Search

Knowledge on the distribution and abundance of organisms is fundamental to understanding their roles within ecosystems and their ecological importance for other taxa. Such knowledge is currently lacking for insects, which have long been regarded as the “little things that run the world”. Even for ubiquitous insects, such as ants, which are of tremendous ecological significance, there is currently neither a reliable estimate of their total number on Earth nor of their abundance in particular biomes or habitats. We compile data on ground-dwelling and arboreal ants to obtain an empirical estimate of global ant abundance. Our analysis is based on 489 studies, spanning all continents, major biomes, and habitats. We conservatively estimate total abundance of ground-dwelling ants at over 3 × 10 15 and estimate the number of all ants on Earth to be almost 20 × 10 15 individuals. The latter corresponds to a biomass of ∼12 megatons of dry carbon. This exceeds the combined biomass of wild birds and mammals and is equivalent to ∼20% of human biomass. Abundances of ground-dwelling ants are strongly concentrated in tropical and subtropical regions but vary substantially across habitats. The density of leaf-litter ants is highest in forests, while the numbers of actively ground-foraging ants are highest in arid regions. This study highlights the central role ants play in terrestrial ecosystems but also major ecological and geographic gaps in our current knowledge. Our results provide a crucial baseline for exploring environmental drivers of ant-abundance patterns and for tracking the responses of insects to environmental change.

Recent concern over increasing loss of biodiversity has prompted considerable interest in the role of urban green spaces as reservoirs of local biodiversity. This study assessed the diversity of three indicator taxa - plants, ants and birds - on golf courses spanning a wide range of environmental variation in terms of climate, elevation, course age, size and connectivity to native woodland. Species richness and community composition was further compared between contrasting on-course habitat types that reflect different management intensities. We identified a set of taxon-specific environmental correlates indicating an intricate interplay of landscape- and local-scale variables that affect local species diversity. Our results show that floristic diversity is positively associated with the amount of rainfall, whereas ant and bird diversity are related to local-scale factors, particularly the number of trees and the size of water features on a site. The amount of on-course native habitat was a strong predictor of plant and ant diversity and was also associated with the number of unique species at the site level; this reinforces the value of remnant habitat patches as local biodiversity reservoirs that represent mini hot-spots in an otherwise species-poor urban landscape. Community composition for all three taxa differed markedly between non-playing and playing areas, with boundary and remnant habitats generally having more diverse, species-rich communities. Our results suggest that local floral and faunal biodiversity on urban golf courses can be enhanced by creating woody non-playing areas and, especially, by preserving, restoring or expanding remnant habitats.

Virtual reality (VR) offers an appealing experimental framework for studying visual performances of insects under highly controlled conditions. In the case of the honeybee Apis mellifera, this possibility may fill the gap between behavioural analyses in free-flight and cellular analyses in the laboratory. Using automated, computer-controlled systems, it is possible to generate virtual stimuli or even entire environments that can be modified to test hypotheses on bee visual behaviour. The bee itself can remain tethered in place, making it possible to record neural activity while the bees is performing behavioural tasks. Recent studies have examined visual navigation and attentional processes in VR on flying or walking tethered bees, but experimental paradigms for examining visual learning and memory are only just emerging. Behavioural performances of bees under current experimental conditions are often lower in VR than in natural environments, but further improvements on current experimental protocols seem possible. Here we discuss current developments and conclude that it is essential to tailor the specifications of the VR simulation to the visual processing of honeybees to improve the success of this research endeavour.

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The movement pattern of a searching animal affects how it interacts with the environment, thus playing an important role in many environmental processes. Social hymenopteran foragers make good models for studying search patterns because their only role is to search for food and bring it back to the nest or hive. Search patterns in ants and bees, the most studied social hymenopterans, consist of loops around the origin of search that expand in size as the search goes on. Different factors influence search structure, including the type and distribution of the resource that is targeted, and the kind and amount of information available for navigation. In the distributions of the lengths of search segments, bees exhibit a Levy walk pattern, which follows a power law in which occasional long segments are interspersed within many short segments, and yet longer segments are interspersed within the long segments. Such a scale-free and heavy-tailed distribution has been shown to be optimal in searching for sparse targets. Ants on the other hand show a composite Brownian search distribution, composed of two exponential distributions at different scales. We argue that such composite Brownian searches serve to approximate a Levy walk. Both Levy walks and composite Brownian searches are found in a range of other animals, including hunter–gatherer humans. These patterns may be ancient, as they have been found in fossils dating to at least 50 million years.

Ants are known to use the terrestrial visual panorama in navigation. Recent evidence has accumulated for the use of the currently perceived visual panorama to determine a direction to head in. The pattern of the height of the terrestrial surround, the skyline, is one key cue for the Central Australian red honey ant Melophorus bagoti in determining a direction of travel. But ants might also possess some mechanism to match the skyline heights encountered during training, which functions to steer away from regions whose skyline is too high and towards regions whose skyline is too low. We made an initial test of this hypothesis by training ants to visit a feeder centred between two experimentally constructed walls of black cloth. Trained ants were then tested for their initial homing direction with the walls retaining their heights as encountered in training (controls), with one of the walls lowered or raised in height, or with one wall lowered and the opposite wall raised. Wall-height manipulations deflected the initial headings of ants towards the lower wall, with combined wall lowering and wall raising changing the initial headings by ~30° when compared with controls. The results suggest that the ants combined the dictates of the panorama in determining the best direction of travel (a heading towards the nest) with some attractor mechanism that functions to establish the skyline heights of training conditions (a heading towards the lower wall).

The Central Australian desert ant Melophorus bagoti is known to use celestial cues for compass orientation. We manipulated the available celestial cues for compass orientation for ants that had arrived at a feeder, were captured and then released at a distant test site that had no useful terrestrial panoramic cues. When tested in an enclosed transparent box that blocked some or most of the ultraviolet light, the ants were still well oriented homewards. The ants were again significantly oriented homewards when most of the ultraviolet light as well as the sun was blocked, or when the box was covered with tracing paper that eliminated the pattern of polarised light, although in the latter case, their headings were more scattered than in control (full-cue) conditions. When the position of the sun was reflected 180° by a mirror, the ants headed off in an intermediate direction between the dictates of the sun and the dictates of unrotated cues. We conclude that M. bagoti uses all available celestial compass cues, including the pattern of polarised light, the position of the sun, and spectral and intensity gradients. They average multiple cues in a weighted fashion when these cues conflict.

In a synthetic approach to studying navigational abilities in desert ants, we review recent work comparing ants living in different visual ecologies. Those living in a visually rich habitat strewn with tussocks, bushes, and trees are compared to those living in visually barren salt pans, as exemplified by the Central Australian Melophorus bagoti and the North African Cataglyphis fortis, respectively. In bare habitats the navigator must rely primarily on path integration, keeping track of the distance and direction in which it has travelled, while in visually rich habitats the navigator can rely more on guidance by the visual panorama. Consistent with these expectations, C. fortis performs better than M. bagoti on various measures of precision at path integration. In contrast, M. bagoti learned a visually based associative task better than C. fortis, the latter generally failing at the task. Both these ants, however, exhibit a similar pattern of systematic search as a 'back up' strategy when other navigational strategies fail. A newly investigated salt-pan species of Melophorus (as yet unnamed) resembles C. fortis more, and its congener M. bagoti less, in its path integration. The synthetic approach would benefit from comparing more species chosen to address evolutionary questions. This article is part of a Special Issue entitled: CO3 2013.

The Australian desert ant Melophorus bagoti (Formicidae) is a thermophilic, solitary foraging ant that inhabits the semi-arid regions of Australia. In recent years, it has become a model species for the study of navigation. However, its ecological traits are not well understood, especially on the level of the entire colony. Here, we investigated this species daily activity schedule and diet composition, and examined its foraging behaviour. Foraging activity is confined to a window of roughly 50-70°C soil surface temperature, and foragers reacted quickly to temperature changes. Consequently, the pattern of daily outbound traffic during summer is unimodal on warm days and bimodal on very hot days. Foragers are opportunistic scavengers; dead insects make up a large proportion of food items, but grass seeds are also occasionally brought back to the nest in large amounts. Diet composition changes with the seasonal availability of certain food groups. Melophorus bagoti foragers have the ability to recruit nestmates to profitable food sources. Recruitment seems to function without the use of pheromone trails, but the exact mechanism requires further investigation.

Foragers of Melophorus bagoti often return to previously rewarding sites to search for more food items. They are opportunistic scavengers that exploit both protein and carbohydrate food sources. Under natural foraging conditions, protein food items are distributed sparsely and randomly, whereas carbohydrates come in patches that are often renewable. This makes for vastly different foraging scenarios that a single forager is confronted with. In theory, foraging performance can be greatly improved if foragers are able to adjust their strategy to different food item distributions. This could be achieved through individual foraging experience or by employing pre-existing, intrinsic foraging strategies. We investigated this by offering both kinds of food with the same distribution: as a renewable food source at a fixed location. After removal of the food source, outbound foragers displayed an area-restricted search centered on its location. Searches for protein had a greater spread than those for carbohydrates, which matches the natural distribution pattern of these food types. However, searches for both kinds of food follow the same general strategy, which is best described as a Brownian-like walk. We suggest that the observed adaptive behavior is a result of differential learning effort.

Scientific Reports 5: Article number: 10747; Published online: 29 May 2015; Updated: 08 July 2016 The authors neglected to cite a published thesis that investigated a related aspect of tandem running in the same study species. This additional reference is listed below as reference 1, and should appear in the text as below.

Ants use the panoramic skyline in part to determine a direction of travel. A theoretically elegant way to define where terrestrial objects meet the sky is to use an opponent-process channel contrasting green wavelengths of light with ultraviolet (UV) wavelengths. Compared with the sky, terrestrial objects reflect relatively more green wavelengths. Using such an opponent-process channel gains constancy in the face of changes in overall illumination level. We tested the use of UV wavelengths in desert ants by using a plastic that filtered out most of the energy below 400 nm. Ants, Melophorus bagoti, were trained to home with an artificial skyline provided by an arena (experiment 1) or with the natural panorama (experiment 2). On a test, a homing ant was captured just before she entered her nest, and then brought back to a replicate arena (experiment 1) or the starting point (the feeder, experiment 2) and released. Blocking UV light led to deteriorations in orientation in both experiments. When the artificial skyline was changed from opaque to transparent UV-blocking plastic (experiment 3) on the other hand, the ants were still oriented. We conclude that UV wavelengths play a crucial role in determining direction based on the terrestrial surround.

Two species of Australian desert ants, one inhabiting a visually rich environment (Melophorus bagoti) and one inhabiting a visually barren environment (as yet unnamed and dubbed Melophorus sp.), were tested on path integration and subsequent search. For each species, prominent landmarks were placed near the nest, and ants were trained to visit a feeder. After training over 2 days, an ant was captured at the feeder when it had grabbed some food and tested, just once individually, in homing with the training landmarks either present or absent. Their subsequent search was also recorded on gridded paper according to a grid marked at the test site. Both species headed initially in the feeder-nest direction, but directional scatter was larger when training landmarks were absent. Melophorus bagoti ran a shorter distance before starting to search on tests with the landmarks absent, but Melophorus sp. ran a similar distance in both conditions. In both species, the variance in distance run was larger when landmarks were absent. In searching, both species expanded their search pattern more when landmarks were absent than when landmarks were present. Contrary to predictions, the distribution of the lengths of search segments was best described as a single exponential function on tests with landmarks absent and as a double exponential function with landmarks present. Two species of Australian desert ants of the same genus but occupying visually different habitats were studied in similar experiments. One inhabits a barren saltpan while the other inhabits a semi-arid environment filled with grass, shrubs and trees. The ants were compared in their return home after a short outbound trip (2 m) to a feeder and the subsequent search for the nest. When the ants were trained in an environment with some prominent artificial landmarks which were subsequently removed in tests, ants living in the visually cluttered habitat cut short their initial run, starting to engage in search earlier than ants living on the salt pan. Search characteristics were, on the other hand, similar in the two species, both expanding their search loops over time, more so when the familiar scene was changed. Visual ecology drives some aspects of navigation, while common task requirements drive other aspects.

Australian desert ants, Melophorus bagoti, return home after foraging by means of path integration and visual navigation. If these mechanisms do not deliver them exactly at the inconspicuous nest entrance, they engage in a systematic search. Here we describe the structure of this search pattern in detail. Trained ants ran home from a feeder in the natural visual setting where they navigated very accurately, and were then captured for tests on a distant test field after they had almost reached their nest. The search pattern consisted of loops and was centred on the position where the nest was most likely to be located. At first, it covered a rather small area, but then gradually extended outwards to cover a larger area. The search density was also adapted to the preceding outbound foraging distance, with longer distances leading to flatter, wider search distributions. Since the visual surround at the time of capture was similar for ants with all outbound distances, we suggest this is an adaptation to the cumulative error of the homing vector. The frequency distribution of segment lengths in the paths of searching ants does not show characteristics of a Levy walk strategy. Instead, it is well described by a double exponential model, lending support to a theoretically optimal strategy that consists of a mixture of two random walks, as in the composite Brownian walk strategy.

In addition to foraging individually several species of ants guide nestmates to a goal by tandem running. We found that the Australian ant, Camponotus consobrinus, forages both individually and by tandem running to head to the same goal, nest-specific native Australian trees on which they forage. While paths of solitary foragers and initial paths of tandem followers showed no differences in heading directions or straightness, tandem followers moved at about half the speed of solitary runs. When leaders were experimentally removed, follower ants initially engaged in a systematic search around the point of interruption, following which they either (a) headed directly towards and successfully reached the foraging trees, or (b) continued searching or (c) returned to the nest. The high incidence of followers that successfully navigated towards the foraging trees on their own provides strong evidence that many tandem followers are in fact experienced foragers. Detailed analysis of the searching behaviour revealed that even seemingly lost followers displayed a directional bias towards the foraging trees in their search path. Our results show that in a foraging context follower ants in a tandem pair are not always naïve.

We suggest that the Australian desert ant Melophorus bagoti approximates a Levy search pattern by using an intrinsic bi-exponential walk and does so when a Levy search pattern is advantageous. When attempting to locate its nest, M. bagoti adopt a stereotypical search pattern. These searches begin at the location where the ant expects to find the nest, and comprise loops that start and end at this location, and are directed in different azimuthal directions. Loop lengths are exponentially distributed when searches are in visually familiar surroundings and are well described by a mixture of two exponentials when searches are in unfamiliar landscapes. The latter approximates a power-law distribution, the hallmark of a Levy search. With the aid of a simple analytically tractable theory, we show that an exponential loop-length distribution is advantageous when the distance to the nest can be estimated with some certainty and that a bi-exponential distribution is advantageous when there is considerable uncertainty regarding the nest location. The best bi-exponential search patterns are shown to be those that come closest to approximating advantageous Levy looping searches. The bi-exponential search patterns of M. bagoti are found to approximate advantageous Levy search patterns.

Honeybees (Apis mellifera) are regularly faced with the task of navigating back to their hives from remote food sources. They have evolved several methods to do this, including compass-directed “vector” flights and the use of landmarks. If these hive-centered mechanisms are disrupted, bees revert to searching for the hive, using an optimal Levy flight searching strategy. The same strategy is adopted when a food source at a known location ceases to be available. Here, we show that the programming for this Levy strategy does not need to be very sophisticated or clever on the bee’s part, as Levy flight patterns can be derived from the Weber–Fechner law in a bee’s odometer. Odometry errors of a different kind occur in desert ants (Cataglyphis spp., Melophorus bagoti). The searching behaviors of these ants are very similar in overall structure to that of honeybees but do not display any Levy flight characteristics. We suggest that errors in the estimation of distance can be implicitly involved in shaping the structure of systematic search behavior and should not be regarded as merely deficiencies in the odometer.

Summary Many animals - including insects - navigate visually through their environment. Solitary foraging desert ants are known to acquire visual information from the surrounding panorama and use it to navigate along habitual routes or to pinpoint a goal like the nest. Returning foragers that fail to find the nest entrance engage in searching behaviour, during which they continue to use vision. The characteristics of searching behaviour have typically been investigated in unfamiliar environments. Here we investigated in detail the nest searching behaviour of Melophorus bagoti foragers within the familiar visual environment of their nest. First, by relating search behaviour to the information content of panoramic (360°) images, we found that searches were more accurate in visually cluttered environments. Second, as observed in unfamiliar visual surrounds, searches were dynamic and gradually expanded with time, showing that nest-pinpointing is not rigidly controlled by vision. Third, contrary to searches displayed in unfamiliar environments, searches observed here could be modelled as a single exponential search strategy, which is similar to a Brownian walk, and there was no evidence of a Lévy walk. Overall, our results revealed that searching behaviour is remarkably flexible and varies according to the relevance of information provided by the surrounding visual scenery.

Terrestrial panoramic cues, path integration and search behavior are the main navigational strategies used by ants to locate food and find their way back to the nest. Searching becomes important when the other navigational cues are either not available or cannot provide sufficient information to pinpoint the goal. When searching in one-dimensional channels Melophorus bagoti ants exhibit a systematic drift in the starting-point-to-goal direction as they turn back and forth, sometimes past the goal location ( Narendra et al., 2008 ). Here we show that this drift in channels is not a stereotypical part of the search behavior in these ants. It rather depends on the conditions of training. In experiments in which the nest entrance is located not at the end but at the side of the channel, forward drift is not always part of the nest search. Experiments on food searches showed that with the food source at the end of the channel, ants performed a linear drift in the starting-point-to-food direction. With food at the side of the channel, they showed a less pronounced drift toward the food source. In this constrained environment, especially with the goal at the end of the channel, ants seem to learn a routine such as ‘run along the channel’, and mix this routine with their usual strategy of turning back and forth in search.

The Australian desert ant Melophorus bagoti often follows stereotypical routes through a cluttered landscape containing both distant panoramic views and obstacles (plants) to navigate around. We created an artificial obstacle course for the ants between a feeder and their nest. Landmarks comprised natural objects in the landscape such as logs, branches, and tussocks. Many ants travelled stereotypical routes home through the obstacle course in training, threading repeatedly the same gaps in the landmarks. Manipulations altering the relations between the landmarks and the surrounding panorama, however, affected the routes in two major ways. Both interchanging the positions of landmarks (transpositions) and displacing the entire landmark set along with the starting position of the ants (translations) (1) reduced the stereotypicality of the route, and (2) increased turns and meanders during travel. The ants might have used the entire panorama in view-based travel, or the distal panorama might prime the identification and use of landmarks en route. Despite the large data set, both options (not mutually exclusive) remain viable.

Over the expanse of the salt lakes of the Lake Eyre basin lives a species of ant of the genus Melophorus (as yet unnamed), both on the edges of the salt-pan and on the salt-pan itself. Most of the foragers are small (~3.0–5.0 mm), but ~10% of the ants seen foraging are much larger (~7.0 mm) and may form a soldier caste. Foragers are thermophilic and show diurnal activity, displaying a single-peaked activity profile across the day, with activity time limited to 2–6 h at most each day (during the late-summer study period, 28 February to 28 March 2012). They forage largely for dead arthropods, but also occasionally bring home plant materials. Foraging success (not considering possible liquid food intake) is ~20%, resembling the success rate of their congener Melophorus bagoti, which inhabits cluttered environments. When displaced with food from a feeder, the ants head systematically and precisely in the feeder-to-nest direction, thus exhibiting path integration abilities involving celestial compass cues. The study of this species provides an interesting comparative perspective in contrasting desert ants of the same genus and thus genetic heritage inhabiting habitats differing in complexity of panoramic terrestrial cues as well as comparing ecologically similar species inhabiting the same type of habitat (in the present case, salt-pans) but differing in their phylogenetic relationships.