Recognition of foreign oviposition-marking pheromone in a multi-trophic context

Both phytophagous and parasitic insects deposit oviposition-marking pheromones (OMPs) following oviposition that function to inform conspecifics of a previously utilized host of reduced suitability. The blueberry maggot fly, Rhagoletis mendax Curran (Diptera: Tephritidae), deposits eggs individually into blueberries and then marks the fruit surface with an OMP which reduces acceptance of fruit for oviposition by conspecifics. Diachasma alloeum (Muesebeck) (Hymenoptera: Braconidae) is a parasitic wasp attacking larval R. mendax which also deposits an OMP, signaling conspecifics of a wasp-occupied host. Behavioral studies were conducted testing the hypothesis that the OMP of the parasitic wasp modifies the oviposition behavior of its host fly. In this study, we show that the OMP of D. alloeum is recognized by R. mendax, and female flies will reject wasp-marked fruit for oviposition. Thus, we present a rare demonstration of pheromonal recognition between animals occupying different taxonomic orders and trophic levels. This chemical eavesdropping may enhance the ability of the fly to avoid fruit unsuitable for larval development.     

Dominance and polyethism in the eusocial wasp Mischocyttarus mastigophorus (Hymenoptera: Vespidae)

Dominance interactions affected patterns of non-reproductive division of labor (polyethism) in the eusocial wasp Mischocyttarus mastigophorus. Socially dominant individuals foraged for food (nectar and insect prey) at lower rates than subordinate individuals. In contrast, dominant wasps performed most of the foraging for the wood pulp used in nest construction. Social dominance also affected partitioning of materials collected by foragers when they returned to the nest. Wood pulp loads were never shared with nest mates, while food loads, especially insect prey, were often partitioned with other wasps. Dominant individuals on the nest were more likely to take food from arriving foragers than subordinate individuals. The role of dominance interactions in regulating polyethism has evolved in the eusocial paper wasps (Polistinae). Both specialization by foragers and task partitioning have increased from basal genera (independent-founding wasps, including Mischo-cyttarus spp.) to more derived genera (swarm-founding Epiponini). Dominance interactions do not regulate forager specialization or task partitioning in epiponines. I hypothesize that these changes in polyethism were enabled by the evolution of increased colony size in the Epiponini. Received: 8 December 1997 / Accepted after revision: 28 March 1998     

Shoal composition, body size and foraging in sticklebacks

Individuals which deviate from the majority in groups are likely to be most vulnerable to predation. This oddity effect, by definition, is frequency dependent, eventually fading at equal frequencies of the phenotypes in a group. It has been hypothesized that the increased predation risk of odd individuals may play an important role in the formation of phenotypically uniform shoals of fish. However, recent work has indicated that individuals may experience, or value, their predation hazard differently depending on their own size in relation to that of other group members: single large fish, but not small ones, appear concerned about their oddity in a shoal. Here I show that the apparent wariness of large fish is also expressed in a frequency-dependent manner, closely conforming to what is predicted if the oddity effect is responsible for their behavior. Using foraging activity of individuals as a means to evaluate their predation risk, I demonstrate with shoals comprising 12 threespine sticklebacks (Gasterosteus aculeatus) that large fish forage least actively when in a shoal consisting of 2 large and 10 small fish. An increase in the number of large fish to 4 among 8 small individuals clearly results in an increase in their foraging activity. However, having reached an equal frequency with small fish in a shoal, large fish do not seem to change their foraging activity much even when their number in a shoal increases further. In contrast, foraging activity of small sticklebacks remains fairly constant throughout the entire range of tested shoal compositions, providing further evidence that small and large fish respond to their oddity differently. Received: 12 February 1998 / Accepted after revision: 7 May 1998     

The role of internal and external information in foraging decisions of Melipona workers (Hymenoptera: Meliponinae)

Social insect foragers have to make foraging decisions based on information that may come from two different sources: information learned and memorised through their own experience (“internal” information) and information communicated by nest mates or directly obtained from their environment (“external” information). The role of these sources of information in decision-making by foragers was studied observationally and experimentally in stingless bees of the genus Melipona. Once a Melipona forager had started its food-collecting career, its decisions to initiate, continue or stop its daily collecting activity were mainly based upon previous experience (activity on previous days, the time at which foraging was initiated the day(s) before, and, during the day, the success of the last foraging flights) and mediated through direct interaction with the food source (load size harvested and time to collect a load). External information provided by returning foragers advanced the start of foraging of experienced bees. Most inexperienced bees initiated their foraging day after successful foragers had returned to the hive. The start of foraging by other inexperienced bees was stimulated by high waste-removal activity of nest mates. By experimentally controlling the entries of foragers (hence external information input) it was shown that very low levels of external information input had large effect on the departure of experienced foragers. After the return of a single successful forager, or five foragers together, the rate of forager exits increased dramatically for 15 min. Only the first and second entry events had large effect; later entries influenced forager exit patterns only slightly. The results show that Melipona foragers make decisions based upon their own experience and that communication stimulates these foragers if it concerns the previously visited source. We discuss the organisation of individual foraging in Melipona and Apis mellifera and are led to the conclusion that these species behave very similarly and that an information-integration model (derived from Fig. 1) could be a starting point for future research on social insect foraging. Received: 16 April 1997 / Accepted after revision: 30 August 1997     

Monopolization in a resource queue: water striders competing for food and mates

The monopolization of resources plays an important theoretical role in the literature on competition for food and mates. We used 12 groups of male water striders (Aquariusremigis) to: (1) test the general prediction that monopolization of both food and mates decreases as the temporal clumping of resources increases, (2) compare the efficiency of two indices of resource monopolization, coefficient of variation and Q (Ruzzante et al. 1996), and (3) quantitatively assess the resource queue model of Blanckenhorn and Caraco (1992). Each group of six males competed for both food items and mates released from the upstream end of a laboratory stream. The mean inter-arrival time for resource units (food or females) was 10 min, with four levels of temporal clumping (variance in inter-arrival time: 0, 25, 50 or 320 min²). As predicted, the monopolization of both food and mates decreased as the temporal clumping of resource arrival increased, although monopolization was greater for food than for mates. Q detected the difference in monopolization of food and mates, whereas the coefficient of variation did not, because Q is independent of mean resource abundance. The resource queue model successfully predicted monopolization of both resource types, explaining 89% and 76% of variation in the proportion of food and mates acquired by the six males. The success of the model suggests that the scaling of handling time to the variance in resource inter-arrival time should play an important role in any general theory of resource monopolization. Received: 28 February 1997 / Accepted after revision: 26 September 1997     

Food-exchange by foragers in the hive – a means of communication among honey bees?

Dancing and trophallactic behaviour of forager honey bees, Apis mellifera ligustica >Spinola, that returned from an automatic feeder with a regulated flow rate of 50% weight-to-weight sucrose solution (range: 0.76–7.65 μl/min) were studied in an observation hive. Behavioural parameters of dancing, such as probability, duration and dance tempo, increased with the nectar flow rate, though with very different response curves among bees. For trophallaxis (i.e. mouth-to-mouth exchange of food), the frequency of giving-contacts and the transfer rate of the nectar increased with the nectar flow rate. After unloading, foragers often approached other nest mates and begged for food before returning to the food source. This behaviour was less frequent at higher nectar flow rates. These results show that the profitability of a food source in terms of nectar flow rate had a quantitative representation in the hive through quantitative changes in trophallactic and dancing behaviour. The role of trophallaxis as a communication channel during recruitment is discussed. Received: 14 January 1995/Accepted after revision: 14 August 1995     

Rank-dependent hoarding effort in willow tits (<Emphasis Type="Italic">Parus montanus</Emphasis>): a test of theoretical predictions

Many birds and mammals store energy as hoarded food supplies. A supply of stored food is beneficial during periods when food is scarce, but building up and managing such a supply also entails costs. The optimal number of caches will be reached when the net benefit is at its maximum. If dominants can steal more stored food from subordinates than the other way around, the optimum will differ between these categories. A previous theoretical model of hoarding in groups with dominant and subordinate members produced three testable predictions: (1) hoarders should store more food as anticipated future conditions get worse; (2) subordinate flock members should store more food than dominants; and (3) dominants should increase hoarding relatively more than subordinates as conditions get worse. Here we present a field experiment on willow tits (Parus montanus) designed to test these predictions. We found support for all three. Hoarding increased as conditions got worse, subordinates stored at a higher rate than dominants, and dominants increased their hoarding effort relatively more than subordinates as conditions worsened. These results support the incorporation of information on dominance and food availability into models predicting food storage behaviour.Communciated by J. Dickinson     

Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a correlation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptions and predictions of the phenotype-limited interference model in Andean condors (Vultur gryphus), a New World vulture with strong sexual size dimorphism (males are 30–40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scavenging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspecific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juvenile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juvenile females were more often observed searching in low-quality areas (plains), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the number of condors at arrival was high. Additionally, low-ranking condors spent less time feeding at carcasses when individuals of higher hierarchical levels were present. On the other hand, the number of condors present had a positive effect on feeding rates of dominant individuals, probably because of a reduction in individual vigilance. These results support most of the assumptions and predictions of the phenotype-limited distribution model, although a spatial truncated distribution between phenotypes was not observed. Asymmetric feeding pay-off, unequal parental roles and sexual selection constraints could favour sexual divergence in body size in Andean condors. Received: 6 April 1998 / Accepted after revision: 11 July 1998     

Adaptive peaks and alternative foraging tactics in brook charr: evidence of short-term divergent selection for sitting-and-waiting and actively searching

Some recently emerged brook charr (Salvelinus fontinalis) inhabiting still-water pools along the sides of streams are sedentary and eat crustaceans from the lower portion of the water column. Others are more active and eat insects from the upper portion of the water column. We provide evidence that this divergent foraging behavior reflects short-term divergent selection brought about by intraspecific competition in the presence of alternative food sources. Rates of encounters and interactions between individuals were density dependent, and encounter and interaction events were closely timed with prey capture attempts. In addition, aggressive fish made more foraging attempts per minute than nonaggressive fish. Aggressive fish were also either inactive or very active, while nonaggressive fish exhibited intermediate levels of activity. Growth rate potential, an important component of fitness during the early life stages of brook charr, was assessed using tissue concentrations of RNA and found to be highest for sedentary fish and for active fish making frequent foraging attempts, and lower for fish exhibiting intermediate levels of activity. Our findings support contentions that individual behavior plays an important role during initial steps in the evolution of resource polymorphisms. Received: 27 July 1998 / Accepted after revision: 16 November 1998     

The influence of prey distribution on the foraging strategy of the lizard Oligosoma grande (Reptilia: Scincidae)

The grand skink, Oligosoma grande, is a diurnal rock-dwelling lizard from the tussock grasslands of Central Otago, New Zealand, whose diet includes a variety of arthropods and fruit. We conducted a field experiment to examine the influence of prey distribution on foraging behavior and spacing patterns. On sites where prey distribution was unaltered (control sites), males and females differed in diet and foraging behavior. Most male feeding attempts were directed at large strong-flying insects, and males used a saltatory search pattern that involved relatively infrequent moves of long duration. Females spent more effort catching small weak-flying insects and visiting fruiting plants. Their search behavior involved frequent moves of short duration. The placement of meat-bait on experimental sites led to a redistribution of large flies without influencing other prey types. Experimental females switched foraging strategy by adopting a search pattern of relatively infrequent moves of long duration, increasing the frequency of attempts to capture large prey, and reducing the importance of fruit in their diet. The experimental manipulation appeared to influence space use. On control sites, both sexes had comparably sized home ranges. On experimental sites, male home ranges were significantly larger than female home ranges. Received: 3 November 1997 / Accepted after revision: 13 December 1998