Foraging costs of a tail ornament: Experimental evidence from two populations of barn swallows Hirundo rustica with different degrees of sexual size dimorphism

Secondary sexual characters are assumed to be costly to produce or maintain. A test of this assumption was performed using the sexually exaggerated outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject of a directional female mate preference. A possible cost of sexual signalling in male barn swallows arises from increased flight cost during foraging in this aerially insectivorous species. A longer tail may impose a greater drag during flight and thereby affect foraging ability. This was tested by determining the relationship between experimentally modified male tail lengths and number and size of prey delivered to offspring in Spain, where sexual size dimorphism in tail length is small, compared to Denmark, where dimorphism is large. Food boluses contained significantly fewer small insects in Spain than in Denmark. Males with elongated tails captured more and smaller insects while males with shortened tails captured fewer and larger prey items at both sites. Males with naturally long tails were less affected by experimental treatment in terms of effects on the number and the size of prey delivered to their offspring, a finding consistent with a long tail being a condition-dependent viability indicator. The effect of a given degree of tail manipulation on prey size and number of prey per bolus was larger in Spain than in Denmark. These results demonstrate that (1) tail length in male barn swallows affects foraging, and (2) larger sexual size dimorphism occurs where the foraging cost of an increment in ornament size is smallest.Communicated by M. Zuk     

Net energetic advantage drives honey bees (Apis mellifera L) to nectar larceny in Vaccinium ashei Reade

Carpenter bees (Xylocopa spp.) act as primary nectar thieves in rabbiteye blueberry (Vaccinium ashei Reade), piercing corollas laterally to imbibe nectar at basal nectaries. Honey bees (Apis mellifera L) learn to visit these perforations and thus become secondary nectar thieves. We tested the hypothesis that honey bees make this behavioral switch in response to an energetic advantage realized by nectar-robbing flower visits. Nectar volume and sugar quantity were higher in intact than perforated flowers, but bees (robbers) visiting perforated flowers were able to extract a higher percentage of available nectar and sugar so that absolute amount of sugar (mg) removed by one bee visit is the same for each flower type. However, because perforated flowers facilitate higher rates of bee flower visitation and the same or higher rates of nectar ingestion, they are rendered more profitable than intact flowers in temporal terms. Accordingly, net energy (J) gain per second flower handling time was higher for robbers on most days sampled. We conclude that the majority evidence indicates an energetic advantage for honey bees that engage in secondary nectar thievery in V. ashei.Communicated by R. Page     

Exploitation of carbohydrate food sources in <Emphasis Type="Italic">Polybia occidentalis</Emphasis>: social cues influence foraging decisions in swarm-founding wasps

An efficient exploitation of carbohydrate food sources would be beneficial for social wasp species that store nectar within their nest. In the swarm-founding polistine wasp Polybia occidentalis, we now demonstrate that the decisions of when and where to forage are influenced by information from conspecifics. Only when foragers had been trained to collect at artificial carbohydrate feeders did newcomers (food-source-naive individuals) continuously arrive at these feeders during 2 h of experiment. In control tests, in which no forager had been trained, not a single newcomer alighted at any of the offered carbohydrate food sources. This indicates that, during the foraging process, a nest-based input provided by successful foragers must have stimulated nestmates to search for food. Once activated, the newcomers’ choice on where to collect was strongly influenced by field-based social information. The mere visual presence of accumulated conspecifics (wasp dummies placed on one of the feeders) attracted newcomers to the food sources. Interestingly, however, visual enhancement was not the only decision-biasing factor at the feeding site. In an experimental series where searching wasps had to choose between the experimental feeder at which 3 foragers continuously collected and the control feeder with nine wasp dummies, only 40% of the wasps chose the visually enhanced feeder. This points to the existence of additional mechanisms of local enhancement. The possibility that, in social wasps, recruitment is involved in the exploitation of carbohydrate food sources is discussed.     

Is the antipredatory response in behaviour reflected in stress measured in faecal corticosteroids in a small rodent?

Predation risk has been shown to alter various behaviours in prey. Risk alters activity, habitat use and foraging, and weight decrease might be a consequence of that. In mammals, studies on physiological measures affected by risk of predation, other than weight, are rare. We studied in two separate laboratory experiments foraging, hoarding behaviour and expression of stress measured non-invasively from the faeces in the bank vole (Clethrionomys glareolus), a common boreal rodent. Voles were exposed to predation risk using odours of the least weasels (Mustela nivalis nivalis). Distilled water served as control. In the first experiment, we found that foraging effort, measured as sunflower seeds taken from seed trays filled with sand, was significantly lower in trays scented with weasel odour. Both immediate consumption of seeds and hoarding were affected negatively by the weasel odour. Females hoarded significantly more than males in autumn. In the second experiment, the negative effect of weasel odour on foraging was consistent over a 3-day experiment, but the strongest effect was observed in the first night. Foraging increased over the time of the experiment, which might reflect either energetic compensation during a longer period of risk, predicted in the predation risk allocation hypothesis, or habituation to the odour-simulated risk. Despite decreased foraging under predation risk, stress measured as corticosteroid metabolite concentration in vole faeces was not affected by the weasel odour treatment. In conclusion, we were able to verify predation-risk-mediated changes in the foraging effort of bank voles but no physiological stress response was measured non-invasively, probably due to great individual variation in secretion of stress hormones.     

Why do honey bees dance?

The honey bee dance language, used to recruit nestmates to food sources, is regarded by many as one of the most intriguing communication systems in animals. What were the ecological circumstances that favoured its evolution? We examined this question by creating experimental phenotypes in which the location information of the dances was obscured. Surprisingly, in two temperate habitats, these colonies performed only insignificantly worse than colonies which were able to communicate normally. However, foraging efficiency was substantially impaired in an Asian tropical forest following this manipulation. This indicates that dance language communication about food source locations may be important in some habitats, but not in others. Combining published data and our own, we assessed the clustering of bee forage sites in a variety of habitats by evaluating the bees’ dances. We found that the indicated sites are more clustered in tropical than in temperate habitats. This supports the hypothesis that in the context of foraging, the dance language is an adaptation to the particular habitats in which the honey bees evolved. We discuss our findings in relation to spatial aggregation patterns of floral food in temperate and tropical habitats.     

Ground gleaning in horseshoe bats: comparative evidence from<Emphasis Type="Italic"> Rhinolophus blasii</Emphasis>,<Emphasis Type="Italic"> R. euryale</Emphasis> and<Emphasis Type="Italic"> R. mehelyi</Emphasis>

The 71 species of horseshoe bat (genus Rhinolophus) use echolocation calls with long constant-frequency (CF) components to detect and localize fluttering insects which they seize in aerial captures or glean from foliage. Here we describe ground-gleaning as an additional prey-capture strategy for horseshoe bats. This study presents the first record and experimental evidence for ground-gleaning in the little-studied Blasius horseshoe bat (Rhinolophus blasii). The gleaning bouts in a flight tent included landing, quadrupedal walking and take-off from the ground. The bats emitted echolocation calls continuously during all phases of prey capture. Both spontaneously and in a choice experiment, all six individuals attacked only fluttering insects and never motionless prey. These data suggest that R. blasii performs ground-gleaning largely by relying on the same prey-detection strategy and echolocation behaviour that it and other horseshoe bats use for aerial hawking.We also studied the Mediterranean horseshoe bat (R. euryale) in the flight tent. All four individuals never gleaned prey from the ground, though they appeared to be well able to detect fluttering moths on the ground. It is not known yet whether ground-gleaning plays a role in Mehelys horseshoe bat (R. mehelyi). In a performance test, we measured the ability of these three European species of middle-sized horseshoe bats (R. euryale, R. mehelyi and R. blasii) to take-off from the ground. All were able to take flight even in a confined space; i.e. the willingness to ground-glean in R. blasii is not related to a superior take-off performance. In contrast to ground-gleaning bats of other phylogenetic lineages, R. blasii appears not to be a specialist, but rather shows a remarkable behavioural flexibility in prey-capture strategies and abilities. We suggest that the key innovation of CF echolocation paired with behavioural flexibility in foraging strategies might explain the evolutionary success of Rhinolophus as the second largest genus of bat.Communicated by T. Czeschlik     

Effects of male removal on female foraging behavior in the Eurasian treecreeper

In old, spruce-dominated forests of central Finland, Eurasian treecreepers Certhia familiaris divide their territories spatially during the breeding season. Females forage primarily on the upper parts of the tree trunks, while males use the lower parts of the tree trunks. In this study we removed males from eight territories in the early nestling period to see if the mate's absence would change the foraging patterns of the resident female. Widowed females foraged at lower heights, thus behaving more like paired males. These females also spent less time on each tree and on each foraging bout than did paired females. We conclude that male removal facilitated the change in a female's foraging niche and foraging time at the trees. Females may re-optimize their foraging site selection owing to the absence of dominant males and a consequent need to increase their parental care. Behavioral plasticity may be the mechanism of niche partitioning between the sexes in this species. Received: 28 June 1996 / Accepted after revision: 27 March 1997     

Parasitism in a social wasp: effect of gregarines on foraging behavior, colony productivity, and adult mortality

Behavior in eusocial insects likely reflects a long history of selection imposed by parasites and pathogens because the conditions of group living often favor the transmission of infection among nestmates. Yet, relatively few studies have quantified the effects of parasites on both the level of individual colony members and of colony success, making it difficult to assess the relative importance of different parasites to the behavioral ecology of their social insect hosts. Colonies of Polybia occidentalis, a Neotropical social wasp, are commonly infected by gregarines (Phylum Apicomplexa; Order Eugregarinida) during the wet season in Guanacaste, Costa Rica. To determine the effect of gregarine infection on individual workers in P. occidentalis, we measured foraging rates of marked wasps from colonies comprising both infected and uninfected individuals. To assess the effect of gregarines on colony success, we measured productivity and adult mortality rates in colonies with different levels of infection prevalence (proportion of adults infected). Foraging rates in marked individuals were negatively correlated with the intensity of gregarine infection. Infected colonies with high gregarine prevalence constructed nests with fewer brood cells per capita, produced less brood biomass per capita, and, surprisingly, experienced lower adult mortality rates than did uninfected or lightly infected colonies. These data strongly suggest that gregarine infection lowers foraging rates, thus reducing risk to foragers and, consequently, reducing adult mortality rates, while at the same time lowering per-capita input of materials and colony productivity. In infected colonies, queen populations were infected with a lower prevalence than were workers. Intra-colony infection prevalence decreased dramatically in the P. occidentalis population during the wet season.An erratum to this article can be found at     

Foraging ability in the scorpionfly <Emphasis Type="Italic">Panorpa vulgaris</Emphasis>: individual differences and heritability

According to indicator models of sexual selection, mates may obtain indirect, i.e. genetic, benefits from choosing partners indicating high overall genetic quality by honest signals. In the scorpionfly Panorpa vulgaris, both sexes show mating preferences on the basis of the condition of the potential partners. Females prefer males that produce nuptial gifts (i.e. salivary secretions) during copulation, while males invest more nuptial gifts in females of high nutritional status. Both characters, males' ability to produce nuptial gifts and high nutritional status of females, are known to be reliable indicators of foraging ability. Thus, besides possible direct benefits, both sexes might also obtain indirect benefits in terms of “good foraging genes” by their choice and thereby increase the fitness of their offspring. A prerequisite for this possibility is the heritability of the respective trait. In the present study, we estimated the repeatability and the heritability of foraging ability. Our results indicate (1) a significant repeatability of individual foraging efficiencies in males and females and (2) a heritable component of this trait by a significant parent–offspring regression. These findings suggest that genetic benefits in terms of increased offspring foraging ability might contribute to selection for mating preferences in both sexes.     

Vibrational communication between hitchhikers and foragers in leaf-cutting ants (Atta cephalotes)

Abstract In a foraging column of the leaf-cutting ant Atta cephalotes, minim workers (the smallest worker subcaste) hitchhike on leaf fragments carried by larger workers. It has been demonstrated that they defend leaf carriers against parasitic phorid flies. The present study examines the cues used by the potential hitchhikers to locate leaf carriers. As recently reported, foraging workers stridulate while cutting a leaf fragment, and the stridulatory vibrations serve as closerange recruitment signals. We tested the hypothesis that these plant-borne stridulatory vibrations are used by the potential hitchhikers to locate workers engaged in cutting. Three different lines of evidence support this view. Firstly, the repetition rate of the stridulations produced by foraging workers increases significantly as foragers maneuver the leaf fragment into the carrying position and walk loaded to the nest. This is the moment when hitchhikers usually climb on the leaf. Although the leaf-borne stridulatory vibrations are considerably attenuated when transmitted through the workers' legs, they can nevertheless be detected at short distances by minims. This subcaste is several times more sensitive to substrate-borne vibrations than larger workers. Secondly, when a stridulating and a silent leaf were simultaneously presented at the foraging site, minim workers spent significantly more time on the stridulating than on the silent leaf. Thirdly, hitchhiking was more frequent in leaf carriers which cut fragments out of the stridulating leaf than in those cutting the silent leaf.Abstract In a foraging column of the leaf-cutting ant Atta cephalotes, minim workers (the smallest worker subcaste) hitchhike on leaf fragments carried by larger workers. It has been demonstrated that they defend leaf carriers against parasitic phorid flies. The present study examines the cues used by the potential hitchhikers to locate leaf carriers. As recently reported, foraging workers stridulate while cutting a leaf fragment, and the stridulatory vibrations serve as closerange recruitment signals. We tested the hypothesis that these plant-borne stridulatory vibrations are used by the potential hitchhikers to locate workers engaged in cutting. Three different lines of evidence support this view. Firstly, the repetition rate of the stridulations produced by foraging workers increases significantly as foragers maneuver the leaf fragment into the carrying position and walk loaded to the nest. This is the moment when hitchhikers usually climb on the leaf. Although the leaf-borne stridulatory vibrations are considerably attenuated when transmitted through the workers' legs, they can nevertheless be detected at short distances by minims. This subcaste is several times more sensitive to substrate-borne vibrations than larger workers. Secondly, when a stridulating and a silent leaf were simultaneously presented at the foraging site, minim workers spent significantly more time on the stridulating than on the silent leaf. Thirdly, hitchhiking was more frequent in leaf carriers which cut fragments out of the stridulating leaf than in those cutting the silent leaf.Communicated by P. Pamilo