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.

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.

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.

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.