Trail‐sharing among tropical ants: interspecific use of trail pheromones?

1. Trail‐sharing between different ant species is rare and restricted to a small number of species pairs. Its underlying mechanisms are largely unknown. For trail‐sharing to occur, two factors are required: (i) one or both species must recognise the other species or its pheromone trails and (ii) both species must tolerate each other to a certain extent to allow joint use of the trail. A species that follows another's trails can efficiently exploit the other's information on food sources contained in the pheromone trails. Hence, food competition and thus aggressive interactions between a species following another's trail and the species being followed, seem likely. 2. In the present study, we investigated interspecific trail following and interspecific aggression in trail sharing associations (i) among Polyrhachis ypsilon, Camponotus saundersi, and Dolichoderus cuspidatus, and (ii) among Camponotus rufifemur and Crematogaster modiglianii. We tested whether trail‐sharing species follow each other's pheromone trails, and whether the ants tolerated or attacked their trail‐sharing partners. In both associations, we confronted workers with pheromone trails of their associated species, and, for the former association, measured interspecific aggression among the trail‐sharing species. 3. In our assays, D. cuspidatus and C. rufifemur regularly followed heterospecific pheromone trails of P. ypsilon and C. modiglianii, respectively. However, only few workers of the remaining species followed heterospecific pheromone trails. Thus, shared trails of P. ypsilon and C. saundersi cannot be explained by interspecific trail‐following. 4. Interspecific aggression among P. ypsilon, C. saundersi, and D. cuspidatus was strongly asymmetric, C. saundersi being submissive to the other two. All three species differentiated between heterospecific workers from the same or another site, suggesting habituation to the respective trail‐sharing partners. We therefore hypothesise that differential tolerance by dominant ant species may be mediated by selective habituation towards submissive species and this way determines the assembly of trail‐sharing associations.     

Monomorium ant's trail pheromones: Glandular source,optimal concentration,longevity and specificity

Many ants use pheromone trails to organize collective foraging. Trail pheromones are produced from different glandular sources and they may be specific to a single species or shared by a number of species. I investigated the source of trail pheromones in three Monomorium ant species: Monomorium niloticum (Emery), M. najrane (Collingwood & Agosti) and M. mayri (Forel). I also examined the optimal concentration, longevity and specificity of the pheromones. M. niloticum and M. najrane secrete trail pheromone from their venom glands, whereas M. mayri secrete trail pheromone from its Dufour's gland. The optimum concentration was 1.0 and 0.1 gaster equivalent (GE)/30 cm trail in M. niloticum, 1.0 GE in M. najrane and 5.0 GE in M. mayri. Longevity of the optimal concentration was about one day for all species. There is no species specificity among the three species of Monomorium in their trail pheromone.     

Fire ant trail pheromones: Analysis of species specificity after gas chromatographic fractionation

The specificity of the trail pheromones of four Solenopsis species was determined using natural trails. Dufour's gland extracts, and purified fractions from Dufour's gland extracts collected after gas-chromatographic separation. S. richteri and S. invicta possess species-specific major trail pheromones, while S. geminata and S. xyloni appear to have a common trail pheromone. Preliminary chemical characterization of the main trail pheromone of S. richteri indicates a M.W. of 218 and empirical formula of C₁₆H₂₆. The trail pheromone system of S. richteri consists of a blend of compounds and this phenomenon may also occur in the other species. The lowest concentration of their trail pheromone that workers of S. richteri could detect was about 10 fg per cm. The significance of blends of pheromones being utilized to generate chemical trails is discussed.     

Individual rules for trail pattern formation in Argentine ants (Linepithema humile)

We studied the formation of trail patterns by Argentine ants exploring an empty arena. Using a novel imaging and analysis technique we estimated pheromone concentrations at all spatial positions in the experimental arena and at different times. Then we derived the response function of individual ants to pheromone concentrations by looking at correlations between concentrations and changes in speed or direction of the ants. Ants were found to turn in response to local pheromone concentrations, while their speed was largely unaffected by these concentrations. Ants did not integrate pheromone concentrations over time, with the concentration of pheromone in a 1 cm radius in front of the ant determining the turning angle. The response to pheromone was found to follow a Weber's Law, such that the difference between quantities of pheromone on the two sides of the ant divided by their sum determines the magnitude of the turning angle. This proportional response is in apparent contradiction with the well-established non-linear choice function used in the literature to model the results of binary bridge experiments in ant colonies (Deneubourg et al. 1990). However, agent based simulations implementing the Weber's Law response function led to the formation of trails and reproduced results reported in the literature. We show analytically that a sigmoidal response, analogous to that in the classical Deneubourg model for collective decision making, can be derived from the individual Weber-type response to pheromone concentrations that we have established in our experiments when directional noise around the preferred direction of movement of the ants is assumed.     

Leptogenys khammouanensis sp. nov. (Hymenoptera: Formicidae). A possible troglobitic species of Laos,with a discussion on cave ants

The new species Leptogenys khammouanensis sp. nov. is described from two caves of the Khammouan karst (Laos). It is characterized by a set of striking morphological characters (reduced eyes, light pigmentation, slender body and very elongated legs and antennae), which recall the troglobiomorphic traits of cave arthropods. Relations between caves and ants are discussed at this occasion, in the light of the recent biological explorations of caves in Southeast Asia. The classical view that ants are rare and unimportant in caves is challenged. Ants are actually major and regular components of guano assemblages in many caves of the region, but none of these guano species exhibit cave-related adaptation in its external morphology. Conversely, ants are very rare in low-resources habitats, where only accidental occurrence of outside species are reported in Southeast Asia. Leptogenys khammouanensis has been found only in such an oligotrophic environment, very deep in the cave and far from any guano deposits. Its presence there, together with its troglobiomorphic traits, support the idea that Leptogenys khammouanensis might be the first truly troglobitic ant.     

Low levels of hybridization in two species of African driver ants

Hybridization in ants can have consequences different from those observed in most other species, with many of the potential deleterious effects being mitigated due to haplodiploidy and eusociality. In some species where colonies are either headed by multiple queens or single queens that mate with many males, hybridization is associated with genetic caste determination, where hybrids develop into workers and purebred individuals develop into queens. A previous study suggested that hybridization occurs between two Dorylus army ant species with multiply mated queens. However, the extent and exact pattern of hybridization have remained unclear, and its possible effect on caste determination has not been investigated. In this study, we aimed to determine the extent and direction of hybridization by measuring how frequently hybrids occur in colonies of both species, and to investigate the possibility of genetic caste determination. We show that hybridization is bidirectional and occurs at equal rates in both species. Hybrid workers make up only 1–2% of the population, and successful interspecific matings represent approximately 2% of all matings in both species. This shows that, although interspecific matings that give rise to worker offspring occur regularly, they are much rarer than intraspecific mating. Finally, we find no evidence of an association between hybridization and genetic caste determination in this population. This means that genetic caste determination is not a necessary outcome of hybridization in ants, even in species where queens mate with multiple males.     

The influence of similar aggregation pheromones on the microhabitat choice of two parapatric species of reptile tick (Acari: Ixodidae)

Summary Non-specificity of part of the sex pheromone system of three species of reptile tick has previously been suggested to result in reproductive interference between the species when they attach to the same host (Andrews et al. 1982). Two of the species, Aponomma hydrosauri and Amblyomma limbatum, have common aggregation pheromones which act when the ticks are detached from their hosts. As each species tends to occupy spatially separated microhabitats, the similarity of the aggregation pheromone could lead to interference between species when off their hosts by inducing one species to move away from its preferred microhabitat into that occupied by the other species. This study demonstrates that coaggregations of both species do occur, but that neither species shows a statistically significant shift in microhabitat preference attributable to interactions between species. We suggest that the behaviours leading to microhabitat choice in each species override the influence of the common aggregation pheromone.     

A comparison of the energetics of foraging of three species of Leptogenys (Hymenoptera, Formicidae)

Abstract. The three Leptogenys species L.nitida, L.schwabi and L.attenuata coexist in the coastal forests of South Africa and exhibit an array of foraging strategies ranging from individual foraging to group raiding. To determine whether there is a physiological basis for these strategies, the energetic cost of locomotion of individuals in these three species was determined. Carbon dioxide emission of voluntary running ants was measured using a flow-through technique, in order to determine their metabolic rate when running. The minimum cost of transport was constant over a range of temperatures (20–35°C), and similar for all three species (common value 212.96 ± 17.35 Jkg^_1m^_1). A comparison of the energy required to sustain representative foragers at 25°C indicated that it was energetically less expensive for L. nitida workers to forage than for the other two species. This may explain why L.nitida uses group raiding, while the other two species use individual foraging with limited recruitment for retrieval of large food items.     

The use of alarm pheromones to enhance bait harvest by grass-cutting ants

The enhancement of bait for the control of grass-cutting ants was investigated using two species of grass-cutting ant, Atta bisphaerica (Forel) and Atta capiguara (Gon?alves) (Hymenoptera: Formicidae). Bait was applied in loose piles to obtain a direct relationship between ant attraction and bait harvest. Enhancement with alarm pheromone compounds significantly increased the attractiveness and harvest of bait under certain conditions. A large proportion of the ants attracted to the enhanced bait were minor workers. These ants rarely transport bait because of their small size, and so it may be possible to increase the effect of bait enhancement by using smaller bait granules. Foragers of A. capiguara were less inclined to transport citrus-pulp bait than were those of Atta laevigata (Fr. Smith), a species that also harvests dicotyledonous plants. This emphasizes the importance of developing a bait matrix that is more acceptable to grass-cutting species. Nevertheless, the results suggest that alarm pheromone compounds have significant potential to improve the efficacy of baits for the control of grass-cutting ants.     

A new ant species of the Leptogenys sulcinoda-group (Hymenoptera: Formicidae) from Saudi Arabia

Leptogenys polaszeki sp. n. is described from the Asir Mountains, Saudi Arabia, based on the worker caste. The new species is a member of the L. sulcinoda-group and appears closest to L. bellii Emery, 1901, originally described from Ethiopia. The likely male caste of L. polaszeki sp. n. is described based on a single specimen collected by a pitfall trap in the type locality.

http://www.zoobank.org/urn:urn:lsid:zoobank.org:pub:5D3BF3AF-5308–48E8–8416–902278909AD5     

Decay rates of attractive and repellent pheromones in an ant foraging trail network

Pharaoh’s ants (Monomorium pharaonis) use at least three types of foraging trail pheromone: a long-lasting attractive pheromone and two short-lived pheromones, one attractive and one repellent. We measured the decay rates of the behavioural response of ant workers at a trail bifurcation to trail substrate marked with either repellent or attractive short-lived pheromones. Our results show that the repellent pheromone effect lasts more than twice as long as the attractive pheromone effect (78 min versus 33 min). Although the effects of these two pheromones decay at approximately the same rate, the initial effect of the repellent pheromone on branch choice is almost twice that of the attractive pheromone (48% versus 25% above control). We hypothesise that the two pheromones have complementary but distinct roles, with the repellent pheromone specifically directing ants at bifurcations, while the attractive pheromone guides ants along the entire trail. Received 15 November 2007; revised 7 March 2008; accepted 18 March 2008.