Different Amounts Of Food Research Articles

Dynamics of extinction in coupled populations of the flour beetle Tribolium castaneum

In this study we analyzed the effect of migration on the persistence time of coupled local populations of Tribolium in different environments. Four treatments were set up to compare different levels of environmental heterogeneity. We established high, low, moderate, and no heterogeneity. These levels were estimated by the different amounts of food offered to each population. To investigate how risk spreading works, a stochastic model for two subpopulations was employed. The high heterogeneity treatment resulted in the longest persistence, even though survival analysis revealed no significant difference among treatments. The magnitude of differences in growth rates among subpopulations is probably associated with persistence.

Foraging behaviour in domestic pigs (Sus scrofa): remembering and prioritizing food sites of different value

This experiment investigated whether domestic pigs can remember the locations of food sites of different relative value, and how a restricted retrieval choice affects their foraging behaviour. Nine juvenile female pigs were trained to relocate two food sites out of a possible eight in a spatial memory task. The two baited sites contained different amounts of food and an obstacle was added to the smaller amount to increase handling time. On each trial, a pig searched for the two baited sites (search visit). Once it had found and eaten the bait, it returned for a second (relocation) visit, in which the two same sites were baited. Baited sites were changed between trials. All subjects learnt the task. When allowed to retrieve both baits, the subjects showed no preference for retrieving a particular one first (experiment 1). When they were allowed to retrieve only one bait, a significant overall preference for retrieving the larger amount emerged across subjects (experiment 2). To test whether this preference reflected an avoidance of the obstacle with the smaller bait, 15 choice-restricted control trials were conducted. In control trials obstacles were present with both baits. Pigs continued to retrieve the larger bait, indicating they had discriminated between the two food sites on the basis of quantity or profitability and adjusted their behaviour accordingly when the relocation choice was restricted. This suggests for the first time that domestic pigs have the ability to discriminate between food sites of different relative value and to remember their respective locations.

Evolution of feeding structure plasticity in marine invertebrate larvae: a possible trade-off between arm length and stomach size

The ability of an organism to alter its morphology in response to environmental conditions (phenotypic plasticity) occurs in several species of marine invertebrates. Examples are sea urchin and sand dollar larvae (plutei). When food is scarce, plutei produce longer food-gathering structures (larval arms and a ciliary band) and smaller stomachs than when food is abundant. However, it is unclear whether stomach size is actually induced through changes in morphogenesis or simply by food distending the stomach. Distinguishing between these two hypotheses is possible because plutei morphologically respond to food concentrations and change the length of their food-gathering structures before they are capable of feeding. More importantly, these two hypotheses provide insights to whether a trade-off exists between the response in food-gathering structures and the response in stomach size—a possible explanation for the evolution of feeding-structure plasticity in marine invertebrate larvae. In this study, I investigated whether sea urchin larvae ( Strongylocentrotus purpuratus and S. franciscanus) reared in different amounts of food produced stomachs of different sizes before they were capable of feeding. Prior to having the ability to ingest food, larvae produced larger stomachs and shorter arms when food was abundant than when food was scarce, consistent with the hypothesis that food induced changes in morphogenesis. In addition, there was a strong negative correlation between the magnitude of plasticity in larval arm length and the magnitude of plasticity in stomach size. These results are consistent with the idea that a trade-off exists between the response in arm length and the response in stomach size, and at least in part, explains the evolution of feeding structure plasticity in plutei. This may also explain why feeding-structure plasticity has evolved in larvae of other taxa (e.g. other echinoderms and gastropods).

Snack Consumption and Waste by Preschool Children Served “Cute” versus Regular Snacks

Objective To determine if preschool children would eat and waste different amounts of food when served snacks in a regular versus a “cute” form. Design Preschool children were served 6 snacks in a cute form and the same snacks in a regular form using the same food components during 24 snack sessions. Children served as their own controls. Subjects/Setting Thirty-nine children ages 36 to 60 months (20 girls, 19 boys) in a university child development laboratory. Main Outcome Measures Snack portions eaten and wasted and time for eating. Analysis Data were analyzed using a 2 × 6 factorial analysis of variance, with snack form (cute or regular) and snack components as the 2 factors. Results There was no significant difference in children's snack consumption or waste between cute and regular versions of snacks. It took children an average of 2 minutes longer to eat the cute snacks. Conclusions and Implications There is no apparent advantage to serving cute snacks in terms of increased consumption or decreased waste. Further research should be conducted on children's mealtime behaviors when served cute foods to learn more about children's responses to the overall appearance of their food.

PARENTAL CARE IN RELATION TO BROOD SIZE IN THE HOUSE FINCH

Studies that quantify parental care in birds are often faced with the confounding effects of variation in brood size. That is, nestlings from broods of varying sizes may receive different quantities of food for reasons not entirely related to the parental quality of the adults. To control for variation in brood size, researchers often divide feeding visitation rates by brood size to yield a per-nestling feeding rate. This presents problems, however, if adults adjust food load size in response to variation in brood size. We examined the relationship between brood size and parental care in the herbivorous House Finch (Carpodacus mexicanus) by considering not only visitation rates but also food load sizes. As brood size increased, the overall visitation rate increased but both per-nestling visitation rate and per-nestling food load rate decreased. The relationship between brood size and per-nestling care was similar regardless of whether we considered visitation rate or actual food loads, suggesting that in the House Finch per-nestling visitation rate serves as a reasonable index of total mass of food received by the nestlings. However, we urge caution in assuming that per-nestling visitation rate is an adequate measure of parental care in other species.

Allocation of Growth in Food-Stressed Atlantic Puffin Chicks

ABsmAcr.-In long-lived seabirds that lay a single-egg clutch, allocation of growth to certain body parts may be advantageous for the chick if food is limited. To investigate this, 40 Atlantic Puffin (Fratercula arctica) hatchlings were distributed in seven groups that were raised on different amounts of food to 38 days of age. When food intake was reduced, growth rates were depressed for all characters measured (i.e. body mass and length of the wing, 2nd primary, forearm, head + bill, culmen, skull, tarsus, and middle toe). Head and wing parts grew preferentially relative to the other characters, and onset of growth was delayed in the primaries. All chicks accumulated significant amounts of subcutaneous fat, whereas internal fat deposits were present only in the chicks that received the most food. Received 14 July 1995, accepted 20 March 1996.

Evaluation of Growth Interruption as a Means of Manipulating Scale Patterns for Mass-Marking Hatchery Trout

Abstract Despite the importance of marking fish, there is substantial need for improvement in types of marks used, methods of administering them, and means of mark detection. Our study was conducted to assess the possibility of mass-marking hatchery-reared cutthroat trout Oncorhynchus clarki by inducing recognizable patterns on scales by manipulating feeding regime and water temperature. Changes in feeding regime and, somewhat less precisely, water temperature effectively altered circulus spacing on trout scales. Models based on scale pattern analysis of fish in experimental groups receiving different amounts of food had mean classification accuracies of 95.6, 80.9, 91.2, and 96.4% correct; in groups exposed to fluctuating water temperature, classifications were 95.2, 79.8, and 78.5% correct. Marks were used to accurately predict fish group origin, making this a simple, inexpensive, efficient, and harmless method of mass-marking hatchery trout. This technique will be useful in evaluating the suitability o...

Plasma Metabolite Levels Predict Individual Body-Mass Changes in a Small Long-Distance Migrant, the Garden Warbler

-Change in body mass is an important measurement in many studies addressing changes in energy stores or condition. Usually, change in body mass is measured in birds caught twice, but this has a number of drawbacks (e.g. low number of retraps, retraps not representative of all first captures, adverse effects of first capture on body-mass development). Therefore, we investigated whether plasma metabolites correlate with body-mass change, and which metabolites could be used to predict body-mass change in birds caught once. In an experiment, 20 Garden Warblers (Sylvia borin) were given different amounts of food to induce stable, increasing, and decreasing body masses. Most of the eight plasma metabolites we examined were significantly correlated with the change in body mass between early morning and midday, the time of blood sampling, but not with body mass or various measures of activity. Metabolites that are known to characterize resorption were elevated in birds gaining body mass and metabolites characteristic of fasting were elevated in birds losing body mass. Triglycerides and ,B-hydroxy-butyrate together explained 61% of the variation in body-mass change (triglycerides alone 44% and ,3-hydroxy-butyrate alone 51%). These metabolites may be used to predict body-mass change in birds caught once, provided that the reliability and sensitivity of this method are checked in field tests. Received 27 September 1993, accepted 30 January 1994. BIRDS MAY FACE DEFICITS in their energy balance during the nonactive period of the day, during periods of several days, or even over a series of weeks. They usually build up energy stores in anticipation of such periods and as an adaptive compromise to different selective pressures (e.g. Lima 1986, Lehikoinen 1987, Moreno 1989, Ekman and Hake 1990, Rogers and Smith 1993). This results in fluctuating energy stores between foraging and nonforaging periods (usually day and night) superimposed on longer periods of net energy storage and net energy expenditure (King 1972). For instance, cold and short winter days, bad weather, incubation, feeding young, and migration may all be associated with decreasing energy stores and preceded by energy storage. Body mass is a crude but widely used estimate of energy stores because it is easy and harmless to measure. If corrected for size, body mass may be used as an indication of condition (e.g. Blem 1990). Changes in body mass may give a fairly accurate picture of the fluctuations in energy stores (e.g. Rogers and Rogers 1990). The measurement of changes in body mass of free-living birds is important in a variety of contexts. For instance, it may allow the assessment of the effects of food availability on energy storage in different habitats, under different weather conditions, and under different degrees of competition or predation (e.g. Bibby et al. 1976, Bibby and Green 1983, Buchanan et al. 1985, Lima 1985, Lindstrom 1990, Moore and Yong 1991). Body-mass changes may indicate periods of constraints and periods of adaptive mass variations during the annual cycle (Moreno 1989). Evaluation of body-mass changes may allow study of the pattern and environmental influences of energy storage during stopovers by migrants (e.g. Bibby et al. 1976, Mehlum 1983a, b, Biebach 1985, Biebach et al. 1986, Alerstam and Lindstrom 1990). In free-living birds, changes in body mass generally are measured in birds caught at least twice, usually at the same place. This, however, has a number of drawbacks: (1) changes in body mass are measured only in the subsample of retraps and this may reduce the sample size drastically, especially when trap shyness is a factor; (2) a certain group of birds may not be recaught and, thus, the sample of retraps may not be representative (e.g. Bibby et al. 1976, Biebach et al. 1986, Veiga 1986, Winker et al. 1992); and (3) first capture may have an adverse

Swimming Speed and Swimming Metabolism of Atlantic Cod (Gadus morhua) in Relation to Available Food: A Laboratory Study

A laboratory study was performed on how young Atlantic cod (Gadus morhua) regulate their swimming speed according to available food. A circular tank 15 m in diameter was divided into six sectors, each containing two cod ranging in length from 29 to 42 cm. Live fish 5 cm long on average were used as prey. For the first 48 d food intake was changed every 1 or 2 wk equally for all six groups. During the last 31 d each group received different amounts of food. The mean swimming speed was highest (0.6 bl/s) when food intake was about half the maximum intake and lowest (0.2 bl/s) when food was unlimited or not available. Swimming metabolism was related to food intake in a similar way, but with a larger difference between the most active (50 mg O2∙kg−1∙h−1) and the least active group (10 mg O2∙kg−1∙h−1). Maximum sustained swimming metabolism was estimated to be about 0.6 times the standard metabolism. The estimated energetic cost of swimming as a percentage of energy obtained from the food decreased with food intake from 24% at maintenance ration to 2% at maximum food intake. On a log–log plot swimming speed decreased linearly with time after encounter of prey.

Resting oxygen consumption in rats during food restriction, starvation and refeeding

Oxygen consumption was measured in male rats during starvation and during different regimens of restricted feeding and refeeding after starvation. Changes in oxygen consumption and body mass were mostly parallel, but rats with a very reduced food intake displayed the same reduction in oxygen consumption as starved rats, despite the smaller reduction in body mass. Also, rats fed different amounts of food after starvation had different oxygen consumptions, but displayed the same changes in body mass. Two different refeeding regimens with restricted food amounts either induced a further depression of oxygen consumption (i.e. below starvation oxygen consumption), or a stabilizing of oxygen consumption on the level of starvation. The changes in oxygen consumption during restriction and feeding after starvation indicate that reductions in resting metabolic rate may not always be predicted from either body mass change or food intake.

HETEROCHRONIC DEVELOPMENTAL PLASTICITY IN LARVAL SEA URCHINS AND ITS IMPLICATIONS FOR EVOLUTION OF NONFEEDING LARVAE.

Preexisting developmental plasticity in feeding larvae may contribute to the evolutionary transition from development with a feeding larva to nonfeeding larval development. Differences in timing of development of larval and juvenile structures (heterochronic shifts) and differences in the size of the larval body (shifts in allocation) were produced in sea urchin larvae exposed to different amounts of food in the laboratory and in the field. The changes in larval form in response to food appear to be adaptive, with increased allocation of growth to the larval apparatus for catching food when food is scarce and earlier allocation to juvenile structures when food is abundant. This phenotypic plasticity among full siblings is similar in direction to the heterochronic evolutionary changes in species that have greater nutrient reserves within the ova and do not depend on particulate planktonic food. This similarity suggests that developmental plasticity that is adaptive for feeding larvae also contributes to correlated and adaptive evolutionary changes in the transition to nonfeeding larval development. If endogenous food supplies have the same effect on morphogenesis as exogenous food supplies, then changes in genes that act during oogenesis to affect nutrient stores may be sufficient to produce correlated adaptive changes in larval development.

Biochemical parameters as a measure of food availability and growth in immature rainbow trout ( oncorhynchus mykiss)

1. 1. Rainbow trout held in brackish water (15 parts per thousand) were starved or fed different amounts of food. 2. 2. A significant correlation was found between the growth rates of the different animals and the feed rates. 3. 3. The RNA : DNA ratio in the white epaxial muscle is lowest in starved fish and increases in proportion to the feed rate and individual specific growth rate. The correlations are significant at the P < 0.01 level. 4. 4. Liver metabolism varies according to food availability. 5. 5. The protein synthesis capacity of the liver (RNA: DNA ratio) and liver somatic index increase as the feeding rate increases. It also correlates significantly with the specific growth rates of the different animals, 6. 6. The intermediary metabolism of the central metabolic organ, the liver, varies in the same way. 7. 7. The activities of the NADPH producing liver enzymes glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), isocitrate dehydrogenase (IDH) and malic enzyme (ME) increase as the feed rate (and therefore the specific growth rate) increases. 8. 8. G6PDH and IDH activity in the kidney is influenced to a much lower degree by food intake. 9. 9. Summarizing, it can be stated that biochemical parameters can be used to describe comprehensively the metabolic status and growth of rainbow trout.

Temperature and Food as Factors Influencing Oxygen Consumption of Intertidal Organisms, Particularly Limpets

SYNOPSIS: In intertidal animals rates of oxygen consumption span an order of magnitude (even when standardised for body size and temperature). This may reflect different adaptations to this variable habitat. Temperatures range widely and potentially increase metabolic expenditure during the heat of the day, while periods of feeding may be restricted. The balance between intake and expenditure of energy may limit zonation, body size, and even mode of reproduction. One pattern is that species suffering shortage of food (“conservers”) have low rates of oxygen consumption while those with abundant food (“exploiters”) have high rates. Conservers appear to have several means of reducing metabolic costs, but exploiters maintain high (seemingly wasteful) rates of oxygen consumption. Oxygen consumption cannot, however, be considered separately, since it reflects turnover,including growth rate and reproductive output. There is also the question whether differences between species are genetically controlled, or simply phenotypic responses to food availability. Comparisons of populations with different amounts of food suggest that exploiters substantially reduce oxygen consumption when food is scarce, but conservers are relatively fixed in their pattern of slow growth and low reproductive output, even if food is experimentally increased. Growth and reproduction are positively correlated, at least within taxonomically related groups (as exemplified by patellacean limpets) but both are inversely related to longevity.Thus growth and reproduction are not necessarily “traded off,” but are both low in “conservers” and both high in “exploiters.”

Microclimatic adaptation and differences in food consumption and assimilation efficiency of different shell colour morphs of the land snail Arianta arbustorum

Food consumption and assimilation efficiency of different shell colour morphs adapted to various microclimatic conditions were determined. Five‐factor analysis of variance (adaptation temperature, relative humidity, phenotypic shell colour, age class and time of acclimatization) was used. There are differences between different levels of adaptation temperature and relative humidity in the effect on food consumption in the two morphs. The interaction of these two factors is also significant. There is no effect of the phenotypic shell colour on the food consumption, but there is a relation between shell colour and adaptation temperature. Food consumption is greater in the juveniles. The interactions between age class and adaptation temperature or relative humidity are relevant. Acclimatization to these conditions shows a highly significant effect on the food consumption. The brown and yellow morphs of Arianta arbustorum consumed different amounts of food in relation to the adaptation temperature.Assimilation efficiency is independent of temperature but declines at high relative humidity. There is a relation between adaptation temperature and relative humidity, but not between the phenotypic shell colour and age class factors. The yellow morphs show higher assimilation efficiencies than the brown morphs during cold adaptation to 5 °C and at the highest level of relative humidity (98%).

The Diet of American Robins: An Analysis of U.S. Biological Survey Records

Abstract I present a quantitative description of the diet of American Robins (Turdus migratorius) and consider how food habits (particularly the proportion of fruit eaten and the diversity of individual meals) are influenced by season, habitat, sex, and time of day. The study is based on an analysis of records of stomach contents compiled by the U.S. Biological Survey and the U.S. Fish and Wildlife Service. Across their entire range, robins ate fruits representing over 50 genera and invertebrates representing over 100 families. Diets were diverse even within local regions, and there was no obvious single feeding niche. The major food classes, consumed in every combination, were soft-bodied invertebrates, hard-bodied invertebrates, and fruits. The same taxa (especially fruits of the family Rosaceae and invertebrates of the orders Coleoptera and Lepidoptera) predominated in robins' diets, irrespective of habitat or geographical location, which presumably reflects both selective foraging and the availability of these widespread taxa. The proportion of fruit (by volume) in the diet was much higher in the fall and winter (median values &gt;90%) than in the spring (&lt;10%); summer values were intermediate. The transition from a diet dominated by invertebrates to a diet dominated by fruits occurred over a 1-2-month period. The number of distinct food items in stomachs, a measure of the species diversity of individual meals, was positively correlated with the fraction of invertebrates in the diet. Thus, at the time of year when robins were dependent on fruits for food, the diversity of their meals was also lowest. The degree of fullness of the stomach showed few consistent trends with season or habitat. Despite different sex roles and nutritional requirements, male and female robins did not differ in the proportion of fruit in the diet in any month or in any region. Nor did their stomachs contain different numbers of distinct food items, different amounts of food, or a different range or distribution of prey taxa. Habitat was an important variable explaining dietary differences. The Biological Survey records have unavoidable shortcomings, most notably problems of sampling biases and the inability to correct for differential digestion of food items. Nonetheless, they are a valuable and underused data base for testing hypotheses, generating new questions of ecological interest, and describing in detail the diets of North American bird species.

Transit of pharmaceutical dosage forms through the small intestine.

The gastrointestinal transit of pharmaceutical dosage forms has been measured in 201 studies in normal subjects using gamma scintigraphy. Solutions, small pellets, and single units (matrix tablets and osmotic pumps) were administered with different amounts of food in the stomach, ranging from fasted state to heavy breakfast. Gastric emptying was affected by the nature of the dosage form and the presence of food in the stomach. Solutions and pellets were emptied even when the stomach was in the digestive mode, while single units were retained for long periods of time, depending on the size of the meal. In contrast, measured intestinal transit times were independent of the dosage form and fed state. The small intestinal transit time of about three hours (mean +/- 1 h SEM) has implications for the design of dosage forms for the sustained release of drugs in specific positions in the gastrointestinal tract.

Intracellular potassium as a possible inducer of amino acid transport across hamster jejunal enterocytes.

Brush border membrane potentials (Vm), intracellular K+ activity (aiK) and alanine uptake were measured in different parts of villi in mid-jejunal tissue taken from hamsters fed different amounts of food at high and low environmental temperatures. Basal villus enterocytes (Y cells) were found to have lower values for Vm and aiK than upper villus enterocytes (O cells). Alanine uptake was confined to O cells. The position on the villus where values for Vm and aiK changed, and where alanine uptake could first be seen to take place, depended on the energy intake and environmental temperature at which hamsters were kept. Na+-dependent alanine uptake and Vm were both higher in O cells of hamsters fed 10 kcal day-1 at 30 degrees C (10 k/30 degrees C) compared with animals fed 30 kcal day-1 at an environmental temperature of 12 degrees C (30 k/12 degrees C hamsters). The rates at which enterocytes migrated along the crypt-villus axis, measured separately in thymidine-labelling experiments, were 6.9 and 16.1 micron h-1 for 10 k/30 degrees C and 30 k/12 degrees C hamsters respectively. Both Vm and aiK were estimated, from these measurements, to have increased significantly by the time enterocytes became 30 h old. Alanine uptake began 15 h later. Neither of these parameters were influenced by previous adaptation conditions. The close temporal and variable positional relationship found between changes in aiK and onset of transport suggests that early changes in electrolyte composition might initiate a second phase of development enabling the enterocyte to absorb nutrients. The possibility that other ions besides K+ might also be involved in this aspect of regulation is also discussed.

Life-history Consequences of Body-size Variation in the Milkweed Leaf Beetle, Labidomera clivicollis (Coleoptera: Chrysomelidae)

Adult body size of milkweed leaf beetles, Labidomera clivicollis rogersii (LeConte), varied greatly among beetles given different amounts of food in the last larval stadium. Larger females laid approximately twice as many eggs as did smaller females, regardless of whether they reproduced before or after diapause, or were maintained on unlimited or limited regimens of food availability. Age at first reproduction was significantly delayed among smaller females when adults were maintained on the limited food regimen. Eggs from larger females were heavier and resulted in higher larval survivorship to pupation than eggs from smaller females. Larger adult beetles survived total starvation nearly twice as long as smaller beetles, but adult size had no consistent effects on survival through summer or winter in outdoor cages.

Microalgal food and the growth of the intertidal gastropods Nerita atramentosa Reeve and Bembicium nanum (Lamarck) at four heights on a shore

The rates of growth of two common intertidal gastropods, Nerita atramentosa Reeve and Bembicium nanum (Lamarck), were measured during ≈ 3 months in cages at four heights on a shore at Cape Banks, New South Wales. Replicate cages at each level were used to keep snails at constant densities throughout the experiments. Amounts of food on the substratum in each cage were measured as the concentration of chlorophyll in samples of the rock, which is known to be a reliable estimator of number of algal cells present. Rates of feeding were measured as the number of radular scrapes per min in snails removed to the laboratory, and the sizes of radulae were measured at the end of the experiments. Nerita did not increase in shell-length at the two higher levels on the shore, but grew at the two lower levels. Rate of growth was positively correlated with the amount of chlorophyll in the substratum, and was not apparently affected by the greater period of submersion (and concomitant time available for feeding) towards the lower levels on the shore. There was no difference in rates of feeding at the four heights. Radulae of snails from the lower levels were longer than those from higher on the shore, but this was attributable to the greater overall size of snails from lower levels (i.e. there was no significant difference in relative size of radula from one height on the shore to another). Bembicium increased in width of shell at all levels on the shore, but grew more quickly at all three lower levels than at the top of the shore. There was no difference in rate of feeding, nor in relative size of the radula from one level to another on the shore. Rate of growth was not significantly correlated with the amount of food available, nor with the period of submersion, nor the product of these two variables. The simplest explanation for these results was that sufficient food was available for maximal growth of Bembicium at the three lower levels, but not at the highest level. Thus, only where the amount of food was inadequate, at the highest level, was rate of growth reduced. These results are discussed with reference to the difficulties of determining the relative importance of influences on the growth of intertidal snails, because of inherent confounding of the period of submersion with different amounts of food at different heights on the shore.

Demonstration of basic concurrent-schedules effects with dogs: Choice between different amounts of food

Two experiments studied the ways dogs perform on concurrent operant schedules when choosing between different amounts of food. Experiment 1 allowed six dogs to choose between obtaining different amounts of food available for responding on two levers. Across phases of the experiment, the food amounts were available in four different weight-to-weight ratios, 1:1, 2:1, 3:1, and 4:1. The operant schedule was concurrent VI 60 sec VI 60 sec. The results showed that the dogs “undermatch” when the ratio of their responses on the two levers is plotted against the ratios of the amounts of food they produced by responding on those levers. When the response ratios were recomputed to include only those lever-presses that occurred outside the change-over delay (C.O.D.) period, the severity of undermatching was reduced, and the data averaged across all six dogs showed a matching function. Experiment 2 studied the effect of varying the duration of the C.O.D. when the dogs chose between food-amounts in a 3:1 weight-to-weight ratio. The number of times the dogs alternated between responding on the two levers per session was inversely related to the duration of the C.O.D., but the approximation to matching seems little affected.