Seasonal changes in sea-water tolerance of Arctic charr (Salvelinus alpinus)

Summary Groups of Arctic charr,Salvelinus alpinus, which had been acclimated to water with a salinity of 7 g·l^–1 and natural temperature and photoperiod, were exposed to water with different salinities and temperatures in June, September and February. At a salinity of 15 g·l^–1, plasma osmolality, plasma Na⁺, Cl^–, Mg²⁺ concentrations and the activity of gill Na-K-ATPase were stable, irrespective of temperature and season. In June, the charr were able to regulate blood plasma ionic levels within narrow limits when exposed to a salinity of 34 g·l^–1 (sea water) and a temperature of 8°C. The hypo-osmoregulatory capacity was less, but sufficient if the temperature was only 1°C during the seawater exposure. At the start of the experiment, the gill Na-K-ATPase activity was significantly higher in June than corresponding enzyme activities in September and February. Furthermore, an increase in gill Na-K-ATPase activity during the seawater exposure (8°C) was seen in June. Irrespective of ambient temperature and salinity, no fish died during the June experiments. In September and February, exposure to sea water produced marked increases in plasma osmolality and plasma ion concentrations. There were no changes in gill Na-K-ATPase activity. Consequently, the fish became dehydrated and were moribund after a short period of seawater exposure. Highest mortality was recorded when charr were exposed to winter sea conditions (34 g·l^–1 and 1°C) in February. The results indicate that an increase in daylength induce a hypo-osmoregulatory capacity in the Arctic charr during summer. In fall and winter, however, reduced daylength are accompanied by poor hypo-osmoregulatory capacity. This leads to high mortality as a result of increased electrolyte levels and dehydration.     

Seasonal and ontogenetic variations in resource use by two sympatric Arctic charr morphs

The study compares the resource utilization of two sympatric Arctic charr morphs over an annual period in a subarctic lake. The two morphs are reproductively isolated in time and place of spawning, and are referred to as the littoral and profundal morphs (L-morph and P-morph) according to their spawning habitats. Fish were sampled monthly (ice-free season) or bimonthly (winter) using gillnets in the main lake habitats. The spatial range of the P-morph was restricted to the profundal zone throughout the whole annual period. The L-morph in contrast utilized all main habitats, exhibiting distinct seasonal and ontogenetic variations in habitat distribution. In the spring, the whole L-morph population was located along the bottom profile of the lake, in profundal and littoral habitats. During summer and autumn, habitat segregation occurred between different life-stages, juveniles mainly utilizing the profundal, pre-adults the pelagic and adult fishes the littoral zone. During winter the whole population was assembled in the littoral habitat. The L-morph also had large seasonal and ontogenetic variations in their feeding ecology, with littoral zoobenthos, zooplankton and surface insects being important prey. The P-morph had a narrower diet niche mainly consisting of chironomid larvae and other profundal zoobenthos. Hence, the two Arctic charr morphs exhibited a consistent resource differentiation during all annual seasons and throughout their life cycles, except for a dietary overlap between P-morph and juvenile L-morph charr in the profundal during summer. The findings are discussed in relation to resource polymorphism and incipient speciation.     

Differences in susceptibility of anadromous and resident stocks of Arctic charr to infections of Gyrodactylus salaris, under experimental conditions

The ability of Gyrodactylus salaris , an important pathogen of the Atlantic salmon Salmo salar , in Norway, to infect anadromous and resident stocks of the Arctic charr, Salvelinus alpinus , has been examined in the laboratory. Resident charr (Korssjoen stock) exposed to heavily infected salmon, were considered innately resistant as they lost their infections within 21 days when individually isolated. Isolated anadromous harr (Hammerfest stock) remained infected for up to 150 days, although most infections disappeared within 30–50 days. In many cases the parasite population grew initially, but growth was limited after 20–30 days and infections subsequently disappeared. At the same time, shoals of 50 anadromous charr, swimming in the tanks containing the individually isolated fish in floating cages, remained infected for up to 280 days. Charr isolated from these shoals after 115 days and subsequently monitored individually lost their infections within 30 days, although the parasite persisted within the shoals for a further 75–135 days. This suggests that G. salaris , persisted on shoaling charr despite an immune response which led to the elimination of parasites from isolated hosts. The Hammerfest stock of anadromous charr supports G. salaris , in the laboratory, and the extended period of survival on this host suggests that charr may be important in the epidemiology of G. salaris , in northern Norway.     

Seasonal variation in the temperature preference of Arctic charr (Salvelinus alpinus)

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Seasonal variation in thermal preference of Arctic charr from a North Norwegian population (70°N) was studied in an electronic shuttlebox, which allowed the fish to control the water temperature by moving between two chambers. The fish were acclimated to 12 °C and a natural photoperiod before measurements. Fish were tested in autumn (September–October), winter (January–February), spring (April–May) and summer (June–August).     

Food borne parasites as indicators of trophic segregation between Arctic charr and brown trout

The habitat and diet choice and the infection (prevalence and abundance) of trophically transmitted parasites were compared in Arctic charr and brown trout living sympatrically in two lakes in northern Norway. Arctic charr were found in all main lake habitats, whereas the brown trout were almost exclusively found in the littoral zone. In both lakes the parasite fauna reflected the niche segregation between trout and charr. Surface insects were most common in the diet of trout, but transmit few parasites, and accordingly the brown trout had a relatively low diversity and abundance of parasites. Parasites transmitted by benthic prey such as Gammarus and insect larva, were common in both salmonid host species. Copepod transmitted parasites were much more common in Arctic charr, as brown trout did not include zooplankton in their diets. Parasite species that may use small fish as transport hosts, were far more abundant in piscivorous fish, especially brown trout. The seasonal dynamics in parasite infection were also consistent with the developments in the diet throughout the year. The study demonstrates that the structure of parasite communities of charr and the trout is highly dependent on shifts in habitat and diet of their hosts both on an annual base and through the ontogeny, in addition to the observed niche segregation between the two salmonid species.     

Upstream migratory behaviour in landlocked Arctic charr

Synopsis Extensive upstream migration of landlocked Arctic charr during spring floods was recorded in several tributaries of an oligotrophic lake in north-west Sweden. Migration was confined to a period of about two weeks and residence in most creeks was of short duration. Only fish migrating to two small productive lakes remained in the new habitat over the summer. Repeated annual migrations were only recorded in the creek leading to these lakes and no straying was observed among repeat migrants. Water temperatures provided the primary cues for initiation and direction of migration, although an ability to detect productive habitats by odour was indicated. Creek size, feeding opportunities during migration and conspeeific odour were subordinate guiding factors.     

Seasonal patterns of growth, lipid deposition and lipid depletion in anadromous Arctic charr

Seasonal patterns of growth, lipid deposition and lipid depletion were studied in anadromous Arctic charr from a north Norwegian population. Samples were collected in late May when fish were migrating between fresh water and the sea, and in mid-July when the fish re-entered fresh water. A sample of maturing fish captured in mid-July was held in captivity until late September to assess lipid mobilization linked to the final stages of maturation. The fish increased substantially in weight during their 40–50-day summer residence in sea water (immatures from c . 300 to 600 g; maturing fish from c . 500 to 800 g), and body lipid stores were increased approximately fivefold. The carcass (head, skeleton and skin) was the major lipid depot, accounting for c . 50% of the total lipid content when the fish re-entered fresh water from the sea. The muscle of the ascending charr contained 35–40% of the body lipids, whereas the gut and liver each held 4–5%. Body lipid decreased 30–40% during the period between the re-entry of the fish to fresh water and spawning; although lipids were depleted from all depots the carcass and muscle were quantitatively the most important. By the time of spawning, the gonads of the females held almost 25% of the body lipid, whereas in the males the gonads accounted for <3% of the total lipids. Females lost c . 80% of their body lipid during spawning and overwintering, and, consequently, the lipid depots were severely depleted by the early spring. By contrast, lipid depletion in the males amounted to 50–55% of total lipid in the same period. These data suggest that the combination of spawning and overwintering in fresh water imposes a greater load on the females than on the males. It may be that the severity of the depletion prevents females from spawning in successive years.     

Thermal habitat use and growth in young-of-the-year Arctic charr from proximal fluvial and lacustrine populations in Labrador,Canada

Predicted increases in temperature associated with climate change are expected to have consequences for fish, in particular for Arctic charr, Salvelinus alpinus, a cold-adapted fish species. Despite differences in predicted hydroecological responses to climate change in fluvial and lacustrine environments, little is known of whether fluvial and lacustrine Arctic charr populations may respond differently to increasing temperatures. In order to compare growth and thermal habitat use between habitat types, otolith-inferred average water temperatures estimated from whole otoliths and fork lengths at capture were measured for young-of-the-year (YOY) Arctic charr obtained from two proximal fluvial and lacustrine sites in Labrador, Canada. Otolith-inferred average experienced water temperatures were not significantly correlated with air temperatures at both sites, suggestive of behavioural thermoregulation by YOY. The majority of Kogluktokoluk Brook (fluvial) YOY were found using water temperatures consistent with laboratory determined preferred temperatures for juvenile Arctic charr, whereas most Tom's Pond (lacustrine) YOY were found using temperatures ranging between preferred temperatures and optimal temperatures for growth. There was no consistent difference between mean water temperatures used between YOY from the two sites. Otolith-inferred average experienced water temperatures were only correlated to fork lengths in Tom's Pond YOY. The lack of correlation in Kogluktokoluk Brook YOY is argued to reflect resource partitioning occurring as a result of territoriality known to occur among stream salmonids. The limited range of temperatures used by fluvial YOY in this study, particularly the lack of cooler temperatures, also suggests that fluvial YOY may face barriers to thermal refugia, and as a result may be particularly vulnerable to climate change.     

Adaptations to Stochastic Environmental Variations: The Effects of Seasonal Temperatures on the Migratory Window of Svalbard Arctic Charr

The circumpolar Arctic charr, Salvelinus alpinus, is ideal for studying how environmental factors affect life history in fishes. Charr populations demonstrate a tremendous ecological plasticity and adaptations to harsh environments. Arctic charr is the only freshwater fish on Svalbard, including anadromous, resident, and landlocked stocks. Freshwater lake systems on Svalbard are characterized by very low water temperatures, long-term or even permanent ice cover, and low levels of nutrients. Food is thus limited and may lead to growth stagnation and early maturity in Arctic charr. The individual growth pattern may alternatively follow a sigmoid-shaped curve, caused by a shift to either cannibalistic or anadromous (migration to sea) behaviour. In lake systems that include migratory charr, the population may consist of a mixture of parr, postsmolt, and adult migratory individuals, as well as small-sized resident, large-sized resident (cannibals), and large formerly resident individuals transformed to anadromy. Our study in the Lake Dieset watercourse (79°N), Svalbard, demonstrates that the annual water flow in the outlet river is strongly correlated to air temperatures and provides a passage to the sea, allowing the charr access to the nutrient-rich seawater environment, during at most two months each year. During one of the years studied, the youngest and small-sized part of the sea-going stock was prevented from ascending the river and probably suffered mortality during winter. The migratory window of the Arctic charr in Lake Dieset is therefore highly variable among years and thus unpredictable. We hypothesize that in worst case scenarios (cold years, low water discharge), climatic variations may occasionally prevent charr from migrating upstream in Svalbard lake systems in late autumn, resulting in high mortality in the population.     

Lack of movement of brook charr and ouananiche in the Caniapiscau River,northern Quebec

Synopsis Marking experiments, 1974–76, showed that brook charr Salvelinus fontinalis and juvenile ouananiche Salmo salar moved very little during the summer or from year to year in the upper reaches of the Caniapiscau River, north central Quebec. The fish were concentrated in rapid sections of the river and there was little exchange between rapids.     

Characterisation and expression of the paired box protein 7 (Pax7) gene in polymorphic Arctic charr (Salvelinus alpinus)

Arctic charr (Salvelinus alpinus L.) from Lake Thingvallavatn, Iceland occur as four distinct morphs: large benthivorous (LB), dwarf benthivorous (DB), piscivorous (PI) and planktonivorous (PL). The morphs differ with respect to body size, head morphology, growth rate, and life history. The aim of this study was to investigate the paired box protein 7 (Pax7) gene as a candidate for such polymorphisms due to its importance in cranio-facial, skeletal muscle, and central nervous system development. No variation in coding and intronic sequences was found between morphs. We identified 10 alternate Pax7 isoforms with insertions/deletions: a four-residue (GNRT) deletion, a GEASS insertion truncated by the first serine residue (GEAS), and a thirteen-residue insertion (GQYA/TGPEYVYCGT). The latter insertion with a threonine (T) contains a putative casein kinase II (CK-2) phosphorylation site. Pax7 spatial expression patterns were identical in embryos of DB-, LB-, and PL-morphs, and were similar to those described for zebrafish Pax7c, but a difference in temporal expression for segmentation was observed between DB and LB morphs. At the end of segmentation, novel expression was observed in the mandibular region as two bilateral domains. The potential role of multiple alternative splicing of the Pax7 gene for the generation of different Arctic charr morphs is briefly discussed.     

Reproduction,condition and food of adult arctic charr (Salvelinus alpinus,L.) in Lake Pavin (Massif Central,France)

RésuméTo determine the role and the importance of adult fish predation directly and indirectly in the food web of oligomesotrophic Lake Pavin, reproduction, condition, and feeding activity of the dominant adult chctic charr (Salvelinus alpinus, L.) were studied. Fish were caught by gillnets of different mesh sizes once a month from April to December 1992. The gonad cycle showed that the spawning period occurred between October to December. Relative fecundity is about 2509 ovules per kg of female fish and ovule mean diameter is 4.7 mm ( 0.6 mm). Seasonal variations of the sex-ratio (Max. in April = 2.67 and Min. in October=0.19) indicated the intralake migrations of females and males. The condition coefficients showed higher values for females than males; the maximum was observed in summer for both sexes and the minimum in April and in October for males and females, respectively. Low values of the condition were also observed during the spawning period. High feeding activity was recorded in spring and in December after reproduction; low feeding activity was observed in October during reproduction. Asellus aquaticus was quantitatively the dominant prey from April to September; during the sp awning period, S. alpinus consumed its own eggs, particularly in large quantity in December. Daphnia longispina was numerically the dominant prey all over the studied cycle and the electivity (Ivlev Index) of arctic charr on the latter prey ranged from +s+0.79 to +0.16. During April, more than 15 500 individuals of D. longispina per kg of fish were found in the gut contents. The selective predation of S. alpinus on benthic (A. aquatics) and zooplankton prey (D. longispina) indicated that the fish immigrate during their feeding activity from benthic/littoral to the pelagic area.Afin de d terminer le rôle et l'importance de la prédation directe et indirecte de l'omble chevalier adulte (Salvelinus alpinus, L.) dans le réseau trophique du lac Pavin, milieu oligo- mésotrophe, nous avons étudié la reproduction, deux facteurs de condition et l'activité alimentaire de ce poisson qui est l'espéce dominante dans ce lac. Les poissons ont été capturés par des filets maillants de différent vide de maille selon un pas d'échantillonnage mensuel d'avril á décembre 1992. Le cycle du développement des gonades montre que la période de reproduction se déroule entre octobre et décembre. La fécondité relative calculée est de 2509 ovules par kg de femelles et le diamétre moyen d'un ovule est de 4,7 mm ( 0,6). Les variations saisonniéres du sex-ratio (Max. en avril = 2,67 et Min. en octobre=0,19) indiquent le déplacement actif et successif des femelles et des mâles. Les variations saisonniéres des coefficients de condition indiquent une copulence plus importante des femelles par rapport aux mâles avec des valeurs maximales en été pour les deux sexes et des valeurs minimales en avril et en octobre pour les mâles et les femelles respectivement. De faibles valeurs des facteurs de condition ont été observées pendant la période de reproduction pour les deux sexes. L'activité alimentaire de l'omble chevalier est intense au printemps et au mois de décembre aprés la fraie et faible au mois d'octobre pendant la reproduction. Asellus aquaticus est, en termes de biomasse, la proie dominante d'avril á septembre. Pendant la période de reproduction, S. alpinus se nourrit de ses propres oeufs sur les zones de frayéres et en décembre la quantité d'oeufs consommée par le Salmonidé est importante. Daphnia longispina est, en termes d'abondance, la proie dominante tout au long de cette étude et l'indice d'électivité (Indice d'Ivlev) de l' ombre chevalier vis-á-vis de cette proie varie entre + 0,79 et + 0,16.De plus, au mois d'avril, plus de 15 500 D. longispina par kg de poisson ont été enregistrées dans les contenus stomacaux. La prédation sélective de S. alpinus sur des proies benthiques (A. aquaticus) et zooplanctoniques (D. longispina) indique que le poisson effectue des déplacements en fonction de son activité alimentaire de la zone littorale/benthique á la zone pélagique.     

Life-history styles and somatic allocation in iteroporous arctic charr and semelparous pink salmon

Synopsis RNA/DNA ratios were used to estimate recent somatic allocation of spawning arctic charr,Salvelinus alpinus, and pink salmon,Oncorhynchus gorbuscha. Smaller arctic charr morphs had lower ratios than larger morphs. Larger male pink salmon had lower ratios than smaller males. Thus recent somatic allocation (anabolic metabolic activity and growth) is coupled with cumulative lifetime growth in iteroparous arctic charr, but is uncoupled in semelparous pink salmon. These data suggest that somatic investment in these species is being differentially effected by the energetic costs of reproduction.     

Development of spinal deformities in Atlantic salmon and Arctic charr fed diets supplemented with oxytetracycline

Some individuals within populations of Atlantic salmon Salmo salar and Arctic charr Salvelinus alpinus fed diets supplemented with oxytetracycline (OTC) developed spinal deformations. Possible differences in feed intake and growth of spinally deformed fish relative to fish without any deformities were investigated. Amongst Atlantic salmon, 17% of the fish fed OTC-supplemented feed developed spinal fractures, whereas none of the fish receiving the basic feed did so. Despite deformation of the spinal column, the injured fish continued to feed and grow, but at lower rates than unaffected individuals. In contrast to Atlantic salmon, Arctic charr showed no signs of spinal fractures at any time during the 65-day experiment.     

Sacciform cells in the epidermis of the brown trout,Salmo trutta,and the Arctic char,Salvetinus alpinus

Summary Sacciform cells containing an acidophilic, proteinaceous secretion, were identified in the epidermis of the brown trout and Arctic char. This cell type increased in number during the chronic stages of infestation by the ectoparasitic flagellate, Ichthyobodo sp., in immature brown trout, and decreased during sexual maturation in male brown trout and char. It is suggested that the salmonid sacciform cell produces a secretion which protects the fish against infestation or damage by skin parasites.     

A Comparison of Floy and Soft VIalpha tags on Hatchery Arctic Charr, with Emphasis on Tag Retention, Growth and Survival

The Floy FTF-69 'fingerling' tag and the soft VIalpha tag are designed to be used on small salmonid fishes (> 100 and > 150mm, respectively). The two tags were compared for 160 days on hatchery-reared Arctic charr, Salvelinus alpinus, (170–209mm). Tag retention and effects on growth rate and survival were analysed. VIalpha-tagged and untagged control fish had similar growth rates and grew significantly faster than Floy-tagged fish during the experiment. Tag retention was significantly higher for Floy tags (94%) than for VIalpha tags (78%). Most VIalpha tag loss (11%) took place within the first 10 days of tagging while there were no Floy tags lost during this period. Survival rates were about equal for all three groups (96–98%), and thus the tags did not seem to affect the survival of charr under hatchery conditions.     

Seasonal variation and sex differences in the nutritional status in two local populations of wild Japanese macaques

Seasonal variations and sex differences in the nutritional status in two local populations of wild Japanese macaques (Macaca fuscata fuscata) were examined. It was hypothesized that the ecological condition and/or reproductive strategies of each sex determine the nutritional condition and its seasonal fluctuation in each sex. Morphometric measures such as body mass, thoracic and femoris circumferences, skinfold thickness in four places (triceps, biceps femoris, subscapular, and abdomen), and wet mass of mesenteric and omental fat were used for comparisons between sexes, seasons, and populations. Animals of the Shimane population were larger than those of the Boso population in most morphometric measures, abdominal skinfold, and mesenteric and omental fat mass, suggesting environmental and/or genetic differences in the two populations. Females of both populations had larger skinfolds and mesenteric and omental fat mass than males, indicating that females had more fat than males. Females showed seasonality in most measures, having two peaks of body mass, thoracic and femoris circumferences, abdominal skinfold, and mesenteric and omental fat masses in early spring and late fall. In contrast, males exhibited no clear seasonal variations for most measurements, except for biceps femoris and subscapular skinfolds, which showed peaks in summer. Most morphometric measurements significantly correlated to each other, particularly in females, but most skinfolds had no correlation with other measurements. These findings suggest that sexual dimorphism in body composition and its fluctuation may be affected by the different reproductive strategies of males and females.     

Variations in juvenile growth, energy allocation and life-history strategies of two populations of Arctic charr in North Norway

Within the populations of Arctic charr Salvelinus alpinus in Storvatn and Rungavatn, Norway, fish that attained the largest size in the late parr stage migrated as 4+ smolts, fish that attained a medium size became 5+ smolts, and the smallest parr became lake residents. Within the last 2 years of the parr stage, those that became anadromous had a lower growth rate during the winter and early summer than those that became resident, and vice versa in late summer. Thus, anadromous fish grew faster in the parr stage, but in years prior to migration, their growth pattern seemed to differ from that of parr that became resident. Due to early size differences between parr that became anadromous or resident, it is suggested that some of the basis for the decision to smoltify or not in charr depends on the growth rate through the whole parr stage, but with significant modification by the local environment. Rungavatn parr had a significantly higher population density (5 ×), higher lipid content and mortality, an earlier maturation, a lower rate of growth, and only one-third the degree of anadromy than those from Storvatn. It is suggested that the two charr populations have evolved different life-history adaptations due to unequal growth, energy allocation and competition opportunities in the lakes.     

Seasonal changes in hypoosmoregulatory ability in landlocked and anadromous populations of Arctic charr,Salvelinus alpinus,and Atlantic salmon,Salmo salar

Synopsis Seasonal changes in hypoosmoregulatory ability were compared in landlocked and anadromous strains of Arctic charr and Atlantic salmon. Seawater adaptability was assessed using periodic 48 h seawater challenge tests with 25. seawater. The landlocked strains of Arctic charr, two from northern Sweden and one from Southern Norway, displayed similar seasonal changes in seawater adaptability as the anadromous strain. Seawater tolerance increased during spring and remained high until the end of July — early August after which it declined. The two strains of Atlantic salmon displayed different seasonal patterns in hypoosmoregulatory ability. The anadromous strain showed a pronounced seasonal pattern with maximal seawater adaptability in early June. In contrast, seawater tolerance in the landlocked strain improved steadily during spring and remained high until late autumn. During the period of enhanced seawater tolerance, hypoosmoregulatory ability increased significantly with body size in both Arctic charr and anadromous Atlantic salmon. The minimum size at which fish were able to regulate plasma sodium following seawater transfer at a level comparable to freshwater levels (<170 mmol I^–1) differed significantly between anadromous Atlantic salmon (ca. 14 cm) and Arctic charr (ca. 22 cm). The results show that seasonal changes in hypoosmoregulatory ability are present in both Atlantic salmon and Arctic charr, and that these physiological traits are retained in the corresponding landlocked strains. However, the seasonal pattern of seawater adaptability as well as the minimum size at which seawater tolerance occurs differs between the two species.     

Seasonal activity and energetics of two species of varanid lizards in tropical Australia

The field metabolic rates (FMR) and rates of water flux were measured in two species of varanid lizards over five periods of the year in tropical Australia. The energetics of these species were further investigated by directly measuring activity (locomotion) and body temperatures of free-ranging animals by radiotelemetry, and by measuring standard metabolic rate (over a range of body temperatures) and activity metabolism in the laboratory. Seasonal differences in the activity and energetics were found in these goannas despite similar, high daytime temperatures throughout the year in tropical Australia. Periods of inactivity were associated with the dry times of the year, but the onset of this period of inactivity differed with respect to habitat even within the same species. Varanus gouldii, which inhabit woodlands only, were inactive during the dry and late dry seasons. V. panoptes that live in the woodland had a similar seasonal pattern of activity, but V. panoptes living near the floodplain of the South Alligator River had their highest levels of activity during the dry season when they walked long distances to forage at the receding edge of the floodplain. However, during the late dry season, after the floodplain had dried completely, they too became inactive. For V. gouldii, the rates of energy expenditure were 196 kJ kg^–1 day^–1 for active animals and 66 kJ kg^–1 day^–1 for inactive animals. The rates of water influx for these groups were respectively 50.7 and 19.5 ml kg^–1 day^–1. For V. panoptes, the rates of energy expenditure were 143 kJ kg^–1 day^–1 for active animals and 56 kJ kg^–1 day^–1 for inactive animals. The rates of water influx for these two groups were respectively 41.4 and 21.0 ml kg^–1 day^–1. We divided the daily energy expenditure into the proportion of energy that lizards used when in burrows, out of burrows but inactive, and in locomotion for the two species during the different seasons. The time spent in locomotion by V. panoptes during the dry season is extremely high for a reptile (mean of 3.5 h/day spent walking), and these results provide an ecological correlate to the high aerobic capacity found in laboratory measurements of some species of varanids.