Inheritance and Mapping of Compact (Cmpt), a New Mutation Causing Hypermuscularity in Mice

During selection for protein content in mice at the Technical University of Berlin, individuals showing high protein content and a compact exterior were noted. Animals showing this ``Compact' phenotype were separated to form a new line. The present investigations were carried out on a Hungarian subpopulation of this line, selected for maximum expression of the Compact phenotype, and apparently at fixation for the relevant genes. Fertility and viability of the Compact subpopulation was normal. As compared to normal mice, carcass percentage values for male and female Compact mice were 9.4 and 6.8% greater, respectively; and the muscle:bone weight ratio in males was 1.61-fold greater. The Compact phenotype showed variable expressivity and was of intermediate dominance in males, but almost fully recessive in females. The hypothesis that a single gene is solely responsible for the Compact phenotype was rejected by maximum likelihood analysis. Linkage mapping using selective DNA pooling located a single locus (denoted Cmpt) strongly associated with the Compact phenotype on mouse chromosome 1. Fine mapping, using individual selective genotyping and haplotype analysis, located Cmpt to the region between D1Mit375 and D1Mit21, approximately one third of the way to D1Mit21.     

Changes in growth,appetite, food conversion efficiency and body composition in mice selected for high post-weaning weight gain on restricted feeding

Summary This study aimed to test the hypothesis that if animals were fed the same amount over the same time period, selection of the fastest growers would result in a change in the partitioning of metabolisable energy toward more protein and less fat deposition. Two mouse lines (S1 and S2) were selected for high 5 to 9 week weight gain corrected to mean 5 week weight. Appetite variation between mice was eliminated by feeding a fixed amount to each mouse daily. After 6 generations of selection, the lines were compared with an unselected control (C) on restricted and ad libitum levels of feeding for growth rate, appetite, food conversion efficiency and chemical body composition.Realised heritabilities of 5 to 9 week gain were 0.36+ 0.05 and 0.19±0.04 for S1 and S2 respectively. Nine week weights were increased by an average of 13% on both feeding levels. Most of this increase, particularly in S2, occurred before 5 weeks and was therefore outside the period of measurement used in selection. On ad libitum feeding, selection increased food intake per unit time by 6% but there was no increase per unit body weight. Food conversion efficiency (gain/food) increased by 12%. Compared with controls at 9 weeks, 3% more of the body weights of selected mice was fat and 1% less was protein. These differences were reduced but were still in the same direction when comparisons were made at the same body weight. Thus the expected change in energy partitioning toward greater protein and less fat deposition in the S lines did not occur.It was concluded that the increased growth and energy retention in the S lines was brought about by a reduction in maintenance requirement. To achieve the desired change in energy partitioning using a similar selection scheme, higher levels of dietary protein should be fed, and some measure of protein deposition rather than growth rate used as the selection criterion.     

Food consumption and body composition in mice selected for high wheel-running activity

The effects of genetic selection for high wheel running (13th generation) and prolonged access (8 weeks) to running wheels on food consumption and body composition were studied in house mice (Mus domesticus). Mice from four replicate lines selected for high wheel-running activity ran over twice as many revolutions per day on activity wheels as did mice from four replicate control lines. At approximately 49 days of age, all mice were placed individually in cages with access to wheels and monitored for 6 days, after which wheels were prevented from rotating for the "sedentary" individuals. During the experiment, five feeding trials were conducted and body mass was measured weekly. After 8 weeks, body composition was measured by hydrogen isotope dilution. Across the five feeding trials, mice in the "active" group (wheels free to rotate) consumed 22.4% more food than mice in the "sedentary" group (wheels locked); mice from the selected lines consumed 8.4% more food than mice from the control lines (average of all trials; body mass-corrected values). In females, but not males, we found a significant interaction between selection and wheel access treatments: within the "active" group the difference in food consumption between selected and control animals was greater than in the "sedentary" group. At the end of the study, mice from the "active" and "sedentary" groups did not differ significantly in body mass; however, mice from the selected lines were approximately 6% smaller in body mass. Estimated lean body mass did not differ significantly either between selected and control lines or between wheel-access groups (P>0.3). Mice from selected lines had lower total body fat compared to mice from control lines (P=0.05; 24.5% reduction; LSMEANS) as did mice from the "active" compared to "sedentary" group (P= 0.03; 29.2% reduction; LSMEANS). Under these conditions, a sufficient explanation for the difference in body mass between the selected and control lines was the difference in fat content.     

Effects of ad libitum and restricted feeding on early production performance and body composition of Yorkshire pigs selected for reduced residual feed intake

Residual feed intake (RFI), defined as the difference between observed and expected feed intake based on growth and backfat, has been used to investigate genetic variation in feed efficiency in cattle, poultry and pigs. However, little is known about the biological basis of differences in RFI in pigs. To this end, the objective of this study was to evaluate the fifth generation of a line of pigs selected for reduced RFI against a randomly selected Control line for performance, carcass and chemical carcass composition and overall efficiency. Here, emphasis was on the early grower phase. A total of 100 barrows, 50 from each line, were paired by age and weight (22.6 ± 3.9 kg) and randomly assigned to one of four feeding treatments in 11 replicates: ad libitum (Ad), 75% of Ad (Ad75), 55% of Ad (Ad55) and weight stasis (WS), which involved weekly adjustments in intake to keep body weight (BW) constant for each pig. Pigs were individually penned (group housing was used for selection) and were on treatment for 6 weeks. Initial BW did not significantly differ between the lines (P > 0.17). Under Ad feeding, the low RFI pigs consumed 8% less feed compared with Control line pigs (P < 0.06), had less carcass fat (P < 0.05), but with no significant difference in growth rate (P > 0.85). Under restricted feeding, low RFI pigs under the Ad75 treatment had a greater rate of gain while consuming the same amount of feed as Control pigs. Despite the greater gain, no significant line differences in carcass composition or carcass traits were observed. For the WS treatment, low RFI pigs had similar BW (P > 0.37) with no significant difference in feed consumption (P > 0.32). Overall, selection for reduced RFI has decreased feed intake, with limited differences in growth rate but reduced carcass fat, as seen under Ad feeding. Collectively, results indicate that the effects of selection for low RFI are evident during the early grower stage, which allows for greater savings to the producer.     

Foraging decisions underlying restricted space use: effects of fire and forage maturation on large herbivore nutrient uptake

Recent models suggest that herbivores optimize nutrient intake by selecting patches of low to intermediate vegetation biomass. We assessed the application of this hypothesis to plains bison (Bison bison) in an experimental grassland managed with fire by estimating daily rates of nutrient intake in relation to grass biomass and by measuring patch selection in experimental watersheds in which grass biomass was manipulated by prescribed burning. Digestible crude protein content of grass declined linearly with increasing biomass, and the mean digestible protein content relative to grass biomass was greater in burned watersheds than watersheds not burned that spring (intercept; F_1,251 = 50.57, P < 0.0001). Linking these values to published functional response parameters, ad libitum protein intake, and protein expenditure parameters, Fryxell's (Am. Nat., 1991, 138 , 478) model predicted that the daily rate of protein intake should be highest when bison feed in grasslands with 400–600 kg/ha. In burned grassland sites, where bison spend most of their time, availability of grass biomass ranged between 40 and 3650 kg/ha, bison selected foraging areas of roughly 690 kg/ha, close to the value for protein intake maximization predicted by the model. The seasonal net protein intake predicted for large grazers in this study suggest feeding in burned grassland can be more beneficial for nutrient uptake relative to unburned grassland as long as grass regrowth is possible. Foraging site selection for grass patches of low to intermediate biomass help explain patterns of uniform space use reported previously for large grazers in fire‐prone systems.     

Selection for increased length of reproductive life in mice

Summary An experiment was conducted in mice to examine whether selection can increase reproductive life and lifetime production of progeny. Mice in two lines with litter size standardized at birth and in two lines without standardization were pair-mated at 7 weeks of age and maintained as long as they produced litters up to 382 days. Progeny from the sixth litters were used to maintain the four selected lines, while progeny from the first litters were bred to maintain unselected control lines. Selected and control lines were compared at five and six generations of the selected lines. Contemporary comparisons revealed that the length of reproductive life and most lifetime production traits were significantly greater in the selected than in control lines. Realized heritability of the length of reproductive life ranged from 0.08 to 0.13. It was concluded that the length of reproductive life and lifetime production in mice can be increased by selection.Animal Research Centre Publication No. 1618     

Growth and body composition of mice selected for high body weight

To elucidate the influence of high body weight selection on body compositional relationships, the accumulation of lipid, protein, and ash was investigated in two lines of mice selected for high 42-day body weight (H lines) and an unselected foundation population (FP). The two H lines differed in population size and were designated as the high-large (HL) and high-small (HS) lines. Logistic body growth curves revealed that HL mice exhibited an accelerated growth rate and reached a higher mature body weight than FP or HS mice. Over the range of body weights examined, HL mice had more lipid, less protein, and less ash than FP or HS mice of the same sex and body weight. However, HL lipid accumulation (relative to body weight increase) was not accelerated in comparison to that of FP mice. This study suggests that the existing model of selection-mediated compositional changes requires expansion to account for the ability of high-growth selection to direct an acceleration of body growth without a correlated enhancement of the relative rate of fat accumulation.     

Improved activity of streptozotocin-selected insulinoma cells following microencapsulation and transplantation into diabetic mice

We have recently shown that repeated streptozotocin (STZ) treatment induces the selection of insulinoma cells (RINmS) with both improved resistance to diabetogenic toxins and functional activity, compared to parental RINm cells. The aim of the present study was to estimate the potential of RINmS cells to maintain their engineered characteristics during in vivo hyperglycemic conditions. It was found that microencapsulation and transplantation into diabetic mice preserved a three-fold higher level of insulin content in selected RINmS cells when compared to the parental ones. Retrieval of transplanted encapsulated cells from the peritoneal cavity of diabetic mice had a significantly higher insulin content and a more intense insulin response to secretogogues in selected RINmS cells when compared to retrieved RINm cells. In conclusion, our results show that RINmS cells do not lose their improved functional characteristics after encapsulation and transplantation into diabetic mice.     

Female-predominant expression of testosterone 16 alpha-hydroxylase ("I"-P-450(16)alpha) and its repression in strain 129/J

Using specific testosterone 16 alpha-hydroxylase activity as the basis for selection of fractions during purification, the cytochrome P-450 ("I"-P-450(16)alpha) has been isolated from livers of phenobarbital-treated 129/J female mice [K. Devore, N. Harada, and M. Negishi (1985) Biochemistry, 24, 5632-5637]. An antibody elicited in rabbits to "I"-P-450(16)alpha was used to determine the amount of hepatic microsomal 16 alpha-hydroxylase activity due to "I"-P-450(16)alpha in untreated females and males of the two mouse strains, 129/J and BALB/cJ. The activities inhibited were 0.03 and 0.3 nmol/min/mg protein in the 129/J and BALB/cJ females, respectively. No significant level of "I"-P-450(16)alpha-dependent activity was detected in the microsomes from males of either mouse strain. Immunoblotting of microsomal proteins with the antibody to "I"-P-450(16)alpha revealed approximately a 10-fold greater amount of a 54-kDa protein in the microsomes from BALB/cJ than from 129/J females (0.03 and 0.26 pmol/micrograms protein, respectively). A cDNA clone (R17) for phenobarbital-inducible rat cytochrome P-450 selected "I"-P-450(16)alpha mRNA of mice, indicating a high degree of homology between the mRNAs of mouse "I"-P-450(16)alpha and phenobarbital-inducible rat cytochrome P-450s. Northern and dot hybridization of total mouse liver poly(A)+ RNA with the R17 cDNA probe indicated that the specific content of the hybridizable mRNA was more than 10 times higher in BALB/cJ females than in males, and that the mRNA level in female 129/J mice was very similar to that of 129/J and BALB/cJ males. The repression of "I"-P-450(16)alpha in 129/J females was inherited as an autosomal recessive trait in 129/J and BALB/cJ pairs as indicated by the levels of mRNA in female F1 offspring and the "I"-P-450(16)alpha-dependent hydroxylase activity. Female and male mice of eight more inbred strains (AKR/J, DBA/2J, C57BL/6J, C3H/HeJ, NZB/J, A/J, CBA/CaJ, and P/J) were tested for levels of mRNA. The results showed that the levels of mRNA were always 5- to 10-fold greater in the females than in the corresponding males, although there was some variation in the mRNA content in the males from the different strains. 129/J females appear to be a genetic variant where the female-predominant expression of the mRNA is repressed.     

Effects of voluntary activity and genetic selection on muscle metabolic capacities in house mice Mus domesticus.

Selective breeding is an important tool in behavioral genetics and evolutionary physiology, but it has rarely been applied to the study of exercise physiology. We are using artificial selection for increased wheel-running behavior to study the correlated evolution of locomotor activity and physiological determinants of exercise capacity in house mice. We studied enzyme activities and their response to voluntary wheel running in mixed hindlimb muscles of mice from generation 14, at which time individuals from selected lines ran more than twice as many revolutions per day as those from control (unselected) lines. Beginning at weaning and for 8 wk, we housed mice from each of four replicate selected lines and four replicate control lines with access to wheels that were free to rotate (wheel-access group) or locked (sedentary group). Among sedentary animals, mice from selected lines did not exhibit a general increase in aerobic capacities: no mitochondrial [except pyruvate dehydrogenase (PDH)] or glycolytic enzyme activity was significantly (P < 0.05) higher than in control mice. Sedentary mice from the selected lines exhibited a trend for higher muscle aerobic capacities, as indicated by higher levels of mitochondrial (cytochrome-c oxidase, carnitine palmitoyltransferase, citrate synthase, and PDH) and glycolytic (hexokinase and phosphofructokinase) enzymes, with concomitant lower anaerobic capacities, as indicated by lactate dehydrogenase (especially in male mice). Consistent with previous studies of endurance training in rats via voluntary wheel running or forced treadmill exercise, cytochrome-c oxidase, citrate synthase, and carnitine palmitoyltransferase activity increased in the wheel-access groups for both genders; hexokinase also increased in both genders. Some enzymes showed gender-specific responses: PDH and lactate dehydrogenase increased in wheel-access male but not female mice, and glycogen phosphorylase decreased in female but not in male mice. Two-way analysis of covariance revealed significant interactions between line type and activity group; for several enzymes, activities showed greater changes in mice from selected lines, presumably because such mice ran more revolutions per day and at greater velocities. Thus genetic selection for increased voluntary wheel running did not reduce the capability of muscle aerobic capacity to respond to training.     

Feed intake and protein skeletal muscle in growing mice treated with growth hormone: time course effects

The exogenous recombinant human growth hormone (rhGH) administration on gastrocnemius muscle growth performance and its contribution to body growth of male and female BALB/c mice fed a 12 % protein diet from 25 to 50 days of age, as well as the mechanism of utilization of feed intake to the lean muscle deposition were studied. Male and female weaning mice (21 days of age) were injected subcutaneously for 29 days with rhGH (74 ng x g(-1)) or saline vehicle (control). Feed intake and body weight (BW) were measured daily. At 25, 30, 35, 40, 45 and 50 days of age twenty mice were killed by cervical dislocation and the gastrocnemius muscle was isolated, weighed and the protein content was measured. The rhGH administration caused a biphasic response of BW and muscle growth as a consequence of age-specific feed intake changes. The initial feed intake fall induced the allometric proportion decreases in both muscle growth versus body growth and protein muscle versus muscle growth. That effect was due to ineffient utilization of energy and protein intake on protein muscle store. Later on, the self-controlled increase of feed intake leads to the recovery of muscle weight to control values, through nutrient partitioning toward non protein tissue showing a compensatory muscle growth. This suggests that a higher dietary protein level should be necessary for promoting the protein anabolic effect of GH during weaning.     

Genetic association between nest building and brown adipose tissue thermogenesis in female house mice

Summary Mice selectively bred for either high or low levels of thermoregulatory nest building were cold-acclimated (5°C) for 3 weeks without nesting material; then body weight and food intake were measured. The mice selected for low nest building (Lows) of both sexes showed lower feed efficiencies than the high nest-building mice (Highs), although their body weights were not significantly different (Table 1). This adds to a large body of evidence which suggests that nest building and feed efficiency were influenced by a common mechanism (Lacy et al. 1978; Sulzbach and Lynch 1984; Lunch et al. 1981; Lynch and Roberts 1984).Brown adipose tissue mitochondrial GDP binding and cytochrome c oxidase activity were measured in the above mice. In females, the Lows had 100% higher levels of total GDP binding than the Highs, while no difference between the lines was seen in males (Fig. 2). Thus in the High females, lower energy expenditure through brown fat thermogenesis may account for their greater feed efficiency. In males, the genetic differences in feed efficiency must be due to differences in either thermogenesis in tissues other than brown fat, or mechanisms which reduce heat loss.Abbreviations Highs Mice from lines selectively bred for high levels of nest-building;Lows mice from the low nest-building selected lines     

Feed selection and digestibility by captive giraffe

A digestion trial with subsequent examination of feed selection was conducted using two captive giraffe fed four feedstuffs. Apparent digestibility coefficients were relatively high, indicating that the animals were efficiently utilizing the feedstuffs. However, values could be affected by the possible ingestion of soil containing acid-insoluble ash. A high fiber pelleted feed was eaten in a greater quantity than a low fiber feed, even though the constituents in each feed were the same. Gross energy content of residual hay was higher than gross energy content of the offered hay. whereas neutral detergent fiber and acid detergent fiber content were lower in the residual than in the offered hay. The animals did not appear to be selecting for a diet high in energy or low in fiber. Based on these results and other information, re-classification of giraffe as a facultative concentrate selector or as an intermediate feeder may be appropriate.     

Effect of reciprocal crossing of selected lines of mice

Summary Mass selection of mice was conducted in populations of various size for 16 generations. Each selected population (E) corresponded to an analogous unselected population (C). The experiment was conducted in three replicates. After the 16th generation the replicates of the selected and control lines were crossed.Reciprocal crossing within the control lines gave better results than reciprocal crossing of the selected lines, despite the fact that the selected mice were characterised by a higher inbreeding coefficient. Larger effects were also obtained when crossing smaller rather than larger populations. This result is understandable since the animals from the smaller populations were characterised by higher inbreeding coefficients.The effect of heterosis was higher upon crossing the control lines rather than the selected ones and this caused a decreased the response to selection in almost all the traits investigated.This work was partly supported by U.S. Department of Agriculture, Grant No. FG-Po-348/JB-13/, Project No. PL-ARS-68     

Changes in nitrogen content and protein profiles following <Emphasis Type="Italic">in vitro</Emphasis> selection of NaCl resistant mung bean and tomato

Mung bean and tomato were in vitro selected on media containing 0, 25, 50, 100 and 150 mM NaCl. Two types of media (hormone supplemented media, CB and hormone free media, MS) were used for mung bean using cotyledon explants whereas two types of explants (cotyledons and shoot apices) were used for tomato on MS media. Total-N, protein content, nitrite reductase (NiR) activity and protein protein profiles were checked in selected plants and compared to original non selected ones. NaCl at low concentrations slightly increased total-N in shoots and roots of in vitro selected mung bean and tomato whereas higher concentrations induced significant reductions. Similar increases in protein content were detected at lower concentrations with no significant effects thereover. On the contrary, NaCl gradually inhibited NiR activity. Similar responses of total-N, protein and NiR activity, but with greater magnitudes, were detected in original plants. In addition, NaCl significantly reduced dry weights of shoots and roots of either in vitro selected or, in particular, original intact plants. Moreover, electrophoresis (SDS-PAGE) of protein from shoots of either in vitro selected or intact plants showed that NaCl induced new protein bands while some others were concomitantly disappeared. The induction of one or more of the 86.4, 79, 77.6, 77 and 71.5 kDa bands following in vitro selection and/or the disappearance of the 86 kDa band from intact plants seemed necessary for mung bean resistance. Also, the presence of 86.2 kDa band and/or the loss of the 85.8 and 57.5 kDa bands might be included in tomato resistance. Of these induced bands in mung bean selected on CB media, only two bands were detected in plants selected on MS media. In tomato, two bands lost following selection from cotyledons but only one band lost following selection from shoot apices. These changes in protein pattern therefore might serve as adaptive regulators for resistance to NaCl.     

Comparison of diets fed to southeast Asian colobines in North American and European zoos,with emphasis on temperate browse composition

In May and June 1994 a survey of diets fed to captive southeast Asian colobines in European (n = 12) and North American (n = 9) zoos was conducted. Most diets were very complex, comprising an average of 25 ingredients; 149 different foods were listed in responses. Comparison of diets fed showed that European zoos feed a greater variety of fruits and vegetables, and fewer browse plants, than North American zoos. No standardized diet recommendations, based on ingredient or nutrient composition, are currently available for colobines in captivity. Foods eaten by these primates in nature appear to contain higher amounts of fiber and lower protein and soluble carbohydrates than current zoo diets. Temperate browse plants (n = 11 spp.; leaves plus twigs) sampled in New York in summer and autumn contained higher fiber and lower protein levels than diets fed in this survey, and may approximate the nutrient content of food items selected by free-ranging colobines. Fast-growing roses grown in greenhouses, fed primarily in Europe, contained substantially less fiber and higher protein concentrations than other browses offered, and may not be an appropriate substitute for native foods. © 1996 Wiley-Liss, Inc.     

Growth Rate, Body Composition, Cellular Growth and Enzyme Activities in Lines of TRIBOLIUM CASTANEUM Selected for 21-Day Pupa Weight

Growth rate, body composition, cell number, cell size, and the activity of four dehydrogenase enzymes were studied from 10 to 25 days of age in one control (1C) and three lines (3, 9, 10) of Tribolium castaneum that had been subjected to long term selection for large 21-day pupae weight.-Selected lines were two- to three-fold larger in size than the control line throughout development. No major differences in percent of protein were detected among the lines but at any particular age, the selected lines were found to have a higher fat content than the control line. The differences in fat content were closely correlated with development such that all the lines reached very similar levels of percent of fat just prior to pupation. Water content showed an inverse relation to percent of fat.-Selection was observed to have caused major changes in the cellular response to growth. The selected lines had an average of from 17% to 48% larger cells (measured as protein/DNA) and were found to have from 37 to 62% more cells (measured as total DNA) than the control line at all ages from 10 to 19 days of age. In addition, the selected lines had a higher RNA content at all ages studied and higher RNA:DNA ratios at the young ages. In contrast the enzyme activities of ICDH and LDH were 60% lower. The results are interpreted as indicating that a more efficient metabolic machinery had evolved in the rapidly growing selected lines.     

The palatability of Arctic willow for greater snow geese: the role of nutrients and deterring factors

Temporary feeding on willow buds and leaves by nesting greater snow geese provided us with an opportunity to test the relative importance of nutrients and deterrents in affecting the palatability for geese of a food plant with a high phenol content. Protein, total phenol and fiber (neutral and acid detergent fiber, and lignin) were analyzed in closed and open buds and in rolled and open leaves. Geese feed on willows at the open-buds and rolled-leaf stages but not at the closed-bud and open-leaf stages. Protein content was higher in open buds and rolled leaves (25–27%) than in closed buds and open leaves (19–21%). Phenol content increased during leaf emergence but was already high (14%) in rolled leaves. All plant fibers were very high in closed buds but declined rapidly during leaf emergence. The increase in phenol: protein ratio appeared to be more important than phenol concentration alone in explaining the cessation of feeding by geese on willow leaves whereas the high fiber content of closed buds may explain why they were not eaten. Our results illustrate the value of a multifactorial approach in the study of the food selection process in herbivores.     

Food Consumption, Feed Efficiency, Metabolic Rate and Utilization of Glucose in Lines of TRIBOLIUM CASTANEUM Selected for 21-Day Pupa Weight

Growth rate, body composition, cell number, cell size, and the activity of four dehydrogenase enzymes were studied from 10 to 25 days of age in one control (1C) and three lines (3, 9, 10) of Tribolium castaneum that had been subjected to long term selection for large 21-day pupae weight.—Selected lines were two- to three-fold larger in size than the control line throughout development. No major differences in percent of protein were detected among the lines but at any particular age, the selected lines were found to have a higher fat content than the control line. The differences in fat content were closely correlated with development such that all the lines reached very similar levels of percent of fat just prior to pupation. Water content showed an inverse relation to percent of fat.—Selection was observed to have caused major changes in the cellular response to growth. The selected lines had an average of from 17% to 48% larger cells (measured as protein/DNA) and were found to have from 37 to 62% more cells (measured as total DNA) than the control line at all ages from 10 to 19 days of age. In addition, the selected lines had a higher RNA content at all ages studied and higher RNA:DNA ratios at the young ages. In contrast the enzyme activities of ICDH and LDH were 60% lower. The results are interpreted as indicating that a more efficient metabolic machinery had evolved in the rapidly growing selected lines.     

The physiological basis for faster growth in the Sydney rock oyster, Saccostrea commercialis

Sydney rock oysters were sampled from a mass selection experiment for growth (the "selected" category) and from a control ("not selected") population and held in the laboratory at three ration levels. We evaluated three models to explain faster rates of growth by selected oysters. Selection resulted in oysters feeding at up to twice the rate and with greater metabolic efficiency than controls. A field experiment confirmed that selection leads to faster rates of feeding across a wide range of food concentrations. Selected oysters also grew more efficiently, at a smaller cost of growth (Cg): mean values for Cg were 0.43 J x J(-1) in selected individuals and 0.81 J x J(-1) in the controls. In contrast, oysters in both categories showed similar metabolic rates at maintenance, i.e., at a ration supporting zero growth. There was no evidence that differential energy allocation affected the balance between total metabolic requirements above and below zero net energy balance. By experimenting with selected and control oysters of different sizes and ages, then standardizing the data for size, we found no effects of age on the differences due to selection. Faster-growing oysters feed more rapidly; invest more energy per joule ingested; show a higher net growth efficiency; and are able to allocate less energy per unit of tissue growth, than slower-growing individuals.