No specialist pheromone-ignoring ants in Lasius niger

In insect societies, the balance between exploitation of known resources and exploration of new ones is important to ensure sufficient resources. Mass recruiting ants, such as Lasius niger, use pheromone trails to recruit nestmates to a newly discovered food source. Pheromone following, however, shows characteristic non-following (lapse) rates among different species, with similar to 20% of L. niger foragers ignoring pheromone. These characteristic lapse rates might simply be 'noise', or they might indicate a subset of specialised explorative foragers, a scouting caste, that consistently ignores pheromone in order to explore. Here we show pheromone ignoring is not a repeatable behaviour in L. niger foragers - ants who did not follow a trail were no more likely to ignore it again an hour later than ants which did follow it. Our findings suggest that there is no subset of specialised pheromone-ignoring L. niger foragers. This may be due to their moderate colony size and strong reliance on individual memories: species with larger colony sizes or a weaker reliance on private information (i.e. memory) may have specialist non-followers. Our work raises the question: what is a scout ant? We encourage future research to investigate the presence of a scouting caste in other ant species using our straightforward methodology, as a social information-ignoring caste may be rarer than expected

Effect of trail bifurcation asymmetry and pheromone presence or absence on trail choice by Lasius niger ants

During foraging, ant workers are known to make use of multiple information sources, such as private information (personal memory) and social information (trail pheromones). Environmental effects on foraging, and how these interact with other information sources, have, however, been little studied. One environmental effect is trail bifurcation asymmetry. Ants forage on branching trail networks and must often decide which branch to take at a junction (bifurcation). This is an important decision, as finding food sources relies on making the correct choices at bifurcations. Bifurcation angle may provide important information when making this choice. We used a Y-maze with a pivoting 90° bifurcation to study trail choice of Lasius niger foragers at varying branch asymmetries (0°, [both branches 45° from straight ahead], 30° [branches at 30° and 60° from straight ahead], 45°, 60° and 90° [one branch straight ahead, the other at 90°]). The experiment was carried out either with equal amounts of trail pheromone on both branches of the bifurcation or with pheromone present on only one branch. Our results show that with equal pheromone, trail asymmetry has a significant effect on trail choice. Ants preferentially follow the branch deviating least from straight, and this effect increases as asymmetry increases (47% at 0°, 54% at 30°, 57% at 45°, 66% at 60° and 73% at 90°). However, when pheromone is only present on one branch, the graded effect of asymmetry disappears. Overall, however, there is an effect of asymmetry as the preference of ants for the pheromone-marked branch over the unmarked branch is reduced from 65%, when it is the less deviating branch, to 53%, when it is the more deviating branch. These results demonstrate that trail asymmetry influences ant decision-making at bifurcations and that this information interacts with trail pheromone presence in a non-hierarchical manner

Irrational risk aversion in an ant

Animals must often decide between exploiting safe options or risky options with a chance for large gains. Both proximate theories based on perceptual mechanisms, and evolutionary ones based on fitness benefits, have been proposed to explain decisions under risk. Eusocial insects represent a special case of risk sensitivity, as they must often make collective decisions based on resource evaluations from many individuals. Previously, colonies of the ant Lasius niger were found to be risk-neutral, but the risk preference of individual foragers was unknown. Here, we tested individual L. niger in a risk sensitivity paradigm. Ants were trained to associate one scent with 0.55 M sucrose solution and another with an equal chance of either 0.1 or 1.0 M sucrose. Preference was tested in a Y-maze. Ants were extremely risk-averse, with 91% choosing the safe option. Based on the psychophysical Weber–Fechner law, we predicted that ants evaluate resources depending on their logarithmic difference. To test this hypothesis, we designed 4 more experiments by varying the relative differences between the alternatives, making the risky option less, equally or more valuable than the safe one. Our results support the logarithmic origin of risk aversion in ants, and demonstrate that the behaviour of individual foragers can be a very poor predictor of colony-level behaviour

Acute exposure to caffeine improves foraging in an invasive ant

Argentine ants, Linepithema humile, are a particularly concerning invasive species. Control efforts often fall short likely due to a lack of sustained bait consumption. Using neuroactives, such as caffeine, to improve ant learning and navigation could increase recruitment and consumption of toxic baits. Here, we exposed L. humile to a range of caffeine concentrations and a complex ecologically relevant task: an open landscape foraging experiment. Without caffeine, we found no effect of consecutive foraging visits on the time the ants take to reach a reward, suggesting a failure to learn the reward’s location. However, under low to intermediate caffeine concentrations ants were 38% faster with each consecutive visit, implying that caffeine boosts learning. Interestingly, such improvements were lost at high doses. In contrast, caffeine had no impact on the ants’ homing behavior. Adding moderate levels of caffeine to baits could improve ant’s ability to learn its location, improving bait efficacy

Positive and negative incentive contrasts lead to relative value perception in ants

Humans usually assess things not according to their absolute value, but relative to reference points - a main tenant of Prospect Theory. For example, people rate a new salary relative to previous salaries and salaries of their peers, rather than absolute income. We demonstrate a similar effect in an insect: ants expecting to find low-quality food showed higher acceptance of medium-quality food than ants expecting medium quality, and vice versa for high expectations. Further experiments demonstrate that these contrast effects arise from cognitive rather than mere sensory or pre-cognitive perceptual causes. Social information gained inside the nest can also serve as a reference point: the quality of food received from other ants affected the perceived value of food found later. Value judgement is a key element in decision making, and thus relative value perception strongly influences which option is chosen and ultimately how all animals make decisions

Not dear neighbours: Antennation and jerking, but not aggression, correlate with genetic relatedness and spatial distance in the ant Lasius niger

1. Neighbour–stranger response differences (NSRDs) are when individuals are either more aggressive (“Nasty Neighbour”) or less aggressive (“Dear Enemy” or “Dear Neighbour”) to direct neighbours than to other competitors perceived as “strangers” by the residents. Such effects are often reported in ants which, being fixed-location central-place foragers, may compete directly with their neighbours for resources or raid each other for brood. Overlayed onto this are potential spatial distance and relatedness effects on aggression, which are often not differentiated from NSRDs. 2. The literature on NSRDs and distance effects in ants does not reveal a systematic pattern across all ants due to their diversity of life histories, requiring each species to be evaluated individually. Lasius niger is a common Eurasian ant species, which can form very dense populations of colonies and shows pronounced nestmate recognition, so may be expected to show NSRDs. 3. Here, we take advantage of a semi-regular colony array to examine the effect of spatial distance and relatedness on aggression and probe for NSRDs. 4. Overt aggression does not vary with relatedness or spatial distance, and there is no evidence that direct neighbours represent a special case in terms of aggression. However, antennation and jerking decrease between less related and more spatially distant pairs, but are almost completely absent from allospecific interactions. 5. We tentatively propose that antennation and jerking together represent a ‘negotiation’ phase, which may either precede or reduce the need for overt aggression. While a Nasty Neighbour effect might occur, a Dear Neighbour effect is unlikely in this species, and overall NSRDs do not play a large role in the ecology of this species. More broadly, this work highlights the importance of considering non-overtly aggressive responses when studying NSRDs

Copy when uncertain: lower light levels increase trail pheromone deposition and reliance on pheromone trails in ants

Animals may gather information from multiple sources, and these information sources may conflict. Theory predicts that, all else being equal, reliance on a particular information source will depend on its information content relative to other sources. Information conflicts are a good area in which to test such predictions. Social insects, such as ants, make extensive use of both private information (e.g. visual route memories) and social information (e.g. pheromone trails) when attempting to locate a food source. Importantly, eusocial insects collaborate on food retrieval, so both information use and information provision may be expected to vary with the information content of alternative information sources. Many ants, such as Lasius niger, are active both day and night. Variation in light levels represents an ecologically important change in the information content of visually-acquired route information. Here, we examine information use and information provision under high light levels (3200 lux, equivalent to a bright but overcast day), moderate light levels simulating dusk (10 lux) and darkness (0.007 lux, equivalent to a moonless night). Ants learn poorly, or not at all, in darkness. As light levels decrease, ants show decreasing reliance on private visual information, and a stronger reliance on social information, consistent with a ‘copy when uncertain’ strategy. In moderate light levels and darkness, pheromone deposition increases, presumably to compensate for the low information content of visual information. Varying light levels for cathemeral animals provides a powerful and ecologically meaningful method for examining information use and provision under varying levels of information content

Data from: Composite collective decision making

Individual animals are adept at making decisions and have cognitive abilities, such as memory, which allow them to hone their decisions. Social animals can also share information. This allows social animals to make adaptive group-level decisions. Both individual and collective decision-making systems also have drawbacks and limitations, and while both are well studied, the interaction between them is still poorly understood. Here, we study how individual and collective decision-making interact during ant foraging. We first gathered empirical data on memory-based foraging persistence in the ant Lasius niger. We used these data to create an agent-based model where ants may use social information (trail pheromones), private information (memories) or both to make foraging decisions. The combined use of social and private information by individuals results in greater efficiency at the group level than when either information source was used alone. The modelled ants couple consensus decision-making, allowing them to quickly exploit high-quality food sources, and combined decision-making, allowing different individuals to specialize in exploiting different resource patches. Such a composite collective decision-making system reaps the benefits of both its constituent parts. Exploiting such insights into composite collective decision-making may lead to improved decision-making algorithms

Data from: Ants adjust their pheromone deposition to a changing environment and their probability of making errors

Animals must contend with an ever-changing environment. Social animals, especially eusocial insects such as ants and bees, rely heavily on communication for their success. However, in a changing environment communicated information can become rapidly outdated. This is a particular problem for pheromone-trail using ants, as once deposited pheromones cannot be removed. Here we study the response of ant foragers to an environmental change. Ants were trained to one feeder location, and the feeder was then moved to a different location. We found that ants responded to an environmental change by strongly up-regulating pheromone deposition immediately after experiencing the change. This may help maintain the colony’s foraging flexibility, and allow multiple food locations to be exploited simultaneously. Our treatment also caused uncertainty in the foragers, by making their memories less reliable. Ants returning to the nest after eventually finding the food source up-regulated pheromone deposition if they had made a wrong choice. Intriguingly, ants on their way towards the food source down-regulated pheromone deposition if they were going to make an error. This may suggest that individual ants can measure the reliability of their own memories and respond appropriately