Navigating desert ants (Cataglyphis fortis) learn to alter their search patterns on their homebound journey

In navigating home, desert ants first run off a global vector estimated on their outbound journey, and then engage in systematic search consisting of ever‐increasing loops interrupted by returns to the starting point of search. Desert ants (Cataglyphis fortis; Wehner, 1983 ) were trained to travel 6 m down a channel to a food source. Different groups of ants were trained to return home in another channel, from distances of 6 m (control), 9 m or 12 m. Ants at the feeder were then tested in a long test channel. The measure of where the ants first turned back on a test gave an estimate of the length of the global vector calculated on their outbound trip. The median distance of search on a 5‐min test gave an estimate of the centre of the search pattern. Relative to controls, the experimental ants did not increase their estimated length of global vector, but changed their search patterns, searching on average further from the start than the controls. Tests of the outbound journey, however, revealed no differences between groups. Desert ants can learn to modify their search pattern based on experience.     

The hidden spiral: systematic search and path integration in desert ants,Cataglyphis fortis

The main navigational mechanism used by foraging desert ants of the genus Cataglyphis is path integration (dead reckoning). Any such egocentric system of navigation is prone to cumulative navigational errors. Hence, while homing Cataglyphis might have reset its path integration system and yet not arrived at the start of its foraging excursion, the nest entrance. Then it resorts to piloting or performs a systematic search for the nest. The search pattern consists of a system of loops of ever increasing size centred about the origin, i.e. the start of the search. Here we show that underlying the system of loops is a spiral search programme that gets transformed into the observed pattern of loops by the ant's idiosyncratic path-integration algorithm. The ant starts to follow a spiral course, then breaks off this course and walks towards the centre, i.e. to what its path-integration system has computed to be the origin of the search. This reset episode is followed by another spiral course, which is terminated by the next reset, and so forth. After each reset, the spiral gets wider, so that the whole pattern expands. Futhermore, every now and then the spiral might change its sign. Computer simulations based on these simple rules lead to search patterns of the kind actually recorded in Cataglyphis ants. These patterns ensure that those parts of the area in which the target (nest entrance) is most likely to be located are searched most heavily; in other words: the search density profile is adapted to the probability density function of the target.     

Calibration processes in desert ant navigation: vector courses and systematic search

This study investigates the ability of desert ants to adapt their path integration system to an "open-jaw" training paradigm, in which the point of arrival (from the nest) does not coincide with the point of departure (to the nest). Upon departure the ants first run off their home vector and then start a systematic search for the nest. Even if they are subjected to this training-around-a-circuit procedure for more than 50 times in succession, they never adopt straight homeward courses towards the nest. Their path integration vector gets slightly recalibrated (pointing a bit closer to the nest), and their search pattern gets asymmetric (with its search density peak shifted towards the nest), but the bipartite structure of the inbound trajectory invariably remains. These results suggest (1). that the ants cannot learn separate inbound and outbound vectors (i.e. vectors that are not 180 degrees reversals of each other), (2). that the recalibrated vector is dominated by the ant's outbound course, (3). that the recalibration of the vector and the modification of the search geometry are fast and flexible processes occurring whenever the ant experiences a mismatch between the stored and actual states of its path integrator.     

How desert ants cope with enforced detours on their way home

Summary We ask whether desert ants (Cataglyphis fortis) perform path integration on their homeward as well as on their outward journey. If path integration does occur on the return journey, then, after an enforced detour, the ant's trajectory should point directly at its nest. To test whether this is so, ants were trained to forage at a spot 25 m from their nest. As an ant began its return journey to the nest, it was caught and transported to a test area where it was released either 2 m or 12 m from a wide barrier which obstructed its homeward path. The direction of the ants' trajectory after detouring around the barrier corresponded closely to that predicted on the assumption that the home vector is accurately updated during the detour.     

Dead reckoning in a small mammal: the evaluation of distance

When hoarding food under infra-red light, golden hamsters Mesocricetus auratus W. return fairly directly from a feeding place to their nest site by evaluating and updating internal signals that they have generated during the previous outward journey to the feeding place. To test more specifically the animals' capacity to evaluate the linear components of the outward journey, the subjects were led from their (cone-shaped) nest to a feeding place along a detour which comprised either 2 (experiment 1) or 5 (experiment 2) segments; adjoining segments were at right angles to each other. In these conditions, the subjects remained significantly oriented towards the nest and therefore were capable of assessing translations as well as rotations during the outward journey. In experiment 3, the nest was removed after the hamsters had started the direct outward journey to the feeding place and the hamsters were rotated during the food uptake. The animals were no longer oriented towards the starting point of their journey, but nonetheless covered, along a fairly straight path, the correct homing distance, and then changed over to a circular search path. These results confirm that mammals can derive the linear components of an outward journey from self-generated signals and therefore are able to judge the homing distance without relying on cues from the environment. For a number of detour outward journeys, our data yield an unexpectedly good fit to Müller and Wehner's (1988) model of dead reckoning in ants. However, this is no longer the case when the outward journey contains an initial loop which brings the subject back to the starting point. These findings are discussed in terms of the biological significance and limitations of an approximate form of path integration.     

Ant navigation en route to the goal: signature routes facilitate way-finding of Gigantiops destructor

We investigated in laboratory conditions how foragers of the tropical ant Gigantiops destructor develop individually distinctive landmark routes. Way-finding along a familiar route involved the recognition of at least two locations, nest and feeding site, and the representation of spatial relations between these places. Familiar visual landmarks were important both at the beginning and at the end of the foraging journey. A motor routine guided the ants at the start of their foraging path towards the first landmarks, which they learnt to pass consistently on the same side, before taking the next direction. At the last stage of the route, landmark recognition allowed them to pinpoint their preferred feeding site without using distant cues or odometric information. By contrast, ants en route to the goal were not systematically guided by a stereotyped sequence of snapshots recalled at each corresponding stage of the route. Each ant slalomed in an idiosyncratic distinctive way around different midway landmarks from a foraging excursion to the next, which induced a variability of the path shapes in their intermediate parts. By reducing the number of landmark recognition-triggered responses, this economical visuomotor strategy may be helpful in the Amazonian forest where many prominent landmarks are alike.     

How does food distance influence foraging in the ant Lasius niger: the importance of home-range marking

We study the influence of food distance on the individual foraging behaviour of Lasius niger scouts and we investigate which cue they use to assess their distance from the nest and accordingly tune their recruiting behaviour. Globally, the number of U-turns made by scouts increases with distance resulting in longer travel times and duration of the foraging cycle. However, over familiar areas, home-range marking reduces the frequency and thereby the impact of U-turns on foraging times leading to a quicker exploitation of food sources than over unmarked set-ups. Regarding information transfer, the intensity of the recruitment trail reaching the nest decreases with increasing food distance for all set-ups and is even more reduced in the absence of home-range marking. Hence, the probability of a scout continuing to lay a trail changes along the homeward journey but in a different way according to home-range marking. Over unexplored setups, at a given distance from the food source, the percentage of returning trail-laying ants remains unchanged for all tested nest-feeder distances. Hence, the tuning of the trail recruiting signal by scouts was not influenced by an odometric estimate of the distance already travelled by the ants during their outward journey to the food. By contrast, over previously explored set-ups, a distance-related factor – that is the intensity of home-range marking – strongly influences their recruiting behaviour. In fact, over a home-range marked bridge, the probability of returning ants maintaining their trail-laying behaviour increases with decreasing food distance while the gradient of home-range marks even induces ants which have stopped laying a trail to resume this behaviour in the nest vicinity. We suggest that home-range marking laid passively by walking ants is a relevant cue for scouts to indirectly assess distance from the nest but also local activity level or foraging risks in order to adaptively tune trail recruitment and colony foraging dynamics. Received 13 July 2004; revised 26 January and 20 May 2005; accepted 2 July 2005.     

Ant behaviour and seed morphology: a missing link of myrmecochory

Seed dispersal by ants (myrmecochory) is mediated by the presence of a lipid-rich appendage (elaiosome) on the seed that induces a variety of ants to collect the diaspores. When seeds mature or fall onto the ground, these ant species transport them to their nest. After eating the elaiosome, the seed is discarded in nest galleries or outside, in the midden or farther away, where seeds can potentially germinate. The final location of seeds with their elaiosomes removed was evaluated to assess the importance of possible handles (structures that ants can grasp to carry) in transporting ants during re-dispersal experiments of seeds from nests of six species of ants. The results indicate that seeds remained within the nest because the ants were not able to transport them out of the nest. As a consequence of the elaiosome being removed, small ant species could not take Euphorbia characias seeds out of their nests. Only large ant species could remove E. characias seeds from their nests. Attaching an artificial handle to E. characias seeds allowed small ant species to redistribute the seeds from their nests. On the other hand, Rhamnus alaternus seeds that have a natural handle after the elaiosome removal were removed from the nests by both groups of ant species. If a seed has an element that acts as a handle, it will eventually get taken out of the nest. The ants’ size and their mandible gap can determine the outcome of the interaction (i.e. the pattern of the final seed shadow) and as a consequence, could influence the events that take place after the dispersal process.     

Simple rules based on pile slope are used in the self organization of sand pile formation by <Emphasis Type="Italic">Pheidole oxyops</Emphasis> ants

We tested the hypothesis that slope influences where worker ants deposit excavated soil on piles near the nest entrance. We predicted that ants will deposit their load near the top of a pile where the slope changes from upward to downward, to prevent material rolling back towards the entrance. We tested this hypothesis by studying five natural colonies of Pheidole oxyops ants at a field site at S?o Sim?o, Brazil. At this site, each colony was dumping sandy soil excavated from its underground nest in a crescent-shaped pile c. 13 cm from and perpendicular to the nest entrance. Each nest was given an experimental sand pile of symmetrical curved cross section on a plywood platform that could be tilted 15 degrees up or down. From videos, the locations where individual ants dumped their soil loads were measured in relation to the inner (position = 0) and outer (position = 1) edges of the pile. When the platform was tilted down the ants deposited their loads significantly closer to the inner edge (0.458 ± 0.007) than when not tilted (0.530 ± 0.006). When the platform was tilted up the ants deposited their loads significantly further from the inner edge (0.626 ± 0.006) than when not tilted (0.522 ± 0.006). These results support the hypothesis that ants use pile slope in deciding where to dump their load. A similar rule is probably used in other ant species that place excavated soil in steep piles near the nest entrance. Received 5 February 2007; revised 10 June 2007; accepted 9 October 2007.     

Landmark cues can change the motivational state of desert ant foragers

Desert ants of the genus Cataglyphis rely on path integration vectors to return to the nest (inbound runs) and back to frequently visited feeding sites (outbound runs). If disturbed, e.g., experimentally displaced on their inbound runs, they continue to run off their home-bound vector, but if disturbed in the same way on their outbound runs, they do not continue their feeder-based vector, but immediately switch on the home-bound state of their path integration vector and return to the nest. Here we show that familiar landmarks encountered by the ants during their run towards the feeder can change the ants’ motivational state insofar that the ants even if disturbed continue to run in the nest-to-feeder direction rather than reverse their courses, as they do in landmark-free situations. Hence, landmark cues can cause the ants to change their motivational state from homing to foraging.     

Soil temperature, digging behaviour, and the adaptive value of nest depth in South American species of Acromyrmex leaf-cutting ants

In leaf-cutting ants, workers are expected to excavate the nest at a soil depth that provides suitable temperatures, since the symbiotic fungus cultivated inside nest chambers is highly dependent on temperature for proper growth. We hypothesize that the different nesting habits observed in Acromyrmex leaf-cutting ants in the South American continent, i.e. superficial and subterranean nests, depend on the occurrence, across the soil profile, of the temperature range preferred by workers for digging. To test this hypothesis, we first explored whether the nesting habits in the genus Acromyrmex are correlated with the prevailing soil temperature regimes at the reported nest locations. Second, we experimentally investigated whether Acromyrmex workers engaged in digging use soil temperature as a cue to decide where to excavate the nest. A bibliographic survey of nesting habits of 21 South American Acromyrmex species indicated that nesting habits are correlated with the soil temperature regimes: the warmer the soil at the nesting site, the higher the number of species inhabiting subterranean nests, as compared to superficial nests. For those species showing nesting plasticity, subterranean nests occurred in hot soils, and superficial nests in cold ones. Experimental results indicated that Acromyrmex lundi workers use soil temperature as an orientation cue to decide where to start digging, and respond to rising and falling soil temperatures by moving to alternative digging places, or by stopping digging, respectively. The soil temperature range preferred for digging, between 20°C and maximally 30.6°C, matched the range at which colony growth would be maximized. It is suggested that temperature-sensitive digging guides digging workers towards their preferred range of soil temperature. Workers’ thermopreferences lead to a concentration of digging activity at the soil layers where the preferred range occurs, and therefore, to the construction of superficial nests in cold soils, and subterranean ones in hot soils. The adaptive value of the temperature-related nesting habits, and the temperature-sensitive digging, is further discussed.     

Sleeping Parties and Nest Distribution of Chimpanzees in the Savanna Woodland, Ugalla, Tanzania

We conducted ecological studies of chimpanzees (Pan troglodytes) in the Ugalla area, western Tanzania. Ugalla is one of the driest habitats of chimpanzees and the Ugalla River is the eastern boundary of chimpanzee distribution. Most of Ugalla is occupied by savanna woodlands dominated by deciduous trees of Brachystegia and Julbernardia. Chimpanzees tended not to make nests in riverine forests in plains, but in small patchy forests dominated by Monopetalanthus richardsiae and valley forests dominated by Julbernardia unijugata on slopes in mountainous areas. We estimated population density of chimpanzees to be 7–9 × 10⁻² individuals/km² based on nest censuses, suggesting that 2–3 × 10² individuals inhabited the 3352 km² area of Ugalla. The size of the largest nest cluster (n=23) suggests that 1 unit group (community) comprised 30–35 individuals. In the daytime, chimpanzees formed small feeding parties (mean 2.0 individuals), but larger ones in the evening (mean 4.8 individuals and 5.2 individuals based on fresh nest clusters). The pattern might reduce the predation risk from large nocturnal carnivores such as lions and leopards. The sleeping sites may function as both a safe sleeping site and a meeting point for chimpanzees with a huge home range that may have difficulty in finding other members of their unit group.     

Effectiveness of protein-rich pea flour for the control of stored-product beetles

Abstract The association between visiting ants and the extrafloral nectaries (EFN)‐bearing shrub Hibiscus pernambucensis Arruda (Malvaceae) was investigated in two different coastal habitats – a permanently dry sandy forest and a regularly inundated mangrove forest. In both habitats the frequency of plants with ants and the mean number of ants per plant were much higher on H. pernambucensis than on non‐nectariferous neighbouring plants. In the sandy forest the proportion of live termite baits attacked by ants on H. pernambucensis was much higher than on plants lacking EFNs. In the mangrove, however, ants attacked equal numbers of termites on either plant class. Ant attendance to tuna/honey baits revealed that overall ant activity in the sandy forest is higher than in the mangrove area. The vertical distribution (ground vs. foliage) of ant activity also differed between habitats. While in the mangrove foraging ants were more frequent at baits placed on foliage, in the sandy forest ant attendance was higher at ground baits. Plants housing ant colonies were more common in the mangrove than in the sandy forest. Frequent flooding in the mangrove may have resulted in increased numbers of ant nests on vegetation and scattered ant activity across plant foliage, irrespective of possession of EFNs. Thus plants with EFNs in the mangrove may not experience increased ant aggression towards potential herbivores relative to plants lacking EFNs. The study suggests that the vertical distribution of ant activity, as related to different nest site distribution (ground vs. foliage) through a spatial scale, can mediate ant foraging patterns on plant foliage and probably affect the ants’ potential for herbivore deterrence on an EFN‐bearing plant species.     

Opening myrmecochory’s black box: what happens inside the ant nest?

In the process of seed dispersal by ants (myrmecochory), foragers bring diaspores back to their nest, then eat the elaiosome and usually reject viable seeds outside the nest. Here, we investigate what happens inside the nest, a barely known stage of the myrmecochory process, for two seed species (Viola odorata, Chelidonium majus) dispersed either by the insectivorous ant Myrmica rubra or by the aphid-tending ant Lasius niger. Globally, elaiosome detachment decreased ants’ interest towards seeds and increased their probability of rejecting them. However, we found marked differences in seed management by ants inside the nest. The dynamics of elaiosome detachment were ant- and plant-specific whereas the dynamic of seed rejection were mainly ant-specific. Seeds remained for a shorter period of time inside the nest of the carnivorous ant Myrmica rubra than in Lasius niger nest. Thus, elaiosome detachment and seed rejection were two competing dynamics whose relative efficiency leads to variable outcomes in terms of types of dispersed items and of nutrient benefit to the ants. This is why some seeds remained inside the nest even without an elaiosome, and conversely, some seeds were rejected with an elaiosome still attached. Fresh seeds may be deposited directly in contact with the larvae. However, the dynamics of larvae-seeds contacts were also highly variable among species. This study illustrates the complexity and variability of the ecological network of ant–seed interactions.     

Performance and fate of tree seedlings on and around nests of the leaf‐cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.     

Garden ant homing behavior in a maze task based on local visual cues

Although it has been shown that visual cues play an essential role in navigation by the garden ant Lasius niger, no previous studies have addressed the way in which information from local visual cues is acquired and utilized in navigation. We found that in the absence of pheromone trails, ants whose homing motivation was triggered by feeding returned to the nest following local visual cues. In our experiments, the ants travelled through a maze to reach a feeder. They explored the maze and sometimes became trapped in its dead ends. We found that the ants more effectively used visual cues during their homeward journey if they experienced a dead end during their outward journey. This result suggested that the ants used the information acquired from visual cues during the outward journey to avoid a dead end on their return journey.     

The relationship between soil variables and leafcutter ant (Atta sexdens) nest distribution in the Colombian Amazon

We determined the relationships between several soil variables and the distribution of leafcutter ant (Atta sexdens) nests on the Colombian shore of the Amazon River. Seven habitats were identified in which nests were consistently present or absent. Soil samples for physiochemical analysis were taken near nest sites in three habitats where nests were present and from randomly selected sites in four other habitats where nests were absent. Percent canopy cover and soil resistance were also measured for each site. Almost all of the 20 identified soil variables were significantly different between habitats with and without A. sexdens nests, and the values of all variables were heterogeneous between the seven habitats. The most important variables correlated with leafcutter ant nest presence were percent canopy cover (positively), percent silt, soil resistance at 0–20 cm, and pH (all negatively). We deduced a binomial and a multinomial logistic regression which showed how each of these variables was related to nest presence and habitat, respectively. We describe how each variable may affect leafcutter ant nest development, especially in its earliest stages, and therefore influences nest distribution. We propose a suitable-soil hypothesis which, complementary to the palatable forage hypothesis, aims to explain patterns in leafcutter ant nest demography based on soil conditions.     

Nest mounds of red wood ants (Formicaaquilonia): hot spots for litter-dwelling earthworms

A previously undocumented association between earthworms and red wood ants (Formicaaquilonia Yarr.) was found during an investigation of the influence of wood ants on the distribution and abundance of soil animals in boreal forest soil. Ant nest mounds and the surrounding soil of the ant territories were sampled. The ant nest mound surface (the uppermost 5-cm layer) harboured a much more abundant earthworm community than the surrounding soil; the biomass of the earthworms was about 7 times higher in the nests than in the soil. Dendrodrilusrubidus dominated the earthworm community in the nests, while in soils Dendrobaenaoctaedra was more abundant. Favorable temperature, moisture and pH (Ca content), together with abundant food supply (microbes and decomposing litter) are likely to make a nest mound a preferred habitat for earthworms, provided that they are not preyed upon by the ants. We also conducted laboratory experiments to study antipredation mechanisms of earthworms against ants. The experiments showed that earthworms do not escape predation by avoiding contact with ants in their nests. The earthworm mucus repelled the ants, suggesting a chemical defence against predation. Earthworms probably prevent the nest mounds from becoming overgrown by moulds and fungi, indicating possible mutualistic relationships between the earthworms and the ants. Received: 21 November 1996 / Accepted: 3 April 1997     

Rubbing behavior and morphology of van der Vecht's gland inBelonogaster petiolata (Hymenoptera: Vespidae)

Females of the social wasp, Belonogaster petiolata,rub the secretion of van der Vecht's gland, located on their terminal gastral sternite, onto the nest pedicel. In bioassays, the secretion was repellent to two species of ants, while shortchain acids were effective releasers of rubbing behavior. Rubbing was associated with pedicel enlargement and departure from the nest in preemergence colonies. Its frequency was high where wasps were often exposed to ants and low where ants were rare or absent. Rubbing also decreased significantly from the pre-to the postemergence stage of the colony cycle. In both stages, subordinate foundresses rubbed more often than queens or workers. These observations support the hypothesis that rubbing behavior and the secretion of van der Vecht's gland function in chemical defense of the nest against ant predation. The general morphology of the gland in B. petiolataresembles that of the four other independent-founding polistine wasp genera.     

Synchronization of the behaviour within nests of the antleptothorax acervorum (fabricius)—I. Discovering the phenomenon and its relation to the level of starvation

Workers of the speciesLeptothorax acervorum show age-polyethism, they start their life as broodworkers and later on they become nestworkers and foragers. Nestworkers and foragers of this ant species are inactive for 72% and 15% of the total time respectively. The short bursts of activity within the nest do not occur randomly but are synchronized so that the whole nest population exhibits nonperiodic pulses of activity: the ants were seen to wake each other actively. In addition starvation experiments were done to assess whether ants react upon food availability. In appeared that during a longlasting period of starvation the proportion of active ants in the nest is at a higher approximately constant level.