An experimental analysis of pleometrotic advantage in the desert seed-harvester antMessor pergandei (Hymenoptera; Formicidae)

Summary Starting colonies of the desert seed-harvester antMessor pergandei are clumped in the field and face severe intraspecific competition through brood raiding. Single foundress laboratory colonies ofM. pergandei are more likely to succeed at brood raiding with conspecific colonies if they are given additional workers and mature pupae several days prior to brood raiding. Per foundress fecundity remains constant across laboratory starting colonies established with 1, 3 and 5 foundresses. These results suggest that the selective advantage of cooperative colony foundation (pleometrosis) in this and similar species may derive directly from the ability of multiple foundresses to produce a larger brood raiding force.     

Relationships between the ant Brachymyrmex obscurior (Hymenoptera, Formicidae) and Acacia pennatula (Fabaceae)

Summary. In central Mexico, the ant Brachymyrmex obscurior Forel feeds on nectar produced by extrafloral nectaries of Acacia pennatula (Schlecht. & Cham.) Benth. However, no studies have determined whether the ants visitation is related to plant nectar availability and whether ants protect A. pennatula from herbivory. The objectives of this 2-yr study (2000–2001) were to assess whether seasonal changes in ant visitation coincide with extrafloral nectar productivity in A. pennatula and to determine whether ants protect the plant. At the end of the dry season (April–June) B. obscurior was the only ant species on A. pennatula and extrafloral nectar production is limited to this period. Exclusion experiments, performed at the end of the dry season showed that A. pennatula did not receive a protective benefit when visited by ants. Branches with ants and branches where ants are excluded had similar numbers of the nonmyrmecophile leafhopper Sibovia sp. which was the only herbivore observed under natural conditions.Received 24 March 2004; revised 4 September 2004; accepted 8 September 2004.     

Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomyrmex subnitidus

Invasive ants often displace native ants, and published studies that focus on these interactions usually emphasize interspecific competition for food resources as a key mechanism responsible for the demise of native ants. Although less well documented, nest raiding by invasive ants may also contribute to the extirpation of native ants. In coastal southern California, for example, invasive Argentine ants (Linepithema humile) commonly raid colonies of the harvester ant, Pogonomyrmex subnitidus. On a seasonal basis the frequency and intensity of raids vary, but raids occur only when abiotic conditions are suitable for both species. In the short term these organized attacks cause harvester ants to cease foraging and to plug their nest entrances. In unstaged, one-on-one interactions between P. subnitidus and L. humile workers, Argentine ants behaved aggressively in over two thirds of all pair-wise interactions, despite the much larger size of P. subnitidus. The short-term introduction of experimental Argentine ant colonies outside of P. subnitidus nest entrances stimulated behaviors similar to those observed in raids: P. subnitidus decreased its foraging activity and increased the number of nest entrance workers (many of which labored to plug their nest entrances). Raids are not likely to be the result of competition for food. As expected, P. subnitidus foraged primarily on plant material (85% of food items obtained from returning foragers), but also collected some dead insects (7% of food items). In buffet-style choice tests in which we offered Argentine ants food items obtained from P. subnitidus, L. humile only showed interest in dead insects. In other feeding trials L. humile consistently moved harvester ant brood into their nests (where they were presumably consumed) but showed little interest in freshly dead workers. The raiding behavior described here obscures the distinction between interspecific competition and predation, and may well play an important role in the displacement of native ants, especially those that are ecologically dissimilar to L. humile with respect to diet. Received 15 July 2005; revised 19 October 2005; accepted 26 October 2005.     

Size and mating success in males of the western harvester ant,Pogonomyrmex occidentalis (Hymenoptera: Formicidae)

Mating success in males of the lek mating ant species,Pogonomyrmex occidentalis, increases with increased body size. We estimated the magnitude of the selection coefficients on components of size by collecting males in copula and comparing their morphology to that of males that were collected at the lek but that were not mating. Four characters, body mass, head width, wing length, and leg length, were measured for a sample of 225 mating and 324 nonmating males and 225 females. Significant direct selection favors increased wing length and leg length. Multiple regression of transformed variables (principal components) indicated that the increased mating success of larger males is a function of all four characters. We found no evidence of positive assortative mating on the basis of any individual character or on the multivariate general size variable (the first principal component).     

Mating for variety increases foraging activity in the harvester ant, Pogonomyrmex occidentalis

Multiple mating by females characterizes most insect species, but is relatively uncommon in social insects. Females may mate with multiple mates because they experience the direct benefits of increased survival or fecundity, to acquire high quality mates, or to lower the risk of reduced fecundity by mating with incompatible males. We used the extensive natural variation in mating frequency in the western harvester ant, Pogonomyrmex occidentalis , to test the hypothesis that increased mating by the queen leads to an increase in colony performance. Colonies with greater genetic diversity began to forage earlier in the day and foraged for longer time periods. The workers which initiated foraging were a nonrandom subset of the genotypes present in the colony. We used a statistical approach to correctly predict the direction and magnitude of the correlation between genetic diversity and colony foraging activity.     

Activity Patterns of Harvester Ants Pogonomyrmex pronotalis and Pogonomyrmex rastratus in the Central Monte Desert, Argentina

Seed-eating ants could have a significant effect on plant communities in deserts and semiarid zones. This effect is mediated through spatial and temporal foraging patterns, and seed selection within patches. Foraging patterns of harvester ants in South American deserts are almost unknown. The purpose of this work is to determine the temporal variations in the activity levels of Pogonomyrmex pronotalis and P. rastratus in the central Monte desert, and how these patterns may be related to abiotic factors, particularly to soil temperature. Activity levels and soil surface temperature were recorded at hourly intervals in five colonies for each species during the activity season (October, December, February, and April) in both 1999–2000 and 2000–2001 periods (except for October 1999). Surface ant activity starts in October, increases between December and February, and then ceases by April. Surface ant activity is diurnal throughout the season and usually has a unique peak during midday in October and April, and two peaks in the morning and the afternoon from December to February. The proportion of the activity budget devoted to nest's maintenance activity was similar for both species. Activity levels of foraging workers tended to be higher in P. pronotalis than in P. rastratus. P. pronotalis is active between 20 and 59°C, with higher levels of activity between 35 and 45°C, whereas P. rastratus shows activity between 18 and 58°C, with higher levels of activity between 30 and 40°C. Our results suggest that temporal changes in surface activity respond mainly to soil temperature fluctuations. However, at intermediate temperatures (those probably encompassing the thermal tolerance range of these ant species), temperature appears not to be a good predictor of daily and seasonal activity fluctuations.     

Respiratory patterns in nurses of the red wood ant Formica polyctena (Hymenoptera, Formicidae)

Summary Cycles of discontinuous gas exchange (DGCs) and abdominal ventilatory movements were studied in nurses of red wood ant, Formica polyctena, using an electrolytic respirometer and an infra-red (IR) gas analyser or flow-through respirometry. Both respirometry systems were combined with an IR actographic device based on IR-emitting and IR-sensor diodes. After recovering from handling and apparatus stress, lasting 1–3 h, completely motionless intact ants displayed regular DGC. After decapitation the ants displayed DGC whose frequency was somewhat lower than that of the intact individuals (7.17 ± 0.79 mHz and 10.43 ± 01.12 mHz, respectively). In headless ants, there occurred continuous slow movements of legs. Bursts of carbon dioxide in the intact and in the headless ants always coincided with a bout of telescoping movements (contractions) of abdominal segments, which was interpreted as active ventilation. During interburst periods, the headless ants exhibited telescoping movements characterised by rapid protraction, lasting 0.07–0.09 s, followed by a slow retraction of segments, suggesting passive ventilation. The intact ants were very sensitive to the flowing air and tended to be continuously active during flow-through respirometry. The decapitated ants, on the contrary, were insensitive to the air current.Received 26 March 2003; revised 30 July 2003; accepted 6 August 2003.     

Seasonal cycle of colony structure in the Ponerine ant Pachycondyla chinensis in western Japan (Hymenoptera, Formicidae)

Investigation of reproductive strategies of ants in the subfamily Ponerinae is important for understanding of the evolution of social structure and of the significance of caste dimorphisms. The biology of species with mated and egg-laying workers (gamergates) has been studied for many species, however, little attention has been paid to species that reproduce via alate queens only. We investigated the seasonal cycle of changes in the colony structure of Pachycondyla chinensis reproduced by alate queens in western Japan, and found the following novel biological characteristics of this species. P. chinensis showed a remarkable caste dimorphism in ovariole numbers: workers had no ovaries while queens had 18 to 36 ovarioles in their ovaries. The nesting system seemed to be polydomous: 266 of 400 nests collected were queenless. The number of queenless nests increased during the reproductive season. Among the 134 queenright nests, 38 had several mated-queens without significant differences in ovary activation and the remaining 96 nests were monogynous. During winter to early spring, most nests were polygynous. After alate production, most of the old queens seemed to die or be expelled and replaced by new queens. Virgin dealated queens were often found and they seemed to have laid eggs. Received 3 August 2007; revised 19 December 2007; accepted 20 December 2007     

Queen polymorphism in the North American harvester ant,Ephebomyrmex imberbiculus

Summary In the North American harvester antEphebomyrmex imberbiculus, in addition to dealate queens, wingless female reproductives occur that have greatly reduced ocelli and thoracic sutures (intermorphic queens). Both queen types are equivalent in function, and do not differ in ovarian morphology. Colonies may contain several inseminated and egg-laying intermorphic queens. We discuss queen polymorphism in respect to the biology of this desert-dwelling species.     

Haplometrosis and pleometrosis in the antAcromyrmex striatus (Hymenoptera: Formicidae)

Summary Laboratory tests with mated females of the leaf-cutting antsAcromyrmex striatus (Myrmicinae, Attini) were conducted to determine if the colony foundation is a solitary or a mutualistic process. We have also tested the effect of foundresses density and number of available areas in this process. Three bioassays were performed: with single foundresses; with paired foundresses; and with groups of 8 to 18 foundresses. The results suggest that the foundation can be by haplometrosis or pleometrosis, possibly depending on physical distances between or densities of foundresses. Foraging activity was common in haplometrotic queens and in foundress pairs, but no foraging occurred in groups of foundresses.     

Male Behavior and Regulation of Worker Mating in a Ponerine Ant, Pachycondyla (Bothroponera) sp. (Hymenoptera: Formicidae)

Male behavior and regulation of worker mating were observed in Indonesian Pachycondyla sp. The colonies had some mated workers but only one gamergate (mated and egg-laying workers). Males frequently tried to mate with workers in their natal nests irrespective of worker age and dominance rank, however, workers never accepted mounting of nestmate males. Worker mating occurred only under the gamergateless condition with alien males.     

Mating Behavior and Colony Founding of the Slave-Making Ant Formica sanguinea (Hymenoptera: Formicidae)

The mating and postmating behavior of reproductives belonging to two sympatric dulotic colonies of the facultative slave-making ant Formica sanguinea was analyzed in the field. Our observations showed that the European blood-red ant adopts a reproductive behavior similar to the male aggregation syndrome. Newly mated females return to a dulotic colony and often wait for a raid. Following a slave raid is an advantageous strategy to locate and invade host nests and to establish a new dulotic colony. In the laboratory, the following modes of colony founding were studied: independent, adoption, alliance, usurpation, and brood raiding. Independent foundation was possible only when several females were kept together. Alliance was obtained with females of two potential slave species (F. cunicularia, F. rufibarbis). Usurpation and adoption were more frequent in the incipient than in the mature host colonies. Mixed colonies were always obtained after the sack of the host pupae. It seems likely that, rather than conspecific adoption followed by budding, F. sanguinea relies on temporary parasitism to start new colonies.     

婚飞行为影响中华蜜蜂性成熟处女蜂王的基因表达

婚飞是性成熟处女蜂王与雄蜂交配过程中的一个重要前奏, 在该过程中蜂王体内伴随着一系列重要的生理变化。为了探究中华蜜蜂Apis cerana cerana处女蜂王婚飞过程中基因表达变化, 本研究利用数字基因表达谱(digital gene expression, DGE) 技术分析了中华蜜蜂性成熟处女蜂王飞行与未飞行之间的基因表达差异。经DGE测序, 分别从两个样品中获得5.98和6.01 百万条Clean标签。通过分析检测到250个基因有差异表达, 其中133个基因在飞行蜂王中上调表达, 117个基因在飞行蜂王中下调表达。这些差异基因可以归类到348个功能性类别和142个生化途径。结果表明中华蜜蜂性成熟处女蜂王在婚飞过程中大量基因的表达发生了变化。这些结果为进一步研究中华蜜蜂蜂王婚飞过程中生理变化的分子机制提供了重要的基因表达信息。     

Sex ratio and nuptial flight pattern of the leaf-cutting antsAcromyrmex heyeri andA. striatus (Hymenoptera,Formicidae)

Summary Data from 22Acromyrmex heyeri and 39A. striatus colonies showed that most were male-biased, while some were female-biased and a few had a sex ratio of 11. The time and pattern of swarming were different in both species and could be seen as strategies to avoid inbreeding.     

Effect of mating stage on water balance, cuticular hydrocarbons and metabolism in the desert harvester ant, Pogonomyrmex barbatus

Water-loss rates increase after mating in queens of the harvester ant Pogonomyrmex barbatus (Formicidae: Myrmicinae), then increase again after the mated queens excavate an incipient nest. We determined the mechanistic basis for these increased water-loss rates by examining cuticular permeability, respiratory water loss, metabolic rates, and cuticular hydrocarbons for queens at three stages in the mating sequence: unmated alate queens, newly mated dealate queens, and mated queens excavated from their incipient nest. Both total water loss and cuticular transpiration increased significantly following mating, with cuticular transpiration accounting for 97% of the increased water loss. In contrast, metabolic rate and respiratory water loss were unaffected by mating stage. The total quantity of cuticular hydrocarbons did not vary by mating stage. However, relative amounts of four of the most abundant cuticular hydrocarbons did vary by mating stage, as did quantities of n-alkanes and methylalkanes. The general pattern was that percent composition of n-alkanes decreased through the mating sequence, while percent composition of methylalkanes increased over the same sequence. We discuss three mechanisms that might cause these post-mating increases in cuticular permeability. Our data support the hypothesis that part of this increase results from soil particles abrading the cuticle during the process of nest excavation.     

Nest-plugging: interference competition in desert ants (Novomessor cockerelli and Pogonomyrmex barbatus)

Summary Pogonomyrmex barbatus and Novomessor cockerelli, sympatric species of harvester ants in the Lower Sonoran desert, compete for seed resources. This study reports on a method of interference competition. Early in the morning, before P. barbatus' activity period, N. cockerelli fills the nest entrances of P. barbatus with sand. This delays the beginning of the P. barbatus activity period for 1–3 h. P. barbatus colonies near N. cockerelli nests were more likely to be plugged. Nest-plugging shifts the typical daily sequence of P. barbatus activities, including the onset of foraging, forward towards midday, when high temperatures force the colony back inside the nest. P. barbatus colonies do not compensate for late emergence or events impeding foraging by increasing foraging rate. Thus nest-plugging by N. cockerelli decreases the foraging capacity of P. barbatus colonies.     

Observation of group recruitment to prey in a primitive ponerine ant,Amblyopone sp. (reclinata group) (Hymenoptera: Formicidae)

Summary Group recruitment during foraging was observed in a primitive ponerine ant,Amblyopone sp. (reclinata group) under laboratory condition. Workers searched for prey singly; however, if a item of prey was stung by a worker, other ants joined the attack. After the prey became immobile, one of the workers laid a trail directly toward the nest. This scout worker recruited additional workers (between 3 and 33). They formed a single file procession to the point of prey capture, and cooperatively transported the prey. A scout worker could stimulate nest workers to leave the nest without direct contact, and the recruited workers could trace the trail without guidance by the scout worker. This is the first report of recruitment behavior during foraging in the primitive antAmblyopone.     

Optimal and central-place foraging theory applied to a desert harvester ant,Pogonomyrmex californicus

Summary Certain predictions of optimal- and central place-foraging theory were tested on the desert harvester ant, Pogonomyrmex californicus. Colonies were offered three different sizes of oat seed and found to maximize net energy intake (ei) over time (t _i ) by harvesting the seed sizes with the highest e _i /t _i rank. Two aspects of t _i were measured that were assumed constant in previous studies. The handling components of t _i (time required to manipulate the seed and travel time back to the colony with the food) were measured and found to be positively correlated with seed size. The manipulation success rate (the percentage of handled seeds successfully picked up) decreased with increased seed size. These results point out how important it is to measure all parameters of e _i /t _i rather than to assume constancy with both prey type and foraging distance. The relative abundance of less preferred food types was important in determining the proportion of preferred types in the diet. The food supply of eight colonies was manipulated experimentally over a 25-day period. Four deprived colonies were constrained within aluminum enclosures to prevented foraging. The remaining four satiated colonies were given food ad libitum. The niche breadths of the treated colonies were then compared to controls, but found not to differ significantly. Seed baits were offered at three distances from the colony to test whether selectivity increased with disance. Contrary to theoretical predictions, all colonies harcested about the same proportion of each seed size at each distance.