Optimal decision rules for herring gulls

A measure of fitness applicable to herring gull (Larus argentatus) life histories is introduced and related to short-term risks which depend on behaviour. This allows calculations of the relationship between fitness and behaviour if one can measure the risks an animal takes behaving in particular ways in specified situations. This method is used to evaluate the fitness of herring gulls during the incubation period at Walney Island, U.K. Two causes of egg mortality are considered, death by predation and death by exposure, and the latter is shown to be less important than the former. The chances of death by predation depend on the number of adults present on the territory and are estimated to be approximately 0·045 eggs per h if no parents are present on territory, 3·5×10^?4 eggs per h if one parent is present and 6·7×10^?4 eggs per h if two parents are present. Adult death by starvation is assumed to occur when energy reserves are exhausted; however a fitness cost is incurred if an animal maintains excess energy reserves. To restrict the number of behaviour sequences examined to manageable proportions, attention is restricted to the behaviour sequences that result from decision rules in which feeding occurs when energy reserves fall below a set point. Then optimal behaviour is characterized by (1) energy reserves maintained between 500 and 1200 kcal; (2) complementarity of mates' feeding preferences such that at least one of the pair feeds at a refuse tip; (3) parental desertion of offspring if energy reserves fall below 200 kcal; (4) parents spending the minimum possible time together on territory. These results are shown by sensitivity analysis to be insensitive to parameter errors in the model. Treating these characteristics of optimal behaviour as predictions about actual behaviour reveals that predictions 1 and 2 are correct, and prediction 4 is wrong. An experiment was designed to test prediction 3 by estimating the fat reserves of life birds and measuring how long each would continue incubation in the absence of its mate. Fatter birds did continue incubation for longer though the quantitative detail did not match prediction 3 exactly. Thus two aspects of behaviour tally with our calculation as to what is optimal, but one does not. This conclusion is discussed.     

Mass-dependent predation risk and lethal dolphin-porpoise interactions

In small birds, mass-dependent predation risk (MDPR) is known to make the trade-off between avoiding starvation and avoiding predation dependent on individual mass. This occurs because carrying increased fat reserves not only reduces starvation risk but also results in a higher predation risk due to reduced escape flight performance and/or the increased foraging exposure needed to maintain a higher body mass. In principle, the theory of MDPR could also apply to any animal capable of storing energy reserves to reduce starvation and whose escape performance decreases with increasing mass. We used a unique situation along certain parts of coastal Britain, where harbour porpoises (Phocoena phocoena) are pursued and killed but crucially not eaten by bottlenose dolphins (Tursiops truncatus), to investigate whether a MDPR effect can occur in non-avian species. We show that where high levels of dolphin 'predation' occur, porpoises carry significantly less energy reserves than would otherwise be expected and this equates to reducing by approximately 37% the length of time that a porpoise could survive without feeding. These results provide the first evidence that a mass-dependent starvation-predation risk trade-off may be a general ecological principle that can apply to widely different animal types rather than, as is currently thought, only to birds.     

捕食风险对不同饥饿状态下艾虎取食行为的影响

在室内条件下,将大鵟作为艾虎的天敌动物,通过双通道选择实验确定6 只成体艾虎在3 个捕食风险水平和4 种饥饿状态条件下的取食行为,探讨艾虎在取食过程中对饥饿风险与捕食风险的权衡策略。研究结果表明:在无捕食风险存在时,艾虎被剥夺食物0 d 和1 d 后对食物量不同的两个斑块中的取食量和利用频次均无明显不同(P > 0. 05),但对高食物量斑块的利用时间均明显高于低食物量斑块的(P <0.05),而艾虎被剥夺食物2 d和3 d后对高食物量斑块中的取食量和利用时间均明显高于低食物量斑块中的(P < 0.05),但在利用频次上均无明显差异(P > 0.05)。在面临低风险时,艾虎在4 种饥饿状态下均只利用无天敌动物存在的低食物量斑块,而基本不利用有天敌动物存在的高食物量斑块。在面临高风险时,艾虎不得不利用有天敌动物存在的食物斑块,被剥夺食物0 d 时艾虎对无风险、无食物量斑块的利用时间基本相同于对高风险、有食物量斑块的利用时间(P>0.05),而被剥夺食物1d、2 d 和3 d 后艾虎对高风险、有食物量斑块的利用时间明显高于无风险、无食物量斑块的(P< 0. 05)。在相同风险条件下,随着饥饿程度增加,艾虎在斑块中的取食量均明显增加(P< 0.05),而对斑块的利用时间和利用频次明显降低(P<0.05)。在相同的饥饿状态下,不同风险水平时,艾虎在斑块中的取食量无明显的差异(P>0.05),但在低风险和高风险时对斑块的利用时间和频次均明显低于无风险时的(P <0.05)。以上结果说明艾虎能够根据食物摄取率和自身的能量需求在捕食风险和饥饿风险之间做出权衡,当饥饿风险小于捕食风险时,艾虎趋于躲避捕食风险,当饥饿风险大于捕食风险时,艾虎趋于面对捕食风险,所采用的取食策略是减少活动时间和能量消耗,最大程度地提高单位时间内获得的能量。     

Winter energy allocation and deficit of juvenile walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> in the Doto area,northern Japan

Seasonal energy allocation and deficits of marine juvenile fishes have considerable effects on their survival. To explore the winter survival mechanism of marine fishes with low lipid reserves in their early life, juvenile walleye pollock Theragra chalcogramma were collected along the continental shelf of northern Japan over a 2-year period, and energy allocation and deficit patterns were compared between wild and laboratory-starved fish. Contrary to expectations, wild fish generally continued to accumulate protein mass and concurrently tended to reduce lipid mass from late autumn through winter. The most plausible explanation for the continuous structural growth is that juvenile pollock give priority to reducing mortality risk from size-selective predators under quasi-prey-limited conditions. Exceptionally, inshore small fish reduced both constituents during a winter. The inshore fish consumed 2.5 times more lipid energy than protein energy in November–December, but protein was more important than lipids as a source of energy in December–January and in February–March. However, dependence upon protein reserves was lower for the wild fish than for the laboratory-starved fish, suggesting milder nutritional stress of the wild fish than that observed in the starvation experiment. Moreover, the lipid contents of mortalities in the starvation experiment were mostly <1%, whereas few wild fish had such lipid contents in the field. These results suggest that juvenile pollock are able to avoid both starvation and predation by accumulating protein reserves.     

Trade‐off of ovarian lipids and total body lipids for fecundity and starvation resistance in tropical populations of Drosophila melanogaster

In Drosophila melanogaster, clines of starvation resistance along a latitudinal gradient (south to north) have been reported in India, which matched with their cline for total body lipids (T_L). Nevertheless, producing too many reserves is likely to be costly and a trade‐off might exist with life‐history traits. Previous studies on starvation resistance and life‐history traits of D. melanogaster have mainly focused on quantification of total body lipids, instead of separating ovarian lipids from total body lipids. In the present study, we have quantified absolute ovarian lipids (O_L) versus absolute body lipids excluding ovarian lipids (B_L) and examined associations with fecundity as well as starvation resistance in two latitudinal populations (8.34 vs. 32.43°N) of D. melanogaster. Firstly, we observed a trade‐off between B_L and O_L that matched the trade‐off of starvation resistance, longevity versus fecundity and development time in latitudinal populations of D. melanogaster. Southern populations had higher starvation resistance, more B_L and lesser O_L, whereas northern populations had enhanced fecundity, O_L and lesser B_L. Secondly, within population, starvation resistance also correlated with B_L, and fecundity with O_L. However, there was no correlation between starvation resistance and O_L. Moreover, there was utilization of B_L and nonutilization of O_L under starvation stress. Therefore, resources invested for fecundity in the form of O_L were independent of evolved starvation resistance in D. melanogaster. Our results suggest that a common pool of energy storage compounds (lipids) are allocated differentially between fecundity and starvation resistance and are consistent with Y‐model of resource allocation.     

Fatness and fitness: exposing the logic of evolutionary explanations for obesity

To explore the logic of evolutionary explanations of obesity we modelled food consumption in an animal that minimizes mortality (starvation plus predation) by switching between activities that differ in energy gain and predation. We show that if switching does not incur extra predation risk, the animal should have a single threshold level of reserves above which it performs the safe activity and below which it performs the dangerous activity. The value of the threshold is determined by the environmental conditions, implying that animals should have variable ‘set points’. Selection pressure to prevent energy stores exceeding the optimal level is usually weak, suggesting that immediate rewards might easily overcome the controls against becoming overweight. The risk of starvation can have a strong influence on the strategy even when starvation is extremely uncommon, so the incidence of mortality during famine in human history may be unimportant for explanations for obesity. If there is an extra risk of switching between activities, the animal should have two distinct thresholds: one to initiate weight gain and one to initiate weight loss. Contrary to the dual intervention point model, these thresholds will be inter-dependent, such that altering the predation risk alters the location of both thresholds; a result that undermines the evolutionary basis of the drifty genes hypothesis. Our work implies that understanding the causes of obesity can benefit from a better understanding of how evolution shapes the mechanisms that control body weight.     

Fat reserves and perceived predation risk in the great tit, Parus major

The fat reserves of small birds are built up daily as insurance against starvation. They are believed to reflect a trade-off between the risks of starvation and predation such that in situations of high predation risk birds are expected either to reduce their fat reserves in response to mass-dependent predation risk or to increase them in response to foraging interruptions. We assessed the effect on fat reserves of experimentally altering the perceived (but not the actual) risk of predation of wild great tits at a winter feeding site. The perceived predation risk was alternated between 'safe' and 'risky'. Increasing the perceived risk of predation involved 'swooping' a model sparrowhawk over the feeder at four unpredictable times each day using a remote mechanism We produce evidence that the experiment was suceessfull in altering the perceived risk of predation. As predicted from the hypothesis of mass-dependent predation risk, great tits (Parus major) carried significantly reduced fat reserves during the 'risky' treatment. Furthermore, dominant individuals were able to reduce their reserves more than subordinates. As birds returned to feeders within seconds after a predator 'attack', the reduction in fat reserves cannot be attributed to an interruption in feeding.     

Body mass variations in disturbed mallards Anas platyrhynchos fit to the mass‐dependent starvation‐predation risk trade‐off

For passerines the starvation‐predation risk theory predicts that birds should decrease their body mass to improve escape flight performance, when predation pressure increases. To investigate whether this theory may apply to large birds, which manage body reserves differently from small passerines, we experimentally increased the predation risk in mallards Anas platyrhynchos. Two groups were disturbed at different frequencies during experimental sessions lasting one week, while a control group was left undisturbed. We found that body mass loss and final wing loading were similar in both disturbed groups and significantly differed from the control group. Food intake in disturbed groups was reduced up to day four of the disturbance session and was lower than in the control group. Altogether our results suggest that disturbed mallards may adjust their body mass to reach a more favorable wing loading, supposedly to improve escape flight performance. Nevertheless, body mass loss in our mallards was double than what has been observed in passerines. This greater mass decrease might be explained by different strategies concerning energy storage. Furthermore, in large birds the predation component of the starvation‐predation trade‐off might be of greater importance. Hence, the observed relevance of this trade‐off over a large size range suggests that the starvation‐predation risk theory is of major ecological significance for many animal species.     

Mass-dependence in the predation risk of unequal competitors; some models

Foragers can monitor their survival through the size of body reserves in a starvation/predation risk trade-off. Energy reserves reduce the risk of energetic shortfall, while survival will be maximised at intermediate reserve levels when there is a cost of carrying mass loads. The size of reserves that will maximise survival may not be identical for unequal competitors, when unequal access to resources will affect the costs and benefits of energy reserves. Here, I evaluate the effect of competitive ability (dominance) for the mass-dependence in predation risk and how it is affected by (1) attack rate (attack rate effect), (2) distance to the emergence of an unconcealed predator attack (attack distance effect) and (3) distance to cover (cover distance effect). This general model is illustrated by empirical data for parameters specific for birds. The effect of competitive ability for the mass-dependence in predation risk is ambiguous and depends on how rank is mediated into mass-dependent predation risk. Dominants pay a lower cost in predation risk for mass loads than sub-ordinates when competitive ability entails that they feed closer to cover (cover distance effect) and when the exposure to attacks and attack rate is lower than for sub-ordinates (attack rate effect) . In contrast, a shorter distance to the emergence of an unconcealed attack (attack distance effect) implies a lower increase in predation risk with mass for sub-ordinates. As a consequence of how the cost of mass load varies with conditions there is no unambiguous relationship for how predation risk can be traded off for starvation risk for individuals with different competitive ability.     

Food supplementation and predation risk in harsh climate: interactive effects on abundance and body condition of tit species

Food availability and predation risk can have drastic impacts on animal behaviour and populations. The tradeoff between foraging and predator avoidance is crucial for animal survival and will strongly affect individual body mass, since large fat reserves are beneficial to reduce starvation but may increase predation risk. However, two‐factor experiments simultaneously investigating the interactive effects of food and predation risk, are still rare. We studied the effects of food supplementation and natural predation risk imposed by pygmy owls Glaucidium passerinum on the abundance and fat reserves of tit species in boreal forests of north Europe, from January to March in 2012 and in 2013. Food supplementation increased the number of individuals present in a given forest patch, whereas the level of predation risk had no clear impact on the abundance of tit species. The stronger impact of food supply respect to predation risk could be the consequence of the harsh winter conditions in north Europe, with constant below‐zero temperatures and only few (5–7 h) daylight hours available for foraging. Predation risk did not have obvious effects on tit abundance but influenced food consumption and, together with food supplementation, affected the deposition of subcutaneous fat in great tits Parus major. High owl predation risk had detrimental effects on body fat reserves, which may reduce over‐winter survival, but the costs imposed by pygmy owl risk were compensated when food was supplemented. The starvation–predation tradeoff faced by great tits in winter may thus be mediated through variation in body fat reserves. In small species living in harsh environment, this tradeoff appeared thus to be biased towards avoidance of starvation, at the cost of increasing predation risk.     

Avian daily foraging patterns: Effects of digestive constraints and variability

Summary Birds show a typical daily pattern of heavy morning and secondary afternoon feeding. We investigate the pattern of foraging by a bird that results in the lowest long-term rate of mortality. We assume the following: mortality is the sum of starvation and predation. The bird is characterized by two state variables, its energy reserves and the amount of food in its stomach. Starvation occurs during the day if the bird's reserves fall to zero. The bird starves during the night if the total energy stored in reserves and the stomach is less than a critical amount. The probability that the bird is killed by a predator is higher if the bird is foraging than if it is resting. Furthermore, the predation risk while foraging increases with the bird's mass. From these assumptions, we use dynamic programming techniques to find the daily foraging routine that minimizes mortality. The principal results are (1) Variability in food finding leads to routines with feeding concentrated early in the day, (2) digestive constraints cause feeding to be spread more evenly through the day, (3) even under fairly severe digestive constraints, the stomach is generally not full and (4) optimal fat reserve levels are higher in more variable environments and under digestive constraints. This model suggests that the characteristic daily feeding pattern of small birds is not due to digestive constraints but is greatly influenced by environmental variability.     

Prey state shapes the effects of temporal variation in predation risk

The ecological impacts of predation risk are influenced by how prey allocate foraging effort across periods of safety and danger. Foraging decisions depend on current danger, but also on the larger temporal, spatial or energetic context in which prey manage their risks of predation and starvation. Using a rocky intertidal food chain, we examined the responses of starved and fed prey (Nucella lapillus dogwhelks) to different temporal patterns of risk from predatory crabs (Carcinus maenas). Prey foraging activity declined during periods of danger, but as dangerous periods became longer, prey state altered the magnitude of risk effects on prey foraging and growth, with likely consequences for community structure (trait-mediated indirect effects on basal resources, Mytilus edulis mussels), prey fitness and trophic energy transfer. Because risk is inherently variable over time and space, our results suggest that non-consumptive predator effects may be most pronounced in productive systems where prey can build energy reserves during periods of safety and then burn these reserves as ‘trophic heat’ during extended periods of danger. Understanding the interaction between behavioural (energy gain) and physiological (energy use) responses to risk may illuminate the context dependency of trait-mediated trophic cascades and help explain variation in food chain length.     

The effect of energy reserves and food availability on optimal immune defence

In order to avoid both starvation and disease, animals must allocate resources between energy reserves and immune defence. We investigate the optimal allocation. We find that animals with low reserves choose to allocate less to defence than animals with higher reserves because when reserves are low it is more important to increase reserves to reduce the risk of starvation in the future. In general, investment in immune defence increases monotonically with energy reserves. An exception is when the animal can reduce its probability of death from disease by reducing its foraging rate. In this case, allocation to immune defence can peak at intermediate reserves. When food changes over time, the optimal response depends on the frequency of changes. If the environment is relatively stable, animals forage most intensively when the food is scarce and invest more in immune defence when the food is abundant than when it is scarce. If the environment changes quickly, animals forage at low intensity when the food is scarce, but at high intensity when the food is abundant. As the rate of environmental change increases, immune defence becomes less dependent on food availability. We show that the strength of selection on reserve-dependent immune defence depends on how foraging intensity and immune defence determine the probability of death from disease.     

Utilization of endogenous reserves and effects of starvation on the health of Prochilodus lineatus (Prochilodontidae)

Prochilodus lineatus Valenciennes (curimbatá) is an important migratory Neotropical fish. It does not feed during spawning migration, and often survives after spawning. The mobilization of energy reserves and some effects of starvation (zero to eight weeks) on fish health were experimentally evaluated. Hepatic glycogen and lipids from the perivisceral fat bodies were the main reserves mobilized during the first four weeks of fasting. During this period, somatic indices and blood parameters showed that fish health was not significantly affected. However, after five weeks of food deprivation, the main energy reserves were depleted and the fish became anaemic. The loss of muscle mass indicates that protein breakdown was an important energy source after the reduction of hepatic and perivisceral reserves. Mortality was increasingly observed from seven weeks of starvation. Prior accumulation of high amounts of reserves is essential to allow movements for long distances during spawning migrations in this species.     

The starvation–predation risk trade-off, body mass and population status in the Common Starling Sturnus vulgaris

It is theoretically and empirically well established that body mass variation in small birds reflects a trade-off between starvation risk and predation risk. This occurs because carrying increased fat reserves reduces starvation risk but also results in a higher predation risk due to reduced escape flight performance and/or the increased foraging exposure needed to maintain a higher body mass. In principle, therefore, the theory of mass-dependent predation risk could be used to understand how a bird perceives and responds to the risks in its environment, because its mass will reflect the predictability of foraging opportunities and predation risk. Mass in birds may then provide a relatively straightforward way of assessing the foraging environment of birds and so the potential conservation problems a species faces. This study tests, for the first time for any species, how body mass changes in response to changing starvation risk, changing predation risk and changing population status. Common Starling Sturnus vulgaris mass varies as predicted by starvation–predation risk trade-off theory: mass is lower when foraging conditions are more favourable and when predation risk is increased. The populations that are declining the most strongly have higher mass, which is most likely indicative of a poor foraging environment, leading to lower relative survival. The results suggest that increased mass in Starlings, and possibly in other species, may provide an indication of the poor quality of the foraging environment and/or rapidly declining populations.     

Effects of “skeleton” photoperiods and high frequency light-dark cycles on the rhythm of cell division in synchronized cultures of Euglena

Summary Two further lines of evidence support the contention (Edmunds, 1966) that the cell cycle in autotrophically grown Euglena can be coupled to an endogenous, circadian biological clock under certain conditions. So-called skeleton photoperiods (LD: 3,6,3:12 and LD: 4,4,4:12) following a complete photoperiod regime entrain the cell division rhythm in the population to a precise 24 hr period, although the step-sizes of the successive fission bursts are always less than 2.00, indicating that not all cells divide in any one 24 hr interval. These findings imply that the continuous action of light is not required for synchronization and suggest that the putative oscillation underlying the rhythm can be phased by discrete light (or dark) pulses or signals.The effects of high frequency LD cycles whose periods were integral submultiples of 24 hr were also investigated. In most regimes (LD:1/4,1/2; LD:1/2,1; LD: 1,2; LD: 1,3; LD: 2,4; LD: 2,6; LD: 4,4) synchronous cell division iccurred in the culture with an average period of 26–27 hr, although only a fraction of the cells divided during any one burst. Similar results were obtained if (i) a synchronized culture was exposed to certain high frequency cycles whose periods were not integral submultiples of 24 hr (e.g., LD: 5,5 or LD: 8,8); (ii) an asynchronous culture (grown in LL) was subsequently exposed to a high frequency cycle; or (iii) a synchronized culture was subjected to a random LD cycle. The synchrony does not break down as long as the given LD regime is imposed and shows some indications of persistence in certain ensuing conditions of continuous illumination.A general formula was derived which predicts the time of division, t _D , for an individual cell: t _D =k+n, where k is the initial phase delay, n is an integer, and is the free-running period of the rhythm observed in the population. These results are interpreted as indicating that the high frequency cycles employed were unable to entrain the circadian oscillation(s) hypothesized to underly and gate cell division, with the result that the rhythm reverted to its free-running period. Exposure to such cycles, however, apparently either initiates a rhythm or synchronizes the phases of the individual oscillations in the populations of cells. The possible direct interaction between energy supply and the observed somewhat variable period lengths is discussed; also, the relevance of stochastic models for the decay of division synchrony in the absence of a recurrent synchronizing procedure is considered.Some of these results were initially reported at the 5th International Congress on Photobiology, Hanover, N.H., U.S.A., August 26–31, 1968.This work was supported by NSF research grants #GB-4140 and #GB-6892 to L. Edmunds.     

State-dependent ideal free distributions

Summary The standard ideal free distribution (IFD) states how animals should distribute themselves at a stable competitive equilibrium. The equilibrium is stable because no animal can increase its fitness by changing its location. In applying the IFD to choice between patches of food, fitness has been identified with the net rate of energetic gain. In this paper we assess fitness in terms of survival during a non-reproductive period, where the animal may die as a result of starvation or predation. We find the IFD when there is a large population that can distribute itself between two patches of food. The IFD in this case is state-dependent, so that an animal's choice of patch depends on its energy reserves. Animals switch between patches as their reserves change and so the resulting IFD is a dynamic equilibrium. We look at two cases. In one there is no predation and the patches differ in their variability. In the other, patches differ in their predation risk. In contrast to previous IFDs, it is not necessarily true that anything is equalized over the two patches.     

Is Foraging Time Limited During Winter? – A Feeding Experiment with Tree Sparrows Under Different Predation Risk

Small passerines are faced with a trade‐off when foraging during winter. Increasing energy reserves makes them more vulnerable to predators, while a low level of reserves exposes them to a high risk of starvation. Whether small birds under these circumstances are allowed to reduce their foraging activity under increased predation risk, for example in feeding sites more exposed to predators, remains controversial in former behavioural and ecological researches. In this study, we investigated the foraging activity of free‐living Tree Sparrow Passer montanus flocks feeding on an artificial feeding platform. The predation risk perceived by the sparrows was manipulated by placing the platform either close to or far from a bushy shelter. Foraging activity, assessed as cumulative activity of sparrows per unit time on the platform, did not differ between the low‐risk and the high‐risk conditions and did not significantly change during the day. Feeding efficiency, assessed as pecking rate, was not either reduced under the high‐risk condition. Our results suggest that sparrows were forced to feed almost continuously during the day in order to maintain their preferred level of energy reserves. However, several behavioural changes helped sparrows to adopt a safer foraging policy when feeding far from cover, as we found in another study. Altogether, sparrow flocks feeding far from cover decreased the overall foraging time (the time when any sparrow stayed on the platform) by approximately 20% as compared to the near cover condition. A possible way to maintain the same level of foraging activity despite of the reduction in overall foraging time is discussed.     

Rice Hypersensitive Induced Reaction Protein 1 (OsHIR1) associates with plasma membrane and triggers hypersensitive cell death

Background  

In plants, HIR (Hypersensitive Induced Reaction) proteins, members of the PID (Proliferation, Ion and Death) superfamily, have been shown to play a part in the development of spontaneous hypersensitive response lesions in leaves, in reaction to pathogen attacks. The levels of HIR proteins were shown to correlate with localized host cell deaths and defense responses in maize and barley. However, not much was known about the HIR proteins in rice. Since rice is an important cereal crop consumed by more than 50% of the populations in Asia and Africa, it is crucial to understand the mechanisms of disease responses in this plant. We previously identified the rice HIR1 (OsHIR1) as an interacting partner of the OsLRR1 (rice Leucine-Rich Repeat protein 1). Here we show that OsHIR1 triggers hypersensitive cell death and its localization to the plasma membrane is enhanced by OsLRR1.     

The Discovery of Intramolecular Stereoelectronic Forces That Drive The Sugar Conformation in Nucleosides and Nucleotides

Abstract

This report summarizes our results⁸ on how the determination of the thermodynamics of the two-state North (N, C2′-exo-C3′-endo) ? South (S,C2′-endo-C3′-exo) pseudorotational equilibrium in aqueous solution (pD 0.6 - 12.0) basing on vicinal ³J_HH extracted from ¹H-NMR spectra measured at 500 MHz from 278K to 358K yields an experimental energy inventory of the unique stereoelectronic forces that dictate the conformation of the sugar moiety in β-D-ribonucleosides (rNs), β-D-nucleotides, in the mirror-image β-D- versus β-L-2′-deoxynucleosides (dNs) as well as in α-D- or L- versus β-D- or L-2′-dNs. Our work shows for the first time that the free-energies of the inherent internal flexibilities of β-D- versus β-L-2′-dNs and α-D- versus α-L-2′-dNs are identical, whereas the aglycone promoted tunability of the constituent sugar conformation is grossly affected in the α-nucleosides compared to the β-counterparts.