小型啮齿动物的繁殖能量代价

繁殖是动物向后代传递和保持遗传信息的方式。因此繁殖的意义是显而易见的,但也需要付出代价。主要代价是能量需求增加。在对小家鼠繁殖能量需求的研究中发现,能量摄入在妊娠期只是稍微增加,而在哺乳期则急剧增加。尽管在妊娠期增加的幅度很小,但这可能反映了消化道和发育的胎儿之间在动物腹中的空间竞争,从而可能使能量摄入受到限制进而影响到繁殖过程。哺乳期间,能量摄入急剧增加,在哺乳后期达到高峰并趋于稳定。对野生鼠的研究也表明,野生鼠妊娠期和哺乳期的能量摄入模式与小家鼠是基本相同的,这样我们在小家鼠研究工作中的发现就具有更普遍的适应意义。对哺乳后期能量摄入的限制机制研究至少已经进行了15 年。能量摄入受消化道消化能力的限制(中心限制假说)或者受乳腺泌乳能力限制(外周限制假说) 的假说,都不能合理地解释一些现有的结果。我们提出了一个新的假说,即能量摄入可能受啮齿动物散热能力的限制(热耗散限制假说)。很久以来,一直认为散热能力是对大型哺乳动物哺乳的一个限制因素,但它在小型啮齿动物中的意义尚不清楚。传统观点认为,啮齿动物哺乳期对褐色脂肪组织产热水平的调节是为了重新分配能量以满足哺乳所需;但现在看来,实际上可能是动物为了避免体温过高而降低其基本的产热水平。我们在这个领域已经有了一些进展,但要利用这些知识来理解即使很简单的生活史权衡等问题也还有很多的工作需要做。     

小型哺乳动物的持续能量收支限制研究进展

综述了小型哺乳动物持续能量收支限制的研究概况和进展。最大持续能量收支在决定物种的地理分布、生存适应、繁殖成功等方面都具有重要意义,但在许多条件下受到限制。食物的丰富度,或者动物自身的摄食、消化和吸收能力似乎不是主要限制因素。持续能量收支可能被外周组织和器官消耗能量的能力限制,即"外周限制"假说;或者机体的散热能力所限制,即"热耗散限制"假说。动物也可能通过衡量季节性繁殖投资的价值,实现最大繁殖输出,即"季节性投资"假说。尽管这些假说得到了一些研究的证实,但仍未阐明持续能量收支限制的机理。本文对相关研究的发展方向进行了展望。     

Limits to sustainable energy budget during lactation in the striped hamster (Cricetulus barabensis) raising litters of different size

A female animal appears to approach an upper limit to the rate of sustained energy intake/metabolic rate (SusEI/MR) during lactation. However, different species of animals may respond differently to the sustainable limit. Here, we measured energy budget during lactation in female striped hamsters raising litters of natural size (Con), and females whose litter size was manipulated during early lactation to support fewer or more pups (minus pups, MP or plus pups, PP). The striped hamsters significantly decreased their body mass and increased food intake from early to late lactation; and MP females had lower weight loss and food intake than the control and PP females. Litter size of the PP group decreased significantly over the period of lactation, and pups were weaned at a similar weight to that of the controls. MP females supported a significantly lower litter mass throughout lactation compared with the control and PP females, but during late lactation the pups from the MP group were significantly heavier. Resting metabolic rate (RMR) did not differ significantly between the three groups and the gross energy intake during peak lactation was 5.0×, 4.2× and 5.0 × RMR for the control, MP and PP females, respectively. Female striped hamsters reached a plateau in food intake at around 14 g/d during peak lactation, which might signify a limit of SusEI at 5.0 × RMR. However, it was not possible to determine whether the limitation on SusEI was imposed centrally by the capacity of the gastrointestinal tract to process food, peripherally by the capacity of the mammary gland to produce milk, or by the capacity of animals to dissipate heat.     

Spare capacity and phenotypic flexibility in the digestive system of a migratory bird: defining the limits of animal design

Flexible phenotypes enable animals to live in environments that change over space and time, and knowing the limits to and the required time scale for this flexibility provides insights into constraints on energy and nutrient intake, diet diversity and niche width. We quantified the level of immediate and ultimate spare capacity, and thus the extent of phenotypic flexibility, in the digestive system of a migratory bird in response to increased energy demand, and identified the digestive constraints responsible for the limits on sustained energy intake. Immediate spare capacity decreased from approximately 50% for birds acclimated to relatively benign temperatures to less than 20% as birds approached their maximum sustainable energy intake. Ultimate spare capacity enabled an increase in feeding rate of approximately 126% as measured in birds acclimated for weeks at −29°C compared with +21°C. Increased gut size and not tissue-specific differences in nutrient uptake or changes in digestive efficiency or retention time were primarily responsible for this increase in capacity with energy demand, and this change required more than 1–2 days. Thus, the pace of change in digestive organ size may often constrain energy intake and, for birds, retard the pace of their migration.     

Energy turnover in European hares is centrally limited during early,but not during peak lactation

We investigated metabolizable energy intake (MEI) and milk energy output in European hares throughout gestation and lactation in females raising three young, i.e., close to maximum litter size in this precocial species. We hypothesized that herbivorous hares may face a central limitation of energy turnover during lactation, imposed by maximum capacity of the gastrointestinal tract. Females were provided with low-energy or high-energy diets, either continually, or during lactation only. Unexpectedly, females on either diet reached identical peak MEIs (>6 times BMR) during late lactation, with females on low-energy diet increasing food intake proportionally. Thus, we reject our hypothesis that in lactating hares, peak MEI is centrally limited. During early lactation, MEI and milk transfer was, however, significantly impaired in females on the low-energy diet, indicating a temporal central limitation due to a time-lag caused by the readjustment of energy intake capacity. Importantly, irrespective of the diet, females significantly increased peak MEI late in the breeding season. Consequently, earlier in the season, when energy reserves are still high, energy throughput was not limited by physiological constraints at all. We conclude that extreme MEI may have fitness costs, and that females maximize lifetime reproductive success by actively down-regulating MEI whenever possible.     

Seasonal intake responses in the nectar-feeding bat Glossophaga soricina

Food intake in nectar-feeding animals is affected by food quality, their energetic demands, and the environmental conditions they face. These animals increase their food intake in response to a decrease in food quality, a behavior named “intake response”. However, their capacity to achieve compensatory feeding, in which they maintain a constant flux of energy, could be constrained by physiological processes. Here we evaluated how both a seasonal change in environmental conditions and physiological constraints affected the food ingestion in the bat Glossophaga soricina. We measured food intake rate during both the wet/warm and dry/cool seasons at sucrose solutions ranging from 146 to 1,022 mmol L⁻¹. We expected that food intake and metabolic demands would be greater during the dry/cool season. Bats ingested ~20% more food in the dry/cool than in the wet/warm season. Regardless of season, bats were unable to achieve a constant flux of energy when facing the different sugar concentrations that we used in our experiments. This suggests that the rate of food intake is physiologically constrained in G. soricina. Using the digestive capacity of bats we modeled their food intake. The analytic model we used predicts that digestive limitations to ingest energy should have an important effect on the ecology of this species.     

Is most glucose absorbed passively in northern bobwhite?

Two hypotheses about gut function have not received general support from studies in birds. Both make assumptions about how natural selection has influenced rates of nutrient uptake. The adaptive modulation hypothesis states that rates of absorption should vary within individuals to accommodate changes in nutrient availability of the diet. The spare capacity hypothesis states that the gut's ability to absorb nutrients should slightly exceed load determined by the organism's food intake. We focus on a recent rejection of these hypotheses in northern bobwhite quail (W. Karasov, personal communication) and demonstrate that a central assumption—that carrier-mediated transport predominates—is not supported. We use a pharmacokinetic technique to show that 52–92% (depending on assumptions of metabolizability and binding) of ingested l-glucose appears in plasma. Because l-glucose is not actively transported, its appearance in plasma must be due to passive absorption. This result suggests that previous studies in birds found uptake capacity to be much less than the observed load because they failed to consider passive absorption. When both passive and carrier-mediated transport are considered, capacity and load are fairly closely matched in quail. Our results also suggest that modulation of carrier-mediated transport may not be selected for in birds, because modulation via passive absorption is faster and requires less energy. An unexplored negative consequence of passive absorption, however, may be nonselective absorption of secondary compounds and toxins.     

Dangerous dive cycles and the proverbial ostrich

Data rarely are available to address the level of predation risk faced by diving animals in different parts of the water column. Consequently, most published research on diving behaviour implicitly assumes – like the proverbial ostrich – that 'unseen' predators are functionally unimportant. We argue that failure to consider diving in a predation risk framework may have precluded many insights into the ecology of aquatic foragers that breathe air. Using existing literature and a simple model, we suggest that fear from submerged predators in several systems might be influencing patch residence time, and therefore the duration of other dive cycle components. These analyses, along with an earlier model of predation risk faced by diving animals at the surface, suggest that dive cycle organisation can be modified to increase safety from predators, but only at the cost of reduced energy gain. Theoretical arguments presented here can seed hypotheses on factors contributing to population declines of diving species. For instance, adjustments to the dive cycle that reduce predation risk might be unaffordable if resources are scarce. Thus, if animals are to avoid imminent starvation or substantial loss of reproductive potential, resource declines might indirectly increase predation rates by limiting the extent to which dive cycles can deviate from those that would maximize energy gain. We hope that ideas presented in this paper stimulate other researchers to further develop theory and test predictions on how predation risk might influence diving behaviour and its ecological consequences.     

Geographic variation in calcium and clutch size

For over a half century numerous hypotheses have surfaced aimed at explaining a key life history trait, the evolution of clutch size in birds. A principal goal has been to explain why clutch size generally increases with latitude both within species and among closely related species. Most hypotheses have stressed food limitation, predation, or seasonality. I present a novel hypothesis to explain geographic variation: a limitation of calcium resulting from broad scale variation in this element. Because the storage capacity of avian medullary bone is limited or nonexistent, during egg formation, female birds must intake supplemental calcium. Yet calcium and other exchangeable bases are much rarer in tropical soils. I briefly review the abundant experimental and observational evidence supporting how calcium limitation affects clutch size and other life history traits, and I present a series of predictions (and apparent support for them), stemming from the calcium variation and limitation hypothesis. The balance of evidence suggests that variability in availability of environmental calcium plays a proximate and ultimate role in the evolution of clutch size. Although this hypothesis is not mutually exclusive with others, it highlights another factor that needs to be considered in studies of the geographic variation in clutch size.     

The role of eyespots as anti-predator mechanisms, principally demonstrated in the Lepidoptera

Eyespots are found in a variety of animals, in particular lepidopterans. The role of eyespots as antipredator mechanisms has been discussed since the 19th Century, with two main hypotheses invoked to explain their occurrence. The first is that large, centrally located eyespots intimidate predators by resembling the eyes of the predators' own enemies; the second, though not necessarily conflicting, hypothesis is that small, peripherally located eyespots function as markers to deflect the attacks of predators to non-vital regions of the body. A third possibility is also proposed; that eyespots intimidate predators merely because they are novel or rarely encountered salient features. These hypotheses are reviewed, with special reference given to avian predators, since these are likely to be the principal visually hunting predators of the lepidopterans considered. Also highlighted is the necessity to consider the potential influence of sexual selection on lepidopteran wing patterns, and the genetics and development of eyespot formation.     

Thermal and digestive constraints to foraging behaviour in marine mammals

While foraging models of terrestrial mammals are concerned primarily with optimizing time/energy budgets, models of foraging behaviour in marine mammals have been primarily concerned with physiological constraints. This has historically centred on calculations of aerobic dive limits. However, other physiological limits are key to forming foraging behaviour, including digestive limitations to food intake and thermoregulation. The ability of an animal to consume sufficient prey to meet its energy requirements is partly determined by its ability to acquire prey (limited by available foraging time, diving capabilities and thermoregulatory costs) and process that prey (limited by maximum digestion capacity and the time devoted to digestion). Failure to consume sufficient prey will have feedback effects on foraging, thermoregulation and digestive capacity through several interacting avenues. Energy deficits will be met through catabolism of tissues, principally the hypodermal lipid layer. Depletion of this blubber layer can affect both buoyancy and gait, increasing the costs and decreasing the efficiency of subsequent foraging attempts. Depletion of the insulative blubber layer may also increase thermoregulatory costs, which will decrease the foraging abilities through higher metabolic overheads. Thus, an energy deficit may lead to a downward spiral of increased tissue catabolism to pay for increased energy costs. Conversely, the heat generated through digestion and foraging activity may help to offset thermoregulatory costs. Finally, the circulatory demands of diving, thermoregulation and digestion may be mutually incompatible. This may force animals to alter time budgets to balance these exclusive demands. Analysis of these interacting processes will lead to a greater understanding of the physiological constraints within which the foraging behaviour must operate.     

Biology is only part of the story ..

The origins and development of human cognition constitute one of the most interesting questions to which archaeology can contribute today. In this paper, we do so by presenting an overview of the evolution of artefact technology from the maker's point of view, and linking that development to some hypotheses on the evolution of human cognitive capacity. Our main hypothesis is that these data indicate that, in the first part of the trajectory, biological limits to cognitive capacity were a major constraint that limited technology, whereas, in the second part, this biological constraint seems to have been lifted and others have come in its place. But these are modifiable by means of conceptual frameworks that facilitate concept innovation and therefore enable learning, thereby permitting acceleration in the pace of change in technology. In the last part of the paper, we elaborate on some of the consequences of that acceleration.     

Reduction of enhanced mammary carcinogenesis in LA/N-cp (corpulent) rats by energy restriction

Restriction of energy intake significantly reduces mammary tumorigenesis in normal rats exposed to carcinogens. Genetically obese LA/N-cp (corpulent) female rats were given 7,12-dimethylbenz[a]anthracene and fed purified diets ad libitum or restricted to 60% of the ad libitum caloric intake. Phenotypically lean littermates were also fed ad libitum. Obese animals developed large mammary tumors more rapidly than genetically normal rats so that 100% of the animals had tumors in less than 16 weeks. Only 21% of the lean animals developed tumors; the energy restricted obese animals had a tumor incidence of 27%. Although obese rats fed the restricted diet weighed significantly less than those fed ad libitum, percent body fat was not reduced, indicating that lean tissue was affected more. Obese animals were markedly hyperinsulinemic (1003 +/- 193 microunits/ml) and energy restriction reduced this to 328 +/- 41; the lean animals had insulin levels of 12 +/- 2. Tumor-bearing rats had higher insulin levels than rats without tumors. These data suggest that body fatness is not directly associated with risk of carcinogenesis. Lean body mass, adipose tissue mass, and their interaction with insulin in its capacity as a growth factor rather than body fatness per se may be determinants of tumor promotion.     

Replacing sweetened caloric beverages with drinking water is associated with lower energy intake

Objective: Reduced intake of sweetened caloric beverages (SCBs) is recommended to lower total energy intake. Replacing SCBs with non‐caloric diet beverages does not automatically lower energy intake, however. Compensatory increases in other food or beverages reportedly negate benefits of diet beverages. The purpose of this study was to evaluate drinking water as an alternative to SCBs. Research Methods and Procedures: Secondary analysis of data from the Stanford A TO Z intervention evaluated change in beverage pattern and total energy intake in 118 overweight women (25 to 50 years) who regularly consumed SCBs (>12 ounces/d) at baseline. At baseline and 2, 6, and 12 months, mean daily beverage intake (SCBs, drinking water, non‐caloric diet beverages, and nutritious caloric beverages), food composition (macronutrient, water, and fiber content), and total energy intake were estimated using three 24‐hour diet recalls. Beverage intake was expressed in relative terms (percentage of beverages). Results: In fixed effects models that controlled for total beverage intake, non‐caloric and nutritious caloric beverage intake (percentage of beverages), food composition, and energy expenditure [metabolic equivalent (MET)], replacing SCBs with drinking water was associated with significant decreases in total energy intake that were sustained over time. The caloric deficit attributable to replacing SCBs with water was not negated by compensatory increases in other food or beverages. Replacing all SCBs with drinking water was associated with a predicted mean decrease in total energy of 200 kcal/d over 12 months. Discussion: The results suggest that replacing SCBs with drinking water can help lower total energy intake in overweight consumers of SCBs motivated to diet.     

Metabolic and hormonal control of the desire for food and sex: implications for obesity and eating disorders

During evolution, the ability to overeat and store the extra energy as glycogen and lipids in specialized tissues must have conferred a reproductive advantage by releasing animals from the need to eat constantly, enabling them to engage in behaviors that improved reproductive success. Mechanisms that inhibited ingestive behavior might have been most adaptive when they caused individuals to stop foraging, hoarding and eating in order to find and court potential mates. Conversely, the ability to abstain from reproductive activities to engage in foraging and eating was probably critical for individual survival during severe energetic challenges because reproductive processes are energetically costly and can be delayed until the energetic conditions improve. The mechanisms that control ingestive behavior most likely evolved under conditions in which both food and mates were available, and thus, our understanding might be limited by our narrow focus on food intake in animals isolated from potential mates, and reproductive behaviors in the absence of food. Our understanding of obesity and eating disorders will be enriched by the study of the choice between ingestive and reproductive behaviors and by a renewed attention to "reproductive" hormones such as gonadal steroids and hypothalamic releasing hormones. Furthermore, leptin and reproductive hormones have both organizational and activational effects on the energy balancing system including those mechanisms that control appetite, body fat content and body fat distribution. Understanding these organizational and activational effects on body fat distribution might lead to a better understanding of sex differences in the propensity to develop obesity, type II diabetes and eating disorders.     

Changes in the Cell Volume of Didinium nasutum During Population Increase*

SYNOPSIS. In 3 species of carnivorous protozoa, the rate of individual food intake per generation declines with an increase in the density of the population. In all, the rate of division remains constant. Three hypotheses may be proposed to explain these phenomena: the individual size decreases, thus bringing about a decline in food need; the individuals in the earlier stages of population growth consume excess food which is passed on to later generations to supplement their food intake; an increase in density within the carnivore population decreases the activity rate, thus reducing the food requirement. Experiments using Didinium nasutum as the carnivore and Paramecium aurelia, syngen 4, as the food source were conducted to test these 3 hypotheses. The results contradicted the 1st hypothesis and part of the 2nd. A new working hypothesis based on the remaining hypotheses is proposed.     

Thermogenesis, energy intake and serum leptin in Apodemus chevrieri in Hengduan Mountains region during cold acclimation

Environmental factors play an important role in the seasonal adaptation of body mass and thermogenesis in small, wild mammals. The purpose of the present study was to test the hypothesis that ambient temperature was a cue to trigger the adjustments in body mass, energy intake, and serum leptin level in Apodemus chevrieri during 42 days of cold exposure. Our data demonstrated that cold acclimation induced a decrease in body mass and a significant increase in energy intake in A. chevrieri. Serum leptin levels were positively correlated with body mass and fat mass. These data suggest that A. chevrieri reduced the body mass and increased energy intake and thermogenic capacity under cold acclimation. Further, serum leptin appears to be involved in the energy intake regulation and thermoregulation.     

Biogeochemistry of selenium and its impact on food chain quality and human health

In areas where soils are low in bioavailable selenium (Se), potential Se deficiencies cause health risks for humans. Though higher plants have been considered not to require this element, the experience with low-Se soils in Finland has provided evidence that the supplementation of commercial fertilizers with sodium selenate affects positively not only the nutritive value of the whole food chain from soil to plants, animals and humans but also the quantity of plant yields. The level of Se addition has been optimal, and no abnormally high concentrations in plants or in foods of animal origin have been observed. Se levels in serum and human milk indicate that the average daily intake has been within limits considered to be safe and adequate. In fact, plants act as effective buffers, because their growth is reduced at high Se levels. They also tend to synthesize volatile compounds in order to reduce excess Se. On the other hand, when added at low concentrations, Se exerts a beneficial effect on plant growth via several mechanisms. As in humans and animals, Se strengthens the capacity of plants to counteract oxidative stress caused by oxygen radicals produced by internal metabolic or external factors. At proper levels it also delays some of the effects of senescence and may improve the utilization of short-wavelength light by plants. High additions are toxic and may trigger pro-oxidative reactions. Thus, the present supplementation of fertilizers with Se can be considered a very effective and readily controlled way to increase the average daily Se intake nationwide.     

Do northern bobwhite quail modulate intestinal nutrient absorption in response to dietary change? A test of an adaptational hypothesis

We acclimated northern bobwhite quail (Colinus virginianus) to either chow (high carbohydrate/low protein) or crickets (low carbohydrate/high protein) and tested predictions of hypotheses based on the premise of the economical design of animals. The adaptive modulation hypothesis predicts that d-glucose uptake would be higher and l-proline uptake lower in bobwhites acclimated to chow. The spare capacity hypothesis predicts that the capacity to absorb d-glucose actively will exceed the estimated nutrient load from daily food intake. There was no significant dietary effect on intestinal d-glucose (P = 0.8) and l-proline (P = 0.7) uptake rates measured in vitro using the everted sleeve technique. In chow eaters maximal mediated d-glucose uptake summed along the entire length of intestine (53 cm) was far too low (7.2 mmol/d) to explain observed rates of glucose absorption in vivo (>35 mmol/d). Hence, both predictions were falsified. In vitro uptake may not be an appropriate measure of the intestine's absorptive capacity because it does not measure possibly important pathways of passive absorption. There is increasing evidence that substantial passive glucose absorption occurs in some birds. If passive absorption predominates the adaptive modulation hypothesis might not apply.