A simple spatially explicit ideal-free distribution: a model and an experiment

This investigation presents a simple spatially explicit analysis of the ideal-free distribution. The traditional ideal-free distribution assumes discrete sites with definite boundaries, and predicts how many individuals should occupy each site. In contrast, the present analysis assumes that a forager’s gains gradually decline with distance from a site, and asks where in space individuals ought to be. Although many interesting situations may arise, the analysis asks how individuals should position themselves as the distance between two identical sources increases. Nash equilibrium positions should follow a pitchfork pattern as the distance between sites is increased; that is, an individual should maintain a position between two sources when they are close together but should move nearer one of the sources when they are far apart. In addition, the text describes an experimental study that parallels the theoretical analysis. The experiment supports the predicted pitchfork pattern, and provides somewhat weaker support for the predicted differences in ”individual” and ”paired” pitchforks. Received: 14 June 2000 / Revised: 20 September 2000 / Accepted: 7 October 2000     

Exploration and exploitation of food sources by social insect colonies: a revision of the scout-recruit concept

Social insect colonies need to explore and exploit multiple food sources simultaneously and efficiently. At the individual level, this colony-level behaviour has been thought to be taken care of by two types of individual: scouts that independently search for food, and recruits that are directed by nest mates to a food source. However, recent analyses show that this strict division of labour between scouts and recruits is untenable. Therefore, a modified concept is presented here that comprises the possible behavioural states of an individual forager (novice forager, scout, recruit, employed forager, unemployed experienced forager, inspector and reactivated forager) and the transitions between them. The available empirical data are reviewed in the light of both the old and the new concept, and probabilities for the different transitions are derived for the case of the honey-bee. The modified concept distinguishes three types of foragers that may be involved in the exploration behaviour of the colony: novice bees that become scouts, unemployed experienced bees that scout, and lost recruits, i.e. bees that discover a food source other than the one to which they were directed to by their nest mates. An advantage of the modified concept is that it allows for a better comparison of studies investigating the different roles performed by social insect foragers during their individual foraging histories. Received: 29 December 1999 / Revised: 25 February 2000 / Accepted: 16 October 2000     

Effects of body mass,age, dominance and parasite load on foraging time of bighorn rams,<Emphasis Type="Italic"> Ovis canadensis</Emphasis>

In sexually dimorphic ungulates, males generally spend less time foraging than females, possibly because of difference in body mass or because of the energetic requirements of lactation. The relationship between body size and foraging time has received little attention at the intra-specific level, because few studies have documented activity budgets for individuals of known size. Bighorn rams are a good model to explore how body mass affects foraging time, because they range in mass from 55 to 140 kg. We examined how the foraging time of bighorn rams varied according to individual characteristics. We observed rams in a marked population and constructed time budgets during the 3 months preceding the rut. We determined ram social rank based on agonistic encounters and collected fecal samples to count lungworm larvae. Time spent foraging was negatively correlated with body mass. After accounting for age differences, larger rams spent less time foraging and more time lying than smaller rams. Among rams aged 6–12 years, dominants spent less time feeding than subordinates, while fecal output of lungworm larvae was negatively correlated with foraging time for rams of all ages. Body mass accounts for much of the individual variation in foraging time, suggesting that sexual dimorphism is important in explaining differences in feeding time between males and females.Communicated by P. Heeb     

Antennal responses of the western pine beetle, <Emphasis Type="Italic">Dendroctonus brevicomis</Emphasis> (Coleoptera: Curculionidae), to stem volatiles of its primary host, <Emphasis Type="Italic">Pinus ponderosa</Emphasis>, and nine sympatric nonhost angiosperms and conifers

Summary. Stem volatile extracts from ten trees that are sympatric with the western pine beetle, Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) were assayed by gas chromatographic-electroantennographic detection analysis (GC-EAD). The extracts were from the primary host, ponderosa pine, Pinus ponderosa Dougl. ex Laws. (Pinaceae); two nonhost angiosperms, California black oak, Quercus kelloggii Newb. (Fagaceae), and quaking aspen, Populus tremuloides Michx. (Salicaceae); and seven nonhost conifers, white fir, Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. (Pinaceae), incense cedar, Calocedrus decurrens (Torr.) Florin (Cupressaceae), Sierra lodgepole pine, P. contorta murrayana Grev. & Balf. (Pinaceae), Jeffrey pine, P. jeffreyi Grev. & Balf. (Pinaceae), sugar pine, P. lambertiana Dougl. (Pinaceae), Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco (Pinaceae), and mountain hemlock, Tsuga mertensiana (Bong.) Carr. (Pinaceae). Sixty-four compounds were identified from the ten trees, 42 of which elicited antennal responses in D. brevicomis, usually in both sexes. In addition, several synthetic compounds, including a number of the antennally-active compounds from the extracted trees and some bark beetle pheromone components, elicited antennal responses in a manner similar to that observed with the extracts. Of the antennally-active compounds known to be present in trees sympatric with D. brevicomis, only geraniol was unique to its host. Four antennally-active compounds were found in the host and in other conifers; five compounds were found only in nonhost conifers; eight compounds were found in either or both of the nonhost angiosperms; eight compounds were found in either or both of the angiosperms and in nonhost conifers, but not in the host; and 19 were found in both the host and in angiosperms and/or nonhost conifers. Several bark beetle pheromone components were found in the stem volatile extracts. Conophthorin was identified from both nonhost angiosperms; exo-brevicomin was identified in A. concolor; verbenone was identified from a number of nonhost conifers; and chalcogran was identified from P. tremuloides. The number of nonhost volatile chemicals that D. brevicomis encounters and is capable of detecting, and the diversity of sources from which they emanate, highlight the complexity of the olfactory environment in which D. brevicomis forages. This provides a basis for further work related to chemically-mediated aspects of foraging in this insect and perhaps other coniferophagous bark beetles, and highlights the need to consider foraging context in the design and implementation of semiochemical-based management tactics for tree protection.     

Client preferences by Caribbean cleaning gobies: food,safety or something else?

Predation risk is amongst the most pervasive selective pressures influencing behaviour and animals have been repeatedly shown to trade-off foraging success for safety. We examined the nature of this trade-off in cleaning symbioses amongst Caribbean coral reef fishes. We predicted that cleaning gobies (Elacatinus evelynae and Elacatinus prochilos) should prefer fish clients that pose a low risk of predation (e.g. herbivores) over clients that may have more ectoparasites but pose a higher risk (e.g. piscivores). Our field observations revealed that cleaners did clean preferentially client species with more parasites but predatory and non-predatory clients had similar ectoparasite loads. Despite the lack of a foraging advantage for inspecting predators, cleaners did not avoid risky clients. On the contrary, a larger proportion of visiting predators than non-predators was inspected, gobies initiated more interactions with predatory clients, and predators were attended to immediately upon arrival at cleaning stations. This preferential treatment of dangerous clients may allow the rapid identification of cleaners as non-prey item or may be due to the effect of predators on the rest of the cleaners’ clientele, which avoided cleaning stations whilst predators were present. Dealing with potentially risky clients may allow gobies to regain access to their main food source: non-predatory clients.     

Foraging areas of king penguins from Macquarie Island in relation to a marine protected area

Twenty-three king penguins (Aptenodytes patagonicus) from Macquarie Island were tracked by satellite during the late incubation period in 1998–1999 to determine the overlap of the foraging zone of king penguins with an area to be declared a marine protected area (MPA) near the island. While all penguins left the colony in an easterly direction and traveled clockwise back to the island, three penguins foraged in the northern parts of the general foraging area and stayed north of 56°S. The remaining 20 penguins ventured south and most crossed 59°S before returning to the island. The total foraging area was estimated to be 156,000 km² with 36,500 km² being most important (where penguins spent >150 hr in total). North-foraging penguins reached on average 331 ± 24 km from the colony compared to 530 ± 76 km for the south-foraging penguins. The latter traveled an average total distance of 1313 ± 176 km, while the northern foragers averaged 963 ± 166 km. Not only did the penguins spend the majority of their foraging time within the boundaries of the proposed MPA, they also foraged chiefly within the boundaries of a highly protected zone. Thus, the MPA is likely to encompass the foraging zone of king penguins, at least during incubation.     

Honey bee foragers as sensory units of their colonies

Forager honey bees function not only as gatherers of food for their colonies, but also as sensory units shaped by natural selection to gather information regarding the location and profitability of forage sites. They transmit this information to colony members by means of waggle dances. To investigate the way bees transduce the stimulus of nectar-source profitability into the response of number of waggle runs, I performed experiments in which bees were stimulated with a sucrose solution feeder of known profitability and their dance responses were videorecorded. The results suggest that several attributes of this transduction process are adaptations to enhance a bee's effectiveness in reporting on a forage site. (1) Bees register the profitability of a nectar source not by sensing the energy gain per foraging trip or the rate of energy gain per trip, but evidently by sensing the energetic efficiency of their foraging. Perhaps this criterion of nectar-source profitability has been favored by natural selection because the foraging gains of honey bees are typically limited by energy expenditure rather than time availability. (2) There is a linear relationship between the stimulus of energetic efficiency of foraging and the response of number of waggle runs per dance. Such a simple stimulus-response function appears adequate because the range of suprathreshold stimuli (max/min ratio of about 10) is far smaller than the range of responses (max/min ratio of about 100). Although all bees show a linear stimulus-response function, there are large differences among individuals in both the response threshold and the slope of the stimulus-response function. This variation gives the colony a broader dynamic range in responding to food sources than if all bees had identical thresholds of dance response. (3) There is little or no adaptation in the dance response to a strong stimulus (tonic response). Thus each dancing bee reports on the current level of profitability of her forage site rather than the changes in its profitability. This seems appropriate since presumably it is the current profitability of a forage site, not the change in its profitability, which determines a site's attractiveness to other bees. (4) The level of forage-site quality that is the threshold for dancing is tuned by the bees in relation to forage availability. Bees operate with a lower dance threshold when forage is sparse than when it is abundant. Thus a colony utilizes input about a wide range of forage sites when food is scarce, but filters out input about low-reward sites when food is plentiful. (5) A dancing bee does not present her information in one spot within the hive but instead distributes it over much of the dance floor. Consequently, the dances for different forage sites are mixed together on the dance floor. This helps each bee following the dances to take a random sample of the dance information, which is appropriate for the foraging strategy of a honey bee colony since it is evidently designed to allocate foragers among forage sites in proportion to their profitability.     

Both evaluation of resource quality and speed of recruited leaf-cutting ants (Acromyrmex lundi) depend on their motivational state

Summary During recruitment, running velocity of both outbound and laden workers of the leaf-cutting ant Acromyrmex lundi depended on the information about resource quality they received from the first successful recruiter. In independent assays, single scout ants were allowed to collect sugar solutions of different concentrations and to recruit nestmates. Recruited workers were presented with standardized paper discs rather than the sugar solution given to the original recruiting ant. Outbound recruited workers were observed to run faster the more concentrated the solution found by the recruiter. Speed of disc-laden workers also depended on the concentration of the solution found by the recruiter, i.e. on the information about food quality they received, since they had no actual contact with the sugar solution. Disc-laden workers ran, as intuitively expected, slower than outbound workers. The reduction in speed, however, could not be attributed to the effects of the load itself, because workers collecting discs of the same weight, but with added sugar, ran as rapidly as outbound, unladen workers. Workers collecting standardized sugared discs reinforced the chemical trail on their way to the nest. The percentage of trail-layers was higher when workers were recruited to 10% than to 1% sugar solution, even though they collected the same kind of discs at the source. Their evaluation of resource quality, therefore, depended on their motivational state, which was modulated by the information they received during recruitment. Using previously published data on energetics of locomotion in leaf-cutting ants, travel costs of A. lundi workers recruited to sugar solutions of different concentration could be estimated. For workers recruited to the more concentrated solution, both speed and oxygen consumption rate increased by a roughly similar factor. Therefore, although workers ran faster to the high-quality resource, their actual energy investment per trip remained similar to that made by workers recruited to the low-quality resource. It is suggested that the more motivated workers reduced travel time without increasing energy costs during the trip. The adaptive value of these responses seems to be related to a rapid transmission of information about a newly discovered food source.