Differences In Feeding Rates Research Articles

It’s Tough Out There: Variation in the Toughness of Ingested Leaves and Feeding Behavior Among Four Colobinae in Vietnam

Colobines are similar in their exploitation of a high percentage of leaf matter. However, this observation obfuscates interesting differences among genera of Southeast Asian colobines in morphology and behavior that may be reflected in the degree to which they rely on mastication or gut volume and gut retention time when ingesting and digesting leaves. We detail the use of a laboratory-based method to measure the mechanical properties of foods selected and processed by 4 captive species of Southeast Asian Colobinae —Pygathrix nemaeus, Pygathrix cinerea, Trachypithecus delacouri, and Trachypithecus laotum hatinhensis— at the Endangered Primate Rescue Center (EPRC), Vietnam. We also detail a field method that quantifies chewing rates and chewing behavior via a consumer-grade video camera and laptop computer. Observations in the captive setting permit a degree of experimental control that is not possible in the wild, and the location of the EPRC in the primates’ habitat country permitted us to provide leaves that they encounter and eat in the wild. We collected toughness data with a portable tester designed by Lucas et al. The average toughness of selected leaves does not differ among the taxa, nor does the length of time spent chewing foods. However, there are differences in feeding rate, with Trachypithecus spp. chewing foods twice as fast as Pygathrix spp. Our findings suggest that Trachypithecus spp. emphasize comminution of food by mastication, while Pygathrix spp. emphasize the comminution of leaf matter in the stomach. The hypothesis is supported by data on molar size, gut mass, and gut morphology. We provide new insights into dietary variation among primate species and detail methods that are typically conducted only in a laboratory setting. We augment the findings with additional data on activity, feeding rates, and tooth morphology.

Different crab species influence feeding of the snail Nucella lapillus through trait‐mediated indirect interactions

AbstractIndirect interactions are among the many important factors that influence the community structure of the rocky intertidal zone. Trait‐mediated indirect interactions, in which the presence of a predator or competitor can influence the relationship between two other species, have emerged as vital for understanding community dynamics. This study examined the effect of different crab species on the feeding habits of an intertidal snail, Nucella lapillus. Crab species were defined as being sympatric predatory (Carcinus maenas and Cancer irroratus), sympatric non‐predatory (Uca pugnax and Pagurus longicarpus), or allopatric predatory (Mithrax sculptus and Percnon gibbesi). Nucella lapillus were potentially exposed to risk cues from each of the crab species. Crabs were kept in perforated boxes, which allowed any chemical cues to be emitted but prevented direct contact. Nucella lapillus had significantly lower feeding rates in the presence of sympatric predatory crab species than N. lapillus exposed to either sympatric non‐predatory crabs or allopatric predatory crabs. There was no difference in feeding rate between N. lapillus exposed to the sympatric non‐predatory crabs and to the allopatric predatory crabs. Nucella lapillus in the presence of sympatric predatory crabs had a feeding rate of only 0.07 barnacles per snail per day, whereas N. lapillus housed with non‐predatory crabs and allopatric predatory crabs had rates of 0.11 and 0.12, respectively, suggesting that N. lapillus alter their behavior in response to chemical risk cues from local predators. These results suggest that the ability to detect and respond to risk cues is a selectively evolved trait.

Pitfalls of measuring feeding rate in the fruit fly Drosophila melanogaster

To the editor: Dietary restriction (DR), a reduction of food intake without malnutrition, extends lifespan in many organisms, including the fruit fly Drosophila, where DR can be implemented by food dilution 1. Flies could compensate for reduced nutrient content of food by increasing their feeding rate. Food intake is difficult to measure in Drosophila due to their small size. A recent study used radioactively labelled food to estimate Drosophila feeding rate, and reported compensation for food dilution 2. The authors assumed that the amount of radioactive label in flies depended only upon rate of ingestion. However, retention time of the food can also vary. Without measurement of this variable, a genuine difference in feeding rate could be undetected, or a spurious difference in feeding rate detected. We illustrate this here, with an experiment using food labelled with a non-absorbed food dye 3, and a mathematical model. We measured dynamics of dye accumulation in flies after transfer to labelled food (Figure 1a). Initially, DR and fully-fed flies accumulated label at similar rates. However, rate of accumulation declined faster and label reached lower equilibrium levels in fully-fed than in DR flies. Had dye accumulation reflected only feeding rate, these results would imply that up to 30 minutes there was no feeding rate difference between the groups but that by 3 hours of feeding there was an approximately 1.5-fold higher feeding rate in the DR flies. We therefore considered the possibility of a difference in retention time for the food. We found that DR flies had 45% larger crop size than fully-fed flies (P < 0.0001, Wilcoxon rank sum test) (Supplementary Methods online). When we exposed flies briefly to dye-labelled food, we found that the dye took less than 50 minutes to start appearing in faeces. Thus, by 30-minutes the amount of dye accumulated in the fly reflected feeding rate alone, while after 50 it reflected the rate of label ingestion, the rate of egestion and the gut capacity, which our crop measurements show was increased by DR 4. The use of radioactive labels 2 involves further potential confounding processes than those for a non-absorbed dye, because the amount of isotope present will also depend upon the capacity of the body for the labelled element 2,5. Figure 1 (a) The amount of labelled food present in DR-fed (green) and fully-fed (red) flies at different times after transfer from unlabelled food to food labelled with blue food-dye. Solid lines represent measured dye accumulation and dashed lines represent ... Using data from Geer et al. (1970) for 14C-choline labelled food accumulation by Drosophila (Supplementary Figure 1 and Supplementary Methods), we generated a model: m(t)=−(cs)×[1−exp(s×t)] where m(t) = amount of label in the fly at time t; c = feeding rate, and ; s = fraction of labeled material removed from the fly (rate of label removal (e) divided by the internal label capacity of the fly (p). We assigned arbitrary values to these parameters and observed their effect on label accumulation (Figure 1b, c, d). The accumulation profile (Figure 1b) consists of an ‘initial’ phase when label is taken in and not egested; an ‘intermediate’ phase where label ingestion rate exceeds egestion rate; and an ‘equilibrium’ phase when label egestion and ingestion rates are equal. The amount of label in the fly gives a reliable estimate of feeding rate only during the ‘initial’ phase. During the ‘intermediate’ phase, the amount of label in the fly will underestimate the extent of a genuine difference in feeding rate (Figure 1b, c and d, green versus blue), and will fail to detect the difference once ‘equilibrium’ is reached. For a fly with a greater internal capacity (red), the amount of dye present will over-estimate feeding rate relative to controls once egestion has started (‘intermediate’ phase), to an extent that reaches a maximum at the ‘equilibrium’ phase (Figure 1c). Fitting this model to the data in Figure 1a, DR and fully-fed flies consumed food at equal rates but fully-fed flies turned over 32% of their gut capacity per hour, DR flies turned over only 14%. At equilibrium, the absolute amount of material egested must equal the amount eaten, and therefore DR flies have an approximately two-fold larger gut capacity for labelled food than do fully-fed flies. Thus the conclusion that fruit flies compensate for DR by increasing their feeding rate 2 was inaccurate due to inappropriate use of the method. For longer-term measurements, we have developed an alternative assay that, when appropriately calibrated, offers an accurate measurement of food intake 6.

Effect of altitude on male parental expenditure in Mountain Bluebirds (Sialia currucoides): are higher-altitude males more attentive fathers?

Male investment of time and energy in caring for offspring varies substantially both between and within bird species. Explaining this variation is of long-standing interest to ornithologists. One factor that may affect male care is breeding site altitude, through its effects on climate. The harsher, less predictable abiotic conditions at higher altitudes are hypothesized to favour increased male investment of time and energy in offspring care. We tested this hypothesis by comparing male parental behaviour in Mountain Bluebirds (Sialia currucoides) nesting at 1500 and 2500 m a.s.l. in the Bighorn Mountains of Wyoming, USA. We compared rates of prey delivery to nestlings at these two altitudes at two times: 1–2 days after hatching, when females spend much of their time brooding young, and 12–13 days later, when brooding has ended and nestling energy demands are peaking. High-altitude males fed nestlings 18 and 28% more often than low-altitude males early and later in the nestling stage, respectively, but only the difference in late-stage feeding rates were significant. Like males, females at the high site also fed nestlings significantly more often than females at the low site later in the nestling stage (45% difference in feeding rates). Consequently, the proportion of all feeding trips made by males at the high site (40%) did not differ significantly from that at the low site (44%). Parents at the high altitude may feed nestlings more often to compensate for their greater thermoregulatory costs. Parents may also be attempting to assist nestlings in storing fat and/or attaining a large size and effective homeothermy as quickly as possible to enhance nestling ability to survive bouts of severe weather which are common at high altitudes.

Replacement of fish meal by meat and bone meal in diets for large yellow croaker, Pseudosciaena crocea

A growth experiment was conducted to investigate the effects of replacement of fish meal (FM) by meat and bone meal (MBM) in diets on the growth and body composition of large yellow croaker ( Pseudosciaena crocea). Six isonitrogenous (43% crude protein) and isoenergetic (20 kJ g − 1 ) diets replacing 0, 15, 30, 45, 60 and 75% FM protein by MBM protein were formulated. Each diet was randomly allocated to triplicate groups of fish in sea floating cages (1.0 × 1.0 × 1.5 m), and each cage was stocked with 180 fish (initial average weight of 1.88 ± 0.02 g). Fish were fed twice daily (05:00 and 17:30) to apparent satiation for 8 weeks. The water temperature ranged from 26.5 to 32.5 °C, salinity from 32 to 36‰, and dissolved oxygen content was approximately 7 mg l − 1 during the experimental period. Survival decreased with increasing dietary MBM and the survival in the fish fed the diet with 75% protein from MBM was significantly lower than other groups ( P < 0.05). There were no significant differences in specific growth rate (SGR) among the fish fed the diets with 0 (the control group), 15, 30 and 45% protein from MBM. However, SGR in the fish fed the diets with 60 and 75% protein from MBM were significantly lower than other groups ( P < 0.05). No significant differences in feeding rate were observed among dietary treatments. The digestibility experiment showed that the apparent digestibility coefficients (ADC) of dry matter, protein, lipid and energy of MBM were significantly lower compared with those of FM ( P < 0.05). Essential amino acid index was found to be correlated positively with SGR in the present study, suggesting that essential amino acid balance was important. Body composition analysis showed that the carcass protein and essential amino acids were not significantly affected by dietary MBM. The lipid and n-3 highly unsaturated fatty acid (n-3 HUFA) in fish muscle, however, significantly decreased with increasing dietary MBM. These results showed that 45% of FM protein could be replaced by MBM protein in diets of large yellow croaker without significantly reducing growth. It was suggested that the reduced growth with higher MBM was due to lower digestibility and imbalance of essential amino acids.

Effects of Replacement of Fish Meal by Soybean Meal and Supplementation of Methionine in Fish Meal/Soybean Meal-based Diets on Growth Performance of the Southern Catfish Silurus meridionalis

Two experiments (Experiment I and 11) were conducted to evaluate the effects of dietary soybean meal (SBM) with or without supplementation of methionine on the growth performance of the southern catfish Silurus meridionalis. Fish were fed isonitrogenous (48% crude protein) and isoenergetic (20 KJ/g gross energy) diets for 8 wk in a recirculating rearing system. In Experiment I, six diets containing 0.0,11.6, 23.1, 34.7, 46.3, and 57.9% solvent-extracted SBM as replacement of 0% (control), 13%, 26%, 39%, 52% and 65% fish meal (FM) protein were formulated to examine the effects of dietary SBM on growth performance. Each of the six diets was randomly fed to triplicate tanks, and each tank was stocked with 10 fish (average initial weight 23.78 ± 0.09 g). Fish fed the diets with 13%, 26%, and 39% protein from SBM had significantly higher or similar specific growth rate (SGR) compared with the control group. However, SGR in fish fed the diets with 52% and 65% protein from SBM was significantly lower than other groups (P < 0.05). These results showed that 39% of FM protein could be replaced by SBM protein in diets of the southern catfish without significantly reducing growth. In Experiment II, seven diets were formulated to examine the effects of supplemental methionine on the growth performance of this fish (initial weight of 19.73 ± 0.36 g) fed diets at the two SBM substitution level (SL) for replacing 39% or 52% FM protein. The control diet was the same as that (0% SL) in Experiment I. Three methionine levels (0.00%, 0.12%, or 0.26% at 39% SL, 0.00%, 0.21%, or 0.33% at 52% SL) were separately supplemented to represent two endogenous controls (no methionine supplementation), and to produce diets with the same methionine content as that found in either the body carcass of this fish or the control diet (0% SBM protein), respectively. The results showed that there were no significant differences in feeding rate, digestibility and SGR between fish fed the diets with and without supplementation of methionine at 39% SL. However, supplementation of methionine at 52% SL results in significantly higher SGR and feed efficiency (FE) than that of fish fed the diet without supplementation (P < 0.05). It is suggested that the endogenous methionine content in the diet at 39% SL could meet the requirement for the growth, but is insufficient at 52% SL. The results of the present study indicate that methionine is one of the limiting factors in SBM-based diets to growth of the southern catfish, and supplementation of methionine into diets containing a high content of SBM could improve the growth performance of this fish.

Interpreting Space Use and Behavior of Blue Tang, Acanthurus coeruleus, in the Context of Habitat, Density, and Intra-specific Interactions

We hypothesized that blue tang, Acanthurus coeruleus, territories on sites with low biogenic structure would be larger than territories on sites with relatively high biogenic structure due to differences in the amount and distribution of resources. We tested this hypothesis by tracking blue tang over uncolonized pavement and reef crest, two habitat types at opposite ends of the habitat structure spectrum. We recorded density, feeding rates and aggression events in order to evaluate our findings in the context of a territory model and the ideal free distribution model. Territories of A. coeruleus averaged nearly four times larger on pavement sites than on reef crest sites. Conversely, densities of A. coeruleus were significantly lower on pavement sites. While there was no significant difference in the average rates of movement between habitats, average turning angles were significantly higher on reef crest. There were no significant differences in feeding rates between habitats, suggesting that higher territory sizes and lower densities may allow fish on uncolonized pavement to match resource acquisition of fish on reef crest. The insignificant difference of aggression encounters between habitats suggests that movement and density differences among habitats are not solely legacies of differential settlement.

Variability in food‐processing behavior among white‐faced capuchins (Cebus capucinus) in Santa Rosa National Park, Costa Rica

Observed patterns of variability in the food-processing behavior of white-faced capuchins (Cebus capucinus) across populations may reflect foraging traditions. However, there has been relatively little attention given to intrapopulation variability in food processing among groups and age/sex classes, making recent cross-population comparisons difficult to interpret. In this paper, we provide data on patterns of object use in foraging that we observed at Santa Rosa National Park, Costa Rica, for comparison with published data from a neighboring research site, Palo Verde National Park. We also describe the techniques used to process two food items consumed by Cebus capucinus at Santa Rosa, and discuss the factors that may underlie observed variability. We conducted a 6-month study on two groups of capuchins in 2001, and collected data on general activity and feeding patterns, rates and forms of object use, and distinctive processing techniques employed for two specific foods (Sloanea terniflora and Luehea candida). Rates of object-use behavior at Santa Rosa were considerably higher than those reported for Palo Verde and showed significant variation between groups and age/sex classes, as did patterns of Sloanea and Luehea processing. Observed differences in feeding rates between groups may reflect food availability or relative profitability, whereas variation between age/sex classes seems to reflect differences in the physical capabilities, foraging strategies, and the relative experience of mature and immature animals. Further research is needed to identify how a social context may influence the acquisition of food-processing techniques in juveniles and the development of foraging traditions in social groups.

The relationship between predator density, community composition, and field predation of Colorado potato beetle eggs

Conservation biological control programs seek to increase natural enemy densities through the adoption of more benign farming practices, under the assumption that higher predator densities will lead to more effective pest suppression. However, predator–predator interference may lead to diminishing returns in improved pest control as predator densities increase. We examined the relationship between predator density and predation rates on Colorado potato beetle eggs in production potato fields. These potato fields naturally spanned a 10-fold range in predator density, due to differences in management practices. Periodically through the growing season we simultaneously measured predator densities and subjected sentinel eggs masses to predation, allowing us to correlate predator density and egg predation for each field on each sample date. Egg predation rates were significantly positively correlated with total predator densities, a correlation that was not improved when predator densities were scaled to reflect differences in feeding rates on potato beetle eggs of the constituent predator taxa. There was no correlation between per-capita egg predation rates and predator density, and so no evidence that predator interference increased with increasing predator density. We divided predators into six dominant taxa—dwarf spiders, crab spiders, minute pirate bugs, big-eyed bugs, damsel bugs, and Lygus bugs (together constituting 93% of all predators collected), and a seventh group, “other predators” that included all other, less common, taxa—and examined correlations between all predator combinations and egg predation rates. The highest correlation was between combined densities of the six most common predator taxa, excluding only the “other predators” grouping. This suggests that predators may be largely equivalent in their impact on Colorado potato beetle eggs, and that field scouts might be able to ignore uncommon predator taxa when sampling for natural enemies.

Behavior of Four Species of Fiddler Crabs, Genus Uca, in Southeast Sulawesi, Indonesia

We studied the behaviors of four species of sympatric fiddler crabs on Kaledupa Island, Indonesia. Species differences in activity level, grooming, burrowing and feeding were related to their habitat and food. Uca chlorophthalmus, living in muddy mangrove areas, were inactive and spent most of the time feeding in place. Females fed 50% faster than males and spent more time feeding. U. vocans was the dominant species at the beach in silty sand and was very active. Its feeding rate was about twice that of the former species, females fed more rapidly than males, and many crabs of both sexes fed in droves at the water’s edge during ebb tides. During ebb tides, they spent most of their time feeding, while at flood tide they engaged in a greater variety of activities, including burrow maintenance. They frequently walked while feeding and interacted aggressively. U. tetragonon lived in a pebbly band along one edge of the beach, by a quay. Their feeding rate was comparable in both sexes and slower than that of U. vocans; they fed largely on filamentous algae growing on the quay, which provides better food, and fed faster during flood tide than ebb tide. They spent more time in waving and other sex-related activities, and were seldom aggressive, except during the week of the full moon. Burrowing activities included placing excavated mud balls some distance away from their burrows and re-arranging them. U. dussumieri inhabited the other end of the beach in muddier substrate. They did not have sex differences in feeding rates and their rate of scooping food into their mouths was slow, but feeding claws made multiple pinches of the substrate, thus accumulating more material in each clawful of food.

A single phase of food deprivation provoked compensatory growth in barramundi Lates calcarifer

Compensatory growth of juvenile barramundi, Lates calcarifer (Bloch) was investigated at 28 °C for 8 weeks. Fish were divided into four feeding groups including one group with continuous feeding (C) and three other groups with food deprivation for 1 week (S1), 2 weeks (S2) and 3 weeks (S3), respectively. All starved fish resumed feeding in week 4. Changes in body weight, specific growth rate, feeding rate, food conversion efficiency and apparent digestion rate were examined weekly during the re-feeding period. Chemical compositions of fish were separately determined at the end of week 3 and week 8. Fish starved for 1 week reached the same weight as the control fish after re-feeding for 3 weeks, indicating that complete compensatory growth occurred. Although the specific growth rate in S2 or S3 fish was greater than that in the control fish after re-feeding, neither S2 nor S3 fish reached the same body weight of the control fish at the end. The feeding rates of S1, S2 and S3 fish were greater than those of control fish for a period of 2, 3 and 4 weeks, respectively. No significant differences in feeding rates were found among S1, S2 and S3 fish in the second week of re-feeding. Neither food conversion efficiency nor apparent digestion rate was significantly different among all treatments except that conversion efficiencies of S1 and the control fish were greater than those of S2 and S3 fish in week 7. At the end of food deprivation, the ratio of lipid to lean body mass (LBM) and the concentration of lipid, energy and moisture in S1, S2 and S3 fish were significantly less than those in the control fish, but the protein concentration in the control fish was greater than that in S2 or S3 fish. After re-feeding for 5 weeks, no difference in the concentrations of protein, ash, energy and moisture were detected among all treatments; however, the lipid concentration in S2 and S3 fish was less than that in the control fish. All fish deprived of food displayed a lower ratio of lipid to lean body weight than that in the control fish. Our results indicate that compensatory growth occurred in barramundi and the magnitude and duration of compensatory growth depended on the length of food deprivation.

The influence of diet properties and feeding rates on PCB toxicokinetics in the ring dove.

The influence of diet properties and feeding rate on the uptake and elimination of polychlorinated biphenyls (PCBs) was investigated in ring doves (Streptopelia rissoria). Elimination rates of PCBs were determined in birds dosed with an Aroclor mixture (1242:1248:1260, 1:1:1) and allowed to depurate on uncontaminated high-lipid or low-lipid/high-fiber diets for 105 days. Uptake rates for seven additional PCBs not found within the above Aroclor mixtures were studied in the same birds by feeding high- or low-lipid PCB-spiked diets for 15 days. The two diet treatments contributed to differences in feeding rates, fecal egestion rates, and total fat volume of the birds. Uptake rate constants of PCBs were higher for the low-lipid diet group, whereas PCB assimilation efficiencies were similar between the two groups. Whole-body elimination rates of Aroclor PCBs were most strongly influenced by the chlorine substitution pattern of the congeners, such that PCBs containing an open meta-para site on one of the phenyl rings were rapidly cleared from the animal. Whole-body elimination rates for persistent PCBs could only be determined for PCB 28; other congeners exhibited negligible elimination over the depuration period. For the latter compounds, fecal elimination rates were estimated using the excreta/carcass distribution coefficient and excreta production rates. For PCB 28, the fecal elimination rate was similar to the whole-body elimination rate, suggesting that fecal elimination of persistent PCBs dominates whole body elimination of these compounds. Diet treatment effects were less evident for PCB elimination rates compared to uptake rates. Steady-state biomagnification factors were estimated to range from 1.0 to 5.1 for rapidly cleared PCBs and from 27 to 97 for persistent congeners.

The influence of culture density and enriched environments on the first stage culture of young cuttlefish, Sepia officinalis (Linnaeus, 1758)

The culture of Sepia officinalis hatchlings and juveniles at different densities and enriched environments was investigated. Experiments were conducted to determine effects of culture density and the use of a substrate on growth and survival. Experiment I studied the effect of three different densities (52, 515 and 1544 hatchlings m−2). Experiment II tested the effects of the enriched environment, using a sandy bottom with pvc shelters. Experiment III tested the effects of density on growth, survival, feeding rates and food conversions. Cuttlefish were fed live grass shrimp at rates of 20% body weight per day (BW d−1). Grass shrimp (Palaemonetes varians) was supplied ad libitum as food in all experiments. In experiment I, growth was different between the three densities, with highest growth for density of 515 hatchlings m−2. IGR was of 8.8, 9.6 and 9.2% BW d−1 for the three densities tested, respectively. Both groups of experiment II had similar growth. IGR was of 10.1 and 9.7% BW d−1 for enriched and non-enriched environments, respectively. Densities of 10, 45 and 120 juvenile m−2 were used in experiment III. Significant differences in feeding rates were only found between densities of 10 and 120 cuttlefish m−2 during the last week. Results indicate that culture of cuttlefish hatchlings could be done in a non-enriched environment, with densities not exceeding 500 hatchlings m−2 and minimum bottom areas of about 600 cm2. Densities of 120 juveniles m−2 in a minimum area of about 1083 cm2 should be considered for juveniles between 5 and 25 g.

Laboratory Measurement of Mowing Machine Material Feed Rate

The main objective of this research was to evaluate two possibilities of material feed rate measurement of modern mowing machines equipped with conditioner. The machines are commonly used for harvest of grasses (Lolium multiflorum, Dactylus glomerata, Phleum pratense, etc.), clover (Trifolium pratense, etc.), alfalfa (Medicago sativa) etc. A mowing machine with finger conditioner was equipped with an electronic measuring unit for the purpose of our measurements. The mowing machine's conditioner shaft was supplied with strain gauges placed on a torque-meter and with a RPM optical sensor counter. Together with torque-meter the mowing machine was equipped with a curved impact plate mounted on the machine's material output. It was decided to arrange a laboratory measurement to obtain information about the dependence of conditioner power input and signals from the impact plate on material mass flow. A mixture of grass and alfalfa was used for our measurement. The measurements carried out proved that a very good linear relationship existed between the conditioner's power input, output frequency of the apparatus measuring impact force by means of the impact plate, and material feed rate through the mowing machine. The calculated coefficients of correlation were about 0.95. It is possible to differentiate a material feed rate difference of 0.5kg s−1 using both methods. This accuracy should be sufficient for practical utilisation such as creating yield maps.

Flexibility of size of gills and palps in the Sydney rock oyster Saccostrea glomerata (Gould, 1850) and the Pacific oyster Crassostrea gigas (Thunberg, 1793)

In some estuaries along the East Coast of Australia, the native Sydney rock oyster, Saccostrea glomerata, and the introduced Pacific oyster, Crassostrea gigas, live together in the same areas. Generally, growth of C. gigas is faster than that of S. glomerata. A study was done to reveal whether morphological differences of the main feeding organs, the gills and palps, could explain differences in rate of growth. Held at three different stocking densities, oysters were regularly sampled during a period of 9 months. On each of four sampling times, the ash-free dry mass (AFDM) of the gills (which is related to gill area and pumping rate), palps and of the remaining tissues was measured. Subsequently, the relative mass of gills (mass of gills/total mass) and palps (mass of palps/total mass) and the gill-to-palp ratio were determined. Density had no effect on these parameters, suggesting that an expected smaller supply of food at high densities did not result in larger gills to be able to collect more food per unit time. Mass of gills and palps, as well as their relative mass, were variable in time; increases as well as decreases were observed, indicating that sizes of gills and palps were flexible and thus were not a constant proportion of total body mass or related to size of shells. Differences between species were most striking. Although both species could change the size of gills and palps, the variability of these over time was much larger for C. gigas than for S. glomerata. This suggests that C. gigas may respond better to changes in its environment than S. glomerata. The gill-to-palp ratio of S. glomerata was constant over time, whereas it was variable for C. gigas. Relatively larger gills mainly caused this variability. It was shown that the ratio of S. glomerata was independent of total body mass, whereas lighter C. gigas had a much larger ratio relative to heavier individuals. This means that light (but not necessarily small) C. gigas are able to collect more food per unit time than larger ones. In both species, a positive correlation between the gill-to-palp ratio and subsequent growth existed. Although not yet fully understood, the observed differences in gill- and palp dimensions between species can possibly explain previously observed differences in feeding rates.

Little evidence for visual monitoring of vigilance in zebra finches

Several models of vigilance assume that it can be adjusted in response to levels maintained by companions. However, the assumption of visual monitoring of vigilance has received little empirical scrutiny. Various treatments in a laboratory study manipulated the ability of zebra finches (Taenopygia guttata) foraging in pairs to monitor visually the vigilance of companions. In one treatment, the insertion of an uncovered partition allowed members of a pair to monitor, if needed, the vigilance of their companion. The addition of a partial cover to the partition in a further treatment eliminated any visual monitoring of vigilance by companions on either side of the partition while leaving the view from the rest of the cage unaltered. The final treatment, without a partition, allowed the feeding behaviour of unrestricted birds to be examined. As birds spent most of their time feeding or vigilant, feeding rate was considered a proxy for vigilance. After an initial difference in feeding rate across treatments, related perhaps to the novelty associated with the insertion of a partition, birds obtained food at a similar rate whether visual monitoring of vigilance was possible or not and whether individuals were physically separated or not. In agreement with recent empirical and theoretical findings, the study provides little evidence for visual monitoring of vigilance in birds.

Feeding rates of Littorina striata and Osilinus atratus in relation to nutritional quality and chemical defenses of seaweeds

Algae frequently suffer attack by herbivores and in some cases complete destruction has been observed. Despite the role of herbivores in structuring marine algal communities, herbivore food choice and the algal traits that herbivores use to choose foods are poorly documented. There has been much discussion and disagreement regarding the best method of assessing food preference. Experiments designed to detect preferential feeding involve offering a selection of food types to one or more individual consumers. Ideally, the potential foods are presented simultaneously to the consumer within a single experimental arena so that the consumer has the opportunity to express a dietary choice. However, some experimental designs allow individual consumers to feed for a constant time on individually presented alternative foods, which permits comparisons of feeding rates on these different foods. Such an experiment is not a true feeding preference experiment because no food choice is provided, but the results can reflect differences in attractiveness or palatability of various potential foods that may also be expressed when choices are offered. In this work, discrimination in selection of algae was tested in two invertebrate species common on intertidal rocks in Gran Canaria (Littorina striata and Osilinus atratus) by measuring their feeding rates on 20 common algal species in no-choice laboratory experiments. The algae represented the major floral elements of the local intertidal community. L. striata and O. atratus exhibited high feeding rates on Enteromorpha muscoides and Ulva rigida, whereas Alsidium corallinum and Laurencia corrallopsis were refused. The majority of algal species were readily consumed by these two generalist herbivores. No clearly defined trends in the concentrations of ash, nitrogen, carbon, soluble carbohydrates, soluble proteins, and caloric content in the seaweeds were found to account for these differences in feeding rates. In contrast, feeding rates varied according to the presence of secondary metabolites. The low susceptibility to grazing by snails in the least-consumed algae seems to be associated with the presence of secondary metabolites. We suggest that feeding rates of L. striata and O. atratus on algae are primarily due to avoidance of algal chemical defenses, whereas positive aspects of food quality probably play a lesser role in determining consumption.

Growth and feed utilization in two strains of gibel carp, Carassius auratus gibelio: paternal effects in a gynogenetic fish

Gibel carp (Carassius auratus gibelio Bloch) is a natural gynogenetic fish which requires sperm of the same or related species to activate egg development. The eggs of one gibel carp were divided into two batches. One batch was ‘fertilized’ with sperm from gibel carp (strain DD), and the other ‘fertilized’ with sperm from red common carp (Cyprinus carpio red variety) (strain DR). The juveniles were transferred to the laboratory 36 days post-hatch. Triplicate groups of each strain were fed a formulated diet at either 3% or satiation ration for 8 weeks. At both the restricted and satiation rations, specific growth rate was significantly higher in strain DR than in strain DD. At the 3% ration, there was no significant difference in feeding rate or feed conversion efficiency between the two strains. At the satiation ration, strain DR had a significantly lower feeding rate but higher feed conversion efficiency than strain DD. At the satiation ration, strain DR had a significantly lower intake protein, but higher recovered protein than strain DD. There was no significant difference in faecal protein loss between the two strains. At the 3% ration, strain had no significant effects on intake protein, faecal protein or recovered protein. Neither faecal energy loss nor recovered energy was affected by strain or ration. At both the 3% and satiation ration, final body contents of dry matter and lipid were significantly lower in strain DR than strain DD, while there was no significant difference in protein and energy content between the two strains at either ration level. The results suggested that gibel carp ‘fertilized’ with sperm of common carp grew faster than those ‘fertilized’ with sperm of gibel carp through increased feed conversion efficiency and protein retention.

Growth and provisioning strategies of Northern Fulmars Fulmarus glacialis

Growth and foraging strategies in procellariiforms show a great deal of variation, but the fulmarine petrels are notable in that chicks are fed frequently and develop unusually rapidly. This study examined age‐related and daily variation in provisioning of the Northern Fulmar Fulmarus glacialis throughout the chick‐rearing period at Fair Isle in 1997. In common with many other petrels, meal mass showed an initial rise with age, probably because of a gradual increase in chick gut capacity, but then levelled off. By comparison, feeding frequency showed little age‐specific variation until chicks reached the oldest age‐class, when the number of meals declined to less than a third of the previous level as chicks underwent mass recession prior to fledging. Compared with the limited day‐to‐day variation in mean provisioning rates for the whole sample, food delivery to particular chicks was much more variable, suggesting that differences in feeding rates were determined by stochastic factors influencing the feeding success of individual parents. The caloric density of feeds and their size in relation to adult mass were lower in Northern Fulmars than in most other Procellariiformes. This implies that adults are not heavily dependent on stomach oil formation to raise the energy content of the payload, but rely on a high feeding frequency to maintain adequate rates of energy transfer to chicks.

Feeding Behavior, Habitat Use, and Abundance of the Angelfish Holacanthus passer (Pomacanthidae) in the Southern Sea of Cortés

Feeding behavior and habitat use of the king angelfish, Holacanthus passer, was studied in the southern Sea of Cortes, Mexico. H. passer fed on benthic communities (algae and sessile invertebrates) and in the water column (mainly feces from the damselfish Chromis atrilobata). Although there were not significant differences in feeding rate between sexes, coprophagy was more common in males, while grazing was more common in females. Spatial distribution of size classes followed a pattern of decreasing size with increasing depth. Feeding rate was significantly different among habitats: small females had a higher feeding rate on the bottom, big females and small males had similar feeding rates from the bottom to 3 m above the bottom, and big males had higher feeding rates from 5 m above the bottom to the surface. Habitat was clearly partitioned, and there was significant habitat overlap only between big females and small males. The abundance of H. passer was partly explained (34% of the total variance) by the abundance of the damselfish C. atrilobata. There was a clear trophic association between C. atrilobata schools and H. passer feeding damselfish feces in the water column. The sex ratio male:female of H. passer populations was >1 at several sites, an unusual pattern for a protogynous fish. The sex ratio on the H. passer water column stock was also biased towards males at most sites. Although there is a positive relationship between C. atrilobata abundance and H. passer, there are factors other than damselfish abundance which cause this dominance of males.