concomitant body growth. In conclusion, Svalbard ptarmigan do not employ extensive local heterothermy for cold protection but instead rely on efficient thermogenesis combined with excellent body insulation. Hence, cold defence in the world's northernmost land bird is not mechanistically much different from that of its lower latitude relatives.

Effects of Polar Bear and Killer Whale Derived Contaminant Cocktails on Marine Mammal Immunity.

Most controlled toxicity studies use single chemical exposures that do not represent the real world situation of complex mixtures of known and unknown natural and anthropogenic substances. In the present study, complex contaminant cocktails derived from the blubber of polar bears (PB; Ursus maritimus) and killer whales (KW; Orcinus orca) were used for in vitro concentration-response experiments with PB, cetacean and seal spp. immune cells to evaluate the effect of realistic contaminant mixtures on various immune functions. Cytotoxic effects of the PB cocktail occurred at lower concentrations than the KW cocktail (1 vs 16 µg/mL), likely due to differences in contaminant profiles in the mixtures derived from the adipose of each species. Similarly, significant reduction of lymphocyte proliferation occurred at much lower exposures in the PB cocktail (EC50: 0.94 vs 6.06 µg/mL; P < 0.01), whereas the KW cocktail caused a much faster decline in proliferation (slope: 2.9 vs 1.7; P = 0.04). Only the KW cocktail modulated natural killer (NK) cell activity and neutrophil and monocyte phagocytosis in a concentration- and species-dependent manner. No clear sensitivity differences emerged when comparing cetaceans, seals and PB. Our results showing lower effect levels for complex mixtures relative to single compounds suggest that previous risk assessments underestimate the effects of real world contaminant exposure on immunity. Our results using blubber-derived contaminant cocktails add realism to in vitro exposure experiments and confirm the immunotoxic risk marine mammals face from exposure to complex mixtures of environmental contaminants.

When the brain goes diving: transcriptome analysis reveals a reduced aerobic energy metabolism and increased stress proteins in the seal brain.

BACKGROUND: During long dives, the brain of whales and seals experiences a reduced supply of oxygen (hypoxia). The brain neurons of the hooded seal (Cystophora cristata) are more tolerant towards low-oxygen conditions than those of mice, and also better survive other hypoxia-related stress conditions like a reduction in glucose supply and high concentrations of lactate. Little is known about the molecular mechanisms that support the hypoxia tolerance of the diving brain. RESULTS: Here we employed RNA-seq to approach the molecular basis of the unusual stress tolerance of the seal brain. An Illumina-generated transcriptome of the visual cortex of the hooded seal was compared with that of the ferret (Mustela putorius furo), which served as a terrestrial relative. Gene ontology analyses showed a significant enrichment of transcripts related to translation and aerobic energy production in the ferret but not in the seal brain. Clusterin, an extracellular chaperone, is the most highly expressed gene in the seal brain and fourfold higher than in the ferret or any other mammalian brain transcriptome. The largest difference was found for S100B, a calcium-binding stress protein with pleiotropic function, which was 38-fold enriched in the seal brain. Notably, significant enrichment of S100B mRNA was also found in the transcriptomes of whale brains, but not in the brains of terrestrial mammals. CONCLUSION: Comparative transcriptomics indicates a lower aerobic capacity of the seal brain, which may be interpreted as a general energy saving strategy. Elevated expression of stress-related genes, such as clusterin and S100B, possibly contributes to the remarkable hypoxia tolerance of the brain of the hooded seal. Moreover, high levels of S100B that possibly protect the brain appear to be the result of the convergent adaptation of diving mammals.

No oxygen? No problem! Intrinsic brain tolerance to hypoxia in vertebrates.

Many vertebrates are challenged by either chronic or acute episodes of low oxygen availability in their natural environments. Brain function is especially vulnerable to the effects of hypoxia and can be irreversibly impaired by even brief periods of low oxygen supply. This review describes recent research on physiological mechanisms that have evolved in certain vertebrate species to cope with brain hypoxia. Four model systems are considered: freshwater turtles that can survive for months trapped in frozen-over lakes, arctic ground squirrels that respire at extremely low rates during winter hibernation, seals and whales that undertake breath-hold dives lasting minutes to hours, and naked mole-rats that live in crowded burrows completely underground for their entire lives. These species exhibit remarkable specializations of brain physiology that adapt them for acute or chronic episodes of hypoxia. These specializations may be reactive in nature, involving modifications to the catastrophic sequelae of oxygen deprivation that occur in non-tolerant species, or preparatory in nature, preventing the activation of those sequelae altogether. Better understanding of the mechanisms used by these hypoxia-tolerant vertebrates will increase appreciation of how nervous systems are adapted for life in specific ecological niches as well as inform advances in therapy for neurological conditions such as stroke and epilepsy.

The effects of season and sex upon the morphology and material properties of keratin in the Svalbard rock ptarmigan (Lagopus muta hyperborea).

The material properties and morphologies of the modified integumentary organs of birds (the keratinous bills, claws and feathers) have evolved to withstand the variety of mechanical stresses imposed by their interaction with the environment. These stresses are likely to vary temporally in seasonal environments and may also differ between the sexes as a result of behavioural dimorphism. Here we investigate the morphology and material properties of the claws of male and female Svalbard ptarmigan (Lagopus muta hyperborea) during the summer and winter using nanoindentation. Despite differences in locomotor demands between the sexes and pronounced seasonal differences in environmental conditions, like ground substrate, ambient temperature and day length, there was no significant difference in Young׳s modulus or hardness between the seasons for each sex. However, when comparing males and females, female claws were significantly harder than those of males and both sexes had significantly wider claws during winter. We propose that wider claws may follow winter claw moulting as the claws are regrown and form an important part of the ptarmigan׳s snowshoe-like foot that is an adaptation to locomotion on snow. Future work focusing on growth rates and more broad measures of material properties in both captive and wild birds is required to determine the extent of seasonal and sex differences in the material properties of their keratinous structures.

Rapid postnatal development of myoglobin from large liver iron stores in hooded seals.

Hooded seals (Cystophora cristata) rely on large stores of oxygen, either bound to hemoglobin or myoglobin (Mb), to support prolonged diving activity. Pups are born with fully developed hemoglobin stores, but their Mb levels are only 25-30% of adult levels. We measured changes in muscle [Mb] from birth to 1 year of age in two groups of captive hooded seal pups, one being maintained in a seawater pool and one on land during the first 2 months. All pups fasted during the first month, but were fed from then on. The [Mb] of the swimming muscle musculus longissimus dorsi (LD) doubled during the month of fasting in the pool group. These animals had significantly higher levels and a more rapid rise in LD [Mb] than those kept on land. The [Mb] of the shoulder muscle, m. supraspinatus, which is less active in both swimming and hauled-out animals, was consistently lower than in the LD and did not differ between groups. This suggests that a major part of the postnatal rise in LD [Mb] is triggered by (swimming) activity, and this coincides with the previously reported rapid early development of diving capacity in wild hooded seal pups. Liver iron concentration, as determined from another 25 hooded seals of various ages, was almost 10 times higher in young pups (1-34 days) than in yearling animals and adults, and liver iron content of pups dropped during the first month, implying that liver iron stores support the rapid initial rise in [Mb].

Probiotic dosing of Ruminococcus flavefaciens affects rumen microbiome structure and function in reindeer.

Highly cellulolytic bacterial species such as Ruminococcus flavefaciens are regarded essential for the microbial breakdown of cellulose in the rumen. We have investigated the effect of ruminal dosing of R. flavefaciens strain 8/94-32 during realimentation of starved reindeer (males, n = 3). Microbiome function measured as in situ digestion of cellulose and food pellets (percent DMD; dry matter disappearance) decreased after probiotic dosing. Microbial community analyses (>100,000 16S rDNA gene sequences for 27 samples) demonstrated that ruminal dosing influenced the microbiome structure; reflected by increased phylogenetic distances from background samples (unweighted UniFrac analysis) and reduced species diversity and evenness. Despite the inability to detect strain 8/94-32 post-dosing, the relative abundance of its affiliate family Ruminococcaceae remained consistent throughout the trial, whilst a dominant peak in the genus Prevotella and decline in uncharacterized Bacteroidetes (uBacNR) were observed in treatment samples. No clear relationships were observed between the relative abundance of Ruminococcaceae, Prevotella and uBacNR with cellulose DMD; however, Prevotella (negative) and uBacNR (positive) exhibited relationships with pellet DMD. These unexpected effects of ruminal dosing of a cellulolytic bacterium on digestibility are relevant for other studies on rumen manipulation.

Pharmacokinetics of a long-acting subcutaneous eprinomectin injection in semi-domesticated reindeer (Rangifer tarandus tarandus) - A pilot study.

Reindeer (Rangifer tarandus tarandus) are exposed to the pathogenic parasitic nematode Elaphostrongylus rangiferi during grazing. The severity of disease is dose-dependent. Prophylactic anthelmintic treatment is needed to improve animal health and reindeer herding sustainability. Herds are traditionally only gathered once during the summer, requiring a drug with a persistent effect. In this study we investigated the suitability of long-acting eprinomectin, given as a single subcutaneous injection at 1 mg/kg bodyweight in adult reindeer and calves. Plasma and faeces concentrations were determined using ultra-high performance liquid chromatography high resolution mass spectrometry (UHPLC-HRMS). Plasma concentrations remained above the presumed effect level of 2 ng/mL for 80 days, demonstrating the drug's potential. Pharmacokinetic parameters were compared to other species using allometric scaling. Calves and adults had slightly different profiles. No viable faecal nematode eggs were detected during treatment. Eprinomectin was measurable in the reindeer faeces up to 100 days, which is of environmental concern.

Tissue distribution of angiotensin-converting enzyme 2 (ACE2) receptor in wild animals with a focus on artiodactyls, mustelids and phocids.

Natural cases of zooanthroponotic transmission of SARS-CoV-2 to animals have been reported during the COVID-19 pandemic, including to free-ranging white-tailed deer (Odocoileus virginianus) in North America and farmed American mink (Neovison vison) on multiple continents. To understand the potential for angiotensin-converting enzyme 2 (ACE2)-mediated viral tropism we characterised the distribution of ACE2 receptors in the respiratory and intestinal tissues of a selection of wild and semi-domesticated mammals including artiodactyls (cervids, bovids, camelids, suids and hippopotamus), mustelid and phocid species using immunohistochemistry. Expression of the ACE2 receptor was detected in the bronchial or bronchiolar epithelium of several European and Asiatic deer species, Bactrian camel (Camelus bactrianus), European badger (Meles meles), stoat (Mustela erminea), hippopotamus (Hippopotamus amphibious), harbor seal (Phoca vitulina), and hooded seal (Cystophora cristata). Further receptor mapping in the nasal turbinates and trachea revealed sparse ACE2 receptor expression in the mucosal epithelial cells and occasional occurrence in the submucosal glandular epithelium of Western roe deer (Capreolus capreolus), moose (Alces alces alces), and alpaca (Vicunga pacos). Only the European badger and stoat expressed high levels of ACE2 receptor in the nasal mucosal epithelium, which could suggest high susceptibility to ACE2-mediated respiratory infection. Expression of ACE2 receptor in the intestinal cells was ubiquitous across multiple taxa examined. Our results demonstrate the potential for ACE2-mediated viral infection in a selection of wild mammals and highlight the intra-taxon variability of ACE2 receptor expression, which might influence host susceptibility and infection.

Understanding sex differences in the cost of terrestrial locomotion.

Little is known regarding the physiological consequences of the behavioural and morphological differences that result from sexual selection in birds. Male and female Svalbard rock ptarmigans (Lagopus muta hyperborea) exhibit distinctive behavioural differences during the breeding season. In particular, males continuously compete for and defend territories in order to breed successfully, placing large demands on their locomotor system. Here, we demonstrate that male birds have improved locomotor performance compared with females, showing both a lower cost of locomotion (CoL) and a higher top speed. We propose that the observed sex differences in locomotor capability may be due to sexual selection for improved male performance. While the mechanisms underlying these energetic differences are unclear, future studies should be wary when pooling male and female data.

Elevated antioxidant defence in the brain of deep-diving pinnipeds.

While foraging, marine mammals undertake repetitive diving bouts. When the animal surfaces, reperfusion makes oxygen readily available for the electron transport chain, which leads to increased production of reactive oxygen species and risk of oxidative damage. In blood and several tissues, such as heart, lung, muscle and kidney, marine mammals generally exhibit an elevated antioxidant defence. However, the brain, whose functional integrity is critical to survival, has received little attention. We previously observed an enhanced expression of several antioxidant genes in cortical neurons of hooded seals (Cystophora cristata). Here, we studied antioxidant gene expression and enzymatic activity in the visual cortex, cerebellum and hippocampus of harp seals (Pagophilus groenlandicus) and hooded seals. Moreover, we tested several genes for positive selection. We found that antioxidants in the first line of defence, such as superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione (GSH) were constitutively enhanced in the seal brain compared to mice (Mus musculus), whereas the glutaredoxin and thioredoxin systems were not. Possibly, the activity of the latter systems is stress-induced rather than constitutively elevated. Further, some, but not all members, of the glutathione-s-transferase (GST) family appear more highly expressed. We found no signatures of positive selection, indicating that sequence and function of the studied antioxidants are conserved in pinnipeds.

Transcriptomes Suggest That Pinniped and Cetacean Brains Have a High Capacity for Aerobic Metabolism While Reducing Energy-Intensive Processes Such as Synaptic Transmission.

The mammalian brain is characterized by high energy expenditure and small energy reserves, making it dependent on continuous vascular oxygen and nutritional supply. The brain is therefore extremely vulnerable to hypoxia. While neurons of most terrestrial mammals suffer from irreversible damage after only short periods of hypoxia, neurons of the deep-diving hooded seal (Cystophora cristata) show a remarkable hypoxia-tolerance. To identify the molecular mechanisms underlying the intrinsic hypoxia-tolerance, we excised neurons from the visual cortices of hooded seals and mice (Mus musculus) by laser capture microdissection. A comparison of the neuronal transcriptomes suggests that, compared to mice, hooded seal neurons are endowed with an enhanced aerobic metabolic capacity, a reduced synaptic transmission and an elevated antioxidant defense. Publicly available whole-tissue brain transcriptomes of the bowhead whale (Balaena mysticetus), long-finned pilot whale (Globicephala melas), minke whale (Balaenoptera acutorostrata) and killer whale (Orcinus orca), supplemented with 2 newly sequenced long-finned pilot whales, suggest that, compared to cattle (Bos taurus), the cetacean brain also displays elevated aerobic capacity and reduced synaptic transmission. We conclude that the brain energy balance of diving mammals is preserved during diving, due to reduced synaptic transmission that limits energy expenditure, while the elevated aerobic capacity allows efficient use of oxygen to restore energy balance during surfacing between dives.

Environmental drivers of population-level variation in the migratory and diving ontogeny of an Arctic top predator.

The development of migratory strategies that enable juveniles to survive to sexual maturity is critical for species that exploit seasonal niches. For animals that forage via breath-hold diving, this requires a combination of both physiological and foraging skill development. Here, we assess how migratory and dive behaviour develop over the first year of life for a migratory Arctic top predator, the harp seal Pagophilus groenlandicus, tracked using animal-borne satellite relay data loggers. We reveal similarities in migratory movements and differences in diving behaviour between 38 juveniles tracked from the Greenland Sea and Northwest Atlantic breeding populations. In both regions, periods of resident and transitory behaviour during migration were associated with proxies for food availability: sea ice concentration and bathymetric depth. However, while ontogenetic development of dive behaviour was similar for both populations of juveniles over the first 25 days, after this time Greenland Sea animals performed shorter and shallower dives and were more closely associated with sea ice than Northwest Atlantic animals. Together, these results highlight the role of both intrinsic and extrinsic factors in shaping early life behaviour. Variation in the environmental conditions experienced during early life may shape how different populations respond to the rapid changes occurring in the Arctic ocean ecosystem.

Regulation of brain temperature in winter-acclimatized reindeer under heat stress.

Reindeer (Rangifer tarandus) are protected against the Arctic winter cold by thick fur of prime insulating capacity and hence have few avenues of heat loss during work. We have investigated how these animals regulate brain temperature under heavy heat loads. Animals were instrumented for measurements of blood flow, tissue temperatures and respiratory frequency (f) under full anaesthesia, whereas measurements were also made in fully conscious animals while in a climatic chamber or running on a treadmill. At rest, brain temperature (T(brain)) rose from 38.5±0.1°C at 10°C to 39.5±0.2°C at 50°C, while f increased from ×7 to ×250 breaths min(-1), with a change to open-mouth panting (OMP) at T(brain) 39.0±0.1°C, and carotid and sublingual arterial flows increased by 160% and 500%, respectively. OMP caused jugular venous and carotid arterial temperatures to drop, presumably owing to a much increased respiratory evaporative heat loss. Angular oculi vein (AOV) flow was negligible until T(brain) reached 38.9±0.1°C, but it increased to 0.81 ml min(-1) kg(-1) at T(brain) 39.2±0.2°C. Bilateral occlusion of both AOVs induced OMP and a rise in T(brain) and f at T(brain) >38.8°C. We propose that reindeer regulate body and, particularly, brain temperature under heavy heat loads by a combination of panting, at first through the nose, but later, when the heat load and the minute volume requirements increase due to exercise, primarily through the mouth and that they eventually resort to selective brain cooling.