How brood influences caste aggregation patterns in the dimorphic ant species Pheidole pallidula

Spatial distribution of ant workers and, notably their aggregation/segregation behaviour, is a key-element of the colony social organization contributing to the efficiency of task performance and division of labour. In polymorphic species, specialized worker castes notably differ in their intrinsic aggregation behaviour. In this context, knowing the preponderant role of minors in brood care, we investigate how a stimulus such as brood can influence the spatial patterns of Pheidole pallidula worker castes. In a homogeneous area without brood, it was shown that minors display only a low level of aggregation while majors form large clusters in the central area. Here we find out that these aggregation patterns of both minors and majors can be deeply influenced by the presence of brood. For minors, it nucleates or enhances the formation of a large stable cluster. Such high sensitivity of minors to brood stimuli fits well with their role as main brood tenders in the colony. For majors, interattraction between individuals still remains the prevailing aggregation factor while brood strongly influences the localisation of their cluster. We discuss how the balance between interattraction and sensitivity to environmental stimuli determines the mobility of each worker castes and, consequently, the availability of minors and majors to participate in everyday colony tasks. Moreover, we will evoke the functional value of majors’ cluster location close to the brood, namely with respect to social regulation of the colony caste ratio. Received 30 May 2005; revised 11 January 2006; accepted 13 January 2006.     

Development of single sequence repeat markers for the ant Aphaenogaster senilis and cross-species amplification in A. iberica, A . gibbosa, A. subterranea and Messor maroccanus

Eight polymorphic microsatellite loci were isolated and characterized for Aphaenogaster senilis, a common ant species distributed in the Western Mediterranean. Characterization of 15 individuals form southern Spain showed moderate to high allelic diversity ranging from 2 to 9 alleles per locus. Cross-species tests on 10 individuals of A. iberica, A. gibbosa, A. subterranea and Messor maroccanus revealed successful amplification for most loci. This set of markers can be useful for population genetic studies and might even prove useful in other phylogenetically close species of the subfamily Myrmicinae.     

Temperature-vegetation structure interaction: the effect on the activity of the ant Messor barbarus (L.)

Several canopy types in an evergreen oak forest with open pastures have been used to examine climatic features of this system related to temperature. Circadian variation in temperature seems to be related to structural characteristics of the vegetation, with an apparent influence of the plant micro- and meso-environments analyzed. The activity of a population of granivorous ants (Messor barbarus (L.)), studied in the same area, showed a clear dependence on ambient temperature, but, although the colonies were always inside one of the vegetation types, there was no significant relationship between the activity of ants and grasscover. This, together with other observations, indicates that the possible controlling effect of the vegetation is limited by the tolerance of ants (when faced with adverse conditions) during each daily foraging period.     

Differential outcrossing rates in dispersing and non-dispersing achenes in the heterocarpic plant Crepis sancta (Asteraceae)

The association between mating systems and dispersal in plants has been studied mostly in cleistogamous species where, generally, seeds produced by cleistogamous (selfed) flowers are less dispersed than seeds produced by chasmogamous (potentially outcrossed) flowers. In heterocarpic Asteraceae, non-dispersing fruits (achenes) are produced at the periphery of the capitulum (outer florets) whereas dispersing achenes are produced by inner florets in the same capitulum. Since all the florets are protandrous, the outer floret developing first are in female phasis when anthesis of inner florets takes place. Thus, outer florets can be potentially selfed by the inner florets of the same capitulum whereas the latter must be pollinated by flowers of other capitula. Therefore outer florets should be more inbred than inner florets. To test this hypothesis, we measured the natural outcrossing rate in outer and inner florets using allozymes in three populations of the heterocarpic Crepis sancta. The results showed that the outcrossing rate was highest for non-dispersed achenes. Moreover, among the outcrossed achenes within a capitulum it was observed that the number of paternal parents of non-dispersing achenes was higher than for dispersing achenes. The pattern observed was therefore the opposite to the pattern of cleistogamous plants and contradicts the putative pollination mechanism we proposed for Asteraceae. The results agree with the predictions of sib competition theory which considers that outcrossing may minimize competitive interactions among relatives (sibs) falling near the mother plant. Higher outcrossing rate in outer florets could also occur because pollinators are more attracted to these florets.     

Isolation of polymorphic microsatellite loci for the study of habitat fragmentation in the harvester ant <Emphasis Type="Italic">Messor structor</Emphasis>

Microsatellite sequences of the harvester ant Messor structor were identified by use of an enriched genomic library. Populations from Austria, Germany and Croatia were screened and 4–21 alleles were identified within seven microsatellite loci. Cross species amplification was evaluated for three other Messor species. ^†W. Arthofer, B. C. Schlick-Steiner and F. M. Steiner contributed equally to this work.     

How photoperiod influences body temperature selection in Lacerta viridis

Summary European green lizards, Lacerta viridis, show a distinct annual cycle in their day and nighttime selected body temperature (T _b) levels when monitored under natural photoperiod. The amplitude between daily photophase and scotophase temperatures varies throughout the year. Highest body temperatures with smallest day/night variation are selected from May through July. Throughout fall, the difference between day and nighttime selected T _b levels increases. Lizards inevitably enter a state of winter dormancy which terminates daily rhythmicity patterns. Under natural photoperiodic conditions, cessation of dormancy occurs spontaneously by mid-March, regardless whether high temperatures are available or not. Lacerta viridis respond to an artificial long photoperiod (16 h light, 8 h dark) at all times of the year with modifications in both diel patterns and levels of selected T _b to summer-like conditions. When, however, the natural photoperiod at different phases in the annual cycle is held constant for six to eight weeks, T _b selection of Lacerta viridis also remains stable at the level corresponding to the prevailing photoperiod. These results implicate that the photoperiod is a more prominent Zeitgeber for seasonal cueing of temperature selection than has been surmised in the past. Further, we suggest that the large variations recorded in daily T _b cycles do not imply that this lizard is an imprecise thermoregulator, but rather indicates an important integral process necessary for seasonal acclimatization.     

Brood translocation and circadian variation of temperature preference in the ant Camponotus mus

Summary 1) When a thermal gradient (20–40° C) was established along a laboratory nest, Camponotus mus nurse workers showed a photoperiodic circadian rhythm of temperature preferences for brood rearing. Two different temperatures were daily selected to translocate the brood, i.e. 30.8° C selected at the middle of the photophase, and 27.5° C selected during the scotophase, 8 h later. 2) The daily temperature response of nurse workers consisted of paired high and low-temperature translocations, with a 8 hs-interval in between: high-temperature translocation was shown to be entrained by the photophase length, whereas low-temperature translocation was shown to be dependent on the precedent one. 3) Prey deprivation to the colony modified the brood transport behaviors resulting in translocations of only cocoons and large (ripe) larvae, stages in which the pupation processes are triggered. Small larvae and eggs remained located at 27.5° C. 4) Evaluation of pupa developmental time as well as percentage of pupa mortality at different temperature regimes allowed to construct an efficiency index relating pupa survival and cocoon developmental time. In the range of temperatures selected by nurses, the index reached its maximal values. 5) The ecological significance of these results is discussed.     

Aggressive interactions between the introduced Argentine ant, Linepithema humile and the native odorous house ant, Tapinoma sessile

The Argentine ant (Linepithema humile) is an invasive species that disrupts the balance of natural ecosystems by displacing indigenous ant species throughout its introduced range. The mechanisms by which Argentine ants effectively compete against native ant species have been previously addressed in field studies that centered on interference and exploitation competition at baits and mainly examined the colony-level performance of Argentine ants. Detailed behavioral observations explaining the basis for the strong competitive ability of L. humile are comparatively rare. To gain a better understanding of the mechanisms by which Argentine ants displace native ants we examined the aggressive interactions between the Argentine ants and the odorous house ant, Tapinoma sessile in four different aggression assays: (1) worker dyad interactions, (2) symmetrical group interactions, (3) intruder introductions into an established resident colony, and (4) a resource competition assay which focused on competition for food and nesting space. Our results demonstrate a clear disparity between worker-level and colony-level fighting ability of Argentine ants and provide behavioral evidence to explain the superior interference ability of Argentine ants in group assays. Argentine ants experienced mixed success in fighting against odorous house ants in dyad interactions, but gradually gained a numerical advantage in symmetrical group interactions by active cooperation among nestmates. Results of the resource competition assay indicate that Argentine ants recruit rapidly, numerically dominate food and nesting sites, and aggressively displace T. sessile from baits. Taken together, the results of these assays allow us to pinpoint the behavioral mechanisms responsible for the remarkable competitive ability of Argentine ants.     

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

Summary The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha^–1 · y^–1;Messor ebeninus: 2823–4437 g · ha^–1 · y^–1;Messor arenarius: 2165–2535 g · ha^–1 · y^–1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha^–1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.     

Responses of the trunk routes of a harvester ant to plant density

Messor barbarus is a Mediterranean harvester ant that constructs physically defined trunk routes on the ground to connect nest entrances with foraging areas. Some responses of these trunk routes to plant density (and therefore resource abundance) were analyzed by testing the preferential allocation of different parts (trunk route ends, segments and branching points) in a patchy environment. Maps of grass density in four categories and Messor barbarus trunk routes were compiled for a Mediterranean pasture in Central Spain over four consecutive years. The proportions of the density categories in each year were used to calculate random expected frequencies of the trunk route points and the predominance of higher or lower grass densities. Trunk route ends discriminate and selectively reach patches with a greater abundance of resources in all study years. Branching points are also allocated preferentially in areas with higher vegetation density, but only in years with a predominance of the higher categories of grass density. In these years, the colonies of Messor barbarus have a phalanx strategy at a colonial level, and branching is more profuse. Finally, trunk route segments do not indicate any preference for crossing determined vegetation densities, but rather connect successive branching points or trunk route ends by the shortest route. These results concur with a model of structural strategy change (guerilla — phalanx) (Hutchings 1988) at the level of trunk routes. They are probably constituted by transitory sections with few branches, that expand other more profusely branched sections which are more dedicated to foraging.     

Honeydew collection by the invasive garden ant Lasius neglectus versus the native ant L. grandis

Honeydew collection performed by the invasive ant Lasius neglectus and by the native ant L. grandis was compared. The invasive ant collected 2.09 kg of honeydew per tree while the native ant collected 0.82 kg. The aphid Lachnus roboris was visited by both ant species. In holm oaks colonized by L. neglectus, aphid abundance tended to increase and its honeydew production increased twofold. The percentage of untended aphids was lower in holm trees occupied by L. neglectus. As tending ants also prey on insects, we estimated the percentage of carried insects. The native ant workers carried more insects than the invasive ant. Both ant species preyed mainly on Psocoptera and the rarely tended aphid, Hoplocallis picta. We conclude that the higher honeydew collection achieved by L. neglectus was the consequence of (1) its greater abundance, which enabled this ant to tend more Lachnus roboris and (2) its greater level of attention towards promoting an increase of honeydew production. Handling editor: Heikki Hokkanen     

Variable thermal sensitivity as output of a circadian clock controlling the bimodal rhythm of temperature choice in the ant Camponotus mus

Along a thermal gradient and under a LD 1212 h cycle, nurse workers of the ant Camponotus mus select for the brood two different temperatures daily: 30.8°C at the middle of the light period (circadian phase = 90°), and 27.5°C 8 h later, during the dark period (CP = 210°). Brood-carrying activity proved to be self-sustained, running its two daily bursts free with a similar period of 23.5 h, under both LL and DD. The LD alternation acted as a strong Zeitgeber. A phase-delay of the LD 1212 h cycle reset the overt rhythm at once, being both daily events locked-on to the delayed light: dark transition. However, changes in expression, non-occurrence, or even splitting of the two daily brood-carrying events during resetting depended on the phase of the delayed DL transition. By comparing the occurrence of activity with predictions based on a threshold curve of thermal sensitivity, results indicated that an immediate resetting of the involved pacemaker actually takes place. Nurse workers do not directly control the total time spent by the brood at the selected temperature. Instead, the endogenously-driven thermal sensitivity triggers their thermal-searching behavior at two critical times of the day, when environmental temperature is expected to reach its maximum and minimum.     

Genetic relationships among populations of the Argentine ant Linepithema humile introduced into Japan

Since some colonies of Argentine ant Linepithema humile were discovered in Japan in 1993, populations of this invasive alien ant species have been expanding their distribution. To resolve the number of invasions and the genetic structure in the early stages of introduction, we inferred the genetic structure and relationships among colonies from eight localities, from analyses of eight nuclear microsatellite DNA markers. F _ST analysis, principal component analysis and assignment test showed that at least three highly genetically differentiated groups of Argentine ants are present in Japan. Populations from Hiroshima Bay were grouped together (Hiroshima, Hatsukaichi, Otake, Iwakuni Central and Iwakuni Kuroiso), while those from Kobe and Yanai were both genetically distant from each other and from the Hiroshima Bay group. Hatsukaichi and Kobe are international seaports, suggesting that the by-ship invasion occurred at least twice. The invasion route of the Yanai population is unknown at this moment. The Aichi population was genetically distant from that of the Hiroshima Bay group by the difference in allele frequencies, and it was plausible that the Aichi population was introduced from the Hiroshima Bay group by human-mediated jump dispersal.     

Intraspecific competition through food robbing in the harvester ant,Messor aciculatus (Fr. Smith), and its consequences on colony survival

Summary Intraspecific interference competition in the harvester ant,Messor aciculatus, was studied. Colonies of this species were found not to have territories. Some nests were located very close to each other, and the foraging areas of the neighbors usually overlapped. Even though the frequency with which alien and resident ants met was very high in the vicinity of the nest entrances, aggressive interactions between them rarely occurred. However, when hostile workers encountered each other, they exhibited a kind of ritualized combat and the winner ejected, but did not injure the loser. If any aliens entered the nest, some of them were pulled out, mainly by the residents.Aliens roaming near a neighbor's nest entrance ferociously attacked the residents carrying seeds in their mandibles and robbed them. On other occasions, aliens entered the nest and stole the collected seed. Although seed robbing and stealing occurred among neighboring colonies, there were remarkable differences in the frequency of their occurrence. The results of field observations and experiments suggest the existence of a dominance order among the neighbors. In one instance, extermination of an inferior colony by its neighbor was observed. The raider colony transferred the stored seeds from the nest of the inferior colony to its own and deposited the larvae and workers some distance away from the nest.The influence of ritualized combat and food robbing on colony activities, and the ecological significance of this interference behavior in terms of spatial distribution and temporal persistence of the nest sites, is discussed.     

针毛收获蚁社群密度与分布及社群各品级在自然穴巢中的季节迁移行为

针毛收获蚁Messoraciculatus(F .Smith)是我国北方特别是西北荒漠草原的优势种蚂蚁 ,亦是重要的种子收获性蚁类 ,收获、贮藏和取食 1 0余种荒漠植物种子。本文以沙坡头自然保护区红卫固定沙丘地段为样地 ,对该蚁的社群密度与分布及社群各品级在自然穴巢中的季节迁移行为作了研究。最近邻体法测定表明 ,社群在新生生境中起初随机定居 ,随社群密度的不断定居 ,空间分布型逐渐向均匀六边形过渡 ;社群密度为成熟社群 34 .98巢 hm2 ,发展中社群 2 6 .99巢 hm2 ,新建社群 4 4.98巢 hm2 ,种群密度 2 .6 8工蚁 m2 。社群结构的季节动态总蚁量和幼蚁为双峰型 ,蚁后和雄蚁为单峰型。室内人工蚁巢饲养表明 ,社群结构约 1月后趋于稳定 ,蚁量 5 0～ 70头 ,单蚁后以 4个品级混合越冬。自然社群的挖掘表明 ,各品级在蚁巢中的季节迁移行为明显呈现上迁、前层持续活动、下迁和深层越冬四期。此外 ,讨论了该蚁对荒漠草原环境的适应性。     

Three queen morphs with alternative nest-founding behaviors in the ant, Temnothorax longispinosus

In many ant species, multiple modes of founding new colonies occur in the same population. These modes include dependent founding, independent founding by haplometrosis (single queen), and independent founding by pleometrosis (multiple queens). In several cases, a dimorphism in queen size has been found, such that each morph specializes in a particular nest-founding behavior. I investigated queen size in the ant Temnothorax longispinosus in several southern Wisconsin populations and found three distinct queen morphs: small queens with very low fat content and short wings, large queens with low fat and long wings, and large queens with high fat and very long wings. Several traits associated with founding behavior correlated with these queen sizes. Small queens were produced in lower numbers, were more common in polygynous nests, and returned to the nest in higher proportions than both large queen morphs. The size ranges and fat levels of each queen morph were similar to those of other species that specialize in either haplometrosis (very large, high fat), pleometrosis (large, low fat), or dependent founding (small and low fat). However, there was extensive overlap in several of the founding behaviors, suggesting that the morphs in these populations have some flexibility in founding behavior. The queen morphs in these populations of T. longispinosus may resemble early stages in the evolution of more specialized dispersal polymorphisms found in other ant species. Received 11 January 2006; revised 15 September 2006; accepted 18 September 2006.     

The effect of queen ants on the survival of Maculinea arion larvae in Myrmica ant nests

Summary The mortality of Maculinea arion caterpillars was measured in both laboratory and wild Myrmica nests, and found to be nearly 3 times higher in nests that had queen ants present. This is attributed to queen effect, which causes worker ants in nests with queens to attack large ant larvae (gynes) that would otherwise develop into new queens. Maculinea arion caterpillars mimic Myrmica larvae, and are usually attacked during the first 10 days after adoption, when they pass through the size range of ant gyne larvae. Caterpillars are also likely to be attacked during this period because their nethod of feeding brings them into close contact with the skins of large ant larvae, which contain gyne larval pheromones; older caterpillars are large enough to eat larvae without their exposed surfaces contacting the larval skin. In the wild, many caterpillars of Maculinea arion die in ant nests, and this has been shown in previous work to be the key factor that determines changes in their abundance from year to year. It is suggested that queen effect can be an important cause of these deaths, and one that particularly affects populations of butterflies that breed on sites with long-established plagioclimaxes of short turf rather than short-lived grass-land successions.     

Spatial and temporal patterns of granivorous ant seed predation in patchy cereal crop areas of central Spain

Summary Patterns of granivorous ant seed predation in extensive cereal croplands of central Spain were investigated by measuring seed removal rates on artificial seed patches. Sampling was designed to cope with the seasonal and daily foraging cycle of ant colonies. Simultaneously with removal rates, I measured seed availability, habitat physiognomy at two spatial scales (landscape and microhabitat), weather variables (temperature and rainfall), and distance to the nearest ant nest. Ant seed predation was concentrated on shrublands, and associated with places with high covers of shrubs, chamaephytes and stones. These results were in close agreement with those obtained by analyzing the spatial distribution of granivorous ant nests (Díaz 1991). Moreover, there was a close relationship between seed removal rates and distance to the nearest ant nest, that fitted the predictions of the optimal foraging model developed by Reyes-López (1987). Seasonal and daily patterns of ant foraging activity seemed to depend more on endogenous factors than on environmental variation. I conclude that ants were not able to track the spatial and temporal variation of their food resources in these man-modified habitats, so that their potential to interact with other members of the granivore system is greatly reduced by human activities.     

Competition/colonization syndrome mediated by early germination in non-dispersing achenes in the heteromorphic species Crepis sancta

Background and Aims

The competition–colonization trade-off theory postulates that the competitive and colonizing abilities of organisms are negatively related; this trade-off has been proposed as a major force in the maintenance of diversity. In plants, the competition–colonization trade-off is often considered to result from variation in resource partitioning, thus generating heavy competitive (non-dispersing) seeds and light (dispersing) non-competitive seeds. Here, the possibility is explored that early germination provides a competitive advantage, thus mediating competitive interactions.

Methods

Using eight populations of the heterocarpic species Crepis sancta (Asteraceae), the possibility was tested that dispersing and non-dispersing achenes differ in germination timing, and the impact of early germination on individual fitness components was analysed in the context of intraspecific competition. To evaluate whether seed reserve varies among achene types, endosperm size was also measured by analysing photographs of cross-sections taken under a binocular microscope.

Key Results and Conclusions

The results show that non-dispersing achenes germinated 4 d earlier (on average) than dispersing achenes. It is also shown that early germination provides a positive advantage for the survival and final biomass of individuals, a pattern that was consistent over the eight populations and independent of achene type. Dispersing and non-dispersing achenes did not differ in terms of seed reserve (endosperm size). It is proposed that germination phenology may mediate the competition–colonization trade-off in Crepis sancta and the evolutionary significance of this phenomenon is discussed.     

Variation in ant aggression and kin discrimination ability within and between colonies

Significant variation in aggressiveness and kin discrimination ability occurs between different laboratory colonies of the ant Rhytidoponera confusa.Different colonies show consistently high (or low) levels of aggression toward nonnestmates over 4–19 weeks. Earlier studies excluded colony size and the natural presence or absence of the queen in colonies and differences in hunger as possible sources of variation. The present study excluded the number of larvae in colonies and the time of the light period of the light cycle when recognition tests were carried out. Highly significant variation occurs between the kin discrimination ability of individual workers in any particular colony. Approximately 28% of the workers in colonies of R. confusashowed very poor kin discrimination. Much of the colony's kin discrimination is carried out by a small number of highly aggressive workers.