Effect of reward level on individual variability in demand feeding activity and growth rate in Arctic charr and rainbow trout

Groups of Arctic charr and rainbow trout were fed by using demand feeders and their individual trigger actuations registered with a PIT-tag (Passive Integrated Transponder) system. Food was supplied at two reward levels, low and high, to five replicate groups of each species for 21 to 27 days. The reward level was defined as the amount of food obtained in response to a single trigger actuation. The effects of reward level on individual demand feeding activity and growth rale were assessed.
As a result of high demand feeding activity, the daily food rations for trout were in excess of their needs at both reward levels. This can be ascribed to the fact that they compensated a low reward level by increasing their bite activity. In contrast, demand feeding activity in charr did not differ significantly between reward levels. Instead, resulting food rations were limiting and excessive, at low and high reward levels, respectively. The variation in bite activity between individuals (measured as their proportional contribution to the total number of trigger actuations within a group) for charr was significantly higher in the low-reward treatment than in the high-reward level. For trout, the variation in bite activity did not differ significantly between treatments. Differences in response to reward level are suggested to be due to the fact that the social hierarchy is weaker in trout than in charr; i.e. the differences in bite activity between dominant and non-dominant individuals are smaller in trout. At both reward levels, the benefit of being dominant, measured in terms of growth rate was significant for charr but non-significant for trout.     

Influence of genetic dissimilarity in the reproductive success and mate choice of brown trout - females fishing for optimal MHC dissimilarity

We examined the reproductive success of 48 adult brown trout (Salmo trutta L.) which were allowed to reproduce in a stream that was controlled for the absence of other trout. Parentage analyses based on 11 microsatellites permitted us to infer reproductive success and mate choice preferences in situ. We found that pairs with intermediate major histocompatibility complex (MHC) dissimilarity mated more often than expected by chance. It appears that female choice was the driving force behind this observation because, compared with other individuals, males with intermediate MHC dissimilarity produced a larger proportion of offspring, whereas female reproductive output did not show this pattern. Hence, rather than seeking mates with maximal MHC dissimilarity, as found in several species, brown trout seemed to prefer mates of intermediate MHC difference, thus supporting an optimality-based model for MHC-dependent mate choice.     

An experimental study of ontogenetic and seasonal changes in the temperature preferences of unfed and fed brown trout,Salmo trutta

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Avoidance of potentially harmful food cannot be socially transmitted between rats

The social transmission of food preferences(STFP) is a behavioural task of olfactory memory, in which an observer rat learns safe food odours from a demonstrator rat, and shows preference for this odour in a subsequent choice test. However, previous studies have failed to detect the transmission of information about food of potential danger and food aversion using STFP test. In this study, we tested how demonstrators' health affects the exchange of odour information and whether observers can learn danger information from an unhealthy demonstrator. As expected, the observer rat formed an odour preference after interacting with a demonstrator rat that had just eaten food containing a new odour, however, odour preference rather than aversion was also formed after interacting with a demonstrator rat injected with LiCl(used to induce gastric malaise). Furthermore, anaesthetized demonstrator rats and half-anaesthetized demonstrator rats, which showed obvious motor deficits suggesting an unhealthy state, also socially transmitted food preferences to observers. These results suggest that the social transmission of food preferences task is independent of a demonstrators' health, and that information about dangerous foods cannot be transmitted using this behavioural task.     

Food preference, keeper ratings, and reinforcer effectiveness in exotic animals: the value of systematic testing

Food preference describes the behavior of selecting between items for consumption; reinforcer effectiveness is the functional effect of that item in controlling behavior. Food preference and reinforcer effectiveness were examined in giant pandas (Ailuropoda melanoleuca) and African elephants (Loxodonta africana). A pairwise comparison between food items was used to assess food preference. High-, moderate-, and low-preference items were selected and tested for reinforcer effectiveness. High-preference items controlled behavior more effectively than less-preferred items. Caregiver ratings of food preferences were also collected for each subject, but these reports did not necessarily coincide with actual subject preferences. Caregiver ratings correlated with the food preferences of only 1 individual of each species; thus, preferences of 1 nonhuman animal may be falsely generalized to all animals of that species. Results suggest that food choice and reinforcer effectiveness should be investigated empirically and not rely on anecdotal reports.     

Evidence for biorhythmicity from the preoptic nucleus of the rainbow trout Salmo gairdnerii under freely swimming conditions--an electrophysiological study

This study describes a technique which allows continuous recording of MUA (Multiple Unit Activity), from the NPO (Preoptic Nucleus), DAP (Dorsal Aortic blood Pressure) and ECG (Electrocardiogram) in freely swimming rainbow trout. From the 21 trout tested, six trout (29%) clearly showed rhythmic patterns of MUA during the five post-operative days (D2-D6). The mean length of rhythmic MUA was about 18 hr (range 6-33 hr) among the six trout during the recording days. Periodic MUA occurred approximately eight times/hr and lasted about 2 min. The maximal frequency of discharges was 20-30 spikes per sec. No change occurred in the mean level of blood pressure from the first operative day to the following post-operative days, where rhythmic MUA appeared or reappeared. These results demonstrate the existence of biorhythmicity within the NPO of freely swimming trout and suggest parallel oscillations in neurohormones secretion.     

The influence of potential predators on the habitat preferenda of emerging brown trout

Much research has focused on the developmental behaviour of fish and it has been shown that their sensory and physical capabilities evolve very quickly during their early life. Thus, ontogenesis could influence fishes preferences for particular environmental factors. Little is known about the habitat preferences of trout during the post-emergence phase and it is not known if they correspond to the preference curves established by the PHABSIM method for the' alevin phase'. Here, the downstream movement and habitat preferences of young emerging trout were studied in a flume. In the absence of predators, alevins preferred a water depth of 20 to 30 cm and pebble rather than gravel substratum. When emergence occurred in an area with 1⁺ trout and sculpin, Cottus gobio , almost all the emergent trout remained cryptic. When visible, most of them were in the shallowest area (10cm depth) where their preference for pebble substratum was less marked. The presence of 1⁺ trout and sculpin increased the movement downstream of young trout by 20% without changing the general and diel patterns of catches. Their presence also reduced the initial growth of 0⁺ trout.     

Plasticity of olfactory orientation to foods in the snailAchatina fulica

Summary Snails were tested in an olfactometer for their orientation to food odours. Measures of odour choice, latency to make a choice and percentage of upwind orientations were obtained after experimental manipulations of dietary experience and hunger motivation. Juvenile snails, given an exclusive diet of a certain food for 12 h or less, oriented preferentially toward the odour of that food when subsequently presented with a choice between it and a novel food odour (Fig. 2). A 48 h period of exposure to a food resulted in a 21 day retention of the odour preference (Fig. 3). There was no evidence for an especially sensitive period for conditioning during the first 50 days of age (Fig. 4); however, adult snails (minimum 1 year old) required longer periods of dietary exposure to establish equivalent food odour memories (Fig. 5). Measures of latency and percentage of upwind orientations confirmed the choice data in demonstrating an effect of dietary experiences on olfactory orientation. Hunger motivation increased the selection for familiar food odours, and reduced the latency (Fig. 6 and Tables 3 and 4). Sensory adaptation appears inadequate to explain the conditioning phenomenon since formation of the preferences was contingent upon ingestion of the foods; exposure to the food odours alone was insufficient (Table 2). The data suggest that orientation preferences derive from a learned association between the odour of a food and its nutritional value.We thank Dr. Donald Kramer for helpful criticism throughout the course of this work. The research was supported financially by the National Research Council of Canada and le Ministère de l'Education du Québec.     

Assassin bugs of the subfamily Diaspidiinae (Heteroptera: Reduviidae): distribution and potential ecological niches

The prediction and definition of the conditions for the potentially suitable ecological niche of the subfamily Diaspidiinae was the main goal of this study. Our research was based on 283 specimens of all known species of assassin bugs belonging to the subfamily Diaspidiinae stored in European museum collections and a set of 21 environmental variables in the form of a 1 × 1 km grid covering Africa and Madagascar. Based on occurrence localities, as well as a digital elevation model and layer of the tree cover‐continuous fields, information about the distribution of each species is given. Using Maxent software, potentially useful ecological niches were modeled, which allowed for the creation of a map of the potential distribution of the members of this subfamily and for determining their climatic preferences. A jackknife test showed that annual precipitation, annual temperature range and tree cover‐continuous fields were the most important environmental variables affecting the distribution of the subfamily Diaspidiinae. An analysis of climatic preferences suggested that the representatives of the subfamily were linked mainly to the tropical climate. An analysis of environmental variables also showed that the subfamily preferred areas with herbaceous vegetation and some trees, and this preference is probably caused by the food preferences of their prey. On the basis of the museum data on the species occurrence, as well as ecological niche modeling methods, we provided new and valuable information on potentially suitable habitat and the possible range of distribution of the subfamily Diaspidiinae along with its climatic preferences.     

Temporal changes in food preferences of wood mice (Apodemus sylvaticus L.)

Diet choice was determined for wild-caught wood mice (Apodemus sylvaticus L.), temporarily confined to cages in the field and offered a choice of 24–26 types of seeds and fruits in 2-h sessions throughout the night. The mice showed an overall preference for some foods over others. The set-up minimized influences on food preferences of predation risk, hunger, food availability and competition. Variation in food preferences was not attributable to differences between individuals, but followed a temporal pattern. The variety of foods eaten showed a bimodal pattern with peaks corresponding to the two most active periods at the beginning and end of the night. Both the amount of food eaten and variation in the amount diminished from the first to the second active period. An expected selection for carbohydrates early in the night and proteins at the end of the night was not found, but sugars were selected for early in the night. These results are discussed in relation to the conflict between an animal's continuous energy requirements and the essentially periodic activity of foraging.     

Effects of trichomes on the behavior and distribution of Platyprepia virginalis caterpillars

Trichomes are an important physical resistance mechanism of plants, as they reduce insect herbivore movement, feeding, and digestion. However, we know little about how trichomes influence herbivore distributions and populations. We conducted laboratory and field experiments to evaluate the preferences of Platyprepia virginalis (Boisduval) (Lepidoptera: Arctiidae) caterpillars to natural and manipulated densities of trichomes on their primary food, Lupinus arboreus Sims (Fabaceae). We then conducted field surveys to determine whether variation in trichome density among lupine bushes affected caterpillar spatial distribution on the landscape. Platyprepia virginalis caterpillars preferred lupine leaves with fewer trichomes in choice and no‐choice experiments. In the field, caterpillar feeding damage was found more often on leaves with fewer trichomes. These preferences scaled up to the level of bushes in the landscape such that more caterpillars were found on bushes with lower trichome densities than on bushes with higher trichome densities. This is one of few studies to show the potential for trichome density to influence herbivore population size and distribution in a natural system at a landscape level. The results are consistent with trichomes functioning as a resistance mechanism with consequences for herbivore choice, performance, and distribution.     

Artificial selection for food colour preferences

Colour is an important factor in food detection and acquisition by animals using visually based foraging. Colour can be used to identify the suitability of a food source or improve the efficiency of food detection, and can even be linked to mate choice. Food colour preferences are known to exist, but whether these preferences are heritable and how these preferences evolve is unknown. Using the freshwater fish Poecilia reticulata, we artificially selected for chase behaviour towards two different-coloured moving stimuli: red and blue spots. A response to selection was only seen for chase behaviours towards the red, with realized heritabilities ranging from 0.25 to 0.30. Despite intense selection, no significant chase response was recorded for the blue-selected lines. This lack of response may be due to the motion-detection mechanism in the guppy visual system and may have novel implications for the evolvability of responses to colour-related signals. The behavioural response to several colours after five generations of selection suggests that the colour opponency system of the fish may regulate the response to selection.     

Food Preference,Keeper Ratings,and Reinforcer Effectiveness in Exotic Animals: The Value of Systematic Testing

Food preference describes the behavior of selecting between items for consumption; reinforcer effectiveness is the functional effect of that item in controlling behavior. Food preference and reinforcer effectiveness were examined in giant pandas (Ailuropoda melanoleuca) and African elephants (Loxodonta africana). A pairwise comparison between food items was used to assess food preference. High-, moderate-, and low-preference items were selected and tested for reinforcer effectiveness. High-preference items controlled behavior more effectively than less-preferred items. Caregiver ratings of food preferences were also collected for each subject, but these reports did not necessarily coincide with actual subject preferences. Caregiver ratings correlated with the food preferences of only 1 individual of each species; thus, preferences of 1 nonhuman animal may be falsely generalized to all animals of that species. Results suggest that food choice and reinforcer effectiveness should be investigated empirically and not rely on anecdotal reports.     

The amount of food ingested in a single meal by rainbow trout offered chopped herring, dry and wet diets

Two-year-old 1·5-kg rainbow trout were held in cages and conditioned by feeding either on low-fat chopped herring (H trout) or dry pellets (P trout) for 15 weeks. Their satiation amounts were then determined under standard conditions. On a wet weight basis H trout ate 2·5-3·5 times more food than P trout; this was sufficient to compensate for the high water content of herring and thereby maintain the dry matter intake. When P trout were offered herring (PH trout) they consumed more food than when offered dry pellets but not as much as H trout. Stomach capacity restricted the intake and their dry matter intake was reduced by c. 40%. When H trout were offered dry pellets (HP trout) they adjusted their intake immediately close to the level of P trout although their larger stomachs could have accommodated more than twice this volume of dry food. The return of appetite after a satiation meal was almost linear with time. Appetite increased at c. 556 mg g^-1 body weight h^-1 for H trout and at 142 mg g^-1 bw h^-1 for P trout. The return of appetite in PH trout was significantly slower (c. 370 mg g^-1 bw h^-1) than in H trout; the previous dietary history of the PH trout limited their capacity to process larger volumes of wet food in a single meal. Fish offered dry diet (P and HP trout) had similar rates of appetite return despite their previous feeding history suggesting that the property of the dry feed itself might limit meal size. The total gastric emptying time of diets of similar dry matter content (with and without large amounts of water) was similar, but the delay time before gastric emptying starts tended to be longer for dry diets. Dry pellets appear to impose a demand for water that prolongs the gastric delay. This water demand is met partly by drinking since the trout fed on dry pellets drank significantly more (436 ± 189 mg kg^-1 h^-1) than unfed and herring-fed trout which drank little or not at all (65 ± 113 and 70 ± 66 mg kg^-1 h^-1 respectively). Dietary water facilitated food processing and increased daily dry matter intake of trout when fed four times a day. When only one satiation meal per day was allowed, dietary water had no effect. It is concluded from this work that, in addition to gastric volume, a short-term limitation on the size of satiation meals in the rainbow trout is the availability of water to moisturize the food and thus to promote gastric digestion and emptying.     

Sympatric relationship between redband trout and non-native brook trout in the Southeastern Oregon Great Basin

Brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) have been widely introduced outside their respective ranges within North America causing declines and displacement of native trout. Yet, successful coexistence of native and non-native trout has received little attention. Here we evaluated the effect of introduced brook trout on the size and density of native redband trout in two invaded sub-basins in southeastern Oregon. In a multi-year study, we investigated whether habitat and fish communities differed between streams and stream reaches where redband trout were allopatric versus where redband trout were sympatric with brook trout. We hypothesized that redband trout would be less dense and have smaller total length in sympatry with brook trout than in allopatry, but that total trout density would not differ. We investigated whether differences in habitat existed between sympatric and allopatric locations that would indicate differentiation in site level habitat preferences for each trout species. We found that sympatric locations had more wood but similar fish community structure. Mean length and densities of redband trout were higher at allopatric locations. However, in most years at sympatric locations total trout density was twice that of allopatric redband trout sites. Using comparable data from an eastern United States system where brook trout are native, sympatric sites had lower densities of brook trout; however, total trout density did not differ. We conclude that invading trout negatively impact native trout densities; but in southeastern Oregon system the negative impact is minimized.     

Timing of predation by rainbow trout controls Daphnia demography and the trophic status of a Minnesota lake

1. Stocking of lakes with rainbow trout is a common practice that presents a potential conflict for lake managers who must balance the interests of anglers with those concerned that zooplanktivory by trout may trigger a trophic cascade and result in decreased water clarity. 2. This study examined how the timing of trout stocking (autumn versus spring) in a Minnesota (U.S.A.) lake affected (i) the population dynamics of their zooplankton food supply (Daphnia pulicaria), (ii) phytoplankton biomass and water clarity and (iii) trout survival. Sizes of both Daphnia and trout populations were estimated acoustically with high‐frequency (192 kHz) sonar. 3. Daphnia were nearly eliminated from the lake during winters after trout were stocked in autumn. In both of these years (1996 and 1997), the Daphnia population was small in the spring, and grew during the summer and into the autumn as the trout population diminished. 4. The lake was then stocked in spring for 2 years (1998 and 1999). This fisheries manipulation alleviated predation over the winter, but increased predation on D. pulicaria during the spring, summer and autumn. However, the high mortality caused by the spring‐stocked trout was offset by even higher rates of reproduction by the relatively large populations of fecund Daphnia that survived the winter in 1998 and 1999. 5. Grazing by these dense populations of Daphnia produced clear‐water phases during May and June that were inhibited in autumn stocking years. In addition, the large Daphnia populations present during the spring and early summer of 1998 and 1999 provided abundant forage for trout. 6. This fisheries manipulation achieved seemingly mutually exclusive management objectives: a robust planktivorous sport fishery, and clear water for other forms of recreation.     

The interactive effects of a gradual temperature decrease and long-term food deprivation on cardiac and hepatic blood flows in rainbow trout (Oncorhynchus mykiss)

The aim of the present study was to determine the extent to which the fish liver is perfused with blood. Transonic? flow probes were therefore implanted around the ventral aorta and hepatic vein(s) to record baseline blood flows in rainbow trout (Oncorhynchus mykiss) previously held under two different feeding regimes (food-deprived or fed to satiation, 8-12 weeks). Fish from both groups were exposed to a gradual temperature decrease (12°C to 5°C) and physical disturbance. Cardiac output (Q), stroke volume (Sv) and hepatic venous blood flow (HVBF) were significantly reduced in food-deprived trout at 12°C. Heart rate was not significantly affected by nutritional status, but was significantly reduced when temperature was decreased to 5°C. Physically disturbing each fish at 12°C and 5°C showed that the performance capacity of the heart was not affected by food deprivation as the capacity to increase Q and Sv was not reduced in the food-deprived group. Overall this study showed that food deprivation in rainbow trout reduced cardiac and hepatic blood flows. However, long-term food deprivation did not affect the capacity of the heart to acutely increase performance.     

Taste attractiveness of different hydrobionts for roach Rutilus rutilus, bitterling Rhodeus sericeus amarus, and rainbow trout Oncorhynchus mykiss

Comparative study of the taste attractiveness of different aquatic invertebrates (daphnids Daphnia longispina and D. pulex, larvae of Chironomidae, whirligig Gyrinus marinus, water skaters Gerris spp.) and plants (duckweed Lemna minor, filamentous alga Cladophora sp.) for roach Rutilus rutilus, bitterling Rhodeus sericeus amarus, and rainbow trout Oncorhynchus mykiss has been performed. Roach’s taste preferences for agar-agar pellets, containing the aqueous extracts of the organisms under study, varies from maximum (daphnids, filamentous alga, larvae of chironomids) to minimum (duckweed) or it can be absent (whirligig). Different taste preferences to pellets containing the extracts of aquatic organisms has been also found in bitterling (daphnids, larvae of chironomids, water skaters) but not in rainbow trout (daphnids, water skaters). It has been suggested that whirligigs and water skaters do not contain any deterrent substances ensuring chemical defense from predators. It has been demonstrated that intraoral gustatory reception of food items follows two alternative behavioral stereotypes that are different from each other in the time during which food items are retained in the oral cavity and the number of manipulations (subsequent grasps of a food item) during the perception process. We discuss the importance of gustatory reception as the major sensory mechanism that ensures feeding selectivity of fish and decreases interspecific competition over food.     

Plant functional traits reveal the relative contribution of habitat and food preferences to the diet of grasshoppers

Food preferences and food availability are two major determinants of the diet of generalist herbivores and of their spatial distribution. How do these factors interact and eventually lead to diet differentiation in co-occurring herbivores? We quantified the diet of four grasshopper species co-occurring in subalpine grasslands using DNA barcoding of the plants contained in the faeces of individuals sampled in the field. The food preferences of each grasshopper species were assessed by a choice (cafeteria) experiment from among 24 plant species common in five grassland plots, in which the four grasshoppers were collected, while the habitat was described by the relative abundance of plant species in the grassland plots. Plant species were characterised by their leaf economics spectrum (LES), quantifying their nutrient vs. structural tissue content. The grasshoppers’ diet, described by the mean LES of the plants eaten, could be explained by their plant preferences but not by the available plants in their habitat. The diet differed significantly across four grasshopper species pairs out of six, which validates food preferences assessed in standardised conditions as indicators for diet partitioning in nature. In contrast, variation of the functional diversity (FD) for LES in the diet was mostly correlated to the FD of the available plants in the habitat, suggesting that diet mixing depends on the environment and is not an intrinsic property of the grasshopper species. This study sheds light on the mechanisms determining the feeding niche of herbivores, showing that food preferences influence niche position whereas habitat diversity affects niche breadth.     

Lek formation by female choice: a simulation study

We used computer simulation to study the formation of matingaggregations under the influence of female choice. Female movementswere not constrained by home range size. Males formed clumpeddispersions if females expressed strong preferences either forlarger leks or for male quality. High travel costs for femalesvisiting leks produced male dispersions that matched the initialdistribution of females. High costs to females for waiting formatings produced evenly dispersed males. Depending on the relativeweights of these factors, a continuous range of outcomes froma single lek per population to evenly dispersed males was produced.The results suggest that effects of female choice on lek evolutionare more likely to be detected by studying the processes bywhich leks form than by characterizing the spatial distributionof displaying males.