Average Prey Size Research Articles

Sexual Size Dimorphism and Feeding Energetics in Anolis carolinensis: Why Do Females Take Smaller Prey than Males?

ABSTRACr. - Costs of feeding were investigated in Anolis carolinensis to determine whether differences in feeding energetics related to sexual dimorphism in head size could explain differences in average prey size taken by male and female lizards in the field. The amount of time required to subdue and swallow prey, as well as the oxygen consumed and lactate produced during feeding, were measured for lizards feeding on a range of prey sizes. There was no difference between the sexes in the amount of lactate produced during feeding. There were however, differences in the amount of time required to subdue and swallow prey and in the amount of oxygen consumed during feeding. Female lizards required more time and more oxygen to consume prey than did males. Energetic considerations, however, are probably not responsible for prey size differentiation as net energy gains are likely to be high and similar for the sexes. While females do spend more time swallowing prey than males, the total amount of time to consume prey is a trivial part of a day. It is probable that the amount of time available to females for activities other than feeding is a less important determinant of fitness than it is for males. However, the longer prey handling times may deter females from selecting large prey because of a probable increase in the risk of predation.

Aspects of the ecology of an introduced anole: Anolis cristatellus in the Dominican Republic

Abstract Anolis cristatellus, a medium-large brown anole native to Puerto Rico, its satellite islands and the Virgin Islands group, has been introduced at La Romana on the south coast of the Dominican Republic. The time and circumstances of introduction are unknown, but it is thought to have occurred before 1920 in the port area southeast of the town. The species has become well established and phenomenally abundant, occupying an area of about 12 km2 and 13 km long. However, it is closely confined to the town of La Romana and adjacent altered areas with parks, gardens and evergreen trees, more mesic in aspect than adjacent relatively natural areas that are characterized by exposed limestone surfaces and chaparral-like formation of thorny shrubs and low, gnarled trees. The introduced A. cristatellus is closely associated with at least three species of native anoles. Anolis distichus is abundant both in the area invaded by A. cristatellus and in the relatively undisturbed habitat where cristatellus is absent. The two species often occur together on the same tree, and their height preferences coincide. Their food habits are similar, but with difference in average size of prey. Ants are a major food source for both. The smaller A. distichus may avoid predation by A. cristatellus through its greater agility. Anolis chlorocyanus is similar in size to A. cristatellus, but where the two co-occur, it is almost confined to trees with smooth and slippery trunks that are avoided by cristatellus; A. cybotes, a large, aggressive, predatory species, similar in habitat preference to A. cristatellus was found only outside the cristatellus area or on its edges. It seems that A. cristatellus is limiting to cybotes perhaps by preying on its hatchlings and outproducing it with shorter generation time and more frequent egg production.

Ecology of gyrfalcons, Falco rusticolus, in the central Canadian Arctic: diet and feeding behaviour

Diet and aspects of feeding behaviour in a population of gyrfalcons (Falco rusticolus) in the Northwest Territories were examined between 1984 and 1986. Three prey species, rock ptarmigan (Lagopus mutus), arctic ground squirrel (Spermophilus parryii), and arctic hare (Lepus arcticus), composed 96.5% of total prey biomass identified. Ptarmigan and hares were taken in May and June of all years (98.2% of biomass). Juvenile squirrels were used extensively in July and August of 1984 and 1985 but not in 1986, when squirrel production fell to almost zero; ptarmigan continued to be the dominant prey species throughout that summer. Because densities of breeding ptarmigan remained relatively constant during the study, but those of juvenile ground squirrels did not, it appeared that gyrfalcons responded functionally to varying availability of prey. Mean weight (250 g) of prey taken by male gyrfalcons was significantly less than the weight (330 g) of prey taken by females. As predicted by optimal foraging theory, average size of prey brought to the nest increased as time away from the nest increased. Conditions of food abundance were observed at most nests, suggesting that the amount of food available during the nestling period was not limiting production. We suggest that annual production is a function of spacing of pairs, which is set during courtship and prelaying, when prey availability is at its yearly low and when males must forage for both members of the pair. The fact that most gyrfalcon pairs initiated laying only after the spring arrival of migrating ptarmigan is consistent with this conclusion.

Synchronous diving by surf scoter flocks

We studied the diving and surfacing synchrony of foraging flocks of wintering surf scoters (Melanitta perspicillata). Our data support the hypothesis that synchronous diving is an adaptation that reduces kleptoparasitism by glaucous-winged gulls (Larus glaucescens), which frequently attend foraging flocks. We developed a statistical method for measuring and comparing synchrony between flocks, and applied it to videotape records of 30 flocks. The results show that diving and surfacing are highly synchronous, and that there is a large variation between flocks in the degree of synchrony exhibited. The most pronounced effect is for surfacing synchrony to be higher in the presence of gulls. This seems to arise because individual birds curtail their dives so that less synchrony is lost between diving and surfacing during a group dive. This curtailment of dive length may lead to a reduction in the average size of prey captured.

The diet of the eastern short-horned lizard (Phrynosoma douglassi brevirostre) in Alberta and its relationship to sexual size dimorphism

The diet of Phrynosoma douglassi brevirostre in Alberta was investigated and potential prey abundance was examined. The Alberta populations are less strongly myrmecophagous than the average for the species, when the diet is broken down by prey item counts. Breakdown of the diet by prey dry weight shows that ants, nonsilphid Coleoptera, and Orthoptera are the major components of the diet. The range of prey sizes taken is large, but the species concentrates on a small range of prey sizes, from 2.1 to 6.0 mm in length. Arthropod abundances are significantly different between all three study sites, and there is always a significant difference in distribution between the pooled pitfall samples and the pooled gut samples of any one study site. The general dissimilarity in distribution between the pooled gut samples of any two study sites suggests some dietary flexibility. The size range of prey items taken by the large lizards includes and slightly exceeds the size range of prey items taken by medium-sized lizards, and there is a significant positive correlation of average prey size with lizard snout–vent length. There are significant differences in the prey sizes, prey dry weights, and prey item counts between large lizards and medium-sized lizards, indicating intersexual dietary partitioning.

Resource seasonality and fish diets in an Illinois stream

The purpose of this study was to evaluate the intensity of competition for food among 9 species of stream fishes that primarily eat aquatic invertebrates. The taxonomic and size composition, and numerical abundance of aquatic invertebrates were monitored for one year using drift and benthic samples. Diet data were obtained from stomachs of fishes captured at the same time and place that invertebrate sampling was done. Diet characteristics examined included taxonomic and size composition, number of prey per fish, and diet breadth. Drifting invertebrates were more abundant early in the year (March–June) than later (July–January). The summer-early fall scarcity of invertebrates was especially notable among those>3.6 mm long, which comprised the bulk of prey found in fish stomachs. Average prey size eaten by a fish species was positively correlated with fish mouth size, but interspecific overlap in prey size was extensive. Cyprinids as a group (5 species) ate proportionally fewer small (< 3.6 mm long) prey from July to January than did the centrarchids and stonecat. Taxonomic compositions of available invertebrates and fish diets varied markedly among sampling dates, but the use of prey taxa by fishes was not correlated with the availability of those taxa. Use of aquatic prey taxa was generally similar among fish species, but cyprinids as a group ate proportionally more terrestrial prey from July to January than did the centrarchids and stonecat. Diet breadths for all species increased as food levels declined, indicating that these fishes experienced resource depression. Food scarcity was evidently more severe for cyprinids since their stomachs contained few prey through the summer and fall relative to the centrarchids and stonecat. Though the fish species studied probably compete for food in the summer and fall, this competition did not account for the community structure observed.

Potential impact of size-selective planktivory on phosphorus release by zooplankton

A laboratory model of phosphorus release by Daphnia rosea was implemented for published zooplankton data from lakes characterized by different abundances of size-selective planktivores. Size-selective feeding reduces average prey size and increases P release per unit biomass. At the system level, decreased prey standing crop associated with higher planktivore abundance could balance the size dependent increase in P release rate. However, estimates of both net reduction and net increase in rate of P release from zooplankton resulted from model application. Size-selective feeding might be important not only in energetic or evolutionary relationships between predator and prey but also in determination of the relative importance of different pathways of phosphorus flow through pelagic systems.

Dietary Relationships among Shrubsteppe Passerine Birds: Competition or Opportunism in a Variable Environment

The suggestion that in less stable environments resource limitation and subsequent interspecific competition may be relatively unimportant in determining bird community structure is explored by examining the dietary relationships within a guild of three ground—foraging passerine birds (Horned Lark, Sage Sparrow, and Western Meadowlark) in the shrubsteppe of southeastern Washington, USA, an area of severe, arid, unstable climate. General dietary analyses indicated a strong temporal component to the organization of bird diets: different species collected at the same time ate much the same things while the same species collected at different times ate different things. This pattern is reinforced by cluster analysis and stepwise discriminant analysis. Similarities in diet extended to other components as well. Dietary diversities tended to be the same for contemporaneous collections of birds, as did average prey sizes, although the latter evidenced a few statistically significant exceptions. Theoretically predicted relationships between diet and trophic structure morphology emerged only at the most general level, and even then were not always observed. In general, differences in body size or bill length were insufficient to account for variations in prey sizes, although meadowlarks did on occasion take significantly larger items than the other, smaller species. Average prey size was significantly correlated with the proportion of seeds in the diet and varied seasonally as seed consumption varied. Several aspects of this study indicate that shrubsteppe passerines are largely opportunistic in their foraging and diet selection, and that the apparent absence of fine tuning to their competitive milieu is most likely a function of the variable environment in which they coexist.

Adaptive Significance of Sexual Dimorphism in Weasels

Field data on Mustela erminea L. were used to examine the adaptive basis for the sexual dimorphism in weasels. Two hypotheses were considered: one proposed by Brown and Lasiewski, and which regards the dimorphism as a result of intersexual selection favouring different food exploitation by males and females; both sexes possibly exploiting and defending an area in common. An alternative hypothesis is suggested in this paper and considers the dimorphism primarily as an adaptation for different roles of males and females in reproduction; small females being selected for on bioenergetic grounds and large males in connection with competitive mating. Intersexual selection favouring different food exploitation has given further advantage to small females and large males and might have increased the dimorphism. Field data showed no long-lasting pair bonds between males and females; movements and behaviour of males during the breeding season indicated a polygynous or promiscuous mating system with the females alone rearing the young. The two sexes exploited different average size of prey and used different hunting techniques (females hunted more in small rodent tunnels). Food of males and females greatly overlapped in autumn and winter when both sexes exploited small rodent populations in the same habitat. In spring and early summer males partly switched to alternative prey, associated with different habitat selection and extensive movements at mating time. Scarcity of basic food small rodents and increased availability of alternative prey tended to encourage habitat and food segregation. The data support the author's hypothesis.

The Factors Influencing the Selection of Food by the House Martin (Delichon urbica (L.))

This paper describes part of a 3-year study in southern England of the breeding biology of the house martin (Delichon urbica (L.)). The food of house martin nestlings and the relationship between the prey taken and the food available to the aerial feeding adult martins is discussed. Lack & Owen (1955) investigated the food and feeding habits of the common swift (Apus apus (L.)) in southern Britain. The diet of the house martin in Germany has been examined by von Gunten (1961). The study of Johnson (1967) on the food of the purple martin (Progne subis (L.)) in North America notably compared the stomach contents of feeding birds with samples of flying insects. In a fundamentally similar study of the feeding of the brown trout (Salmo trutta (L.)), the stomach contents showed a high correlation with the occurrence of invertebrates in freshwater drift samples (Elliot 1970). This study also demonstrated the phenomenon of diel variation in the predator's diet associated with diel changes in the composition of the available food. The analogous phenomenon of diel changes in the composition of the aerial fauna, associated with differences in flight periodicity of the constituent insects (Lewis & Taylor 1965), results in different food availabilities at different times of day to aerially feeding birds. This emphasizes the necessity for a continuous diurnal sample of both the available food and the diet. Such a sample was collected in this study, which consequently differs in this respect from earlier work on the diet of swallows (Hirundinidae) and swifts (Apodidae). Hespenheide (1971) described the food preferences of a number of insectivorous birds, including several hirundines. He concluded that the average size of prey taken is always larger than that available. Gibb & Betts (1963) also demonstrated selection of larger prey than those generally available by tits (Parus spp.) feeding on lepidopterous larvae in a pine forest. The purpose of this study was to investigate whether the food taken by the house martin was the result of a similar selection of food items from the available resources, and to assess the influence of the abundance and quality of airborne insects and concurrent weather conditions on food selection.