Prey Eaten Research Articles

Ontogenetic change in blood oxygen capacity and maximum activity in garter snakes (Thamnophis sirtalis)

There is an ontogenetic increase in the time that garter snakes (Thamnophis s. sirtalis) can maintain maximum activity at 25°C. Newborn snakes are exhausted by 3–5 min of activity while adults can be active for 20–25 min. The increased endurance of adult snakes results from ontogenetic increases in both aerobic and anaerobic energy generation. At rest juvenile and adult snakes have the same whole-body lactic acid concentrations, but at exhaustion adult lactic acid concentrations are 1.5 times those of juveniles. This increase in anaerobic energy production accounts for part of the endurance of adult snakes, but increased aerobic metabolism appears to be more important. Among the mechanisms increasing aerobic metabolism are more effective pulmonary ventilation and a 3-fold ontogenetic increase in blood oxygen capacity. The rapid exhaustion of small garter snakes probably limits the microhabitats they can occupy and the sorts of hunting methods they can employ. Small garter snakes feed only on small prey that are easily subdued. There is an ontogenetic increase in the relative size of prey eaten by garter snakes that parallels the ontogenetic increase in endurance. Adult feeding habits are adopted at the same body size at which adult blood oxygen capacity and endurance are attained.

Predation by the salt marsh killifish Fundulus heteroclitus (L.) in relation to prey size and habitat structure: Consequences for prey distribution and abundance

Laboratory feeding preference experiments show that the maximum size of prey eaten (the snail Melampus bidentatus (Say) and the amphipod Orchestia grillus (Bosc)) increases with increasing size of the predator, Fundulus heteroclitus (L.). Melampus > 7 mm in shell height escape predation by even the largest killifish. In the laboratory, consumption of prey is reduced in high marsh habitat relative to low marsh, particularly in the case of larger fish. Low marsh has few grass stems per unit area, while high marsh is considerably more complex, with dense small stems providing cover for prey and reducing successful fish hunting. The population of Melampus in low marsh within Great Sippewissett salt marsh consists mainly of large snails but this size is rare in high marsh. The size-distribution is inverse for Orchestia, with large amphipods more abundant in high marsh. The construction of fences excluding Fundulus from the marsh surface led to low marsh size-distributions of Melampus and Orchestia resembling those of high marsh, in agreement with the laboratory results. Killifish predation seems to be an important factor regulating the abundance and size-distribution of the two prey species in the two marsh habitats.

Prey-lithium aversions. II: laboratory rats and ferrets

Lithium-induced prey aversions were studied in the laboratory rat and ferret. Both species acquired aversions, blocking consumption of flavored foods and the flesh of mice. In the rat, attack was also blocked when illness immediately followed mouse-killing, when mice were dipped in a strongly aversive flavor, and when illness followed killing and eating of prey dipped in an artificial flavor with strong olfactory and gustatory properties. Testing context was of some importance. The ferret, on the other hand, continued to attack, killing mice with its feet rather than with a bite to the neck. Strong footshock produced a transient inhibition of attack that was specific to the training situation. These results with laboratory species are in distinct contrast to those with wild predators.

Size Selective Predation by Juvenile Chum Salmon (Oncorhynchus keta) on Epibenthic Prey in Puget Sound

Analysis of gut contents shows that juvenile (30–50 mm) chum salmon (Oncorhynchus keta) in Puget Sound select epibenthic organisms as their primary prey. Harpacticoid copepods numerically comprised over 80% of their natural diet in two areas studied, while terrestrial insects and cladocerans were most important in a third area. Calculation of Ivlev (1961) electivity coefficients indicated high selectivity factors for harpacticoids at one site (+0.59 to +0.94). Comparison of fish gut contents with quantitative epibenthic pump samples of available prey shows that prey selection was size related, but opposite that currently reported in the literature (e.g. Brooks and Dodson 1965); that is, the smaller of the available prey was preferred. This was true for both the total available prey size spectrum and the harpacticoid copepod fraction of the prey spectrum. Large numbers of prey eaten per fish suggest that juvenile chum salmon may exert high predation pressure on nearshore epibenthic organisms in Puget Sound during spring.

Food Overlap of Lake-Dwelling Triclads in the Field

In an earlier paper (Reynoldson & Davies 1970), the ability of the four common British species of triclads to co-exist in lotic habitats was explained in terms of 'food refuges' and found to be related to the productivity of the water-body. A food refuge was regarded as that part of the diet on which a particular triclad species was competitively superior. This Sexplanation has been tested by studying the performance of laboratory populations provided with varying combinations of the specific food refuges; prediction of the outcome of interspecific competition based on the food refuge concept was confirmed in all six of the experimental regimes which were completed (Reynoldson & Bellamy 1973). In the 1970 paper which dealt with differences rather than overlap, each food item was expressed as a percentage of the diet so that variations in absolute amounts of prey eaten were not taken into account. In analysing contrasts no use was made of the various indices which have been developed (e.g. Horn 1966; Ricklefs 1966; MacArthur, Recher & Cody 1966). Here, niche overlap is examined using the index of MacArthur & Levins (1967) as described by Ricklefs (1973) for comparison with the earlier results; the competition coefficient of species j with species i is given as

Food Specialization By Individual Trout

Analyses of stomach contents showed that the kinds of prey eaten by brook trout (Salvelinus fontinalis), cutthroat trout (Salmo clarki), and rainbow trout (Salmo gairdneri) were seldom distributed at random among the individuals. Repeated observation of food eaten by individuals in a stream and ponds showed that prey types were eaten in proportions which were characteristic for an individual.Specialization occurred on several different kinds of prey. Although the degree of specialization was higher during shorter intervals, the data suggested that some specialization persisted for half a year. There were no striking correlations between degree of specialization and other individual properties such as size, growth rate, weight of food, number of food items, previous specialization, or area of recapture.In addition to the observations on trout in relatively undisturbed habitats, a field experiment was conducted using laboratory-reared rainbow trout held in small ponds. The food of each trout in the experiment was sampled repeatedly. In analysis of variance, interaction among the individuals and kinds of prey eaten showed that food specialization occurred. Both the absolute and relative abundance of potential prey were constant during the experiment.

Prey selection in naive Elaphe obsoleta (Squamata: serpentes) — A reappraisal

Naive hatchling Elaphe obsoleta were exposed to extracts prepared from prey eaten by young (lizard, frog) and adults (mouse, bird, rat). A preference was shown for the frog (Hyla) extract, a favorite food of young of this species. Young will also accept adult prey species provided they are cut into small pieces. This suggests that prey size is an important determinant of the feeding response in this species.

A predator-prey size relationship for Plaice larvae feeding on Oikopleura

SUMMARYPelagic plaice larvae in the southern North Sea normally feed exclusively on the appendicularian Oikopleura dioica, and the size of prey eaten can be estimated from the Oikopleura faecal pellets found in the larval gutPredator-prey size measurements show that smaller plaice larvae are restricted to smaller prey, and that, as larval growth proceeds, so their feeding versatility increases, to include all sizes of Oikopleura at a late pelagic stage of development. Competition for food will therefore be keenest in the early larval stages, when prey-size restriction prevails. This accords with Hjort's hypothesis regarding brood strengths.As growth proceeds, so plaice larvae begin to discriminate for prey size. At the close of the pelagic phase, the degree of size selection is roughly equal to that of a 6o-mesh to i in. plankton net.