Within-colony genetic diversity differentially affects foraging, nest maintenance, and aggression in two species of harvester ants

The ontogeny of selection on genetic diversity in harvester ants

Selection may favour traits throughout an individual's lifetime or at a particular life stage. In many species of social insects, established colonies that are more genetically diverse outperform less diverse colonies with respect to a variety of traits that contribute to fitness, but whether selection favours high diversity in small colonies is unknown. We tested the hypothesis that selection favours genetically diverse colonies during the juvenile period using a multi-year field experiment with the harvester ant, Pogonomyrmex occidentalis. We used controlled matings to generate colonies that varied in genetic diversity and transplanted them into the field. We monitored their survival for seven (the 2015 cohort, n = 149) and six (the 2016 cohort, n = 157) years. Genetically more diverse colonies had greater survival, resulting in significant viability selection. However, in both cohorts survival was not influenced by genetic diversity until colonies were three years old. We suggest that changes in their internal organization enabled colonies to use the benefits of multiple genotypes, and discuss possible mechanisms that can generate this pattern.

Individual Variation Does Not Regulate Foraging Response to Humidity in Harvester Ant Colonies

Differences among groups in collective behavior may arise from responses that all group members share, or instead from differences in the distribution of individuals of particular types. We examined whether the collective regulation of foraging behavior in colonies of the desert red harvester ant (Pogonomyrmex barbatus) depends on individual differences among foragers. Foragers lose water while searching for seeds in hot, dry conditions, so colonies regulate foraging activity in response to humidity. In the summer, foraging activity begins in the early morning when humidity is high, and ends at midday when humidity is low. We investigated whether individual foragers within a colony differ in the decision whether to leave the nest on their next foraging trip as humidity decreases, by tracking the foraging trips of marked individuals. We found that individuals did not differ in response to current humidity. No ants were consistently more likely than others to stop foraging when humidity is low. Each day there is a skewed distribution of trip number: only a few individuals make many trips, but most individuals make few trips. We found that from one day to the next, individual foragers do not show any consistent tendency to make a similar number of trips. These results suggest that the differences among colonies in response to humidity, found in previous work, are due to behavioral responses to current humidity that all workers in a colony share, rather than to the distribution within a colony of foragers that differ in response.

Biology, Ecology, and Management of the Invasive Longlegged Ant, Anoplolepis gracilipes

The longlegged ant (Anoplolepis gracilipes) is one of the most damaging invasive tramp ants globally. It is generally found between latitudes 27°N and 27°S in Asia, although it has been introduced to other continents. Its native range remains debatable, but it is believed to be in Southeast Asia. Anoplolepis gracilipes invasion has many serious ecological consequences, especially for native invertebrate, vertebrate, and plant communities, altering ecosystem dynamics and functions. We examine and synthesize the literature about this species' origin and distribution, impacts on biodiversity and ecosystems, biology and ecology, chemical control, and potential biocontrol agents. We highlight emerging research needs on the origin and invasion history of this species, its reproductive mode, its relationship with myrmecophiles, and its host-microbial interactions, and we discuss future research directions.

Non-spatial information on the presence of food elevates search intensity in ant workers, leading to faster maze solving in a process parallel to spatial learning

Experience can lead to faster exploitation of food patches through spatial learning or other parallel processes. Past studies have indicated that hungry animals either search more intensively for food or learn better how to detect it. However, fewer studies have examined the contribution of non-spatial information on the presence of food nearby to maze solving, as a parallel process to spatial learning. We exposed Cataglyphis niger ant workers to a food reward and then let them search for food in a maze. The information that food existed nearby, even without spatial information, led to faster maze solving compared to a control group that was not exposed to the food prior to the experiment. Faster solving is probably achieved by a higher number of workers entering the maze, following the information that food is present nearby. In a second experiment, we allowed the ants to make successive searches in the maze, followed by removing them after they had returned to the nest and interacted with their naïve nestmates. This procedure led to a maze-solving time in-between that displayed when removing the workers immediately after they had reached the food and preventing their return to the colony, and that of no removal. The workers that interacted upon returning to the nest might have transferred to naïve workers information, unrelated to spatial learning, that food existed nearby, and driven them to commence searching. Spatial learning, or an increase in the correct movements leading to the food reward relative to those leading to dead-ends, was only evident when the same workers were allowed to search again in the same maze. However, both non-spatial information on the presence of food that elevated search intensity and spatial learning led to faster maze solving.

The effect of maze complexity on maze-solving time in a desert ant

One neglected aspect of research on foraging behavior is that of the effect of obstacles that increase habitat complexity on foraging efficiency. Here, we explored how long it takes individually foraging desert ant workers (Cataglyphis niger) to reach a food reward in a maze, and examined whether maze complexity affects maze-solving time (the time elapsed till the first worker reached the food reward). The test mazes differed in their complexity level, or the relative number of correct paths leading to the food reward, vs. wrong paths leading to dead-ends. Maze-solving time steeply increased with maze complexity, but was unaffected by colony size, despite the positive correlation between colony size and the number of workers that searched for food. The number of workers observed feeding on the food reward 10 min after its discovery decreased with complexity level but not colony size. We compared our experimental results to three simulation models, applying different search methods, ranked them according to their fit to the data and found the self-avoiding random search to fit the best. We suggest possible reasons for the model deviations from the observational findings. Our data emphasize the necessity to refer to habitat complexity when studying foraging behavior.

Comparing ant behaviour indices for fine-scale analyses

Animal behaviour often is characterised by standardised assays. In social insects such as ants, behaviour assays are for example used to characterise aggressive and peaceful behaviour. Such assays differ in the number of individuals, the duration and place of assays, and the scoring scales. Also the behaviour indices used to summarise the results differ. Here, we compared five behaviour indices (Aggression Index, Mean Maximum Aggression Index; and the newly introduced Mean Maximum Peace Index, Mean Behaviour Index aggressive, and Mean Behaviour Index peaceful) using a scoring scale that comprises peaceful and aggressive behaviour. The indices were applied on eight simulations and three observed data sets. The five indices were correlated but frequently differed in their means. Multiple indices were needed to capture the complete behaviour range. Furthermore, subtle differences in workers' behaviour, that is, differences that go beyond the presence/absence of aggression, were only identified when considering multiple indices. We infer that the indices applied are differently suited for different analyses. Fine-scale analyses of behavioural variation profit from using more than one index. The particular choice of index or indices likely influences the interpretation of behaviour and should be carefully done in the light of study species and research question.