Daily energy expenditure of singing great reed warblers Acrocephalus arundinaceus

According to honest signalling theory, signals must be costly to produce to retain information about the signaller's quality. The song produced by male birds during breeding is a vocal "ornament" used for intra- and inter-sexual purposes. The energetic cost of this vocal signal remains a contentious issue. We used the doubly labelled water method to measure field metabolic rate by estimating CO₂ production and then convert this to daily energy expenditure (DEE) in great reed warbler males singing under natural conditions (10 at low to moderate intensity and 7 at very high intensity from dawn to dusk). There was a significant positive relationship between singing intensity and DEE. From this relationship we extrapolated the average DEE for intensely singing males (i.e., males producing song sounds 50% of the time and hence sitting at their elevated song post in the top of a reed stem more or less continuously throughout the ∼20 h of daylight) to 3.3×BMR (basal metabolic rate) and for non-singing males to 2.2×BMR. The mean DEE measured for the seven males singing with very high intensity was 3.1×BMR. The maximum measured DEE for a single male was 3.9×BMR, i.e. close to the maximum sustainable DEE (4×BMR), and the minimum DEE was 2.1×BMR for a male singing at very low intensity. These results imply that producing intensive advertising song in birds may incur a substantial cost in terms of increased energy expenditure.     

Seasonal sex–specific energy expenditure in breeding and non-breeding Palestine sunbirds Nectarinia osea

The timing of the chick‐rearing phase is known to have a profound effect on the reproductive success of birds. However, little is known about the energetic costs faced by the parents during different periods of the breeding season. These costs may have vital consequences for both their survival and future reproduction. In most studies, daily energy expenditure (DEE) of breeding and non‐breeding birds has been compared, without controlling for the effect of season. In the present study, we examined the energy demands of breeding compared to non‐breeding Palestine sunbirds Nectarinia osea and whether there were sex‐specific differences in DEE within and between different seasons. We predicted that DEE would be elevated when birds rear chicks, especially at cooler ambient temperatures. Time‐energy budgets were constructed for pairs of sunbirds, rearing chicks, or not breeding, in spring and summer. There were significant seasonal differences in estimates of DEE in non‐breeders that were 21% higher in spring than in summer. We attributed these to increases in non‐flight metabolic rate rather than changes in time spent on different activities. Our estimates of DEE for the birds that were rearing chicks were higher than non‐breeding adults. In females the increase in DEE when breeding, compared to when not breeding, was similar in both spring and summer, while males increased their DEE much less when breeding in spring. The differences in estimated DEE, however, were not significant between male and female birds in any season. Between seasons, female breeders had 17.1% higher DEE in spring than in summer, while male breeders showed no difference in DEE when rearing chicks in different seasons. Accordingly, our initial prediction was supported, as DEE in chick‐rearing adults was higher than in non‐breeding adults. In addition, although temperatures are lower in spring, breeding in the spring is only more costly than breeding in summer for females. Apparently, males are more flexible in reallocating their time and energy spent on different activities.     

内蒙古草原布氏田鼠代谢率与身体器官的关系

动物代谢率存在差异的原因及其意义是进化生理学的一个核心问题。为了解代谢率的影响因素和功能意义, 我们测定了不同驯化条件下布氏田鼠(Microtus brandti) 的基础代谢率(basal metabolic rate , BMR) 、日能量消耗(daily energy expenditure , DEE) 和冷诱导的最大代谢率(maximum metabolic rate , MMR) , 分析了动物体内11 种器官、组织的重量与代谢率的关系。结果显示, 排除温度、光照、食物质量和体重的影响后, BMR 与心脏、肝脏、肾脏、胃和盲肠相关; DEE与心脏、肾脏、胃和盲肠相关; MMR 与脑重显著负相关。这表明: 在布氏田鼠体内存在着代谢活性器官, 主要包括心脏、肝脏、肾脏、胃和盲肠, 这些器官对BMR 有较大的贡献。动物的能量周转水平与体内“代谢机器” (metabolic machinery) 的大小相关连, 主要受到心脏、肾脏、胃和盲肠的影响。最大代谢率受脑重的影响。BMR 与MMR 的相关性不显著, 而BMR 与DEE 的相关性显著, 说明较高的BMR 有助于维持较高的DEE , 但不能维持较高的MMR。     

Variable energetic responses to clutch size manipulations in white-throated dippers Cinclus cinclus

We used the doubly-labelled water technique to measure daily energy expenditure (DEE) of a free-living uniparental incubator, the white-throated dipper Cinclus cinclus , in Scotland. DEE was 205±8 (s.e.m.) kJ d⁻¹ for 17 females incubating their natural clutch sizes, equivalent to 3.2±0.1×basal metabolic rate (BMR). To investigate the influence of clutch size on the energy budget, we measured the DEE of 14 females with clutches increased or reduced by a single egg. Birds with reduced clutch sizes had an energy expenditure with a mean and variance that did not differ from those of birds with unmanipulated clutches. Enlarging the clutch led to an increase in energy expenditure to over 4×BMR for some individuals but not for others, resulting in greater variance in energy expenditure for birds with enlarged clutches. Individual variation in energy expenditure could not be fully explained by environmental conditions, by patterns of behaviour or clutch size. Incubating females received a maximum of only 4 kJ d⁻¹ (2% of DEE) from provisioning by the male, and mobilised up to 6 kJ d⁻¹ (3% of DEE) from reserves. Females spent 2.9±0.2 h (n=20) away from the nest each day, so a foraging rate of 95 kJ h⁻¹ was required during incubation recesses to balance DEE. This 'required foraging rate' is double previous estimates of the maximum rates of energy acquisition for birds of this size. We suggest that the greater likelihood of a raised energy expenditure associated with larger clutches, combined with the difficulties in maintaining energy supplies, may constitute a constraint on avian clutch size.     

A seasonal difference of daily energy expenditure in a free-living subterranean rodent,the silvery mole-rat (Heliophobius argenteocinereus; Bathyergidae)

In seasonal climatic regimes, animals have to deal with changing environmental conditions. It is reasonable to expect that seasonal changes are reflected in animal overall energetics. The relation between daily energy expenditure (DEE) and seasonally variable ecological determinants has been studied in many free-living small mammals; however with inconsistent results. Subterranean mammals, i.e. fossorial (burrowing) mammals which live and forage underground, live in a seasonally and diurnally thermally stable environment and represent a suitable model to test seasonality in DEE in respect to seasonal changes, particularly those in soil characteristics and access to food supply. Both factors are affected by seasonal rainfall and are supposed to fundamentally determine activity of belowground dwellers. These ecological constraints are pronounced in some tropical regions, where two distinct periods, dry and rainy seasons, regularly alternate. To explore how a tropical mammal responds to an abrupt environmental change, we determined DEE, resting metabolic rate (RMR) and sustained metabolic scope (SusMS) in a solitary subterranean rodent, the silvery mole-rat, at the end of dry season and the onset of rainy season. Whereas RMR did not differ between both periods, mole-rats had 1.4 times higher DEE and SusMS after the first heavy rains. These findings suggest that rainfall is an important environmental factor responsible for higher energy expenditure in mole-rats, probably due to increased burrowing activity. SusMS in the silvery mole-rat is comparable to values in other bathyergids and all bathyergid values rank among the lowest SusMS found in endothermic vertebrates.     

Seasonal variation in the range areas of the diurnal rodent Octodon degus

Both breeding activity and abundance and quality of available food are expected to influence daily movements of animals. Animals are predicted to range over large areas to meet high energy demands associated with reproduction (females) or to increase mating success (males). However, animals should expand their range areas whenever food conditions deteriorate. To examine the extent to which breeding activity versus food availability influence space use, we compared the size and location of range areas (home ranges) of the degu (Octodon degus), a diurnal rodent from semiarid environments of north-central Chile, during the austral winter and summer seasons. Degus produce young during the austral spring (September-October) when high-quality food is readily available. In contrast, degus do not breed during the austral summer (January-March) when food is scarce and of low quality. We predicted that degus would range over smaller areas in winter if the availability of food has a greater influence on space than breeding activity. Individuals were radiotracked in winter and the following summer over a 3-year period. Surveys of herbaceous cover were conducted during winter and summer to determine seasonal changes in the abundance and quality of primary food. In summer degus expanded and moved the location of their range areas to locations with available food. Given that preferred food was less abundant in summer than winter, we suggest that degu range areas are strongly influenced by food conditions.     

Annual cycle of energy and time expenditure in a golden-mantled ground squirrel population

Summary We have analyzed seasonal shifts of energy and time allocation in a population of golden-mantled ground squirrels (Spermophilus saturatus) by directly measuring total daily energy expenditure (DEE) with an isotopic technique (doubly labeled water=dlw), and by estimating components of total DEE through an integration of field behavioral observations with laboratory-measured rates of energy expenditure (oxygen consumption) associated with major behavioral and physiological states. Hibernation laster about 7 1/2 months, and the 4 1/2-month activity season consisted of mating, a 28-d gestation of 3–5 young, 5 1/2 weeks of postnatal growth building to a peak in lactation just before the young emerged above ground, an additional 2–3-week period of maternal care before dispersal, and finally restoration of body mass preceding hibernation. Although the hibernation season comprised nearly two-thirds of the year, it involved only 13–17% of annual energy expenditure, leaving about 85% of energy expenditure for the active season. Ground squirrels were actually present on the surface for only about 11% of the year's time, and the foraging time required to obtain the total annual energy supply amounted to only about 2% of the year's time. The squirrels fed mainly on herbs in the early season and hypogeous fungi later; both were used extensively during peak lactation when female energy expenditure and demand were maximal. Average daily foraging time increased steadily throughout the season to a maximum of 28% of aboveground time as availability of greens diminished and fungus predominated in the diet; time availability did not limit foraging since the animals sat on average for 65% of the daily surface time of about 7 h. Timing of reproduction is apparently optimized such that peak reproductive energy demands are matched with maximal food availability and moderate thermal conditions that minimize energy demand. Despite the greater body mass of males, the greatest total DEE (measured by dlw) of any squirrels at any time of year was that of females during peak lactation. For production of young and lactation through above-ground emergence of an average litter of 2.7, females required a total energy increase of 24% above annual nonreproductive metabolism. Yearling females all bred and performed similarly to older females, yet some costs were greater because the yearlings began and ended hibernation at smaller mass, compensated by giving birth later, and finally showed a greater absolute increase in body mass over the active season than older females. Annual metabolic energy expenditure of breeding males was about 18% greater than that of females, due to greater male body mass. Yet the annual energy intake requirement for both sexes was essentially identical (about 42MJ) due to the greater reproductive export by females in the form of newborn and milk. During the mating season males showed wide-ranging exploratory behavior and social interactions, including aggression, that involved considerable locomotory energy expenditures. Although we did not directly account for the energetics of these specific reproductive behaviors, they are critical to male reproductive success and on a daily basis they probably involved much greater energy expenditure than sperm production. Some yearling males avoided these costs by foregoing testicular development, yet they allocated four times as much energy to growth as older males, thereby increasing somatic condition for the future.     

Energy expenditure and testosterone in free-living male yellow-pine chipmunks

The onset of mating in yellow-pine chipmunks (Tamias amoenus) follows emergence from a prolonged period of energy conservation during hibernation. Energy expenditures are greatly accelerated to meet the demands of the reproductive season. When emerging from hibernation, typical male chipmunks (breeders) have enlarged testes and a high level of plasma testosterone (T). However, certain males that do not participate in reproduction (nonbreeders) maintain small testes and low plasma T levels and emerge several weeks later than the breeders. The timing of the terminal arousal from hibernation and onset of mating are associated with increased plasma T levels. Experimental elevation of T levels in T. amoenus outside the mating season has been associated with a decrease in body mass, further suggesting an effect of T on energy balance. To test this hypothesis, we measured daily energy expenditure (DEE) in free-living, nonbreeding male chipmunks in the presence and absence of a T-implant. We also measured DEE in breeding males when endogenous T levels were high. DEE of the nonbreeders was not affected by our manipulation of plasma T, and the DEE of breeding males did not differ from that of nonbreeders. We conclude that energy expenditure on a daily basis in male yellow-pine chipmunks is not influenced by levels of T. However, on a seasonal basis, the earlier emergence from hibernation by breeding males, which appears to be influenced by T, represents an overall seasonal energy expenditure that exceeds that of nonbreeding males.     

Seasonal and sex variation in physical activity levels among agro-pastoralists in Nepal

Considerable attention has been devoted to variation in levels of energy expenditure between and within populations; these are commonly evaluated following international guidelines for grading light, moderate, and heavy physical activity levels (PAL). This study presents activity profiles by season and sex for subsistence agro-pastoralists in Nepal, comparing data for a sample of 20 men observed four times across the year with previously published data on women. Total energy expenditure (TEE) was estimated from direct minute-by-minute observation (totaling 1,679 h for men, 3,601 h for women) and measures of the energy cost of single tasks (117 for men, 168 for women). PAL were calculated and graded as multiples of predicted basal metabolic rate (BMR). Despite an explicitly egalitarian organization of labor, men achieved higher PAL than women (P < .0001), although according to international gradings, both men and women assume moderately heavy PAL in the winter and very heavy PAL in the monsoon. PAL were 1.88 and 2.22 × BMR for men in respective seasons (P < .005; TEE, 11.8 MJ/d and 13.9 MJ/d) and 1.77 and 2.0 × BMR for women (TEE, 9.1 MJ/d and 10.5 MJ/d). High TEE values result from time-consuming work in subsistence tasks, most of which are of moderate energy cost. Results show that the international guideline (FAO/WHO/UNU [1985]) for grading levels of energy expenditure, which adopts discrepant sex-specific values to define thresholds for moderate or heavy PAL, can mask significant gender variation. Male/female ratios of PAL values are suggested instead for population-level comparisons. © 1996 Wiley-Liss, Inc.     

Evidence for an intrinsic energetic ceiling in free-ranging kittiwakes Rissa tridactyla

1. The rate at which free-living animals can expend energy is limited but the causes of this limitation are not well understood. Theoretically, energy expenditure may be intrinsically limited by physiological properties of the animal constraining its capacity to process energy. Alternatively, the limitation could be set extrinsically by the amount of energy available in the environment or by a fitness trade-off in terms of reduced future survival associated with elevated metabolism.
2. We measured daily energy expenditure (DEE) using the doubly labelled water method in chick-rearing black-legged kittiwakes ( Rissa tridactyla ) at a study site close to the northern limit of their breeding range over 5 years. We measured breeding success, foraging trip duration and diet composition as proxies of resource availability during these years and estimated the probability of parent kittiwakes to return to the colony in relation to their energy expenditure in order to determine whether kittiwakes adjust their DEE in response to variation in prey availability and whether elevated DEE is associated with a decrease in adult survival.
3. We found that DEE was strikingly similar across all five study years. There was no evidence that energy expenditure was limited by resource availability that varied considerably among study years. Furthermore, there was no evidence of a negative effect of DEE on adult return rate, which does not support the hypothesis of a survival cost connected to elevated energy expenditure.
4. The additional lack of variation in DEE with respect to ambient temperature, brood size or between sexes suggests that kittiwakes at a time of peak energy demands may operate close to an intrinsic metabolic ceiling independent of extrinsic factors.     

Energy expenditure during egg laying is equal for early and late breeding free-living female great tits

In many bird populations, variation in the timing of reproduction exists but it is not obvious how this variation is maintained as timing has substantial fitness consequences. Daily energy expenditure (DEE) during the egg laying period increases with decreasing temperatures and thus perhaps only females that can produce eggs at low energetic cost will lay early in the season, at low temperatures. We tested whether late laying females have a higher daily energy expenditure during egg laying than early laying females in 43 great tits (Parus major), by comparing on the same day the DEE of early females late in their laying sequence with DEE of late females early in their egg laying sequence. We also validated the assumption that there are no within female differences in DEE within the egg laying sequence. We found a negative effect of temperature and a positive effect of female body mass on DEE but no evidence for differences in DEE between early and late laying females. However, costs incurred during egg laying may have carry-over effects later in the breeding cycle and if such carry-over effects differ for early and late laying females this could contribute to the maintenance of phenotypic variation in laying dates.     

Energetics of the annual cycle of Dippers Cinclus cinclus

Time-activity budgets and energy expenditure of Dippers Cinclus cinclus were studied in all months of the year and for every stage of the annual cycle. The commonest daytime activity was feeding (54%), then resting (43%) and flying (4%). On a 24-hr day basis the most marked changes in activity followed from changing daylengths. DEE (Daily Energy Expenditure), derived from time-activity budgets through the year and laboratory estimates of metabolism, averaged 201 kj d ^-1 in females and 228 kj d ^-1 in the larger males. Over a more restricted range of circumstances, direct estimates of DEE obtained from 77 Dippers using the doubly labelled water technique averaged 205 ± 43 kj d^-1 and 251 ± 55 kj d^-1 in females and males, respectively. Overall, the correspondence between these largely independent estimates of energy expenditure was reasonably close. DEE was highest during breeding (laying-females; rearing-males) and in late winter for both sexes. The lowest energy expenditures occurred during moult, amongst juveniles and in early winter. Incubating females and mate-guarding males also had low energy costs. Across all stages of the annual cycle body size, activity patterns, ambient temperature and river flow had significant effects on energy expenditure. The rate at which food was gathered to meet these changing energy demands varied widely. While some of this variation was imposed by a seasonal environment, it was also likely to reflect adaptive shifts in rates of food gathering, in some cases consequent upon the changing fitness benefits of various non-feeding activities.     

Early postnatal physical and behavioural development of degus (Octodon degus).

6 litters of Octodon degus were studied from birth to 10 days of age. Newly-born degus (mean weight 14.6 g) had open eyes, upper and lower pigment, fur, and teeth. Within 3-4 hours of birth they were able to walk supporting their full weight, right themselves rapidly, sit upright on their haunches, or rear upright with support, and vocalize. By the 1st or 2nd day, the young animals displayed functional grooming (face washing, hind-paw scratching, rapid head-shake). Solid food was ingested from day 6, although newborns chewed wood chips and 3-day olds gnawed dried faeces. Young degus were tested daily in an open field apparatus and showed increased activity and exploration, with repeated testing while decreasing distress vocalization after the 4th or 5th day. Degus are proposed for the study of developmental topics since their degree of development at birth allows for immediate testing. The degus studied here seem to be more fully developed at birth than those studied in Britain.     

Social cues and hormone levels in male Octodon degus (Rodentia): a field test of the Challenge Hypothesis

Social interactions are important factors determining and regulating individual behaviors. Testosterone has been related to agonistic interactions, while glucocorticoids have been related to social stress, especially during interactions of dominance. We compared testosterone and cortisol concentrations in male degus (Octodon degus, Rodentia) under laboratory conditions without male social interactions, with data from wild males in nature. Under natural conditions, males should present higher levels of testosterone during the breeding season due to social interactions (Challenge Hypothesis). Alternatively, intense social instability could act as a stressing environment, raising glucocorticoids, which inhibit testosterone concentrations. Our results show a significant increase in agonistic interactions between males during the breeding season, and disappearance of non-agonistic male interactions during this period. Hormone levels in breeding season show nonsignificant differences between laboratory groups, but testosterone concentrations in field males were significantly higher than in laboratory males. Testosterone levels were similar among pre-breeding and breeding periods, but in field animals the concentration was approximately 30% higher than in laboratory degus. In field animals, we found two different mating strategies: resident males, with territorial behavior, and transient males, displayed an opportunistic approach to females. Finally, cortisol presents a similar pattern in both laboratory and field animals; pre-breeding values of cortisol are higher than during the breeding season. This suggests that social interactions in O. degus activate a rise in testosterone, supporting the Challenge Hypothesis, and could be considered as partial support of the Social Stress Hypothesis.     

Evaluating the prudence of parents: daily energy expenditure throughout the annual cycle of a free-ranging bird, the macaroni penguin Eudyptes chrysolophus

We measured daily energy expenditure (DEE) continuously for a whole year in a free ranging bird, the macaroni penguin Eudyptes chrysolophus . We combined these measurements with concurrently recorded foraging behaviour, and literature information on body mass and dietary factors to estimate prey consumption rates and foraging success. DEE was at a maximum during late chick-rearing but was equally high during all other active phases of the breeding season. DEE was approximately 4×resting metabolic rate, which accords with established theory and suggests a common 'energetic ceiling' throughout the summer period. However, whether this represents a maximum in physiological capacity, or a rate which optimises fitness is still unclear. Rates of prey consumption and foraging success followed different patterns from daily energy expenditure. Daily prey consumption was high as the penguins prepared for long fasts associated with moulting and incubation but relatively low during chick-rearing, when foraging areas were restricted and foraging success lower. It appears that the energy intake of macaroni penguins is subject to extrinisic or environmental constraints rather than to intrinsic physiological limits.     

Male Weasels Decrease Activity and Energy Expenditure in Response to High Ambient Temperatures

The heat dissipation limit (HDL) hypothesis suggests that the capacity of endotherms to dissipate body heat may impose constraints on their energy expenditure. Specifically, this hypothesis predicts that endotherms should avoid the detrimental consequences of hyperthermia by lowering their energy expenditure and reducing their activity in response to high ambient temperatures (T_a). We used an extensive data set on the daily energy expenditure (DEE, n = 27) and the daily activity time (AT, n = 48) of male weasels (Mustela nivalis) during the spring and summer breeding season to test these predictions. We found that T_a was related in a “hump-shaped” (i.e. convex) manner to AT, DEE, resting metabolic rate (RMR) and metabolic scope (the ratio of DEE to RMR). These results support the HDL hypothesis because in response to warm T_as male weasels reduced their AT, DEE, and RMR. Although the activity and energy expenditure of large endotherms are most likely to be constrained in response to warm T_as because they are less able to dissipate heat, our results suggest that small endotherms may also experience constraints consistent with the HDL hypothesis.     

Differential energy costs of winter acclimatized common spiny mice Acomys cahirinus from two adjacent habitats

The common spiny mouse Acomys cahirinus, of Ethiopian origin, has a widespread distribution across arid, semi-arid and Mediterranean parts of the Arabian sub-region. We compared the daily energy expenditure (DEE), water turnover (WTO) and sustained metabolic scope (SusMS=DEE/resting metabolic rate) of two adjacent populations during the winter. Mice were captured from North- and South- facing slopes (NFS and SFS) of the same valley, comprising mesic and xeric habitats, respectively. Both DEE and SusMS winter values were greater in NFS than SFS mice and were significantly greater than values previously measured in the summer for these two populations in the same environments. However, WTO values were consistent with previously established values and were not significantly different from allometric predictions for desert eutherians. We suggest that physiological plasticity in energy expenditure, which exists both temporally and spatially, combined with stable WTO, perhaps reflecting a xeric ancestry, has enabled A. cahirinus to invade a wide range of habitats.     

Optimal body size and energy expenditure during winter: why are voles smaller in declining populations?

Winter is energetically challenging for small herbivores because of greater energy requirements for thermogenesis at a time when little energy is available. We formulated a model predicting optimal wintering body size, accounting for the scaling of both energy expenditure and assimilation to body size, and the trade-off between survival benefits of a large size and avoiding survival costs of foraging. The model predicts that if the energy cost of maintaining a given body mass differs between environments, animals should be smaller in the more demanding environments, and there should be a negative correlation between body mass and daily energy expenditure (DEE) across environments. In contrast, if animals adjust their energy intake according to variation in survival costs of foraging, there should be a positive correlation between body mass and DEE. Decreasing temperature always increases equilibrium DEE, but optimal body mass may either increase or decrease in colder climates depending on the exact effects of temperature on mass-specific survival and energy demands. Measuring DEE with doubly labeled water on wintering Microtus agrestis at four field sites, we found that DEE was highest at the sites where voles were smallest despite a positive correlation between DEE and body mass within sites. This suggests that variation in wintering body mass between sites was due to variation in food quality/availability and not adjustments in foraging activity to varying risks of predation.     

Photic phase response curve in Octodon degus: assessment as a function of activity phase preference

Light exposure during the early and late subjective night generally phase delays and advances circadian rhythms, respectively. However, this generality was recently questioned in a photic entrainment study in Octodon degus. Because degus can invert their activity phase preference from diurnal to nocturnal as a function of activity level, assessment of phase preference is critical for computations of phase reference [circadian time (CT) 0] toward the development of a photic phase response curve. After determining activity phase preference in a 24-h light-dark cycle (LD 12:12), degus were released in constant darkness. In this study, diurnal (n = 5) and nocturnal (n = 7) degus were randomly subjected to 1-h light pulses (30-35 lx) at many circadian phases (CT 1-6: n = 7; CT 7-12: n = 8; CT 13-18: n = 8; and CT 19-24: n = 7). The circadian phase of body temperature (Tb) onset was defined as CT 12 in nocturnal animals. In diurnal animals, CT 0 was determined as Tb onset + 1 h. Light phase delayed and advanced circadian rhythms when delivered during the early (CT 13-16) and late (CT 20-23) subjective night, respectively. No significant phase shifts were observed during the middle of the subjective day (CT 3-10). Thus, regardless of activity phase preference, photic entrainment of the circadian pacemaker in Octodon degus is similar to most other diurnal and nocturnal species, suggesting that entrainment mechanisms do not determine overt diurnal and nocturnal behavior.     

Energetics of the acrobatic courtship in male golden-collared manakins (Manacus vitellinus)

In lek mating systems, females choose mates through indicators of quality, which males may exhibit by their performance of courtship displays. In temperate regions, displaying seasons are brief (one to two months), whereas in the tropics courtship seasons may be prolonged. Moreover, in temperate-breeding animals lekking behaviour can be energetically demanding, but little is known about the energy costs of lekking in tropical animals. Daily, over the course of a nearly seven-month-long breeding season, male golden-collared manakins (Manacus vitellinus) of Panamanian rainforests perform acrobatic courtship displays that markedly elevate heart rates, suggesting that they require high energy investment. Typically, animals of tropical lowland forests (such as manakins) exhibit a ‘slow pace of life’ metabolic strategy. We investigated whether male manakin courtship is indeed metabolically costly or whether the birds retain a low daily energy expenditure (DEE), as seen in other tropical species. To assess these questions, we calibrated manakin heart rate against metabolic rate, examined daily lek activity and, using telemetry, obtained heart rates of individual wild, lekking male manakins. Although metabolic rates peak during courtship displays, we found that males actually invest minimal time (only approx. 5 min d⁻¹) performing displays. As a consequence, the DEE of approximately 39 kJ d⁻¹ for male manakins is comparable to other lowland tropical species. The short, intense bursts of courtship by these birds make up only approximately 1.2% of their total DEE. Presumably, this cost is negligible, enabling them to perform daily at their arenas for months on end.